Patent Publication Number: US-2023157597-A1

Title: Transcutaneous analyte sensor systems and methods

Description:
INCORPORATION BY REFERENCE TO RELATED APPLICATIONS 
     Any and all priority claims identified in the Application Data Sheet, or any correction thereto, are hereby incorporated by reference under 37 CFR 1.57. This application is a continuation of U.S. application Ser. No. 15/387,088, filed Dec. 21, 2016, which, in turn, claims the benefit of U.S. Provisional Application No. 62/272,983, filed Dec. 30, 2015 and U.S. Provisional Application No. 62/412,100, filed Oct. 24, 2016. Each of the aforementioned applications is incorporated by reference herein in its entirety, and each is hereby expressly made a part of this specification. 
    
    
     FIELD 
     Various embodiments disclosed herein relate to measuring an analyte in a person. Certain embodiments relate to systems and methods for applying a transcutaneous analyte measurement system to a person. 
     BACKGROUND 
     Diabetes mellitus is a disorder in which the pancreas cannot create sufficient insulin (Type I or insulin dependent) and/or in which insulin is not effective (Type 2 or non-insulin dependent). In the diabetic state, the victim suffers from high blood sugar, which can cause an array of physiological derangements associated with the deterioration of small blood vessels, for example, kidney failure, skin ulcers, or bleeding into the vitreous of the eye. A hypoglycemic reaction (low blood sugar) can be induced by an inadvertent overdose of insulin, or after a normal dose of insulin or glucose-lowering agent accompanied by extraordinary exercise or insufficient food intake. 
     Conventionally, a person with diabetes carries a self-monitoring blood glucose monitor, which typically requires uncomfortable finger pricking methods. Due to the lack of comfort and convenience, a person with diabetes normally only measures his or her glucose levels two to four times per day. Unfortunately, such time intervals are so far spread apart that the person with diabetes likely finds out too late of a hyperglycemic or hypoglycemic condition, sometimes incurring dangerous side effects. Glucose levels may be alternatively monitored continuously by a sensor system including an on-skin sensor assembly. The sensor system may have a wireless transmitter which transmits measurement data to a receiver which can process and display formation based on the measurements. 
     The process of applying the sensor to the person is important for such a system to be effective and user friendly. The application process can result in the sensor assembly being attached to the person in a state where it is capable of sensing glucose level information, communicating the glucose level information to the transmitter, and transmitting the glucose level information to the receiver. 
     The analyte sensor can be placed into subcutaneous tissue. A user can actuate an applicator to insert the analyte sensor into its functional location. This transcutaneous insertion can lead to incomplete sensor insertion, improper sensor insertion, exposed needles, or unnecessary pain. Thus, there is a need for a system that more reliably enables transcutaneous sensor insertion while being easy to use and relatively pain-free. 
     SUMMARY 
     Various systems and methods described herein enable reliable, simple, and pain-minimizing transcutaneous insertion of analyte sensors. Some embodiments are a system for applying an on-skin sensor assembly to skin of a host. Systems can comprise a telescoping assembly having a first portion configured to move distally relative to a second portion from a proximal starting position to a distal position along a path; a sensor module coupled to the first portion, the sensor module including a sensor, electrical contacts, and a seal; and/or a base coupled to the second portion such that the base protrudes from a distal end of the system. The base can comprise an adhesive configured to couple the sensor module to the skin. The moving of the first portion to the distal position can couple the sensor module to the base. The sensor can be an analyte sensor; a glucose sensor; any sensor described herein or incorporated by reference; and/or any other suitable sensor. 
     In some embodiments (i.e., optional and independently combinable with any of the aspects and embodiments identified herein), the sensor module can include a sensor module housing. The sensor module housing can include a first flex arm. 
     In some embodiments (i.e., optional and independently combinable with any of the aspects and embodiments identified herein), the sensor can be located within the second portion while the base protrudes from the distal end of the system such that the system is configured to couple the sensor to the base via moving the first portion distally relative to the second portion. 
     In several embodiments (i.e., optional and independently combinable with any of the aspects and embodiments identified herein), the sensor can be coupled to the sensor module while the first portion is located in the proximal starting position. 
     In some embodiments, a needle is coupled to the first portion (of the telescoping assembly) such that the sensor and the needle move distally relative to the base and relative to the second portion. The system can comprise a needle release mechanism configured to retract the needle proximally. 
     In several embodiments, the base comprises a distal protrusion having a first hole. The distal protrusion can be configured to reduce a resistance of the skin to piercing. The sensor can pass through the first hole of the distal protrusion. 
     In some embodiments, a needle having a slot passes through the first hole of the distal protrusion. A portion of the sensor can be located in the slot such that the needle is configured to move distally relative to the base without dislodging the portion of the sensor from the slot. 
     In several embodiments, the distal protrusion is convex such that the distal protrusion is configured to tension the skin while the first portion moves distally relative to the second portion to prepare the skin for piercing. The distal protrusion can be shaped like a dome. 
     In some embodiments, the adhesive comprises a second hole. The distal protrusion can be located at least partially within the second hole such that the distal protrusion can tension at least a portion of the skin beneath the second hole. 
     In several embodiments, the adhesive covers at least a majority of the distal protrusion. The adhesive can cover at zero percent, at least 30 percent, at least 70 percent, and/or less than 80 percent of the distal protrusion. The distal protrusion can protrude at least 0.5 millimeters, less than 3 millimeters, and/or less than 5 millimeters. 
     In some embodiments, a sensor module is coupled to the first portion and is located at least 3 millimeters and/or at least 5 millimeters from the base while the first portion is in the proximal starting position. The system can be configured such that moving the first portion to the distal position couples the sensor module to the base. 
     In several embodiments, the sensor is already coupled to the sensor module while the first portion is located in the proximal starting position. For example, the sensor can be coupled to the sensor module at the factory (e.g., prior to the user opening a sterile barrier). The sensor can be located within the second portion while the base protrudes from the distal end of the system. 
     In some embodiments, the sensor is coupled to a sensor module. During a first portion of the path, the sensor module can be immobile relative to the first portion, and the base can be immobile relative to the second portion. During a second portion of the path, the system can be configured to move the first portion distally relative to the second portion to move the sensor module towards the base, couple the sensor module to the base, and/or enable the coupled sensor module and the base to detach from the telescoping assembly. 
     In several embodiments, a sensor module is coupled to the sensor. The system comprises a vertical central axis oriented from a proximal end to the distal end of the system. The sensor module can comprise a first flex arm that is oriented horizontally and is coupled to the base. The first flex arm can extend from an outer perimeter of the sensor module. 
     In some embodiments, the base comprises a first proximal protrusion coupled to the first flex arm to couple the sensor module to the base. A first horizontal locking protrusion can be coupled to an end portion of the first flexible arm. A second horizontal locking protrusion can be coupled to the first proximal protrusion of the base. The first horizontal locking protrusion can be located distally under the second horizontal locking protrusion to secure the sensor module to the base. The system can be configured such that moving the first portion of the telescoping assembly to the distal position causes the first flex arm to bend to enable the first horizontal locking protrusion to move distally relative to the second horizontal locking protrusion. 
     In several embodiments, the base comprises a second proximal protrusion coupled to a second flex arm of the sensor module. The first flex arm can be located on an opposite side of the sensor module relative to the second flex arm. 
     In some embodiments, a sensor module is coupled to the sensor. The system can comprise a vertical central axis oriented from a proximal end to the distal end of the system. The base can comprise a first flex arm that is oriented horizontally and is coupled to the sensor module. The sensor module can comprise a first distal protrusion coupled to the first flex arm to couple the sensor module to the base. 
     In several embodiments, a first horizontal locking protrusion is coupled to an end portion of the first flexible arm, a second horizontal locking protrusion is coupled to the first distal protrusion of the sensor module, and the second horizontal locking protrusion is located distally under the first horizontal locking protrusion to secure the sensor module to the base. The system can be configured such that moving the first portion of the telescoping assembly to the distal position causes the first flex arm to bend to enable the second horizontal locking protrusion to move distally relative to the first horizontal locking protrusion. 
     In some embodiments, the sensor module comprises a second distal protrusion coupled to a second flex arm of the base. The first distal protrusion can be located on an opposite side of the sensor module relative to the second distal protrusion. 
     In several embodiments, a sensor module is coupled to the sensor. The first portion can comprise a first flex arm and a second flex arm that protrude distally and latch onto the sensor module to releasably secure the sensor module to the first portion while the first portion is in the proximal starting position. The sensor module can be located remotely from the base while the first portion is in the proximal starting position (e.g., such that the sensor module does not touch the base). 
     In some embodiments, the sensor module is located within the second portion while the base protrudes from the distal end of the system such that the system is configured to couple the sensor module to the base via moving the first portion distally relative to the second portion. 
     In several embodiments, the system comprises a vertical central axis oriented from a proximal end to the distal end of the system. The first and second flex arms of the first portion can secure the sensor module to the first portion such that the sensor module is releasably coupled to the first portion with a first vertical holding strength. The sensor module can comprise a third flex arm coupled with a first proximal protrusion of the base such that the sensor module is coupled to the base with a second vertical holding strength. 
     In some embodiments, the second vertical holding strength is greater than the first vertical holding strength such that continuing to push the first portion distally once the sensor module is coupled to the base overcomes the first and second flex arms of the first portion to detach the sensor module from the first portion. The third flex arm can extend from an outer perimeter of the sensor module. 
     In several embodiments, the base protrudes from the distal end of the system while the first portion of the telescoping assembly is located in the proximal starting position and the sensor is located remotely relative to the base such that the system is configured to couple the sensor to the base via moving the first portion distally relative to the second portion. The base can comprise a first radial protrusion releasably coupled with a first vertical holding strength to a second radial protrusion of the second portion of the telescoping assembly. 
     In some embodiments, the first radial protrusion protrudes inward and the second radial protrusion protrudes outward. The system can be configured such that moving the first portion to the distal position moves the second radial protrusion relative to the first radial protrusion to detach the base from the telescoping assembly. 
     In several embodiments, the first portion of the telescoping assembly comprises a first arm that protrudes distally, the second portion of the telescoping assembly comprises a second flex arm that protrudes distally, and the system is configured such that moving the first portion from the proximal starting position to the distal position along the path causes the first arm to deflect the second flex arm and thereby detach the second flex arm from the base to enable the base to decouple from the telescoping assembly. When the first portion is in the proximal starting position, the first arm of the first portion can be at least partially vertically aligned with the second flex arm of the second portion to enable the first arm to deflect the second flex arm as the first portion is moved to the distal position. 
     In some embodiments, when the first portion is in the proximal starting position, at least a section of the first arm is located directly over the second flex arm to enable the first arm to deflect the second flex arm as the first portion is moved to the distal position. 
     In several embodiments, the second flex arm comprises a first horizontal protrusion, and the base comprises a second horizontal protrusion latched with the first horizontal protrusion to couple the base to the second portion of the telescoping assembly. The first arm of the first portion can deflect the second flex arm of the second portion to unlatch the base from the second portion of the telescoping assembly. 
     In some embodiments, the system is configured to couple the sensor to the base at a first position, and the system is configured to detach the base from the telescoping assembly at a second position that is distal relative to the first position. 
     In several embodiments, a third flex arm couples the sensor to the base at a first position, the second flex arm detaches from the base at a second position, and the second position is distal relative to the first position such that the system is configured to secure the base to the telescoping assembly until after the sensor is secured to the base. 
     In some embodiments, the base protrudes from the distal end of the system while the first portion of the telescoping assembly is located in the proximal starting position and the sensor is located remotely relative to the base. The system can further comprise a spring configured to retract a needle. The needle can be configured to facilitate inserting the sensor into the skin. When the first portion is in the proximal starting position, the spring can be in a first compressed state. The system can be configured such that moving the first portion distally from the proximal starting position further increases a compression of the spring. The first compressed state places the first and second portions in tension. 
     In several embodiments, a system is configured to apply an on-skin sensor assembly to the skin of a host (i.e., a person). The system can include a telescoping assembly having a first portion configured to move distally relative to a second portion from a proximal starting position to a distal position along a path; a sensor coupled to the first portion; and/or a latch configurable to impede a needle from moving proximally relative to the first portion. The sensor can be an analyte sensor; a glucose sensor; any sensor described herein or incorporated by reference; and/or any other suitable sensor. 
     In some embodiments, the first portion is releasably secured in the proximal starting position by a securing mechanism that impedes moving the first portion distally relative to the second portion. The system can be configured such that prior to reaching the distal position, moving the first portion distally relative to the second portion releases the latch thereby causing the needle to retract proximally into the system. The system can be configured such that moving the first portion distally relative to the second portion (e.g., moving the first portion to the distal position) releases the latch thereby causing the needle to retract proximally into the system. The securing mechanism can be an interference between the first portion and the second portion of the telescoping assembly. 
     In several embodiments, a first force profile is measured along the path. The first force profile can comprise a first magnitude coinciding with overcoming the securing mechanism; a third magnitude coinciding with releasing the latch; and a second magnitude coinciding with an intermediate portion of the path that is distal relative to overcoming the securing mechanism and proximal relative to releasing the latch. 
     In some embodiments, the second magnitude is less than the first and third magnitudes such that the system is configured to promote needle acceleration during the intermediate portion of the path to enable a suitable needle speed (e.g., a sufficiently high needle speed) at a time the needle first pierces the skin. 
     In several embodiments, the first magnitude is at least 100 percent greater than the second magnitude. The first magnitude can be greater than the third magnitude such that the system is configured to impede initiating a sensor insertion cycle unless a user is applying enough force to release the latch. The first magnitude can be at least 50 percent greater than the third magnitude. 
     In some embodiments, an intermediate portion of the path is distal relative to overcoming the securing mechanism and proximal relative to releasing the latch. The system can further comprise a second force profile coinciding with the intermediate portion of the path. A proximal millimeter of the second force profile can comprise a lower average force than a distal millimeter of the second force profile in response to compressing a spring configured to enable the system to retract the needle into the telescoping assembly. 
     In several embodiments, a first force profile is measured along the path. The first force profile can comprise a first average magnitude coinciding with moving distally past a proximal half of the securing mechanism and a second average magnitude coinciding with moving distally past a distal half of the securing mechanism. The first average magnitude can be greater than the second average magnitude such that the system is configured to impede initiating a sensor insertion cycle unless a user is applying enough force to complete the sensor insertion cycle (e.g., drive the needle and/or the sensor to the intended insertion depth). 
     In some embodiments, a first force peak (coinciding with moving distally past the proximal half of the securing mechanism) is at least 25 percent higher than the second average magnitude. 
     In several embodiments, a first force profile is measured along the path. The first force profile can comprise a first magnitude coinciding with overcoming the securing mechanism and a subsequent magnitude coinciding with terminating the securing mechanism. The first magnitude can comprise a proximal vector and the subsequent magnitude can comprise a distal vector. 
     In some embodiments, the securing mechanism can comprise a radially outward protrusion extending from the first portion. The radially outward protrusion can be located proximally relative to a proximal end of the second portion while the telescoping assembly is in the proximal starting position. The radially outward protrusion can be configured to cause the second portion to deform elliptically to enable the first portion to move distally relative to the second portion. 
     In several embodiments, the securing mechanism comprises a radially outward protrusion of the first portion that interferes with a radially inward protrusion of the second portion such that the securing mechanism is configured to cause the second portion to deform elliptically to enable the first portion to move distally relative to the second portion. 
     In some embodiments, the needle is retractably coupled to the first portion by a needle holder configured to resist distal movement of the first portion relative to the second portion. The securing mechanism can comprise a flexible arm of the second portion. The flexible arm can be releasably coupled to the needle holder to releasably secure the first portion to the second portion in the proximal starting position. 
     In several embodiments, the securing mechanism comprises a frangible coupling between the first portion and the second portion while the first portion is in the proximal starting position. The system can be configured such that moving the first portion to the distal position breaks the frangible coupling. 
     In some embodiments, the securing mechanism comprises a magnet that releasably couples the first portion to the second portion while the first portion is in the proximal starting position. The magnet can be attracted to a metal element coupled to the first portion or the second portion of the telescoping assembly. 
     In several embodiments, an electric motor drives the first portion distally relative to the second portion. The electric motor can be configured to move the needle in the skin. 
     In some embodiments, an on-skin sensor system is configured for transcutaneous glucose monitoring of a host. The system can comprise a sensor module housing, in which the sensor module housing can include a first flex arm; a sensor having a first section configured for subcutaneous sensing and a second section mechanically coupled to the sensor module housing; an electrical interconnect mechanically coupled to the sensor module housing and electrically coupled to the sensor; and/or a base coupled to the first flex arm of the sensor module housing. The base can have an adhesive configured to couple the base to the skin of the host. The sensor can be an analyte sensor; a glucose sensor; any sensor described herein or incorporated by reference; and/or any other suitable sensor. 
     In several embodiments, the electrical interconnect comprises a spring. The spring can comprise a conical portion and/or a helical portion. 
     In some embodiments, the sensor module housing comprises at least two proximal protrusions located around a perimeter of the spring. The proximal protrusions can be configured to help orient the spring. A segment of the sensor can be located between the proximal protrusions. 
     In several embodiments, the sensor module housing is mechanically coupled to a base having an adhesive configured to couple the base to skin of the host. 
     In some embodiments, the proximal protrusions orient the spring such that coupling an electronics unit to the base presses the spring against a first electrical contact of the electronics unit and a second electrical contact of the sensor to electrically couple the sensor to the electronics unit. 
     In several embodiments, the sensor module housing comprises a first flex arm that is oriented horizontally and is coupled to the base. The first flex arm can extend from an outer perimeter of the sensor module housing. The base can comprise a first proximal protrusion coupled to the first flex arm to couple the sensor module housing to the base. 
     In some embodiments, the electrical interconnect comprises a leaf spring, which can include one metal layer or multiple metal layers. The leaf spring can be a cantilever spring. 
     In some embodiments, the sensor module housing comprises a proximal protrusion having a channel in which at least a portion of the second section of the sensor is located. The channel can position a first area of the sensor such that the first area is electrically coupled to the leaf spring. 
     In some embodiments, the leaf spring arcs away from the first area and protrudes proximally to electrically couple with an electronics unit. At least a portion of the leaf spring can form a “W” shape. At least a portion of the leaf spring forms a “C” shape. 
     In several embodiments, the leaf spring bends around the proximal protrusion. The leaf spring can bend at least 120 degrees and/or at least 160 degrees around the proximal protrusion. The leaf spring can protrude proximally to electrically couple with an electronics unit. 
     In some embodiments, a seal is configured to impede fluid ingress to the leaf spring. The sensor module housing can be mechanically coupled to a base. The base can have an adhesive configured to couple the base to skin of the host. 
     In several embodiments, the leaf spring is oriented such that coupling an electronics unit to the base presses the leaf spring against a first electrical contact of the electronics unit and against a second electrical contact of the sensor to electrically couple the sensor to the electronics unit. A proximal height of the seal can be greater than a proximal height of the leaf spring such that the electronics unit contacts the seal prior to contacting the leaf spring. 
     In some embodiments, the sensor module housing comprises a first flex arm that is oriented horizontally and is coupled to the base. The first flex arm can extend from an outer perimeter of the sensor module housing. The base can comprise a first proximal protrusion coupled to the first flex arm to couple the sensor module housing to the base. 
     In several embodiments, the sensor module housing comprises a channel in which at least a portion of the second section of the sensor is located. A distal portion of the leaf spring can be located in the channel such that a proximal portion of the leaf spring protrudes proximally out the channel. The sensor module housing can comprise a groove that intersects the channel. The leaf spring can comprise a tab located in the groove to impede rotation of the leaf spring. 
     In some embodiments, the sensor module housing is mechanically coupled to a base that has an adhesive configured to couple the base to skin of the host. The sensor module housing can comprise a first flex arm that is oriented horizontally and is coupled to the base. The first flex arm can extend from an outer perimeter of the sensor module housing. The base can comprises a first proximal protrusion coupled to the first flex arm to couple the sensor module housing to the base. 
     In several embodiments, electrical interconnects (such as springs or other types of interconnects) comprises a resistance of less than 100 ohms and/or less than 5 ohms. Electrical interconnects can comprise a compression force of less than one pound over an active compression range. 
     In some embodiments, electrical interconnects may require a compression force of less than one pound to compress the spring 20 percent from a relaxed position, which is a substantially uncompressed position. In some embodiments, electrical interconnects may require a compression force of less than one pound to compress the spring 25 percent from a relaxed position, which is a substantially uncompressed position. In some embodiments, electrical interconnects may require a compression force of less than one pound to compress the spring 30 percent from a relaxed position, which is a substantially uncompressed position. In some embodiments, electrical interconnects may require a compression force of less than one pound to compress the spring 50 percent from a relaxed position, which is a substantially uncompressed position. 
     In several embodiments, the spring is configured such that compressing the spring 25 percent from a relaxed position requires a force of at least 0.05 pounds and less than 0.5 pounds, and requires moving an end of the spring at least 0.1 millimeter and less than 1.1 millimeter. 
     In some embodiments, a system for applying an on-skin sensor assembly to a skin of a host comprises a telescoping assembly having a first portion configured to move distally relative to a second portion from a proximal starting position to a distal position along a path; a sensor coupled to the first portion; and a base comprising adhesive configured to couple the sensor to the skin. The telescoping assembly can further comprise a third portion configured to move distally relative to the second portion. 
     In some embodiments, a first spring is positioned between the third portion and the second portion such that moving the third portion distally relative to the second portion compresses the first spring. In the proximal starting position of the telescoping assembly, the first portion can be locked to the second portion. The system can be configured such that moving the third portion distally relative to the second portion unlocks the first portion from the second portion. 
     In several embodiments, a first proximal protrusion having a first hook passes through a first hole in the second portion to lock the first portion to the second portion. The third portion can comprise a first distal protrusion. The system can be configured such that moving the third portion distally relative to the second portion engages a ramp to bend the first proximal protrusion to unlock the first portion from the second portion. 
     In some embodiments, the sensor is located within the second portion while the base protrudes from the distal end of the system such that the system is configured to couple the sensor to the base by moving the first portion distally relative to the second portion. 
     In several embodiments, a sensor module is coupled to a distal portion of the first portion such that moving the first portion to the distal position couples the sensor module to the base. The sensor can be coupled to the sensor module while the first portion is located in the proximal starting position. 
     In some embodiments, the system is configured such that moving the third portion distally relative to the second portion unlocks the first portion from the second portion and locks the third portion to the second portion. 
     In several embodiments, the system comprises a first protrusion that couples with a hole of at least one of the second portion and the third portion to lock the third portion to the second portion. 
     In some embodiments, the system comprises a second protrusion that couples with a hole of at least one of the first portion and the second portion to lock the first portion to the second portion in response to moving the first portion distally relative to the second portion. 
     In several embodiments, a first spring is positioned between the third portion and the second portion such that moving the third portion distally relative to the second portion compresses the first spring and unlocks the first portion from the second portion, which enables the compressed first spring to push the first portion distally relative to the second portion, which pushes at least a portion of the sensor out of the distal end of the system and triggers a needle retraction mechanism to enable a second spring to retract a needle. 
     In some embodiments, a system for applying an on-skin assembly to a skin of a host is provided. Advantageously, the system includes a sensor inserter assembly having a needle assembly, a sensor module, a base, an actuation member, and a retraction member, the sensor inserter assembly having an initial configuration in which at least the sensor module is disposed in a proximal starting position, the sensor inserter assembly further having a deployed configuration in which at least the sensor module and the base are disposed at a distal applied position. Preferably, the actuation member is configured to, once activated, cause the needle assembly to move a proximal starting position to a distal insertion position, and the retraction member is configured to, once activated, cause the needle assembly to move from the distal insertion position to a proximal retracted position. 
     The sensor module may comprise a sensor and a plurality of electrical contacts. In the initial configuration, the sensor can be electrically coupled to at least one of the electrical contacts. Optionally, in the initial configuration, the actuation member is in an unenergized state. In some embodiments, the actuation member can be configured to be energized by a user before being activated. In alternative embodiments, in the initial configuration, the actuation member is in an energized state. 
     In several embodiments the actuation member can include a spring. In an initial configuration, the spring can be in an unstressed state. In alternative embodiments, in the initial configuration, the spring is in a compressed state. 
     In some embodiments, the sensor inserter assembly may include a first portion and a second portion, the first portion being fixed, at least in an axial direction, with respect to the second portion at least when the sensor inserter assembly is in the initial configuration, the first portion being movable in at least a distal direction with respect to the second portion after activation of the actuation member. The first portion may be operatively coupled to the needle assembly so as to secure the needle assembly in the proximal starting position before activation of the actuation member and to urge the needle assembly toward the distal insertion position after activation of the actuation member. 
     In several embodiments, the retraction member is in an unenergized state when in the initial configuration. Advantageously, the retraction member is configured to be energized by the movement of the needle assembly from the proximal starting position to the distal insertion position. In the initial configuration, the retraction member may be in an energized state. 
     In still other embodiments, the retraction member comprises a spring. The spring may be integrally formed with the needle assembly. The spring may be operatively coupled to the needle assembly. In the initial configuration, the spring may be in an unstressed state. In other embodiments, in the initial configuration, the spring is in compression. 
     In some aspects, in the second configuration, the spring is in compression. In still other embodiments, in the second configuration, the spring is in tension. 
     In some embodiments, the sensor inserter assembly can further include a third portion, the third portion being operatively coupled to the first portion. The actuation member may be integrally formed with the third portion in certain embodiments. Optionally, the actuation member is operatively coupled to the third portion. 
     In some embodiments, the sensor inserter assembly includes interengaging structures configured to prevent movement of the first portion in the distal direction relative to the second portion until the interengaging structures are decoupled. Advantageously, the decoupling of the interengaging structures may activate the actuation member. In other embodiments, the interengaging structures may include a proximally extending tab of the first portion and a receptacle of the second portion configured to receive the proximally extending tab. Optionally, the sensor inserter assembly can include a decoupling member configured to decouple the interengaging structures. The decoupling member may have a distally extending tab of the third portion. 
     In yet other embodiments, the sensor inserter assembly can include interengaging structures configured to prevent proximal movement of the third portion with respect to the first portion. These interengaging structures may include a distally-extending latch of the third portion and a ledge of the first portion configured to engage the distally-extending latch. 
     In certain embodiments, the sensor inserter assembly can include interengaging structures configured to prevent proximal movement of the needle assembly at least when the needle assembly is in the distal insertion position. The interengaging structures can have radially-extending release features of the needle assembly and an inner surface of the first portion configured to compress the release features. Optionally, the sensor inserter assembly includes a decoupling member configured to disengage the interengaging structures of the first portion and the needle assembly. The decoupling member may include an inner surface of the second portion configured to further compress the release features. Advantageously, the system may further include a trigger member configured to activate the actuation member. The trigger member may be operatively coupled to the third portion. The trigger member may be integrally formed with the third portion. The trigger member may include a proximally-extending button. Alternatively, the trigger member may include a radially-extending button. The trigger member may be configured to decouple the interengaging structure of the first portion and the third portion. 
     In some embodiments, the system may further include a releasable locking member configured to prevent activation of the actuation member until the locking member is released. The releasable locking member may be configured to prevent proximal movement of the third portion with respect to the first portion until the locking member is released. The releasable locking member may include a proximally-extending tab of the first portion and a latch feature of the third portion configured to receive the proximally-extending tab. Advantageously, the releasable locking member is configured to prevent energizing of the sensor inserter assembly. In other aspects, the releasable locking member is configured to prevent energizing of the actuation member. 
     Embodiments may further include a system for applying an on-skin component to a skin of a host, the system may include a sensor inserter assembly having an on-skin component being movable in at least a distal direction from a proximal position to a distal position, a first securing feature configured to releasably secure the on-skin component in the proximal position, a second securing feature configured to secure the on-skin component in the distal position, and a first resistance configured to prevent movement of the on-skin component in a proximal direction at least when the on-skin component is in the distal position. 
     The first resistance feature can be configured to prevent movement of the on-skin component in a proximal direction when the on-skin component is secured in the distal position. In some embodiments, the first securing feature is configured to releasably secure the on-skin component to a needle assembly. The on-skin component may have a sensor module. The sensor module may include a sensor and a plurality of electrical contacts. Optionally, the sensor is electrically coupled to at least one of the electrical contacts, at least when the sensor inserter assembly is in the first configuration. 
     In some embodiments, the on-skin component comprises a base. The on-skin component may include a transmitter. The second securing feature can be configured to secure the on-skin component to a second on-skin component. 
     In other embodiments, the sensor inserter assembly includes at least one distally-extending leg, and wherein the first securing feature comprises an adhesive disposed on a distally-facing surface of the leg. The sensor inserter assembly may include at least one distally-extending member, and wherein the first securing feature comprises a surface of the distally-extending member configured to frictionally engage with a corresponding structure of the on-skin component. The corresponding structure of the on-skin component may include an elastomeric member. Optionally, the distally-extending member includes at least one leg of the sensor inserter assembly. The distally-extending member may include a needle. 
     In some embodiments of the system, the second securing feature includes an adhesive disposed on a distally-facing surface of the on-skin component. The second securing feature may have an elastomeric member configured to receive the on-skin component. 
     In other embodiments, the first resistance feature includes a distally-facing surface of the sensor inserter assembly. The first resistance feature may be distal to an adhesive disposed on a distally-facing surface of the on-skin component. 
     The system may further include a pusher configured to move the on-skin component from the proximal position to the distal position. Optionally, the system can further include a decoupling feature configured to decouple the pusher from the on-skin component at least after the on-skin component is in the distal position. The decoupling feature may have a frangible portion of the pusher. Optionally, the decoupling feature comprises a frangible portion of the on-skin component. 
     The system may further comprise a sensor assembly configured to couple with the on-skin component, wherein a third securing feature is configured to releasably secure the sensor assembly in a proximal position, and wherein a fourth securing feature is configured to secure the sensor assembly to the on-skin component. 
     Any of the features of each embodiment is applicable to all aspects and embodiments identified herein. Moreover, any of the features of an embodiment is independently combinable, partly or wholly with other embodiments described herein in any way, e.g., one, two, or three or more embodiments may be combinable in whole or in part. Further, any of the features of an embodiment may be made optional to other aspects or embodiments. Any aspect or embodiment of a method can be performed by a system or apparatus of another aspect or embodiment, and any aspect or embodiment of a system can be configured to perform a method of another aspect or embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages are described below with reference to the drawings, which are intended to illustrate, but not to limit, the invention. In the drawings, like reference characters denote corresponding features consistently throughout similar embodiments. 
         FIG.  1    illustrates a schematic view of a continuous analyte sensor system, according to some embodiments. 
         FIG.  2    illustrates a perspective view of an applicator system, according to some embodiments. 
         FIG.  3    illustrates a cross-sectional side view of the system from  FIG.  2   , according to some embodiments. 
         FIG.  4    illustrates a perspective view of an on-skin sensor assembly, according to some embodiments. 
         FIGS.  5  and  6    illustrate perspective views of a transmitter coupled to a base via mechanical interlocks, according to some embodiments. 
         FIGS.  7 - 11    illustrate cross-sectional side views of the applicator system from  FIG.  3   , according to some embodiments. 
         FIG.  12 A  illustrates a cross-sectional side view of a portion of the applicator system from  FIG.  3   , according to some embodiments. 
         FIG.  12 B  illustrates a cross-sectional side view of a base that can be used with the applicator system shown in  FIG.  3   , according to some embodiments. 
         FIG.  13    illustrates a perspective view of a portion of the adhesive from  FIG.  4   , according to some embodiments. 
         FIG.  14    illustrates a perspective view of a portion of the applicator system from  FIG.  3   , according to some embodiments. 
         FIGS.  15  and  16    illustrate perspective views of cross sections of portions of the system shown in  FIG.  7   , according to some embodiments. 
         FIG.  17    illustrates a cross-sectional view of the first portion of the telescoping assembly from  FIG.  7   , according to some embodiments. 
         FIGS.  18  and  19    illustrate perspective views of portions of the applicator system from  FIG.  7   , according to some embodiments. 
         FIGS.  20  and  21    illustrate perspective views of the needle after being removed from the telescoping assembly of  FIG.  7   , according to some embodiments. 
         FIG.  22    illustrates a perspective view of a cover of the telescoping assembly of  FIG.  7   , according to some embodiments. 
         FIG.  23    illustrates a schematic view of force profiles, according to some embodiments. 
         FIG.  24    illustrates a cross-sectional side view of a portion of an applicator system, according to some embodiments. 
         FIG.  25    illustrates a cross-sectional side view of a portion of a securing mechanism, according to some embodiments. 
         FIG.  26    illustrates a top view of a ring, according to some embodiments. 
         FIG.  27    illustrates a perspective view of a securing mechanism, according to some embodiments. 
         FIG.  28    illustrates a cross-sectional perspective view of telescoping assembly with a motor, according to some embodiments. 
         FIGS.  29  and  30    illustrate cross-sectional side views of telescoping assemblies with a motor, according to some embodiments. 
         FIG.  31    illustrates a side view of a telescoping assembly that causes rotational movement, according to some embodiments. 
         FIG.  32    illustrates a cross-sectional perspective view of a telescoping assembly with a downward locking feature, according to some embodiments. 
         FIG.  33    illustrates a perspective view of an on-skin senor assembly just before the electronics unit is coupled to the base, according to some embodiments. 
         FIGS.  34  and  35    illustrate perspective views of sensor modules that have springs, according to some embodiments. 
         FIG.  36    illustrates a cross-sectional perspective view of a portion of a sensor module, according to some embodiments. 
         FIG.  37    illustrates a perspective view of a sensor module that has springs, according to some embodiments. 
         FIG.  38    illustrates a cross-sectional perspective view of a portion of a sensor module, according to some embodiments. 
         FIG.  39    illustrates a perspective view of a sensor module, according to some embodiments. 
         FIG.  40    illustrates a cross-sectional perspective view of assembly that has an offset, according to some embodiments. 
         FIG.  41    illustrates a side view of a sensor, according to some embodiments. 
         FIG.  42    illustrates a bottom view of a needle, according to some embodiments. 
         FIG.  43    illustrates a front view of a needle, according to some embodiments. 
         FIG.  44    illustrates a cross-sectional perspective view of an applicator system, according to some embodiments. 
         FIG.  45    illustrates a cross-sectional perspective view of a portion of an applicator system, according to some embodiments. 
         FIG.  46    illustrates a perspective view of a portion of an applicator system, according to some embodiments. 
         FIG.  47    illustrates a perspective view of a sensor module, according to some embodiments. 
         FIG.  48    illustrates a cross-sectional perspective view of an applicator system, according to some embodiments. 
         FIG.  49    illustrates a cross-sectional perspective view of a proximal portion of a telescoping assembly, according to some embodiments. 
         FIG.  50    illustrates a perspective view of a distal portion of a telescoping assembly, according to some embodiments. 
         FIG.  51    illustrates a perspective view of a needle with adhesive, according to some embodiments. 
         FIG.  52    illustrates a perspective view of a needle that has two separate sides, according to some embodiments. 
         FIG.  53    illustrates a cross-sectional top view of the needle shown in  FIG.  52   , according to some embodiments. 
         FIG.  54    illustrates a perspective view of a needle that has a ramp, according to some embodiments. 
         FIG.  55    illustrates a cross-sectional top view of four needles, according to some embodiments. 
         FIGS.  56 - 58    illustrate cross-sectional side views of a system that is similar to the embodiment shown in  FIG.  7    except that the system does not include a needle, according to some embodiments. 
         FIG.  59    illustrates a cross-sectional side view of a system that is similar to the embodiment shown in  FIG.  7    except for the starting position and the movement of the base, according to some embodiments. 
         FIG.  60    illustrates a perspective view of a system having a cover, according to some embodiments. 
         FIGS.  61 - 63    illustrate cross-sectional perspectives views of a system that is similar to the embodiment shown in  FIG.  7    except that the telescoping assembly includes an extra portion, according to some embodiments. 
         FIG.  64    illustrates a cross-sectional side view of the system shown in  FIGS.  61 - 63   , according to some embodiments. 
         FIG.  65    illustrates a perspective view of portions of a sensor module, according to some embodiments. 
         FIG.  66    illustrates a cross-sectional side view of the sensor module shown in  FIG.  65   , according to some embodiments. 
         FIG.  67    illustrates a perspective view of portions of a sensor module, according to some embodiments. 
         FIG.  68    illustrates a top view of the sensor module shown in  FIG.  67   , according to some embodiments. 
         FIGS.  69  and  70    illustrate perspective views of an electronics unit just before the electronics unit is coupled to a base, according to some embodiments. 
         FIG.  71    illustrates a cross-sectional perspective view of an applicator system, according to some embodiments, in a resting state. 
         FIG.  72    illustrates a cross-sectional perspective view of the applicator system of  FIG.  71   , with the actuation member energized. 
         FIG.  73    illustrates a rotated cross-sectional perspective view of the applicator system of  FIG.  72   . 
         FIG.  74    illustrates a cross-sectional perspective view of the applicator system of  FIG.  71   , with the actuation member activated and with the needle assembly deployed in an insertion position. 
         FIG.  75    illustrates a cross-sectional perspective view of the applicator system of  FIG.  71   , with the on-skin component in a deployed position and the needle assembly retracted. 
         FIG.  76    illustrates a cross-sectional side view of another applicator system, according to some embodiments, in a resting state. 
         FIG.  77    illustrates a cross-sectional side view of the applicator system of  FIG.  76   , with the actuation member energized. 
         FIG.  78    illustrates a cross-sectional side view of the applicator system of  FIG.  76   , with the actuation member activated and with the needle assembly deployed in an insertion position. 
         FIG.  79    illustrates a cross-sectional side view of the applicator system of  FIG.  76   , with the on-skin component in a deployed position and the needle assembly retracted. 
         FIG.  80    illustrates a cross-sectional side view of another applicator system, according to some embodiments, in a resting state. 
         FIG.  81    illustrates a cross-sectional side view of the applicator system of  FIG.  80   , with the actuation member energized. 
         FIG.  82    illustrates a cross-sectional side view of the applicator system of  FIG.  80   , with the actuation member activated and with the needle assembly deployed in an insertion position. 
         FIG.  83    illustrates a cross-sectional side view of the applicator system of  FIG.  80   , with the on-skin component in a deployed position and the needle assembly retracted. 
         FIG.  84    illustrates a perspective view of the applicator system of  FIG.  80   , with the first and third portions shown in cross section to better illustrate certain portions of the system, and in a resting state. 
         FIG.  85    illustrates a perspective view of the applicator system of  FIG.  80   , with the first and third portions shown in cross section to better illustrate certain portions of the system, and with the actuation member energized. 
         FIG.  86    illustrates a cross-sectional side view of another applicator system, according to some embodiments, in a resting state in which the actuation member is already energized. 
         FIG.  87    illustrates a cross-sectional side view of the applicator system of  FIG.  86   , with the actuation member activated and with the needle assembly deployed in an insertion position. 
         FIG.  88    illustrates a cross-sectional side view of the applicator system of  FIG.  86   , with the on-skin component in a deployed position and the needle assembly retracted. 
         FIG.  89    illustrates a cross-sectional side view of another applicator system, according to some embodiments, in a resting state in which the actuation member is already energized. 
         FIG.  90    illustrates a cross-sectional side view of the applicator system of  FIG.  86   , with the actuation member activated and with the needle assembly deployed in an insertion position. 
         FIG.  91    illustrates a cross-sectional side view of the applicator system of  FIG.  86   , with the on-skin component in a deployed position and the needle assembly retracted. 
         FIG.  92    illustrates a side view of another applicator system, according to some embodiments, with a top trigger member, in a resting state. 
         FIG.  93    illustrates a side view of the applicator system of  FIG.  92   , after being cocked but before being triggered. 
         FIG.  94    illustrates a cross-sectional perspective view of the applicator system of  FIG.  92   , in a resting state. 
         FIG.  95    illustrates a cross-sectional perspective view of the applicator system of  FIG.  92    while being cocked. 
         FIG.  96    illustrates a cross-sectional perspective view of the applicator system of  FIG.  92   , after being cocked but before being triggered. 
         FIG.  97    illustrates a cross-sectional side view of the applicator system of  FIG.  96   . 
         FIG.  98    illustrates a cross-sectional side view of the applicator system of  FIG.  92   , during triggering. 
         FIG.  99    illustrates a cross-sectional side view of the applicator system of  FIG.  92   , after being triggered and with the needle assembly deployed in an insertion position. 
         FIG.  100    illustrates a cross-sectional side view of the applicator system of  FIG.  92   , with the on-skin component in a deployed position and the needle assembly retracted. 
         FIG.  101    illustrates a side view of another applicator system, according to some embodiments, with a side trigger member. 
         FIG.  102    illustrates another side view of the applicator system of  FIG.  101   , with the first and third portions shown in cross-section to illustrate the trigger mechanism. 
         FIG.  103    illustrates a side view of another applicator system, according to some embodiments, with an integrated side trigger. 
         FIG.  104    illustrates another side view of the applicator system of  FIG.  103   , with the first and third portions shown in cross-section and with a portion of the second portion removed to illustrate the trigger mechanism. 
         FIG.  105    illustrates a perspective view of another applicator system, according to some embodiments, with a safety feature. 
         FIG.  106    illustrates a cross-sectional perspective view of a portion of the applicator system of  FIG.  105   , with the safety feature in a locked configuration. 
         FIG.  107    illustrates an enlarged view of the portion of the applicator system of  FIG.  106   , with the safety feature in a locked configuration. 
         FIG.  108    illustrates a cross-sectional perspective view of a portion of the applicator system of  FIG.  105   , with the safety feature in a released configuration. 
         FIG.  109    illustrates a cross-sectional perspective view of a portion of the applicator system of  FIG.  105   , with the safety feature in a released configuration and with the third portion moved distally relative to the first portion. 
         FIG.  110    illustrates a cross-sectional perspective view of an applicator system, according to some embodiments, in a resting and locked state, with the on-skin component secured in a proximal position. 
         FIG.  111    illustrates a cross-sectional perspective view of the applicator system of  FIG.  110   , with the safety feature unlocked. 
         FIG.  112    illustrates a cross-sectional perspective view of the applicator system of  FIG.  110   , with the actuation member energized. 
         FIG.  113    illustrates a cross-sectional perspective view of the applicator system of  FIG.  110   , with the actuation member activated and with the needle assembly and on-skin component deployed in a distal position. 
         FIG.  114    illustrates a cross-sectional perspective view of the applicator system of  FIG.  110   , with the on-skin component in a deployed position and separated from the retracted needle assembly. 
         FIG.  115    illustrates a perspective view of the needle assembly from the system of  FIG.  110   , shown securing the on-skin component during deployment, with the base removed for purposes of illustration. 
         FIG.  116    illustrates another perspective view of the needle assembly from the system of  FIG.  110   , shown separated from the on-skin component, with the base removed for purposes of illustration. 
         FIG.  117    illustrates a perspective view of a portion of the system of  FIG.  100   . 
         FIG.  118    illustrates a perspective view of the sensor module of  FIG.  100   , before being coupled to the base. 
         FIG.  119    illustrates a perspective view of the sensor module of  FIG.  100   , after being coupled to the base. 
         FIG.  120    illustrates a side view of an on-skin component and base, according to some embodiments, prior to coupling of the on-skin component to the base. 
         FIG.  121    illustrates a perspective view of the on-skin component and base of  FIG.  120   , prior to coupling of the on-skin component to the base. 
         FIG.  122    illustrates a side view of the on-skin component and base of  FIG.  120   , after coupling of the on-skin component to the base. 
         FIG.  123    illustrates a perspective view of a portion of another applicator system, according to some embodiments, with an on-skin component coupled to a needle assembly in a proximal position. 
         FIG.  124    illustrates a perspective view of the on-skin component and the needle assembly of  FIG.  123   . 
         FIG.  125    illustrates a perspective view of a portion of the applicator system shown in  FIG.  123   , with the on-skin component separated from the needle assembly. 
         FIG.  126    illustrates a perspective view of a portion of a securing member, shown securing an on-skin component. 
         FIG.  127    illustrates a perspective view of a portion of the securing member of  FIG.  126   , with the sensor module of the on-skin component shown in cross section, and illustrated with a decoupling feature of an applicator assembly, according to some embodiments. 
         FIG.  128    illustrates a perspective view of the on-skin component of  FIG.  126   , after decoupling of the on-skin component from the securing member. 
         FIG.  129    illustrates a perspective view of a portion of an applicator assembly, according to some embodiments, with the second portion shown in cross section, and with a securing member shown securing an on-skin component in a proximal position. 
         FIG.  130    illustrates a perspective view of a portion of the applicator assembly of  FIG.  129   , shown with a portion of the securing member cut away to better illustrate the configuration of the securing member. 
         FIG.  131    illustrates a perspective view of a portion of the applicator assembly of  FIG.  129   , after decoupling of the on-skin component from the needle assembly, shown with portions of the on-skin component and the securing member cut away. 
         FIG.  132    illustrates a perspective view of a portion of an applicator assembly, according to some embodiments, with the second portion shown in cross section, and with a securing member shown securing an on-skin component in a proximal position. 
         FIG.  133    illustrates a perspective view of the needle assembly and on-skin component of  FIG.  132   , after decoupling of the on-skin component from the needle assembly. 
         FIG.  134    illustrates an exploded perspective view of a portion of an applicator assembly, according to some embodiments, with a securing member configured to releasably couple an on-skin component to a needle assembly. 
         FIG.  135    illustrates a perspective view of a portion of the applicator assembly of  FIG.  134   , with the needle assembly coupled to the on-skin component. 
         FIG.  136    illustrates a perspective view of a portion of the applicator assembly of  FIG.  134   , with the needle assembly decoupled from the on-skin component. 
         FIG.  137    illustrates a perspective view of an applicator assembly, according to some embodiments, with an on-skin component releasably secured in a proximal position within the applicator assembly. 
         FIG.  138    illustrates a perspective view of the applicator assembly of  FIG.  137   , with the on-skin component released from securement. 
         FIG.  139    illustrates a perspective view of the on-skin component of  FIG.  137   , with the securing feature in a secured configuration. 
         FIG.  140    illustrates a perspective view of the on-skin component of  FIG.  137   , with the securing feature in a released configuration. 
         FIG.  141    illustrates a cross-sectional perspective view of a portion of an applicator assembly, according to some embodiments, with the second and third portions shown in cross section, and showing a base coupled to an applicator. 
         FIG.  142    illustrates a perspective view of another applicator assembly, according to some embodiments, showing a patch coupled to an applicator. 
         FIG.  143    illustrates a perspective view of the applicator assembly of  FIG.  142   , the patch decoupled from the applicator. 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS 
     Although certain embodiments and examples are disclosed below, inventive subject matter extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses, and to modifications and equivalents thereof. Thus, the scope of the claims appended hereto is not limited by any of the particular embodiments described below. For example, in any method or process disclosed herein, the acts or operations of the method or process may be performed in any suitable sequence and are not necessarily limited to any particular disclosed sequence. Various operations may be described as multiple discrete operations in turn, in a manner that may be helpful in understanding certain embodiments; however, the order of description should not be construed to imply that these operations are order dependent. Additionally, the structures, systems, and/or devices described herein may be embodied as integrated components or as separate components. 
     For purposes of comparing various embodiments, certain aspects and advantages of these embodiments are described. Not necessarily all such aspects or advantages are achieved by any particular embodiment. Thus, for example, various embodiments may be carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other aspects or advantages as may also be taught or suggested herein. 
     System Introduction 
     U.S. Patent Publication No. US-2013-0267811-A1, the entire contents of which are incorporated by reference herein, explains how  FIG.  1    is a schematic of a continuous analyte sensor system  100  attached to a host (e.g., a person). The analyte sensor system  100  communicates with other devices  110 - 113  (which can be located remotely from the host). A transcutaneous analyte sensor system  102  comprising an on-skin sensor assembly  600  is fastened to the skin of a host via a base (not shown), which can be a disposable housing. 
     The system  102  includes a transcutaneous analyte sensor  200  and an electronics unit (referred to interchangeably as “sensor electronics” or “transmitter”)  500  for wirelessly transmitting analyte information to a receiver. The receiver can be located remotely relative to the system  102 . In some embodiments, the receiver includes a display screen, which can display information to a person such as the host. Example receivers include computers such as smartphones, smartwatches, tablet computers, laptop computers, and desktop computers. In some embodiments, receivers can be Apple Watches, iPhones, and iPads made by Apple Inc. In still further embodiments, the system  102  can be configured for use in applying a drug delivery device, such an infusion device, to the skin of a patient. In such embodiments, the system can include a catheter instead of, or in addition to, a sensor, the catheter being connected to an infusion pump configured to deliver liquid medicines or other fluids into the patient&#39;s body. In embodiments, the catheter can be deployed into the skin in much the same manner as a sensor would be, for example as described herein. 
     In some embodiments, the receiver is mechanically coupled to the electronics unit  500  to enable the receiver to receive data (e.g., analyte data) from the electronics unit  500 . To increase the convenience to users, in several embodiments, the receiver does not need to be mechanically coupled to the electronics unit  500  and can even receive data from the electronics unit  500  over great distances (e.g., when the receiver is many feet or even many miles from the electronics unit  500 ). 
     During use, a sensing portion of the sensor  200  can be under the host&#39;s skin and a contact portion of the sensor  200  can be electrically connected to the electronics unit  500 . The electronics unit  500  can be engaged with a housing (e.g., a base) which is attached to an adhesive patch fastened to the skin of the host. 
     The on-skin sensor assembly  600  may be attached to the host with use of an applicator adapted to provide convenient and secure application. Such an applicator may also be used for attaching the electronics unit  500  to a base, inserting the sensor  200  through the host&#39;s skin, and/or connecting the sensor  200  to the electronics unit  500 . Once the electronics unit  500  is engaged with the base and the sensor  200  has been inserted into the skin (and is connected to the electronics unit  500 ), the sensor assembly can detach from the applicator. 
     The continuous analyte sensor system  100  can include a sensor configuration that provides an output signal indicative of a concentration of an analyte. The output signal including (e.g., sensor data, such as a raw data stream, filtered data, smoothed data, and/or otherwise transformed sensor data) is sent to the receiver. 
     In some embodiments, the analyte sensor system  100  includes a transcutaneous glucose sensor, such as is described in U.S. Patent Publication No. US-2011-0027127-A1, the entire contents of which are hereby incorporated by reference. In some embodiments, the sensor system  100  includes a continuous glucose sensor and comprises a transcutaneous sensor (e.g., as described in U.S. Pat. No. 6,565,509, as described in U.S. Pat. No. 6,579,690, as described in U.S. Pat. No. 6,484,046). The contents of U.S. Pat. Nos. 6,565,509, 6,579,690, and 6,484,046 are hereby incorporated by reference in their entirety. 
     In several embodiments, the sensor system  100  includes a continuous glucose sensor and comprises a refillable subcutaneous sensor (e.g., as described in U.S. Pat. No. 6,512,939). In some embodiments, the sensor system  100  includes a continuous glucose sensor and comprises an intravascular sensor (e.g., as described in U.S. Pat. No. 6,477,395, as described in U.S. Pat. No. 6,424,847). The contents of U.S. Pat. Nos. 6,512,939, 6,477,395, and 6,424,847 are hereby incorporated by reference in their entirety. 
     Various signal processing techniques and glucose monitoring system embodiments suitable for use with the embodiments described herein are described in U.S. Patent Publication No. US-2005-0203360-A1 and U.S. Patent Publication No. US-2009-0192745-A1, the contents of which are hereby incorporated by reference in their entirety. The sensor can extend through a housing, which can maintain the sensor on the skin and can provide for electrical connection of the sensor to sensor electronics, which can be provided in the electronics unit  500 . 
     In several embodiments, the sensor is formed from a wire or is in a form of a wire. A distal end of the wire can be sharpened to form a conical shape (to facilitate inserting the wire into the tissue of the host). The sensor can include an elongated conductive body, such as a bare elongated conductive core (e.g., a metal wire) or an elongated conductive core coated with one, two, three, four, five, or more layers of material, each of which may or may not be conductive. The elongated sensor may be long and thin, yet flexible and strong. For example, in some embodiments, the smallest dimension of the elongated conductive body is less than 0.1 inches, less than 0.075 inches, less than 0.05 inches, less than 0.025 inches, less than 0.01 inches, less than 0.004 inches, and/or less than 0.002 inches. 
     The sensor may have a circular cross section. In some embodiments, the cross section of the elongated conductive body can be ovoid, rectangular, triangular, polyhedral, star-shaped, C-shaped, T-shaped, X-shaped, Y-shaped, irregular, or the like. In some embodiments, a conductive wire electrode is employed as a core. To such an electrode, one or two additional conducting layers may be added (e.g., with intervening insulating layers provided for electrical isolation). The conductive layers can be comprised of any suitable material. In certain embodiments, it may be desirable to employ a conductive layer comprising conductive particles (i.e., particles of a conductive material) in a polymer or other binder. 
     In some embodiments, the materials used to form the elongated conductive body (e.g., stainless steel, titanium, tantalum, platinum, platinum-iridium, iridium, certain polymers, and/or the like) can be strong and hard, and therefore can be resistant to breakage. For example, in several embodiments, the ultimate tensile strength of the elongated conductive body is greater than 80 kPsi and less than 500 kPsi, and/or the Young&#39;s modulus of the elongated conductive body is greater than 160 GPa and less than 220 GPa. The yield strength of the elongated conductive body can be greater than 60 kPsi and less than 2200 kPsi. 
     The electronics unit  500  can be releasably coupled to the sensor  200 . The electronics unit  500  can include electronic circuitry associated with measuring and processing the continuous analyte sensor data. The electronics unit  500  can be configured to perform algorithms associated with processing and calibration of the sensor data. For example, the electronics unit  500  can provide various aspects of the functionality of a sensor electronics module as described in U.S. Patent Publication No. US-2009-0240120-A1 and U.S. Patent Publication No. US-2012-0078071 -A1, the entire contents of which are incorporated by reference herein. The electronics unit  500  may include hardware, firmware, and/or software that enable measurement of levels of the analyte via a glucose sensor, such as an analyte sensor  200 . 
     For example, the electronics unit  500  can include a potentiostat, a power source for providing power to the sensor  200 , signal processing components, data storage components, and a communication module (e.g., a telemetry module) for one-way or two-way data communication between the electronics unit  500  and one or more receivers, repeaters, and/or display devices, such as devices  110 - 113 . Electronics can be affixed to a printed circuit board (PCB), or the like, and can take a variety of forms. The electronics can take the form of an integrated circuit (IC), such as an Application-Specific Integrated Circuit (ASIC), a microcontroller, and/or a processor. The electronics unit  500  may include sensor electronics that are configured to process sensor information, such as storing data, analyzing data streams, calibrating analyte sensor data, estimating analyte values, comparing estimated analyte values with time-corresponding measured analyte values, analyzing a variation of estimated analyte values, and the like. Examples of systems and methods for processing sensor analyte data are described in more detail in U.S. Pat. Nos. 7,310,544, 6,931,327, U.S. Patent Publication No. 2005-0043598-A1, U.S. Patent Publication No. 2007-0032706-A1, U.S. Patent Publication No. 2007-0016381-A1, U.S. Patent Publication No. 2008-0033254-A1, U.S. Patent Publication No. 2005-0203360-A1, U.S. Patent Publication No. 2005-0154271-A1, U.S. Patent Publication No. 2005-0192557-A1, U.S. Patent Publication No. 2006-0222566-A1, U.S. Patent Publication No. 2007-0203966-A1 and U.S. Patent Publication No. 2007-0208245-A1, the contents of which are hereby incorporated by reference in their entirety. 
     One or more repeaters, receivers and/or display devices, such as a key fob repeater  110 , a medical device receiver  111  (e.g., an insulin delivery device and/or a dedicated glucose sensor receiver), a smartphone  112 , a portable computer  113 , and the like can be communicatively coupled to the electronics unit  500  (e.g., to receive data from the electronics unit  500 ). The electronics unit  500  can also be referred to as a transmitter. In some embodiments, the devices  110 - 113  transmit data to the electronics unit  500 . The sensor data can be transmitted from the sensor electronics unit  500  to one or more of the key fob repeater  110 , the medical device receiver  111 , the smartphone  112 , the portable computer  113 , and the like. In some embodiments, analyte values are displayed on a display device. 
     The electronics unit  500  may communicate with the devices  110 - 113 , and/or any number of additional devices, via any suitable communication protocol. Example communication protocols include radio frequency; Bluetooth; universal serial bus; any of the wireless local area network (WLAN) communication standards, including the IEEE 802.11, 802.15, 802.20, 802.22 and other 802 communication protocols; ZigBee; wireless (e.g., cellular) telecommunication; paging network communication; magnetic induction; satellite data communication; and/or a proprietary communication protocol. 
     Additional sensor information is described in U.S. Pat. Nos. 7,497,827 and 8,828,201. The entire contents of U.S. Pat. Nos. 7,497,827 and 8,828,201 are incorporated by reference herein. 
     Any sensor shown or described herein can be an analyte sensor; a glucose sensor; and/or any other suitable sensor. A sensor described in the context of any embodiment can be any sensor described herein or incorporated by reference. Thus, for example, the sensor  138  shown in  FIG.  7    can be an analyte sensor; a glucose sensor; any sensor described herein; and any sensor incorporated by reference. Sensors shown or described herein can be configured to sense, measure, detect, and/or interact with any analyte. 
     As used herein, the term “analyte” is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to a substance or chemical constituent in a biological fluid (for example, blood, interstitial fluid, cerebral spinal fluid, lymph fluid, urine, sweat, saliva, etc.) that can be analyzed. Analytes can include naturally occurring substances, artificial substances, metabolites, or reaction products. 
     In some embodiments, the analyte for measurement by the sensing regions, devices, systems, and methods is glucose. However, other analytes are contemplated as well, including, but not limited to ketone bodies; Acetyl Co A; acarboxyprothrombin; acylcarnitine; adenine phosphoribosyl transferase; adenosine deaminase; albumin; alpha-fetoprotein; amino acid profiles (arginine (Krebs cycle), histidine/urocanic acid, homocysteine, phenylalanine/tyrosine, tryptophan); andrenostenedione; antipyrine; arabinitol enantiomers; arginase; benzoylecgonine (cocaine); biotinidase; biopterin; c-reactive protein; carnitine; carnosinase; CD4; ceruloplasmin; chenodeoxycholic acid; chloroquine; cholesterol; cholinesterase; cortisol; testosterone; choline; creatine kinase; creatine kinase MM isoenzyme; cyclosporin A; d-penicillamine; de-ethylchloroquine; dehydroepiandrosterone sulfate; DNA (acetylator polymorphism, alcohol dehydrogenase, alpha 1-antitrypsin, cystic fibrosis, Duchenne/Becker muscular dystrophy, glucose-6-phosphate dehydrogenase, hemoglobin A, hemoglobin S, hemoglobin C, hemoglobin D, hemoglobin E, hemoglobin F, D-Punjab, beta-thalassemia, hepatitis B virus, HCMV, HIV-1, HTLV-1, Leber hereditary optic neuropathy, MCAD, RNA, PKU, Plasmodium vivax, sexual differentiation, 21-deoxycortisol); desbutylhalofantrine; dihydropteridine reductase; diptheria/tetanus antitoxin; erythrocyte arginase; erythrocyte protoporphyrin; esterase D; fatty acids/acylglycines; triglycerides; glycerol; free β-human chorionic gonadotropin; free erythrocyte porphyrin; free thyroxine (FT4); free tri-iodothyronine (FT3); fumarylacetoacetase; galactose/gal-1-phosphate; galactose-1-phosphate uridyltransferase; gentamicin; glucose-6-phosphate dehydrogenase; glutathione; glutathione perioxidase; glycocholic acid; glycosylated hemoglobin; halofantrine; hemoglobin variants; hexosaminidase A; human erythrocyte carbonic anhydrase I; 17-alpha-hydroxyprogesterone; hypoxanthine phosphoribosyl transferase; immunoreactive trypsin; lactate; lead; lipoproteins ((a), B/A-1, β); lysozyme; mefloquine; netilmicin; phenobarbitone; phenytoin; phytanic/pristanic acid; progesterone; prolactin; prolidase; purine nucleoside phosphorylase; quinine; reverse tri-iodothyronine (rT3); selenium; serum pancreatic lipase; sissomicin; somatomedin C; specific antibodies (adenovirus, anti-nuclear antibody, anti-zeta antibody, arbovirus, Aujeszky&#39;s disease virus, dengue virus,  Dracunculus medinensis, Echinococcus granulosus, Entamoeba histolytica , enterovirus,  Giardia duodenalisa, Helicobacter pylori , hepatitis B virus, herpes virus, HIV-1, IgE (atopic disease), influenza virus,  Leishmania donovani, leptospira , measles/mumps/rubella,  Mycobacterium leprae, Mycoplasma pneumoniae, Myoglobin, Onchocerca volvulus , parainfluenza virus,  Plasmodium falciparum , poliovirus,  Pseudomonas aeruginosa , respiratory syncytial virus,  Rickettsia  (scrub typhus),  Schistosoma mansoni, Toxoplasma gondii, Trepenoma pallidium, Trypanosoma cruzi/rangeli , vesicular stomatis virus,  Wuchereria bancrofti , yellow fever virus); specific antigens (hepatitis B virus, HIV-1); acetone (e.g., succinylacetone); acetoacetic acid; sulfadoxine; theophylline; thyrotropin (TSH); thyroxine (T4); thyroxine-binding globulin; trace elements; transferrin; UDP-galactose-4-epimerase; urea; uroporphyrinogen I synthase; vitamin A; white blood cells; and zinc protoporphyrin. 
     Salts, sugar, protein, fat, vitamins, and hormones naturally occurring in blood or interstitial fluids can also constitute analytes in certain embodiments. The analyte can be naturally present in the biological fluid or endogenous, for example, a metabolic product, a hormone, an antigen, an antibody, and the like. Alternatively, the analyte can be introduced into the body or exogenous, for example, a contrast agent for imaging, a radioisotope, a chemical agent, a fluorocarbon-based synthetic blood, or a drug or pharmaceutical composition, including but not limited to insulin; glucagon; ethanol; cannabis (marijuana, tetrahydrocannabinol, hashish); inhalants (nitrous oxide, amyl nitrite, butyl nitrite, chlorohydrocarbons, hydrocarbons); cocaine (crack cocaine); stimulants (amphetamines, methamphetamines, Ritalin, Cylert, Preludin, Didrex, PreState, Voranil, Sandrex, Plegine); depressants (barbiturates, methaqualone, tranquilizers such as Valium, Librium, Miltown, Serax, Equanil, Tranxene); hallucinogens (phencyclidine, lysergic acid, mescaline, peyote, psilocybin); narcotics (heroin, codeine, morphine, opium, meperidine, Percocet, Percodan, Tussionex, Fentanyl, Darvon, Talwin, Lomotil); designer drugs (analogs of fentanyl, meperidine, amphetamines, methamphetamines, and phencyclidine, for example, Ecstasy); anabolic steroids; and nicotine. The metabolic products of drugs and pharmaceutical compositions are also contemplated analytes. Analytes such as neurochemicals and other chemicals generated within the body can also be analyzed, such as, for example, ascorbic acid, uric acid, dopamine, noradrenaline, 3-methoxytyramine (3MT), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 5-hydroxytryptamine (5HT), 5-hydroxyindoleacetic acid (FHIAA), and intermediaries in the Citric Acid Cycle. 
     Many embodiments described herein use an adhesive (e.g., the adhesive  126  in  FIG.  7   ). One purpose of the adhesive can be to couple a base, a sensor module, and/or a sensor to a host (e.g., to skin of the host). The adhesive can be configured for adhering to skin. The adhesive can include a pad (e.g., that is located between the adhesive and the base). Additional adhesive information, including adhesive pad information, is described in U.S. patent application Ser. No. 14/835,603, which was filed on Aug. 25, 2015. The entire contents of U.S. patent application Ser. No. 14/835,603 are incorporated by reference herein. 
     Distal Base Location 
     As noted above, systems can apply an on-skin sensor assembly to the skin of a host. The system can include a base that comprises an adhesive to couple a glucose sensor to the skin. 
     In some applicators, the base is hidden deep inside the applicator until the user moves the needle distally with the base. One challenge with this approach is that the insertion site (on the skin of the host) is not ideally prepared for sensor and/or needle insertion. For example, the distal end of the applicator may be a hoop that presses against the skin. The pressure of the applicator on the skin can cause the area of the skin within the hoop to form a convex shape. In addition, the skin within the hoop can be too easily compressed such that the skin lacks sufficient resilience and firmness. In this state, the sensor and/or needle may press the skin downward without immediately piercing the skin, which may result in improper sensor and/or needle insertion. 
     In several embodiments, the base is coupled to a telescoping assembly such that the base protrudes from the distal end of the system while the glucose sensor is located remotely from the base and is located within the telescoping assembly. This configuration enables the base to prepare the insertion site of the skin for sensor and/or needle insertion (e.g., by compressing the skin). Thus, these embodiments can dramatically improve the reliability of sensor and/or needle insertion while reducing pain associated with sensor and/or needle insertion. 
     The system can hold the base in a position that is distal relative to a glucose sensor module such that a glucose sensor is not attached to the base and such that the glucose sensor can move relative to the base. Moving the glucose sensor module distally towards the base can attach the glucose sensor to the base. This movement can occur as a result of compressing an applicator. 
       FIG.  2    illustrates a perspective view of an applicator system  104  for applying at least portions of an on-skin sensor assembly  600  (shown in  FIG.  4   ) to skin of a host (e.g., a person). The system can include a sterile barrier having a shell  120  and a cap  122 . The cap  122  can screw onto the shell  120  to shield portions of the system  104  from external contaminants. 
     The electronics unit  500  (e.g., a transmitter having a battery) can be detachably coupled to the sterile barrier shell  120 . The rest of the applicator system  104  can be sterilized, and then the electronics unit  500  can be coupled to the sterile barrier shell  120  (such that the electronics unit  500  is not sterilized with the rest of the applicator system  104 ). 
     The user can detach the electronics unit  500  from the sterile barrier shell  120 . The user can also couple the electronics unit  500  to the base  128  (as shown in  FIG.  6   ) after the applicator system  104  places at least a portion of a sensor in a subcutaneous position (for analyte sensing). 
     Many different sterilization processes can be used with the embodiments described herein. The sterile barrier  120  and/or the cap  122  can block gas from passing through (e.g., can be hermetically sealed). The hermetic seal can be formed by threads  140  (shown in  FIG.  3   ). The threads  140  can be compliant such that they deform to create a seal. The threads  140  can be located between the sterile barrier shell  120  and the cap  122 . 
     The cap  122  can be made polypropylene and the shell  120  can be made from polycarbonate (or vice versa) such that one of the cap  122  and the shell  120  is harder than the other of the cap  122  and the  120 . This hardness (or flexibility) difference enables one of the components to deform to create the thread  140  seal. 
     In some embodiments, at least one of the shell  120  and the cap  122  includes a gas-permeable material to enable sterilization gases to enter the applicator system  104 . For example, as explained in the context of  FIG.  60   , the system can include a cover  272   h.    
     Referring now to  FIG.  3   , the threads  140  can be configured such that a quarter rotation, at least 15 percent of a full rotation, and/or less than 50 percent of a full rotation uncouples the cap  122  from the shell  120 . Some embodiments do not include threads  140 . The cap  122  can be pushed onto the shell  120  (e.g., during assembly) even in some threaded embodiments. 
     A cap  122  can be secured to the shell  120  by a frangible member  142  configured such that removing the cap  122  from the shell  120  brakes the frangible member  142 . The frangible member  142  can be configured like the safety ring (with a frangible portion) of a plastic soda bottle. Unscrewing the cap from the plastic soda bottle breaks the safety ring from the soda bottle&#39;s cap. This approach provides evidence of tampering. In the same way, the applicator system  104  can provide tamper evidence (due to the frangible member  142  being broken by removing the cap  122  from the shell  122 ). 
     U.S. Patent Publication No. US-2013-0267811-A1; U.S. Patent Application No. 62/165,837, which was filed on May 15, 2015; and U.S. Patent Application No. 62/244,520, which was filed on Oct. 21, 2015, include additional details regarding applicator system embodiments. The entire contents of U.S. Patent Publication No. US-2013-0267811-A1; U.S. Patent Application No. 62/165,837; and U.S. Patent Application No. 62/244,520 are incorporated by reference herein. 
       FIG.  3    illustrates a cross-sectional view of the system  104 . A glucose sensor module  134  is configured to couple a glucose sensor  138  to the base  128  (e.g., a “housing”). The telescoping assembly  132  is located in a proximal starting position such that the glucose sensor module  134  is located proximally relative to the base  128  and remotely from the base  128 . The telescoping assembly  132  is configured such that collapsing the telescoping assembly  132  connects the glucose sensor module  134  to the base  128  via one or more mechanical interlocks (e.g., snap fits, interference features). 
     The sterile barrier shell  120  is coupled to a telescoping assembly  132 . After removing the cap  122 , the system  104  is configured such that compressing the sterile barrier shell  120  distally (while a distal portion of the system  104  is pressed against the skin) can insert a sensor  138  (shown in  FIG.  4   ) into the skin of a host to place the transcutaneous, glucose analyte sensor  138 . In many figures shown herein, the sterile barrier shell  120  and cap  122  are hidden to increase the clarity of other features. 
     Collapsing the telescoping assembly  132  also pushes at least 2.5 millimeters of the glucose sensor  138  out through a hole in the base  128  such that at least 2.5 millimeters of the glucose sensor  138  that was previously located proximally relative to a distal end of the base protrudes distally out of the base  128 . Thus, in some embodiments, the base  128  can remain stationary relative to a distal portion of the telescoping assembly  132  while the collapsing motion of the telescoping assembly  132  brings the glucose sensor module  134  towards the base  128  and then couples the sensor module  134  to the base  128 . 
     This relative motion between the sensor module  134  and the base  128  has many benefits, such as enabling the base to prepare the insertion site of the skin for sensor and/or needle insertion (e.g., by compressing the skin). The starting position of the base  128  also enables the base  128  to shield people from a needle, which can be located inside the applicator system  104 . For example, if the base  128  were directly coupled to the sensor module  134  in the proximal starting position of the telescoping assembly, the needle may protrude distally from the base  128 . The exposed needle could be a potential hazard. In contrast, the distal starting position of the base  128  enables the base  128  to protect people from inadvertent needle insertion. Needle protection is especially important for caregivers (who are not the intended recipients of the on-skin sensor assembly  600  shown in  FIG.  4   ). 
       FIG.  4    illustrates a perspective view of the on-skin sensor assembly  600 , which includes the base  128 . An adhesive  126  can couple the base  128  to the skin  130  of the host. The adhesive  126  can be a foam adhesive suitable for skin adhesion. A glucose sensor module  134  is configured to couple a glucose sensor  138  to the base  128 . 
     The applicator system  104  (shown in  FIG.  2   ) can couple the adhesive  126  to the skin  130 . The system  104  can also secure (e.g., couple via mechanical interlocks such as snap fits and/or interference features) the glucose sensor module  134  to the base  128  to ensure the glucose sensor  138  is coupled to the base  128 . Thus, the adhesive  126  can couple the glucose sensor  138  to the skin  130  of the host. 
     After the glucose sensor module  134  is coupled to the base  128 , a user (or an applicator) can couple the electronics unit  500  (e.g., a transmitter) to the base  128  via mechanical interlocks such as snap fits and/or interference features. The electronics unit  500  can measure and/or analyze glucose indicators sensed by the glucose sensor  138 . The electronics unit  500  can transmit information (e.g., measurements, analyte data, glucose data) to a remotely located device (e.g.,  110 - 113  shown in  FIG.  1   ). 
       FIG.  5    illustrates a perspective view of the electronics unit  500  coupled to the base  128  via mechanical interlocks such as snap fits and/or interference features. Adhesive  126  on a distal face of the base  128  is configured to couple the sensor assembly  600  to the skin.  FIG.  6    illustrates another perspective view of the electronics unit  500  coupled to the base  128 . 
     Any of the features described in the context of  FIGS.  1 - 6    can be applicable to all aspects and embodiments identified herein. For example, many embodiments can use the on-skin sensor assembly  600  shown in  FIG.  4    and can use the sterile barrier shell  120  shown in  FIG.  2   . Moreover, any of the features of an embodiment is independently combinable, partly or wholly with other embodiments described herein in any way, e.g., one, two, or three or more embodiments may be combinable in whole or in part. Further, any of the features of an embodiment may be made optional to other aspects or embodiments. Any aspect or embodiment of a method can be performed by a system or apparatus of another aspect or embodiment, and any aspect or embodiment of a system can be configured to perform a method of another aspect or embodiment. 
       FIGS.  7 - 11    illustrate cross-sectional views of the applicator system  104  from  FIG.  3   . The sterile barrier shell  120  and the cap  134  are hidden in  FIGS.  7 - 11    to facilitate viewing the telescoping assembly  132 . 
     The telescoping assembly  132  is part of a system for applying an on-skin sensor assembly  600  to skin of a host (shown in  FIG.  4   ). The telescoping assembly  132  can apply portions of the system to the host. Additional portions of the system can be added to the on-skin sensor assembly  600  after the applicator system  104  couples initial portions of the sensor assembly  600  to the host. For example, as shown in  FIG.  4   , the electronics unit  500  (e.g., a transmitter) can be coupled to the on-skin sensor assembly  600  after the applicator system  104  (shown in  FIG.  3   ) couples the base  128 , the glucose sensor module  134 , and/or the glucose sensor  138  to the skin  130  of the host. 
     In some embodiments, the applicator system  104  (shown in  FIG.  3   ) couples at least one, at least two, at least three, at least four, and/or all of the following items to the skin of the host: the electronics unit  500 , the glucose sensor module  134 , the glucose sensor  138 , the base  128 , and the adhesive  126 . The electronics unit  500  can be located inside the applicator system  104  such that the applicator system  104  is configured to couple the electronics unit  500  to the skin of the host. 
       FIG.  7    illustrates a telescoping assembly  132  having a first portion  150  (e.g., a “pusher”) configured to move distally relative to a second portion  152  (e.g., a “needle guard”) from a proximal starting position to a distal position along a path  154 .  FIG.  7    illustrates the telescoping assembly  132  in the proximal starting position.  FIG.  8    illustrates the telescoping assembly  132  moving between the proximal starting position and the distal position.  FIG.  11    illustrates the telescoping assembly  132  in the distal position. The path  154  (shown in  FIG.  7   ) represents the travel between the proximal starting position and the distal position. 
     A first set of items can be immobile relative to the first portion  150 , and a second set of items can be immobile relative to the second portion  152  while the first set of items move relative to the second set of items. 
     Referring now to  FIG.  7   , the glucose sensor  138  and the sensor module  134  are coupled to the first portion  150  (e.g., such that they are immobile relative to the first portion  150  during a proximal portion of the path  154 ). The base  128  is coupled to the second portion  152  such that the base  128  protrudes from a distal end of the system (e.g., the base protrudes from a distal end of the telescoping assembly  132 ). The base  128  comprises adhesive  126  configured to eventually couple the glucose sensor  138  to the skin (e.g., after at least a portion of the glucose sensor  138  is rigidly coupled to the base  128 ). 
     In  FIG.  7   , the glucose sensor  138  and the sensor module  134  are located within the second portion  152  while the base  128  protrudes from the distal end of the system (e.g., from the distal end of the telescoping assembly  132 ) such that the system is configured to couple the glucose sensor  138  to the base  128  via moving the first portion  150  distally relative to the second portion  152 . The progression shown in  FIGS.  7 - 11    illustrates moving the first portion  150  distally relative to the second portion  152 . 
     The sensor module  138  is coupled to a distal portion of the first portion  150  such that moving the first portion  150  to the distal position (as described above) couples the sensor module  134  to the base  128 . The glucose sensor  138  is coupled to the sensor module  134  (e.g., immobile relative to the sensor module  134 ) while the first portion  150  is located in the proximal starting position. The glucose sensor  138  can include a distally protruding portion and a proximal portion. The proximal portion can be rigidly coupled to the sensor module  134  such that the proximal portion cannot move relative to the sensor module  134  even though the distally protruding portion may bend relative to the sensor module  134 . 
     A needle  156  (e.g., a “C-shaped” needle) is coupled to the first portion  150  such that the glucose sensor  138  and the needle  156  move distally relative to the base  128  and relative to the second portion  152 . The system can further comprise a needle release mechanism  158  configured to retract the needle  156  proximally. 
     The needle  156  can have many different forms. Many different types of needles  156  can be used with the embodiments described herein.  FIGS.  51 - 55    illustrate various needle embodiments that can be used with any of the embodiments described herein. 
     The needle  156  can guide the sensor  138  into the skin of the host. A distal portion of the sensor  138  can be located in a channel of the needle  156  (as shown in  FIG.  42   ). Sometimes, a distal end of the sensor  138  sticks out of the needle  156  and gets caught on tissue of the host as the sensor  138  and needle  156  are inserted into the host. As a result, the sensor  138  may buckle and fail to be inserted deeply enough into the subcutaneous tissue. In other words, in some embodiments, the sensor wire must be placed within the channel of the C-shaped needle  156  to be guided into the tissue and must be retained in the channel  330  during deployment. 
     The risk of the sensor  138  sticking out of the channel  330  (and thereby failing to be property inserted into the host) can be greatly diminished by the embodiment illustrated in  FIG.  51   . In this embodiment, adhesive  376  bonds a distal portion of the glucose sensor  138  into the channel  330  of the needle  156 . Retracting the needle  156  can break the bond of the adhesive  376  to enable a distal portion of the sensor  138  to stay in a subcutaneous location while the needle  156  is retracted (and even after the needle  156  is retracted). 
     The risk of the sensor  138  sticking out of the channel  330  (and thereby failing to be property inserted into the host) can be greatly diminished by the embodiment illustrated in  FIGS.  52  and  53   . In this embodiment, the needle  156   a  comprises two sides, which can be separated by slots  378 . The sensor  138  can have a width that is larger than the width of the slots  378  such that the sensor  138  cannot come out of the channel  330   a  until the two sides of the needle  156   a  are moved apart (to widen the slots  378 ). 
     The embodiment illustrated in  FIG.  54    can be used with any of the other embodiments described herein. The needle  156   b  includes a ramp  380  at the distal end of the channel  330   b . The distal end of the needle  156   b  can include a conical tip  382 . The ramp  380  can be configured to push the sensor  138  out of the channel  330   b  of the needle  156   b  as the needle  156   b  is retracted into the telescoping assembly  132  (shown in  FIG.  7   ). 
       FIG.  55    illustrates cross sectional views of different needles  156   c ,  156   d ,  156   e ,  156   f , which can be used as needle  156  in  FIG.  7    or in any other embodiment described herein. Needle  156   c  includes an enclosed channel  330   c . Needles  156   d ,  156   e ,  156   f  are C-needles, although many other C-needle shapes can be used in several embodiments. The ends of the needle  156   d can be angled relative to each other. In some embodiments, the ends of the needle can be angled away from each other, in an opposite fashion as shown by  156   d . In some embodiments, the ends of the needle can have flared edges, in which the flared edges are rounded to prevent the sensor from contacting sharp edges. The ends of the needle  156   e  can be parallel and/or flat relative to each other. The outside portion of the channel  330   f  can be formed by walls that are straight and/or parallel to each other (rather than by curved walls as is the case for other needles  156   d ,  156   e ). Some needles  156   d  can be manufactured via laser cutting, some needles  156   e  can be manufactured via wire electrical discharge machining (“EDM”), and some needles  156   f  can be manufactured via stamping. 
     As shown in  FIG.  7   , a needle hub  162  is coupled to the needle  156 . The needle hub includes release features  160  that protrude outward. In some embodiments, the release features can comprise one, two, or more flexible arms. Outward ends  164  of the release features  160  catch on inwardly facing overhangs  166  (e.g., undercuts, detents) of the first portion  150  such that moving the first portion  150  distally relative to the second portion  152  causes the needle retraction mechanism  158  to move distally until a release point. 
     At the release point, proximal protrusions  170  of the second portion  152  engage the release features  160  (shown in  FIG.  9   ), which forces the release features  160  to bend inward until the release features  160  no longer catch on the overhangs  166  of the first portion  150  (shown in  FIG.  10   ). Once the release features  160  no longer catch on the overhangs  166  of the first portion  150 , the spring  234  of the needle retraction mechanism  158  pushes the needle  156  and the needle hub  162  proximally relative to the first portion  150  and relative to the second portion  152  until the needle no longer protrudes distally from the base  128  and is completely hidden inside the telescoping assembly  132  (shown in  FIG.  11   ). 
     The needle  156  can be removed from the embodiment illustrated in  FIG.  7    to make a needle-free embodiment. Thus, a needle  156  is not used in some embodiments. For example, a distal end of the glucose sensor  138  can be formed in a conical shape to enable inserting the glucose sensor  138  into the skin without using a needle  156 . Unless otherwise noted, the embodiments described herein can be formed with or without a needle  156 . 
     In several embodiments, a needle can help guide the glucose sensor  138  (e.g., at least a distal portion of the glucose sensor) into the skin. In some embodiments, a needle is not part of the system and is not used to help guide the glucose sensor  138  into the skin. In needle embodiments and needle-free embodiments, skin piercing is an important consideration. Failing to properly pierce the skin can lead to improper placement of the glucose sensor  138 . 
     Tensioning the skin prior to piercing the skin with the glucose sensor  138  and/or the needle  156  can dramatically improve the consistency of achieving proper placement of the glucose sensor  138 . Tensioning the skin can be accomplished by compressing the skin with a distally protruding shape (e.g., a convex shape) prior to piercing the skin and at the moment of piercing the skin with the glucose sensor  138  and/or the needle  156 . 
       FIG.  12 A  illustrates a portion of the cross section shown in  FIG.  7   . The base  128  includes an optional distally facing protrusion  174  located distally relative to the second portion  152  (and relative to the rest of the telescoping assembly  132 ). The distal protrusion  174  is convex and is shaped as a dome. In some embodiments, the distal protrusion  174  has block shapes, star shapes, and cylindrical shapes. Several base  128  embodiments do not include the protrusion  174 . 
     The distal protrusion  174  can be located farther distally than any other portion of the base  128 . The distal protrusion  174  can extend through a hole  176  in the adhesive  126  (as also shown in  FIG.  5   ). A distal portion of the convex protrusion  174  can be located distally relative to the adhesive  126  while a proximal portion of the convex protrusion  174  is located proximally relative to the adhesive  126 . 
     The distal protrusion  174  has a hole  180  through which the needle  156  and/or the glucose sensor  138  can pass. The distal protrusion  174  can compress the skin such that the distal protrusion  174  is configured to reduce a resistance of the skin to piercing. 
       FIG.  12 B  illustrates a cross sectional view of a base  128   b  that is identical to the base  128  illustrated in  FIGS.  7  and  12 B  except for the following features: The base  128   b  does not include a protrusion  174 . The base  128   b  includes a funnel  186  (e.g., a radius) on the distal side of the hole  180   b.    
     Like the embodiment shown in  FIG.  12 A , the sensor  138  (e.g., an analyte sensor) and/or the needle  156  (shown in  FIG.  12 A ) can pass through the hole  180   b  (shown in  FIG.  12 B ). The funnels  182 ,  186  can be mirror images of each other or can be different shapes. The base  128   b  can be used with any of the embodiments described herein. 
       FIG.  13    illustrates a perspective view of a portion of the adhesive  126 . The needle  156  can have many different shapes and cross sections. In some embodiments, the needle  156  includes a slot  184  (e.g., the channel  330  shown in  FIGS.  42  and  43   ) into which at least a portion of the glucose sensor  138  can be placed. 
     The needle  156  having a slot  184  passes through the hole  180  of the distal protrusion and through the hole  176  of the adhesive  126 . A portion of the glucose sensor  138  is located in the slot  184  such that the needle  156  is configured to move distally relative to the base  128  (shown in  FIG.  12 A ) without dislodging the portion of the glucose sensor  138  from the slot  184 . The distal protrusion  174  is convex such that the distal protrusion  174  is configured to tension the skin while the first portion  150  moves distally relative to the second portion  152  of the telescoping assembly  132  (shown in  FIG.  7   ) to prepare the skin for piercing. 
     As mentioned above, the adhesive  126  comprises a hole  176  through which at least a portion of the distal protrusion  174  of the base  128  can pass. The distal protrusion  174  is located within the hole  176  of the adhesive  126  such that the distal protrusion  174  can tension at least a portion of the skin within the second hole (e.g., located under the hole  176 ). The hole  176  can be circular or any other suitable shape. The hole  176  can be sized such that at least a majority of the distal protrusion  174  extends through the hole  176 . A perimeter of the hole  176  can be located outside of the distal protrusion  174  such that the perimeter of the hole  176  is located radially outward relative to a perimeter of the protrusion  174  where the protrusion  174  connects with the rest of the base  128 . 
     In some embodiments, the hole  176  of the adhesive  126  is large enough that the adhesive  126  does not cover any of the distal protrusion  174 . In some embodiments, the adhesive  126  covers at least a portion of or even a majority of the distal protrusion  174 . Thus, the adhesive  126  does not have to be planar and can bulge distally in an area over the distal protrusion  174 . 
     In several embodiments, the adhesive  126  has a non-uniform thickness such that the thickness of the adhesive  126  is greater in an area surrounding a needle exit area than in other regions that are farther radially outward from the needle exit area. Thus, the distal protrusion  174  can be part of the adhesive  126  rather than part of the base  128 . However, in several embodiments, the base  128  comprises the adhesive  126 , and the distal protrusion  174  can be formed by the plastic of the base  128  or by the foam adhesive  126  of the base  128 . 
     The needle  156  includes a distal end  198  and a heel  194 . The heel  194  is the proximal end of the angled portion of the needle&#39;s tip. The purpose of the angled portion is to form a sharp end to facilitate penetrating tissue. The sensor  138  has a distal end  208 . 
     During insertion of the needle  156  and the sensor  138  into the tissue; as the needle  156  and the sensor  138  first protrude distally from the system; and/or while the needle  156  and the sensor  138  are located within the telescoping assembly, the end  208  of the sensor  138  can be located at least 0.1 millimeter proximally from the heel  194 , less than 1 millimeter proximally from the heel  194 , less than 3 millimeters proximally from the heel  194 , and/or within plus or minus 0.5 millimeters of the heel  194 ; and/or the end  208  of the sensor  138  can be located at least 0.3 millimeters proximally from the distal end  198  of the needle  156  and/or less than 2 millimeters proximally from the distal end  198 . 
     Referring now to  FIG.  12 A , the distal protrusion  174  can protrude at least 0.5 millimeters and less than 5 millimeters from the distal surface of the adhesive  126 . In embodiments where the adhesive  126  has a non-planar distal surface, the distal protrusion  174  can protrude at least 0.5 millimeters and less than 5 millimeters from the average distal location of the adhesive  126 . 
     As described above, in some embodiments the base is coupled to a telescoping assembly such that the base protrudes from the distal end of the system while the glucose sensor is located remotely from the base and is located within the telescoping assembly. In other embodiments, however, the base is coupled to a telescoping assembly such that the base is located completely inside the telescoping assembly and the base moves distally with the sensor as the first portion is moved distally relative to the second portion of the telescoping assembly. 
     For example,  FIG.  59    illustrates a base  128  coupled to the sensor module  134  and to the sensor  138  while the first portion  150  of the telescoping assembly  132   f  is located in the proximal starting position. The base  128  moves distally as the first portion  150  is moved distally relative to the second portion  152 . The base  128  can be coupled to a distal end portion of the first portion  150  while the first portion  150  is located in the proximal starting position. All of the features and embodiments described herein can be configured and used with the base  128  positioning described in the context of  FIG.  59   . 
     All of the embodiments described herein can be used with the base coupled to a telescoping assembly such that the base is located completely inside the telescoping assembly and the base moves distally with the sensor as the first portion is moved distally relative to the second portion of the telescoping assembly. All of the embodiments described herein can be used with the base coupled to a telescoping assembly such that the base protrudes from the distal end of the system while the glucose sensor is located remotely from the base and is located within the telescoping assembly. 
     Sensor Module Docking and Base Detachment 
     As explained above, maintaining the base against the skin during insertion of the sensor and/or needle enables substantial medical benefits. Maintaining the base against the skin, however, can necessitate moving the sensor relative to the base during the insertion process. Once inserted, the sensor needs to be coupled to the base to prevent the sensor from inadvertently dislodging from the base. Thus, there is a need for a system that enables the sensor to move relative to the base and also enables locking the sensor to the base (without being overly burdensome on users). 
     Maintaining the base against the skin during the distal movement of the sensor and/or needle is enabled in many embodiments by unique coupling systems that secure the sensor (and the sensor module) to a first portion of a telescoping assembly and secure the base to a second portion of the telescoping assembly. Moving the first portion towards the second portion of the telescoping assembly can align the sensor with the base while temporarily holding the sensor. Then, the system can couple the sensor to the base. Finally, the system can detach the base and sensor from the telescoping assembly (which can be disposable or reusable with a different sensor). 
     As illustrated in  FIG.  4   , the sensor module  134  and the glucose sensor  138  are not initially coupled to the base  128 . Coupling the sensor module  134  and the glucose sensor  138  to the base  128  via compressing the telescoping assembly  132  and prior to detaching the base  128  from the telescoping assembly  132  can be a substantial challenge, yet is enabled by many of the embodiments described herein. 
     As illustrated in  FIGS.  7  and  14   , the sensor module  134  (and the glucose sensor  138 ) can be located remotely from the base  128  even though they are indirectly coupled via the telescoping assembly  132 . In other words, the sensor module  134  (and the glucose sensor  138 ) can be coupled to the first portion  150  of the telescoping assembly  132  while the base  128  is coupled to the second portion  152  of the telescoping assembly  132 . In this state, the sensor module  134  and the glucose sensor  138  can move relative to the base  128  (e.g., as the sensor module  134  and the glucose sensor  138  move from the proximal starting position to the distal position along the path to “dock” the sensor module  134  and the glucose sensor  138  to the base  128 ). 
     After the sensor module  134  and the glucose sensor  138  are “docked” with the base  128 , the system can detach the base  128  from the telescoping assembly  132  to enable the sensor module  134 , the glucose sensor  138 , and the base  128  to be coupled to the skin by the adhesive  126  while the telescoping assembly  132  and other portions of the system are discarded. 
     As shown in  FIG.  7   , the sensor module  134  is coupled to the first portion  150  and is located at least 5 millimeters from the base  128  while the first portion  150  is in the proximal starting position. The system is configured such that moving the first portion  150  to the distal position couples the sensor module  134  to the base  128  (as shown in  FIG.  11   ). The glucose sensor  138  is coupled to the sensor module  134  while the first portion  150  is located in the proximal starting position. The glucose sensor  138  is located within the second portion  152  while the base  128  protrudes from the distal end of the system. 
     Arrow  188  illustrates the proximal direction in  FIG.  7   . Arrow  190  illustrates the distal direction in  FIG.  7   . Line  172  illustrates a horizontal orientation. As used herein, horizontal means within plus or minus 20 degrees of perpendicular to the central axis  196 . 
       FIG.  15    illustrates a perspective view of a cross section of portions of the system shown in  FIG.  7   . The cross section cuts through the hole  180  of the base  128 . Visible portions include the sensor module  134 , the sensor  138 , a seal  192 , the needle  156 , the base  128 , and the adhesive  126 . The sensor module  134  is in the proximal starting position. The seal  192  is configured to block fluid (e.g., bodily fluid) from entering the glucose sensor module  134 . 
     The glucose sensor  138  is mechanically coupled to the sensor module  134 . The glucose sensor  138  runs into an interior portion of the sensor module  134  and is electrically coupled to interconnects in the interior portion of the sensor module  134 . The interconnects are hidden in  FIG.  15    to facilitate seeing the proximal portion of the glucose sensor  138  inside the interior portion of the sensor module  134 . Many other portions of the system are also hidden in  FIG.  15    to enable clear viewing of the visible portions. 
     In many embodiments, the sensor module  134  moves from the position shown in  FIG.  15    until the sensor module  134  snaps onto the base  128  via snap fits that are described in more detail below.  FIG.  11    illustrates the sensor module  134  snapped to the base  128 . This movement from the proximal starting position to the “docked” position can be accomplished by moving along the path  154  (shown in  FIG.  7    and illustrated by the progression in  FIGS.  7 - 11   ). (The arrow representing the path  154  is not necessarily drawn to scale.) 
     Referring now to  FIGS.  7  and  15   , during a first portion of the path  154 , the sensor module  134  is immobile relative to the first portion  150 , and the base  128  is immobile relative to the second portion  152  of the telescoping assembly  132 . During a second portion of the path  154 , the system is configured to move the first portion  150  distally relative to the second portion  152 ; to move the sensor module  134  towards the base  128 ; to move at least a portion of the sensor  138  through a hole  180  in the base  128 ; to couple the sensor module  134  to the base  128 ; and to enable the coupled sensor module  134  and the base  128  to detach from the telescoping assembly  132 . 
       FIG.  7    illustrates a vertical central axis  196  oriented from a proximal end to the distal end of the system. (Part of the central axis  196  is hidden in  FIG.  7    to avoid obscuring the arrow that represents the path  154  and to avoid obscuring the needle  156 .) 
       FIG.  15    illustrates a flex arm  202  of the sensor module  134 . The flex arm  202  is oriented horizontally and is configured to secure the sensor module  134  to a protrusion of the base  128 . In some embodiments, the flex arm  202  is an alignment arm to prevent and/or impede rotation of the sensor module  134  relative to the base  128 . 
       FIG.  16    illustrates a perspective view of a cross section in which the sensor module  134  is coupled to the base  128  via flex arms  202 . Interconnects  204  protrude proximally to connect the sensor module  134  to the electronics unit  500  (e.g., a transmitter). 
     Referring now to  FIGS.  15  and  16   , the flex arms  202  extend from an outer perimeter of the sensor module  134 . The base  128  comprises protrusions  206  that extend proximally from a planar, horizontal portion of the base  128 . 
     Referring now to  FIG.  16   , each of the proximal protrusions  206  of the base  128  are coupled to a flex arm  202  of the sensor module  134 . Thus, the coupling of the proximal protrusions  206  to the flex arms  202  couples the sensor module  134  to the base  128 . 
     Each proximal protrusion  206  can include a locking protrusion  212  that extends at an angle of at least 45 degrees from a central axis of each proximal protrusion  206 . In some embodiments, the locking protrusions  212  extend horizontally (e.g., as shown in  FIG.  15   ). Each horizontal locking protrusion  212  is coupled to an end portion  210  of a flexible arm  202 . 
     The end portion  210  of each flexible arm  202  can extend at an angle greater than 45 degrees and less than 135 degrees relative to a central axis of the majority of the flexible arm  202 . The end portion  210  of each flexible arm  202  can include a horizontal locking protrusion (e.g., as shown in  FIG.  15   ). 
     In  FIGS.  15  and  16   , a first horizontal locking protrusion is coupled to an end portion  210  of the first flexible arm  202 . A second horizontal locking protrusion  212  is coupled to the first proximal protrusion  206  of the base  128 . In  FIG.  16   , the first horizontal locking protrusion is located distally under the second horizontal locking protrusion  212  to secure the sensor module  134  to the base  128 . The system is configured such that moving the first portion  150  of the telescoping assembly  132  to the distal position (shown in  FIG.  11   ) causes the first flex arm  202  to bend to enable the first horizontal locking protrusion of the flex arm  202  to move distally relative to the second horizontal locking protrusion  212 . Thus, the flex arm  202  is secured between the locking protrusion  212  and the distal face of the base  128 . 
     At least a portion of the flex arm  202  (e.g., the end portion  210 ) is located distally under the horizontal locking protrusion  212  of the base  128  to secure the sensor module  134  to the base  128 . The system is configured such that moving the first portion  150  of the telescoping assembly  132  to the distal position causes the flex arm  202  (e.g., the end portion  210 ) to bend away (e.g., outward) from the rest of the sensor module  134  to enable the horizontal locking protrusion of the flex arm  202  to go around the locking protrusion  212  of the proximal protrusion  206 . Thus, at least a portion of the flex arm  202  can move distally relative to the horizontal locking protrusion  212  of the proximal protrusion  206  of the base  128 . 
     The sensor module  134  can have multiple flex arms  202  and the base can have multiple proximal protrusions  206  configured to couple the sensor module  134  to the base  128 . In some embodiments, a first flex arm  202  is located on an opposite side of the sensor module  134  relative to a second flex arm  202  (e.g., as shown in  FIGS.  15  and  16   ). 
     In some embodiments, the base  128  comprises flex arms (e.g., like the flex arms  202  shown in  FIGS.  15  and  16   ) and the sensor module  134  comprises protrusions that couple to the flex arms of the base  128 . The protrusions of the sensor module  134  can be like the protrusions  206  shown in  FIGS.  15  and  16    except that, in several embodiments, the protrusions extend distally towards the flex arms of the base  128 . Thus, the base  128  can be coupled to the sensor module  134  with flex arms and mating protrusions regardless of whether the base  128  or the sensor module  134  includes the flex arms. 
     In several embodiments, a sensor module is coupled to the glucose sensor. The system comprises a vertical central axis oriented from a proximal end to the distal end of the system. The base comprises a first flex arm that is oriented horizontally and is coupled to the sensor module. The sensor module comprises a first distal protrusion coupled to the first flex arm to couple the sensor module to the base. A first horizontal locking protrusion is coupled to an end portion of the first flexible arm. A second horizontal locking protrusion is coupled to the first distal protrusion of the sensor module. The second horizontal locking protrusion is located distally under the first horizontal locking protrusion to secure the sensor module to the base. The system is configured such that moving the first portion of the telescoping assembly to the distal position causes the first flex arm to bend to enable the second horizontal locking protrusion to move distally relative to the first horizontal locking protrusion. The sensor module comprises a second distal protrusion coupled to a second flex arm of the base. The first distal protrusion is located on an opposite side of the sensor module relative to the second distal protrusion. 
     Docking the sensor module  134  to the base  128  can include securing the sensor module  134  to the first portion  150  of the telescoping assembly  132  while the first portion  150  moves the sensor module  134  towards the base  128 . This securing of the sensor module  134  to the first portion  150  of the telescoping assembly  132  needs to be reliable, but temporary so the sensor module  134  can detach from the first portion  150  at an appropriate stage. The structure that secures the sensor module  134  to the first portion  150  of the telescoping assembly  132  generally needs to avoid getting in the way of the docking process. 
       FIG.  17    illustrates a cross-sectional view of the first portion  150  of the telescoping assembly  132 .  FIG.  17    shows the glucose sensor module  134  and the needle  156 . Some embodiments do not include the needle  156 . Many items are hidden in  FIG.  17    to provide a clear view of the flex arms  214 ,  216  of the first portion  150 . 
     The first portion  150  comprises a first flex arm  214  and a second flex arm  216  that protrude distally and latch onto the sensor module  134  to releasably secure the sensor module  134  to the first portion  150  while the first portion  150  is in the proximal starting position (shown in  FIG.  7   ). The flex arms  214 ,  216  can couple to an outer perimeter of the sensor module  134  such that distal ends of the flex arms  214 ,  216  wrap around a distal face of the sensor module  134 . In some embodiments, the distal ends of the flex arms  214 ,  216  are located distally of the sensor module  134  while the first portion  150  is in the proximal starting position. 
     The base  128  is hidden in  FIG.  17   , but in the state illustrated in  FIG.  17   , the sensor module  134  is located remotely from the base  128  to provide a distance of at least 3 millimeters from the sensor module  134  to the base  128  while the first portion  150  is in the proximal starting position. This distance can be important to enable the base to rest on the skin as the needle  156  and/or the glucose sensor  138  pierce the skin and advance into the skin during the transcutaneous insertion. 
     Referring now to  FIGS.  7  and  17   , the sensor module  134  is located within the second portion  152  while the base  128  protrudes from the distal end of the system such that the system is configured to couple the sensor module  134  to the base  128  via moving the first portion  150  distally relative to the second portion  152 . The sensor module  134  is located within the second portion  152  while the base  128  protrudes from the distal end of the system even though the sensor module  134  is movable relative to the second portion  152  of the telescoping assembly  132 . Thus, the first portion  150  moves the sensor module  134  through an interior region of the second portion  152  of the telescoping assembly  132  without moving the base  128  through the interior region of the second portion  152 . 
     The system comprises a vertical central axis  196  oriented from a proximal end to the distal end of the system. The first flex arm  214  and the second flex arm  216  of the first portion  150  secure the sensor module  134  to the first portion  150  such that the sensor module  134  is releasably coupled to the first portion  150  with a first vertical holding strength (measured along the vertical central axis  196 ). 
     As shown in  FIGS.  15  and  16   , the sensor module  134  is coupled to the base  128  via at least one flex arm  202  such that the sensor module  134  is coupled to the base  128  with a second vertical holding strength. The flex arms  202  can extend from an outer perimeter of the sensor module  134 . The flex arms  202  can be part of the base  128 . 
     Referring now to  FIG.  17   , in some embodiments, the second vertical holding strength is greater than the first vertical holding strength such that continuing to push the first portion  150  distally once the sensor module  134  is coupled to the base  128  overcomes the first and second flex arms  214 ,  216  of the first portion  150  to detach the sensor module  134  from the first portion  150 . 
     In some embodiments, the second vertical holding strength is at least 50 percent greater than the first vertical holding strength. In several embodiments, the second vertical holding strength is at least 100 percent greater than the first vertical holding strength. In some embodiments, the second vertical holding strength is less than 400 percent greater than the first vertical holding strength. 
       FIG.  6    illustrates the on-skin sensor assembly  600  in a state where it is attached to a host. The on-skin sensor assembly  600  can include the glucose sensor  138  and/or the sensor module  134  (shown in  FIG.  7   ). In some embodiments, the on-skin sensor assembly  600  includes the needle  156 . In several embodiments, however, the on-skin sensor assembly  600  does not include the needle  156 . 
     As explained above, maintaining the base against the skin during insertion of the sensor and/or needle enables substantial medical benefits. Maintaining the base against the skin, however, can complicate detaching the base from the applicator. For example, in some prior-art systems, the base detaches after the base moves downward distally with a needle. This relatively long travel can enable several base detachment mechanisms. In contrast, when the base is maintained in a stationary position as the needle moves towards the base, releasing the base can be problematic. 
     Many embodiments described herein enable maintaining the base  128  against the skin during insertion of the sensor  138  and/or the needle  156 . As mentioned above in the context of  FIGS.  7 - 11   , after the sensor module  134  is coupled to the base  128 , the sensor module  134  and the base  128  need to detach from the telescoping assembly  132  to secure the glucose sensor  138  to the host and to enable the telescoping assembly to be thrown away, recycled, or reused. 
     As shown in  FIGS.  7 - 11   , several embodiments hold the base  128  in a stationary position relative to the second portion  152  of the telescoping assembly  132  as the sensor module  134  moves towards the base  128 . Referring now to  FIG.  18   , once the sensor module  134  is attached to the base  128 , the system can release the base  128  by bending flex arms  220  that couple the base  128  to the second portion  152 .  FIG.  18    shows the system in a state prior to the sensor module  134  docking with the base  128  to illustrate distal protrusions  222  of the first portion  150  aligned with the flex arms  220  such that the distal protrusions  222  are configured to bend the flex arms  220  (via the distal protrusions  222  contacting the flex arms  220 ). 
     The distal protrusions  222  bend the flex arms  220  to detach the base  128  from the telescoping assembly  132  (shown in  FIG.  7   ) after the sensor module  134  is coupled to the base  128  (as shown in  FIGS.  11  and  16   ). The flex arms  220  can include a ramp  224 . A distal end of the distal protrusions  222  can contact the ramp  224  and then can continue moving distally to bend the flex arm  220  as shown by arrow  228  in  FIG.  18   . This bending can uncouple the flex arm  220  from a locking feature  230  of the base  128 . This unlocking is accomplished by the first portion  150  moving distally relative to the second portion  152 , which causes the distal protrusions  222  to move as shown by arrow  226 . 
     An advantage of the system shown in  FIG.  18    is that the unlocking movement (of the arm  220  bending as shown by arrow  228 ) is perpendicular (within plus or minus 20 degrees) to the input force (e.g., as represented by arrow  226 ). Thus, the system is designed such that the maximum holding capability (e.g., of the locking feature  230 ) can be many times greater than the force necessary to unlock the arm  220  from the base  128 . As a result, the system can be extremely reliable and insensitive to manufacturing variability and normal use variations. 
     In contrast, if the holding force and the unlocking force were oriented along the same axis (e.g., within plus or minus 20 degrees), the holding force would typically be equal to or less than the unlocking force. However, the unique structure shown in  FIG.  18    allows the holding force to be at least two times larger (and in some cases at least four times larger) than the unlocking force. As a result, the system can prevent inadvertent unlocking of the base  128  while having an unlocking force that is low enough to be easily provided by a user or by another part of the system (e.g., a motor). 
     Another advantage of this system is that it controls the locking and unlocking order of operation. In other words, the structure precludes premature locking and unlocking. In a medical context, this control is extremely valuable because reliability is so critical. For example, in several embodiments, the process follows this order: The sensor module  134  couples to the base  128 . Then, the first portion  150  releases the sensor module  134 . Then, the second portion  152  releases the base  128 . In several embodiments, the vertical locations of various locking and unlocking structures are optimized to ensure this order is the only order that is possible as the first portion  150  moves from the proximal starting position to the distal position along the path described previously. (Some embodiments use different locking and unlocking orders of operation.) 
       FIG.  7    illustrates the base  128  protruding from the distal end of the system while the first portion  150  of the telescoping assembly  132  is located in the proximal starting position. The sensor module  134  and at least a majority of the glucose sensor  138  are located remotely relative to the base  128 . The system is configured to couple the sensor module  134  and the glucose sensor  138  to the base  128  via moving the first portion  150  distally relative to the second portion  152 . 
     Referring now to  FIGS.  18  and  19   , the base  128  comprises a first radial protrusion  230  (e.g., a locking feature) releasably coupled with a first vertical holding strength to a second radial protrusion  232  (e.g., a locking feature) of the second portion  152  of the telescoping assembly  132  (shown in  FIG.  7   ). The first radial protrusion  230  protrudes inward and the second radial protrusion protrudes outward  232 . The system is configured such that moving the first portion  150  to the distal position moves the second radial protrusion  232  relative to the first radial protrusion  230  to detach the base  128  from the telescoping assembly  132 . 
     The first portion  150  of the telescoping assembly  132  comprises a first arm  222  that protrudes distally. The second portion  152  of the telescoping assembly  132  comprises a second flex arm  220  that protrudes distally. The first arm  222  and the second flex arm  220  can be oriented within 25 degrees of each other (as measured between their central axes). The system is configured such that moving the first portion  150  from the proximal starting position to the distal position along the path  154  (shown in  FIG.  7   ) causes the first arm  222  to deflect the second flex arm  220 , and thereby detach the second flex arm  220  from the base  128  to enable the base  128  to decouple from the telescoping assembly  132  (shown in  FIG.  7   ). Thus, the flex arm  220  is configured to releasably couple the second portion  152  to the base  128 . 
     When the first portion  150  is in the proximal starting position, the first arm  222  of the first portion  150  is at least partially vertically aligned with the second flex arm  220  of the second portion  152  to enable the first arm  222  to deflect the second flex arm  220  as the first portion is moved to the distal position. 
     The first arm  222  and the second arm  220  can be oriented distally such that at least a portion of the first arm  222  is located proximally over a protrusion (e.g., the ramp  224 ) of the second arm  220 . This protrusion can be configured to enable a collision between the first arm  222  and the protrusion to cause the second arm  220  to deflect (to detach the base  128  from the second portion  152 ). 
     In the embodiment illustrated in  FIG.  18   , when the first portion  150  is in the proximal starting position, at least a section of the first arm  222  is located directly over at least a portion of the second flex arm  220  to enable the first arm  222  to deflect the second flex arm  220  as the first portion  150  is moved to the distal position described above. The second flex arm  220  comprises a first horizontal protrusion (e.g., the locking feature  232 ). The base  128  comprises a second horizontal protrusion (e.g., the locking feature  230 ) latched with the first horizontal protrusion to couple the base  128  to the second portion  152  of the telescoping assembly  132 . The first arm  222  of the first portion  150  deflects the second flex arm  220  of the second portion  152  to unlatch the base  128  from the second portion  152 , which unlatches the base  128  from the telescoping assembly  132 . 
     Referring now to  FIG.  7   , the system is configured to couple the glucose sensor  138  to the base  128  at a first position. The system is configured to detach the base  128  from the telescoping assembly  132  at a second position that is distal relative to the first position. 
     A third flex arm (e.g., flex arm  202  in  FIG.  15   ) couples the glucose sensor  138  to the base  128  at a first position. The second flex arm (e.g., flex arm  220  in  FIG.  18   ) detaches from the base at a second position. The second position is distal relative to the first position such that the system is configured to secure the base  128  to the telescoping assembly  132  until after the glucose sensor  138  is secured to the base  128 . 
     Spring Compression 
     Needles used in glucose sensor insertion applicators can be hazardous. For example, inadvertent needle-sticks can transfer diseases. Using a spring to retract the needle can reduce the risk of needle injuries. 
     Referring now to  FIG.  7   , a spring  234  (e.g., a coil spring) can be used to retract the needle hub  162  that supports the c-shaped needle  156 . The needle hub  162  can be released at the bottom of insertion depth (to enable the needle  156  to retract). For example, when the needle  156  reaches a maximum distal position, a latch  236  can release to enable the spring  234  to push the needle  156  proximally into a protective housing (e.g., into the first portion  150 , which can be the protective housing). 
     Many applicators use pre-compressed springs. Many applicators use substantially uncompressed springs that are compressed by a user as the user compresses the applicator. One disadvantage of a pre-compressed spring is that the spring force can cause the components to creep (e.g., change shape over time), which can compromise the reliability of the design. One disadvantage of an uncompressed spring is that the first and second portions of the telescoping assembly can be free to move slightly relative to each other (when the assembly is in the proximal starting position). This “chatter” of the first and second portions can make the assembly seem weak and flimsy. 
     Many of the components described herein can be molded from plastic (although springs are often metal). Preventing creep in plastic components can help ensure that an applicator functions the same when it is manufactured and after a long period of time. One way to reduce the creep risk is to not place the parts under a load (e.g., in storage) that is large enough to cause plastic deformation during a storage time. 
     Generating the retraction energy by storing energy in a spring during deployment limits the duration of load on the system. For example, the retraction force of the spring can be at least partially generated by collapsing the telescoping assembly (rather storing the system with a large retraction force of a fully pre-compressed spring). 
     Transcutaneous and implantable sensors are affected by the in vivo properties and physiological responses in surrounding tissues. For example, a reduction in sensor accuracy following implantation of the sensor is one common phenomenon commonly observed. This phenomenon is sometimes referred to as a “dip and recover” process. Dip and recover is believed to be triggered by trauma from insertion of the implantable sensor, and possibly from irritation of the nerve bundle near the implantation area, resulting in the nerve bundle reducing blood flow to the implantation area. 
     Alternatively, dip and recover may be related to damage to nearby blood vessels, resulting in a vasospastic event. Any local cessation of blood flow in the implantation area for a period of time leads to a reduced amount of glucose in the area of the sensor. During this time, the sensor has a reduced sensitivity and is unable to accurately track glucose. Thus, dip and recover manifests as a suppressed glucose signal. The suppressed signal from dip and recover often appears within the first day after implantation of the signal, most commonly within the first 12 hours after implantation. Dip and recover normally resolves within 6-8 hours. 
     Identification of dip and recover can provide information to a patient, physician, or other user that the sensor is only temporarily affected by a short-term physiological response, and that there is no need to remove the implant as normal function will likely return within hours. 
     Minimizing the time the needle is in the body limits the opportunity for tissue trauma that can lead to phenomena such as dip and recover. Quick needle retraction helps to limit the time the needle is in the body. A large spring retraction force can quickly retract the needle. 
     The embodiment illustrated in  FIG.  7    solves the “chatter” problem, avoids substantial creep, and enables quick needle retraction. The embodiment places the spring  234  in a slight preload between the first portion  150  and the second portion  152  of the telescoping assembly  132 . In other words, when the first portion  150  is in the proximal starting position, the spring  234  is in a slightly compressed state due to the relaxed length of the spring  234  being longer than the length of the chamber in which the spring  234  resides inside the telescoping assembly  132 . 
     In some embodiments, the relaxed length of the spring  234  is at least 4 percent longer than the length of the chamber. In several embodiments, the relaxed length of the spring  234  is at least 9 percent longer than the length of the chamber. In some embodiments, the relaxed length of the spring  234  is less than 18 percent longer than the length of the chamber. In several embodiments, the relaxed length of the spring  234  is less than 30 percent longer than the length of the chamber. 
     The spring  234  is compressed farther when the first portion  150  is moved distally relative to the second portion  152 . In some embodiments, this slight preload has a much shorter compression length than the compression length of typical fully pre-compressed springs. In several embodiments, the preload causes a compression length of the spring  234  that is less than 25 percent of the compression length of the fully compressed spring  234 . In some embodiments, the preload causes a compression length of the spring  234  that is greater than 3 percent of the compression length of the fully compressed spring  234 . The slight preload eliminates the “chatter” while having a force that is too small to cause substantial creep of non-spring components in the system. 
     The spring  234  can be inserted into the first portion  150  via a hole  238  in the proximal end of the first portion  150 . Then, the needle hub  162  (and the attached C-shaped needle  156 ) can be loaded through the proximal side of the first portion  150  of the telescoping assembly (e.g., via the hole  238  in the proximal end of the first portion  150 ). 
     The needle hub  162  is slid through the first portion  150  until radial snaps (e.g., the release feature  160  of the needle hub  162 ) engage a section of the first portion  150  (see the latch  236 ). Thus, the spring  234  is placed with a slight preload between the needle hub  162  and a distal portion of the first portion  150  of the telescoping assembly  132 . 
     During applicator activation and the telescoping (e.g., collapsing of the first portion  150  into the second portion  152 ), the spring  234  is compressed farther. At the bottom of travel (e.g., at the distal ending position), the radial snaps of the needle hub  162  are forced radially inward by features (e.g., the protrusions  170 ) in the telescoping assembly  132  (as shown by the progression of  FIGS.  7 - 11   ). This releases the needle hub  162  and allows the spring  234  to expand to drive the needle  156  proximally out of the host (and into the first portion  150  and/or the second portion  152 ). 
     As shown in  FIG.  7   , the base  128  protrudes from the distal end of the system while the first portion  150  of the telescoping assembly  132  is located in the proximal starting position and the glucose sensor  138  is located remotely relative to the base  128 . The glucose sensor  138  is moveably coupled to the base  128  via the telescoping assembly  132  because the glucose sensor  138  is coupled to the first portion  150  and the base  128  is coupled to the second portion  152  of the telescoping assembly  132 . 
     The system includes a spring  234  configured to retract a needle  156 . The needle  156  is configured to facilitate inserting the glucose sensor  138  into the skin. In some embodiments, the system does not include the needle  156 . 
     When the first portion  150  is in the proximal starting position, the spring  234  is in a first compressed state. The system is configured such that moving the first portion  150  distally from the proximal starting position increases a compression of the spring  234 . The first compressed state places the first portion  150  and second portion  152  in tension. Latching features hold the first portion  150  and second portion  152  in tension. In other words, in the proximal starting position, the latching features are configured to prevent the spring  234  from pushing the first portion  150  proximally relative to the second portion  152 . The latching features resist the first compressed state. 
     In several embodiments, the potential energy of the first compressed state is less than the amount of potential energy necessary to retract the needle  156 . This low potential energy of the partially pre-compressed spring  234  is typically insufficient to cause creep, yet is typically sufficient to eliminate the “chatter” described above. 
     Redundant systems can help ensure that the needle  156  (and in some cases the sensor  138 ) can always be removed from the host after they are inserted into the host. If in extreme cases the necessary needle removal force is greater than the spring retraction force, the user can pull the entire telescoping assembly  132  proximally to remove the needle  156  and/or the sensor  138  from the host. 
     Some embodiments include a secondary retraction spring. In other words, in some embodiments, the spring  234  in  FIG.  7    is actually two concentric springs. (In several embodiments, the spring  234  is actually just one spring.) The secondary spring can be shorter than the primary retraction spring. The secondary retraction spring can provide additional needle retraction force and can enable additional tailoring of the force profile. 
     Many users desire to minimize the amount of material they throw away (as trash). Moving the needle  156  to the back of the applicator post deployment enables easy access to remove the needle  156  post deployment. 
       FIG.  20    illustrates a perspective view of the needle  156 , the needle hub  162 , and the spring  234  just after they were removed proximally from the hole  238  in a proximal end of the first portion  150  of the telescoping assembly  132 . 
     The hole  238  is an opening at a proximal end of the applicator. The hole  238  is configured to enable removing the needle  156 , the needle hub  162 , and/or the spring  234 . This opening can be covered by a removable cover (e.g., a sticker, a hinged lid). 
       FIGS.  21  and  22    illustrate perspective views where a removable cover  272  is coupled to the first portion  150  to cover the hole  238  through which the needle  156  can be removed from the telescoping assembly  132 . A hinge  274  can couple the cover  272  to the first portion  150  such that the cover  272  can rotate to close the hole  238  (as shown in  FIG.  22   ) and rotate to open the hole  238  (as shown in  FIG.  21   ). 
     Removing the cover  272  can enable a user to remove the needle  156  from the applicator (e.g., the telescoping assembly  132 ) such that the user can throw the needle  156  in a sharps container and reuse the applicator with a new needle. Removing the needle  156  from the applicator can also enable throwing the rest of the applicator into a normal trash collector to reduce the amount of trash that needs to be held by the sharps container. 
     The features described in the context of  FIGS.  20 - 22  and  60    can be combined with any of the embodiments described herein. 
       FIG.  60    illustrates a perspective view of another telescoping assembly embodiment  132   h . The cover  272   h  is adhered to a proximal end of the telescoping assembly  132   h  to cover a hole configured to retrieve a needle after the needle retracts (e.g., as described in the context of  FIGS.  21  and  22   ). Peeling the cover  272   h  from the telescoping assembly  132   h  can enable a user to dump the needle  156  (shown in  FIG.  7   ) into a sharps container. 
     In this embodiment, the cover  272   h  is a flexible membrane such as a Tyvek label made by E. I. du Pont de Nemours and Company (“DuPont”). The cover  272   h  can include an adhesive to bond the cover  272   h  to the proximal end of the telescoping assembly  132   h.    
     In some embodiments, a second cover  272  is adhered to a distal end of the telescoping assembly  132   h  to cover the end of the telescoping assembly  132   h  through which the sensor  138  (shown in  FIG.  7   ) passes. The distal end of the telescoping assembly  132   h  can also be covered by a plastic cap  122   h.    
     The cover  272   h  can be configured to enable sterilization processes to pass through the material of the cover  272   h  to facilitate sterilization of the interior of the telescoping assembly  132   h . For example, sterilization gases can pass through the cover  272   h.    
     Any of the features described in the context of  FIG.  60    can be applicable to all aspects and embodiments identified herein. For example, the embodiments described in the context of  FIG.  60    can be combined with the embodiments described in the context of  FIGS.  1 - 59  and  61 - 70   . 
     The telescoping assembly  132   h  can use the same interior features and components as described in the context of  FIG.  7   . One important difference is that the first portion  150   h  slides on an outer surface of the second portion  152  (rather than sliding inside part of the second portion  152  as shown in  FIG.  7   ). Also, the telescoping assembly  132   h  does not use a sterile barrier shell  120  (as shown in  FIG.  2   ). 
     Any of the features described in the context of  FIGS.  7 - 22    can be applicable to all aspects and embodiments identified herein. For example, the embodiments described in the context of  FIGS.  7 - 22    can be combined with the embodiments described in the context of  FIGS.  23 - 70   . Moreover, any of the features of an embodiment is independently combinable, partly or wholly with other embodiments described herein in any way, e.g., one, two, or three or more embodiments may be combinable in whole or in part. Further, any of the features of an embodiment may be made optional to other aspects or embodiments. Any aspect or embodiment of a method can be performed by a system or apparatus of another aspect or embodiment, and any aspect or embodiment of a system can be configured to perform a method of another aspect or embodiment. 
     Force Profiles 
     Referring now to  FIG.  7   , in some embodiments, moving the first portion  150  of the telescoping assembly  132  distally relative to the second portion  152  typically involves placing the distal end of the system against the skin of the host and then applying a distal force on the proximal end of the system. This distal force can cause the first portion  150  to move distally relative to the second portion  152  to deploy the needle  156  and/or the glucose sensor  138  into the skin. 
     The optimal user force generated axially in the direction of deployment is a balance between preventing accidental premature deployment and ease of insertion. A force that is ideal at a certain portion of distal actuation may be far less than ideal at another portion of distal actuation. 
     The user places the applicator (e.g., the telescoping assembly  132 ) against the skin surface and applies a force distally on the applicator (e.g., by pushing down on the proximal end of the applicator). When the user-generated force exceeds a threshold, the applicator collapses (e.g., telescopes distally) and the user drives the sensor into the body. 
     Several embodiments include unique force profiles that reduce accidental premature deployment; dramatically increase the likelihood of complete and proper deployment; and reduce patient discomfort. Specific structures enable these unique force profiles. For example, the following structures can enable the unique force profiles described herein: structures that hold the telescoping assembly  132  in the proximal starting position; structures that attach the sensor module  134  to the base  128 ; structures that release the sensor module  134  from the first portion  150 ; structures that prevent the needle  156  from retracting prematurely; structures that retract the needle  156 ; structures that release the base  128  from the second portion  152 ; structures that pad the collision at the distal position; and structures that hold the telescoping assembly  132  in a distal ending position. These structures are described in various sections herein. 
     Several embodiments include a system for applying an on-skin sensor assembly  600  to a skin  130  of a host (shown in  FIG.  4   ). Referring now to  FIG.  7   , the system can comprise a telescoping assembly  132  having a first portion  150  configured to move distally relative to a second portion  152  from a proximal starting position to a distal position along a path  154 ; a glucose sensor  138  coupled to the first portion  150 ; and a latch  236  configurable to impede a needle  156  from moving proximally relative to the first portion. 
     The first portion  150  is releasably secured in the proximal starting position by a securing mechanism (e.g., the combination of  240  and  242  in  FIG.  7   ) that impedes moving the first portion  150  distally relative to the second portion  152 . The system is configured such that prior to reaching the distal position and/or by reaching the distal position, moving the first portion  150  distally relative to the second portion  152  releases the latch  236  thereby causing the needle  156  to retract proximally into the system. 
     In several embodiments, the securing mechanism is formed by an interference between the first portion  150  and the second portion  152 . The interference can be configured to impede the first portion  150  from moving distally relative to the second portion  152 . For example, a radially outward protrusion  240  of the first portion  150  can collide with a proximal end  242  of the second portion  152  such that moving the first portion  150  distally requires overcoming a force threshold to cause the first portion  150  and/or the second portion  152  to deform to enable the radially outward protrusion  240  to move distally relative to the proximal end  242  of the second portion  152 . 
     The system can include a first force profile measured along the path  154 . As shown in  FIG.  23   , the force profile  244  can include force on the Y axis and travel distance on the X axis. Referring now to  FIGS.  7  and  23   , the force profile  244  can be measured along the central axis  196 . 
     One way in which the force profile  244  can be measured is to place the telescoping assembly  132  against the skin; place a force gauge such as a load cell on the proximal end of the telescoping assembly  132 ; calibrate the measurement system to account for the weight of the force gauge; and then press on the proximal side of the force gauge to drive the telescoping assembly  132  from the proximal starting position to the distal position along the path  154 .  FIG.  23    illustrates force versus distance from the proximal starting position based on this type of testing procedure. 
     The first force profile  244  can comprise a first magnitude  246  coinciding with overcoming the securing mechanism (e.g.,  240  and  242 ), a third magnitude  250  coinciding with releasing the latch  236  (e.g., releasing the needle retraction mechanism), and a second magnitude  248  coinciding with an intermediate portion of the path  154  that is distal relative to overcoming the securing mechanism and proximal relative to releasing the latch  236 . 
     In several embodiments, the second magnitude  248  is a peak force associated with compressing a needle retraction spring (e.g., the spring  234  in  FIG.  7   ) prior to beginning to release the latch  236 . This peak force can be at least 0.5 pounds, at least 1.5 pounds, less than 4 pounds, and/or less than 6 pounds. 
     In several embodiments, the third magnitude  250  is a peak force associated with releasing the needle retraction mechanism. This peak force can be at least 1 pound, at least 2 pounds, less than 4 pounds, and/or less than 6 pounds. 
     In some embodiments, the second magnitude  248  is less than the first magnitude  246  and the third magnitude  250  such that the system is configured to promote needle acceleration during the intermediate portion of the path  154  to enable a suitable needle speed at a time the needle  156  (or the glucose sensor  138 ) first pierces the skin. 
     The first magnitude  246  can be the peak force required to overcome the securing mechanism (e.g.,  240  and  242 ). This peak force can be at least 5 pounds, at least 6 pounds, less than 10 pounds, and/or less than 12 pounds. The first magnitude  246  can be at least 100 percent greater than the second magnitude  248 . The first magnitude  246  can be at least 200 percent greater than the second magnitude  248 . The second magnitude  248  can be during a portion of the force profile  244  where the compression of the spring  234  is at least 50 percent of the maximum spring compression reached just before the needle  156  begins to retract proximally. The slope of the force profile  244  can be positive for at least 1 millimeter during the time at which the second magnitude  248  is measured (due to the increasing spring force as the spring compression increases). 
     The first magnitude  246  can be greater than the third magnitude  250  (and/or greater than the second magnitude  248 ) such that the system is configured to impede initiating a glucose sensor insertion cycle unless a user is applying enough force to release the latch  236 . For example, the force necessary for the protrusion  240  to move distally relative to the proximal end  242  can deliberately be designed to be greater than the force necessary to retract the needle  156 . 
     To provide a sufficient safety margin, the first magnitude  246  can be at least 50 percent greater than the third magnitude  250 . In some embodiments, the first magnitude  246  is at least 75 percent greater than the third magnitude  250 . To avoid a system where the first magnitude  246  is unnecessarily high in light of the forces required along the path  154  distally relative to the first magnitude  246 , the first magnitude  246  can be less than 250 percent greater than the third magnitude  250 . 
     A second force profile  252  can coincide with the intermediate portion of the path  154 . For example, the second magnitude  248  can be part of the second force profile  252 . This second force profile  252  can include a time period in which the slope is positive for at least 1 millimeter, at least 2.5 millimeters, less than 8 millimeters, and/or less than 15 millimeters (due to the increasing spring force as the spring compression increases). 
     A proximal millimeter of the second force profile  252  comprises a lower average force than a distal millimeter of the second force profile  252  in response to compressing a spring  234  configured to enable the system to retract the needle  156  into the telescoping assembly  132 . 
     The system also includes a first force profile  254  (measured along the path  154 ). The first force profile  254  comprises a first average magnitude coinciding with moving distally past a proximal half of the securing mechanism and a second average magnitude coinciding with moving distally past a distal half of the securing mechanism. The first average magnitude is greater than the second average magnitude such that the system is configured to impede initiating a glucose sensor insertion cycle unless a user is applying enough force to complete the glucose sensor insertion cycle. 
     A first force peak  256  coincides with moving distally past the proximal half of the securing mechanism. The first force peak  256  is at least 25 percent higher than the second average magnitude. 
     The first force profile  254  comprises a first magnitude  246  coinciding with overcoming the securing mechanism and a subsequent magnitude coinciding with terminating the securing mechanism (e.g., moving past the distal portion of the securing mechanism). The first magnitude  246  comprises a proximal vector and the subsequent magnitude comprises a distal vector.  FIG.  23    is truncated at zero force, so the distal vector appears to be have a magnitude of zero in  FIG.  23   , although the actual value is negative (e.g., negative 2 pounds). 
     The proximal vector means the system is resisting the distal movement of the first portion  150  relative to the second portion  152 . The distal vector means that the second half of the securing mechanism can help propel the needle  156  and the sensor  138  towards the skin and/or into the skin. In other words, the distal vector assists the distal movement of the first portion  150  relative to the second portion  152 . 
     The third force profile  260  can include many peaks and values due to the following events: the sensor module  134  docking to the base  128 ; the base detaching from the second portion  152  (and thus detaching from the telescoping assembly  132 ); the release feature  160  of the needle hub  162  defecting inward due to the proximal protrusions  170  of the second portion  152 ; the latch  236  releasing; the needle  156  retracting into an inner chamber of the first portion  150 ; and/or the first portion  150  hits the distal position (e.g., the end of travel). 
     As shown in  FIG.  7   , the securing mechanism can be a radially outward protrusion  240  (of the first portion  150 ) configured to collide with a proximal end  242  of the second portion  152  such that moving the first portion  150  distally requires overcoming a force threshold to cause the first portion  150  and/or the second portion  152  to deform to enable the radially outward protrusion  240  to move distally relative to the proximal end  242  of the second portion  152 . The radially outward protrusion  240  is configured to cause the second portion  152  to deform elliptically to enable the first portion  150  to move distally relative to the second portion  152 . 
       FIG.  24    illustrates another securing mechanism. At least a section of the first portion  150  interferes with a proximal end  242  of the second portion  152  such that pushing the first portion  150  distally relative to the second portion  152  requires a force greater than a force threshold. The force threshold is the minimum force necessary to deform at least one of the first portion  150  and the second portion  152  to overcome the interference  266 , which is shown inside a dashed circle in  FIG.  24   . 
     Many different interference geometries and types are used in various embodiments. The interference can be between the first portion  150  and the second portion  152 . The interference can be between the needle hub  162  and the second portion  152 . For example, the interference can resist the distal movement of the needle hub  162 . 
     In some embodiments, the first portion  150  includes a taper  262 . Once an interfering section of the first portion  150  moves distally past the interference area  266 , the taper  262  makes the system such that the interference  266  no longer impedes distal movement of the first portion  150 . 
     The second portion  152  can also have a taper  263 . The taper  263  can be on an interior surface of the second portion  152  such that the interior size gets larger as measured proximally to distally along the taper  263 . 
     The interfering portion  242  of the second portion  152  can include a ramp (as shown in  FIG.  24   ) to aid the deformation described above. The interfering section of the first portion  150  is located proximally relative to the interfering section of the second portion  152 . 
     The securing mechanism can comprise a radially outward protrusion (e.g.,  240  in  FIG.  7   ) of the first portion  150  that interferes with a radially inward protrusion of the second portion  152  (e.g., as shown by the interference  266  in  FIG.  24   ) such that the securing mechanism is configured to cause the second portion  152  to deform elliptically to enable the first portion  150  to move distally relative to the second portion  152 . 
     Any of the features described in the context of  FIGS.  24 - 32    can be applicable to all aspects and embodiments identified herein. For example, the embodiments described in the context of  FIGS.  24 - 32    can be combined with the embodiments described in the context of  FIGS.  1 - 23  and  33 - 70   . Moreover, any of the features of an embodiment is independently combinable, partly or wholly with other embodiments described herein in any way, e.g., one, two, or three or more embodiments may be combinable in whole or in part. Further, any of the features of an embodiment may be made optional to other aspects or embodiments. Any aspect or embodiment of a method can be performed by a system or apparatus of another aspect or embodiment, and any aspect or embodiment of a system can be configured to perform a method of another aspect or embodiment. 
       FIG.  25    illustrates a cross sectional view of a portion of an embodiment in which the needle holder (e.g., the needle hub  162 ) is configured to resist distal movement of the first portion  150  relative to the second portion  152   b . The second portion  152   b  is like other second portions  150  described herein (e.g., as shown in  FIG.  7   ) except that the second portion  152   b  includes flex arms  276  that are at least part of the securing mechanism. The flex arms  276  are releasably coupled to the needle holder to releasably secure the first portion  150  to the second portion  152   b  in the proximal starting position (as shown in  FIG.  7   ). 
     The needle  156  (shown in  FIG.  7   ) is retractably coupled to the first portion  150  by the needle holder  162 . The needle holder  162  is configured to resist distal movement of the first portion  150  relative to the second portion  152   b  due to a chamber and/or a ramp  278  interfering with flex arms  276 . Pushing the first portion  150  distally requires overcoming the force necessary to deflect the flex arms  276  outward such that the flex arms  276  move out of the way of the ramp  278 . 
       FIG.  27    illustrates a perspective view of another securing mechanism, a frangible release  280 .  FIG.  26    illustrates a top view of a frangible ring  282 . The ring  282  includes two frangible tabs  284  that protrude radially inward. In some embodiments, the tabs  284  are radially inward protrusions on opposite sides of the ring  282  relative to each other. The frangible member (e.g., the ring  282 ) can be part of the first portion  150 , the second portion  152 , or any other portion of the system. For example, the frangible member can be a feature of a molded second portion  152 . 
     The ring  282  can be made of a brittle material configured to enable the tabs  284  to break when the first portion  150  is pushed distally relative to the second portion  152 . For example, a section of the first portion  150  can be located proximally over the tab  284  when the first portion  150  is in the proximal starting position (as shown in  FIG.  27    by the frangible release  280 ). Moving the first portion  150  distally can cause the section of the first portion  150  to bend and/or break the tab  284 . 
     In some embodiments, a radially outward protrusion  286  of the first portion  150  is configured to bend and/or break the tab  284 . The ring  282 , the tab  284 , and the other components described herein can be molded from a plastic such as acrylonitrile butadiene styrene, polyethylene, and polyether ether ketone. (Springs, interconnects, and needles can be made of steel.) In some embodiments, the ring  282  is at least 0.2 millimeters thick, at least 0.3 millimeters thick, less than 0.9 millimeters thick, and/or less than 1.5 millimeters thick. 
     The ring  282  can be secured between the first portion  150  and the second portion  152  of the telescoping assembly  132 . The ring  282  can wrap around a perimeter of the first portion  150  and can be located proximally relative to the second portion  152  such that the ring  282  rests against a proximal end of the second portion  152 . 
     The ring  282  enables a frangible coupling between the first portion  150  and the second portion  152  while the first portion  150  is in the proximal starting position. In  FIG.  27   , the system is configured such that moving the first portion  150  to the distal position breaks the frangible coupling (e.g., the frangible release  280 ). 
     In some embodiments, the tabs  284  are not part of a ring  282 . The tabs  284  can be part of the second portion  152  or part of the first portion  150 . 
       FIG.  27    also includes a magnet system  290 . The magnet system  290  includes a magnet and a metal element in close enough proximity that the magnet is attracted to the metal element (e.g., a metal disk). For example, the second portion  152  can include a magnet, and the first portion  150  can include the metal element. In several embodiments, the second portion  152  can include a metal element, and the first portion  150  can include the magnet. 
     The magnet and metal element can be located such that they are located along a straight line oriented radially outward from the central axis  196  (shown in  FIG.  7   ). This configuration can position the magnet for sufficient attraction to the metal element to resist movement of the first portion  150 . For example, when the first portion  150  is in the proximal starting position, the magnetic force of the magnet system  290  can resist distal movement of the first portion. Thus, the magnet releasably couples the first portion  150  to the second portion  152  while the first portion  150  is in the proximal starting position. 
     In several embodiments, a user can compress an internal spring or the spring can be pre-compressed (e.g., compressed fully at the factory). The telescoping assembly can include a button  291  configured to release the spring force to cause the needle and/or the sensor to move into the skin. 
     The cover  272   h  described in the context of  FIG.  60    can be adhered to the proximal end of the first portion  150  shown in  FIG.  27   . The cover  272   h  can be used with any of the embodiments described herein. 
       FIG.  31    illustrates a side view of a telescoping assembly  132   e  having a first portion  150   e  and a second portion  152   e . The first portion  150   e  includes a radially outward protrusion  286   e  configured to engage a radially inward ramp  296  located on an interior wall of the second portion  152   e . When a user applies a distal, axial force on the first portion  150   e , the protrusion  286   e  collides with the ramp  296 . The angle of the ramp causes the first portion  150   e  to rotate relative to the second portion  152   e . This rotation resists the distal force and acts as a securing mechanism. Once the protrusion  286   e  moves beyond the distal end of the ramp  296 , the ramp  296  no longer causes rotation, and thus, no longer acts as a securing mechanism. 
     Many of the embodiments described herein rely on a compressive force of a person. Many unique structures enable the force profiles described herein. The structures help ensure the compressive force caused by a person pushing distally on a portion of the system results in reliable performance. One challenge of relying on people to push downward on the system to generation appropriate forces is that the input force can vary substantially by user. Even a single user can apply different input forces on different occasions. 
     One solution to this variability is to replace the need for a user-generated input force with a motor-generated force. The motor can provide reliable input forces. Motors also enable varying the force at different sections of the path from the proximal starting position to the distal position. 
       FIGS.  28 - 30    illustrates embodiments of telescoping assemblies  132   c ,  132   d  that include motors  290   c ,  290   d  to drive a needle  156  and/or a glucose sensor  138  into the skin. The motors  290   c ,  290   d  can be linear actuators that use an internal magnetic system to push a rod distally and proximally. The linear actuators can also convert a rotary input into linear motion to push a rod distally and proximally. The movement of the rod can move various portions of the system including the needle  156 , the needle hub  162   c , the first portion  150   c ,  150   d  of the telescoping assembly  132   c ,  132   d , the sensor module  134 , and/or the sensor  138 . The motors  290   c ,  290   d  can include internal batteries to supply electricity for the motors  290   c ,  290   d.    
       FIG.  28    illustrates a perspective, cross-sectional view of an embodiment in which the motor  290   c  pushes the needle hub  162   c  distally relative to the motor  290   c  and relative to the second portion  152   c . The needle hub  162   c  can include a rod that slides in and out of the housing of the motor  292   c . The distal movement of the needle hub  162   c  can push at least a portion of the needle  156  and/or the sensor  138  (shown in  FIG.  7   ) into the skin. The distal movement of the needle hub  162   c  can move the sensor module  134  distally such that the sensor module  134  docks with the base  128 . This coupling can precede the detachment of the base  128  from the telescoping assembly  132   c.    
       FIGS.  29  and  30    illustrate side, cross-sectional views of another motor embodiment. In this embodiment, the rod  294  of the motor  292   d  is coupled to and immobile relative to the second portion  152   d  of the telescoping assembly  132   d . The motor  292   d  is coupled to and immobile relative to the first portion  150   d  of the telescoping assembly  132   d . As a result, pulling the rod  294  into the housing of the motor  292   d  causes the first portion  150   d  to move distally relative to the second portion  152   d . The glucose module  134  is coupled to a distal portion of the first portion  150   d  (as described herein). Thus, the glucose sensor  138  is moved distally into the skin of the host and the glucose module  134  is coupled to the base  128 . As illustrated in  FIGS.  29  and  30   , the embodiment does not include a needle. Similar embodiments can include a needle. 
       FIG.  32    illustrates a perspective, cross-section view of the telescoping assembly  132 . In some embodiments, a protrusion  302  of the first portion  150  couples with a hole  304  of the second portion  152 . The protrusion  302  can be oriented distally to latch with the hole  304  in response to the first portion  150  reaching the distal position. 
     In several embodiments, a protrusion  302  of the second portion  152  couples with a hole  304  of the first portion  150 . The protrusion  302  can be oriented proximally to latch with the hole  304  in response to the first portion  150  reaching the distal position. 
     The protrusion  302  can be a flex arm that is at least 10 millimeters long, at least 15 millimeters long, and/or less than 50 millimeters long. The protrusion  302  can include an end portion that protrudes at an angle relative to the central axis of the majority of the protrusion  302 . This angle can be at least 45 degrees, at least 75 degrees, less than 110 degrees, and/or less than 135 degrees. 
     Coupling the protrusion  302  to the hole  304  can permanently lock the first portion  150  in a downward position (that is distal to the proximal starting position and is within 3 millimeter of the distal position) while the needle  156  is in a retracted state. This locking can prevent the system from being reused and can prevent needle-stick injuries. 
     Any of the features described in the context of  FIG.  23    can be applicable to all aspects and embodiments identified herein. For example, the embodiments described in the context of  FIG.  23    can be combined with the embodiments described in the context of  FIGS.  1 - 22  and  24 - 70   . Moreover, any of the features of an embodiment is independently combinable, partly or wholly with other embodiments described herein in any way, e.g., one, two, or three or more embodiments may be combinable in whole or in part. Further, any of the features of an embodiment may be made optional to other aspects or embodiments. Any aspect or embodiment of a method can be performed by a system or apparatus of another aspect or embodiment, and any aspect or embodiment of a system can be configured to perform a method of another aspect or embodiment. 
     Interconnects 
     Referring now to  FIG.  4   , in many embodiments, the electronic unit  500  drives a voltage bias through the sensor  138  so that current can be measured. Thus, the system is able to analyze glucose levels in the host. The reliability of the electrical connection between the sensor  138  and the electronics unit  500  is critical for accurate sensor data measurement. 
     In many embodiments, the host or a caregiver create the electrical connection between the sensor  138  and the electronics unit  500 . A seal  192  can prevent fluid ingress as the electronics unit  500  is pressed onto the glucose sensor module  134 . Oxidation and corrosion can change electrical resistance of the system and are sources of error and noise in the signal. 
     The electrical connections should be mechanically stable. Relative movement between the parts of the electrical system can cause signal noise, which can hinder obtaining accurate glucose data. 
     A low-resistance electrical connection is more power efficient. Power efficiency can help maximize the battery life of the electronics unit  500 . 
     In embodiments where the host or caregiver must compress the electrical interconnect and/or seal  192 , minimizing the necessary force increase user satisfaction. Lowering the user-applied force makes the transmitter easier to install. If the necessary force is too great, users and caregivers may inadvertently fail to apply adequate force, which can jeopardize the reliability and performance of the system. The force that the user needs to apply to couple the electronics unit  500  to the base  128  and sensor module  134  is strongly influenced by the force necessary to compress the interconnect. Thus, there is a need for an electrical interconnect with a lower compression force. 
     Manufacturing variability, host movement, and temperature variations while the host is using the on-skin sensor assembly  600  necessitate providing a robust electrical connection throughout an active compression range (which encompasses the minimum and maximum compression states reasonably possible). Thus, there is a need for electrical connections that are tolerant of compression variation within the active compression range. 
     Metallic springs (e.g., coil or leaf springs) can be compressed between the sensor  138  and the electronics unit  500  to provide a robust, reliable electrical connection that requires a low compression force to couple the electronics unit  500  to the base  128 . 
       FIG.  33    illustrates a perspective view of an on-skin senor assembly just before the electronics unit  500  (e.g., a transmitter) is snapped onto the base  128 . Coupling the electronics unit  500  to the base  128  can compress the seal  192  to prevent fluid ingress and can compress an interconnect (e.g., springs  306 ) to create an electrical connection  310  between the glucose sensor  138  and the electronics unit  500 . 
     Creating the electrical connection  310  and/or coupling the electronics unit  500  to the base  128  can cause the electronics unit  500  (e.g., a transmitter) to exit a sleep mode. For example, conductive members (e.g., of the sensor module  134  and/or of the base  128 ) can touch electrical contacts of the electronics unit  500  (e.g., electrical contacts of a battery of the electronics unit  500 ), which can cause the electronics unit  500  to exit a sleep mode. The conductive member of the sensor module  134  and/or of the base  128  can be a battery jumper that closes a circuit to enable electricity from the battery to flow into other portions of the electronics unit  500 . 
     Thus, creating the electrical connection  310  and/or coupling the electronics unit  500  to the base  128  can “activate” the electronics unit  500  to enable and/or to prepare the electronics unit  500  to wirelessly transmit information to other devices  110 - 113  (shown in  FIG.  1   ). U.S. Patent Publication No. US-2012-0078071-A1 includes additional information regarding transmitter activation. The entire contents of U.S. Patent Publication No. US-2012-0078071-A1 are incorporated by reference herein. 
     The distal face of the electronics unit  500  can include planar electrical contacts that touch the proximal end portions of the springs  306 . The distal end portions of the springs  306  can contact various conductive elements of the glucose sensor  138 . Thus, the springs  306  can electrically couple the electronics unit  500  to the various conductive elements of the glucose sensor  138 . In the illustrated embodiment, two metallic springs  306  electrically connect the glucose sensor  138  and the electronics unit  500 . Some embodiments use one spring  306 . Other embodiments use three, four, five, ten, or more springs  306 . 
     Metallic springs  306  (e.g., gold-plated springs) are placed above the sensor wire  138  in the sensor module  134 . The sensor  138  is located between a rigid polymer base  128  and the bottom surface of the spring  306 . The top surface of the spring  306  contacts a palladium electrode located in the bottom of the electronics module  500 . The rigid electronics module  500  and the rigid polymer base  128  are brought together creating a compressed sandwich with the sensor  138  and the spring  306 . 
     The springs  306  can be oriented such that their central axes are within 25 degrees of the central axis  196  of the telescoping assembly  132  (shown in  FIG.  7   ). The springs  306  can have a helical shape. The springs  306  can be coil springs or leaf springs. 
     Springs  306  can have ends that are plain, ground, squared, squared and ground, or any other suitable configuration. Gold, copper, titanium, and bronze can be used to make the springs  306 . Springs  306  can be made from spring steel. In several embodiments, the steels used to make the springs  306  can be low-alloy, medium-carbon steel or high-carbon steel with a very high yield strength. The springs  306  can be compression springs, torsion springs, constant springs, variable springs, helical springs, flat springs, machined springs, cantilever springs, volute springs, balance springs, leaf springs, V-springs, and/or washer springs. 
     Some embodiments use a spring-loaded pin system. The spring system can include a receptacle. A pin can be located partially inside the receptacle such that the pin can slide partially in and out of the receptacle. A spring can be located inside the receptacle such that the spring biases the pin outward towards the electronics unit  500 . The receptacle can be electrically coupled to the sensor  138  such that pressing the electronics unit  500  onto the spring-loaded pin system electrically couples the electronics unit  500  and the sensor  138 . 
     Mill-Max Mfg. Corp. of Oyster Bay, N.Y., U.S.A. (“Mill-Max”) makes a spring-loaded pin system with a brass-alloy shell that is plated with gold over nickel. One Mill-Max spring-loaded pin system has a stainless steel spring and an ordering code of 0926-1-15-20-75-14-11-0. 
     In several embodiments, the electronics unit  500  includes a battery to provide electrical power to various electrical components (e.g., a transmitter) of the electronics unit  500 . 
     In some embodiments, the base  128  can include a battery  314  that is located outside of the electronics unit  500 . The battery  314  can be electrically coupled to the electrical connection  310  such that coupling the electronics unit  500  to the base  128  couples the battery  314  to the electronics unit  500 .  FIGS.  22 B and  22 C  of U.S. Patent Publication No. US-2009-0076360-A1 illustrate a battery  444 , which in some embodiments, can be part of the base (which can have many forms including the form of base  128  shown in  FIG.  33    herein). The entire contents of U.S. Patent Publication No. US-2009-0076360-A1 are incorporated by reference herein. 
       FIG.  34    illustrates a perspective view of the sensor module  134 . Protrusions  308  can secure the springs  306  to the sensor module  134 . (Not all the protrusions  308  are labeled in order to increase the clarity of  FIG.  34   .) The protrusions  308  can protrude distally. 
     At least three, at least four, and/or less than ten protrusions  308  can be configured to contact a perimeter of a spring  306 . The protrusions  308  can be separated by gaps. The gaps enable the protrusions  308  to flex outward as the spring  306  is inserted between the protrusions  308 . The downward force of coupling the electronics unit  500  to the base  128  can push the spring  306  against the sensor  138  to electrically couple the spring  306  to the sensor  138 . The sensor  138  can run between at least two of the protrusions  308 . 
       FIG.  33    illustrates an on-skin sensor system  600  configured for transcutaneous glucose monitoring of a host. The on-skin sensor system  600  can be used with the other components shown in  FIG.  7   . The sensor module  134  can be replaced with the sensor modules  134   d ,  134   e  shown in  FIGS.  35  and  37   . Thus, the sensor modules  134   d ,  134   e  shown in  FIGS.  35  and  37    can be used with the other components shown in  FIG.  7   . 
     Referring now to  FIGS.  33  and  34   , the system  600  can include a sensor module housing  312 ; a glucose sensor  138   a ,  138   b  having a first section  138   a  configured for subcutaneous sensing and a second section  138   b  mechanically coupled to the sensor module housing  312 ; and an electrical interconnect (e.g., the springs  306 ) mechanically coupled to the sensor module housing  312  and electrically coupled to the glucose sensor  138   a ,  138   b . The springs can be conical springs, helical springs, or any other type of spring mentioned herein or suitable for electrical connections. 
     The sensor module housing  312  comprises at least two proximal protrusions  308  located around a perimeter of the spring  306 . The proximal protrusions  308  are configured to help orient the spring  306 . A segment of the glucose sensor  138   b  is located between the proximal protrusions  308  (distally to the spring  306 ). 
     The sensor module housing  312  is mechanically coupled to the base  128 . The base  128  includes an adhesive  126  configured to couple the base  128  to skin of the host. 
     The proximal protrusions  308  orient the spring  306  such that coupling an electronics unit  500  to the base  128  presses the spring  306  against a first electrical contact of the electronics  500  unit and a second electrical contact of the glucose sensor  138   b  to electrically couple the glucose sensor  138   a ,  138   b  to the electronics unit  500 . 
     Referring now to  FIGS.  33  and  35 - 38   , the system  600  can include a sensor module housing  312   d ,  312   e ; a glucose sensor  138   a ,  138   b  having a first section  138   a  configured for subcutaneous sensing and a second section  138   b  mechanically coupled to the sensor module housing  312   d ,  312   e ; and an electrical interconnect (e.g., the leaf springs  306   d ,  306   e ) mechanically coupled to the sensor module housing  312   d ,  312   e  and electrically coupled to the glucose sensor  138   a ,  138   b . The sensor modules  134   d ,  134   e  can be used in place of the sensor module  134  shown in  FIG.  7   . The leaf springs  306   d ,  306   e  can be configured to bend in response to the electronics unit  500  coupling with the base  128 . 
     As used herein, cantilever springs are a type of leaf spring. As used herein, a leaf spring can be made of a number of strips of curved metal that are held together one above the other. As used herein in many embodiments, leaf springs only include one strip (e.g., one layer) of curved metal (rather than multiple layers of curved metal). For example, the leaf spring  306   d  in  FIG.  35    can be made of one layer of metal or multiple layers of metal. In some embodiments, leaf springs include one layer of flat metal secured at one end (such that the leaf spring is a cantilever spring). 
     As shown in  FIGS.  35  and  36   , the sensor module housing  312   d  comprises a proximal protrusion  320   d  having a channel  322   d  in which at least a portion of the second section of the glucose sensor  138   b  is located. The channel  322   d  positions a first area of the glucose sensor  138   b  such that the area is electrically coupled to the leaf spring  306   d.    
     As shown in the cross-sectional, perspective view of  FIG.  36   , the leaf spring  306   d  arcs away from the first area and protrudes proximally to electrically couple with an electronics unit  500  (shown in  FIG.  33   ). At least a portion of the leaf spring  306   d  forms a “W” shape. At least a portion of the leaf spring  306   d  forms a “C” shape. The leaf spring  306   d  bends around the proximal protrusion  320   d . The leaf spring  306   d  protrudes proximally to electrically couple with an electronics unit  500  (shown in  FIG.  33   ). The seal  192  is configured to impede fluid ingress to the leaf spring  306   d.    
     The leaf spring  306   d  is oriented such that coupling an electronics unit  500  to the base  128  (shown in  FIG.  33   ) presses the leaf spring  306   d  against a first electrical contact of the electronics unit  500  and a second electrical contact of the glucose sensor  138   b  to electrically couple the glucose sensor  138   a ,  138   b  to the electronics unit  500 . The proximal height of the seal  192  is greater than a proximal height of the leaf spring  306   d  such that the electronics unit  500  contacts the seal  192  prior to contacting the leaf spring  306   d.    
     Referring now to  FIGS.  33  and  37 - 38   , the sensor module housing  312   e  comprises a channel  322   e  in which at least a portion of the second section of the glucose sensor  138   b  is located. A distal portion of the leaf spring  306   e  is located in the channel  322   e  such that a proximal portion of the leaf spring  306   e  protrudes proximally out the channel  322   e.    
     The sensor module housing  312   e  comprises a groove  326   e  that cuts across the channel  322   e  (e.g., intersects with the channel  322   e ). The leaf spring  306   e  comprises a tab  328  located in the groove to impede rotation of the leaf spring. At least a portion of the leaf spring  306   e  forms a “C” shape. 
       FIGS.  36  and  38    illustrate two leaf spring shapes. Other embodiments use other types of leaf springs. Elements shown in  FIGS.  33 - 38    can be combined. 
     Referring now to  FIGS.  33 - 38   , interconnects  306 ,  306   d ,  306   e  can comprise a palladium contact, an alloy, a clad material, an electrically conductive plated material, gold plated portions, silver material, and/or any suitable conductor. Interconnects  306 ,  306   d ,  306   e  described herein can have a resistance of less than 5 ohms, less than 20 ohms, and/or less than 100 ohms. Many interconnect embodiments enable a resistance of approximately 2.7 ohms or less, which can significantly increase battery life compared to higher resistance alternatives. 
     Reducing the force necessary to compress an interconnect  306 ,  306   d ,  306   e  (e.g., as an electronics unit  500  is coupled to the base  128 ) can reduce coupling errors and difficulties. For example, if the necessary force is high, odds are substantial that users will inadvertently fail to securely couple the electronics unit  500  to the base  128 . In some cases, if the necessary force is too high, some users will be unable to couple the electronics unit  500  to the base  128 . Thus, there is a need for systems that require less force to couple the electronics unit  500  to the base  128 . 
     Many embodiments described herein (e.g., spring embodiments) dramatically reduce the force necessary to couple the electronics unit  500  to the base  128 . The interconnects  306 ,  306   d ,  306   e  can have a compression force of at least 0.05 pounds; less than 0.5 pounds, less than 1 pound, less than 3 pounds; and/or less than 4.5 pounds over an active compression range. 
     In some embodiments, the interconnects  306 ,  306   d ,  306   e  may require a compression force of less than one pound to compress the spring 20 percent from a relaxed position, which is a substantially uncompressed position. In some embodiments, the interconnects  306 ,  306   d ,  306   e  may require a compression force of less than one pound to compress the spring 25 percent from a relaxed position, which is a substantially uncompressed position. In some embodiments, the interconnects  306 ,  306   d ,  306   e  may require a compression force of less than one pound to compress the spring 30 percent from a relaxed position, which is a substantially uncompressed position. In some embodiments, the interconnects  306 ,  306   d ,  306   e  change dependency to independent claim) may require a compression force of less than one pound to compress the spring 50 percent from a relaxed position, which is a substantially uncompressed position. 
     Springs  306 ,  306   d ,  306   e  can have a height of 2.6 millimeters, at least 0.5 millimeters, and/or less than 4 millimeters. The seal  192  can have a height of 2.0 millimeters, at least 1 millimeter, and/or less than 3 millimeters. In some embodiments, in their relaxed state (i.e., a substantially uncompressed state), springs  306 ,  306   d ,  306   e  protrude (e.g., distally) at least 0.2 millimeters and/or less than 1.2 millimeters from the top of the seal  192 . 
     When the electronics unit  500  is coupled to the base  128 , the compression of the springs  306 ,  306   d ,  306   e  can be 0.62 millimeters, at least 0.2 millimeters, less than 1 millimeter, and/or less than 2 millimeters with a percent compression of 24 percent, at least 10 percent, and/or less than 50 percent. Active compression range of the springs  306 ,  306   d ,  306   e  can be 16 to 40 percent, 8 to 32 percent, 40 to 57 percent, 29 to 47 percent, at least 5 percent, at least 10 percent, and/or less than 66 percent. 
     In some embodiments, the electrical connection between the sensor  138  and the electronics unit  500  is created at the factory. This electrical connection can be sealed at the factory to prevent fluid ingress, which can jeopardize the integrity of the electrical connection. 
     The electrical connection can be made via any of the following approaches: An electrode can pierce a conductive elastomer (such that vertical deformation is not necessary); the sensor can be “sandwiched” (e.g., compressed) between adjacent coils of a coil spring; conductive epoxy; brazing; laser welding; and resistance welding. 
     Referring now to  FIGS.  4 ,  6 ,  7 , and  33   , one key electrical connection is between the electronics unit  500  (e.g., a transmitter) and the sensor module  134 . Another key electrical connection is between the sensor module  134  and the glucose sensor  138 . Both connections should be robust to enable connecting the sensor module  134  to the base  128 , and then connecting the base  128  and sensor module  134  to the electronics unit  500  (e.g., a transmitter). A stable sensor module  134  allows the sensor module  134  to couple to the base  128  without causing signal noise in the future. 
     These two key electrical connections can be made at the factory (e.g., prior to the host or caregiver receiving the system). These electrical connections can also be made by the host or caregiver when the user attaches the electronics unit  500  to the base  128  and/or the sensor module  134 . 
     In some embodiments, the connection between the glucose sensor  138  and the sensor module  134  can be made at the factory (e.g., prior to the user receiving the system), and then the user can couple the electronics unit  500  to the sensor module  134  and/or the base  128 . In several embodiments, the electronics unit  500  can be coupled to the sensor module  134  and/or to the base  128  at the factory (e.g., prior to the user receiving the system), and then the user can couple this assembly to the glucose sensor  138 . 
     Any of the features described in the context of  FIGS.  33 - 38    can be applicable to all aspects and embodiments identified herein. For example, the embodiments described in the context of  FIGS.  33 - 38    can be combined with the embodiments described in the context of  FIGS.  1 - 32  and  39 - 70   . Moreover, any of the features of an embodiment is independently combinable, partly or wholly with other embodiments described herein in any way, e.g., one, two, or three or more embodiments may be combinable in whole or in part. Further, any of the features of an embodiment may be made optional to other aspects or embodiments. Any aspect or embodiment of a method can be performed by a system or apparatus of another aspect or embodiment, and any aspect or embodiment of a system can be configured to perform a method of another aspect or embodiment. 
     Referring now to  FIG.  33   , the battery  314  can be located inside the electronics unit  500  or can be part of the base  128 . Maximizing the life of the battery  314  is important to many reasons. For example, the electronics unit  500  may be in storage for months or even years before it is used. If the battery  413  is substantially depleted during this storage, the number of days that a host can use the electronics unit (e.g., to measure an analyte) can be dramatically diminished. 
     In some embodiments, the electronics unit  500  is in a low-power-consumption state (e.g., a “sleep” mode) during storage (e.g., prior to being received by the host). This low-power-consumption state can drain the battery  314 . Thus, there is a need for a system that reduces or even eliminates battery power consumption during storage and/or prior to the electronics unit  500  being coupled to the base  128 . 
     As described in the context of  FIG.  33   , creating the electrical connection  310  and/or coupling the electronics unit  500  to the base  128  can cause the electronics unit  500  (e.g., a transmitter) to exit a sleep mode. For example, conductive members (e.g., of the sensor module  134  and/or of the base  128 ) can touch electrical contacts of the electronics unit  500  (e.g., electrical contacts of a battery of the electronics unit  500 ), which can cause the electronics unit  500  to exit a sleep mode and/or can begin the flow of electrical power from the battery. The conductive member of the sensor module  134  and/or of the base  128  can be a battery jumper that closes a circuit to enable electricity from the battery to flow into other portions of the electronics unit  500 . 
     Thus, creating the electrical connection  310  and/or coupling the electronics unit  500  to the base  128  can “activate” the electronics unit  500  to enable and/or to prepare the electronics unit  500  to wirelessly transmit information to other devices  110 - 113  (shown in  FIG.  1   ). U.S. Patent Publication No. US-2012-0078071-A1 includes additional information regarding electronics unit  500  activation (e.g., transmitter activation). The entire contents of U.S. Patent Publication No. US-2012-0078071-A1 are incorporated by reference herein. 
       FIG.  65    illustrates a perspective view of portions of a sensor module  134   j . Some items, such as springs and sensors, are hidden in  FIG.  65    to clarify that the sensor module  134   j  can use any spring or sensor described herein. The sensor module  134   j  can use any of the springs  306 ,  306   d ,  306   e ; sensors  138 ,  138   a ,  138   b ; protrusions  308 ; channels  322   d ,  322   e ; and grooves  326   e  described herein (e.g., as shown in  FIGS.  34 - 40   ). The sensor module  134   j  can be used in the place of any other sensor module described herein. The sensor module  134   j  can be used in the embodiment described in the context of  FIG.  7    and can be used with any of the telescoping assemblies described herein. 
       FIG.  66    illustrates a cross-sectional side view of the sensor module shown in  FIG.  65   . Referring now to  FIGS.  65 - 70   , the sensor module  134   j  includes a conductive jumper  420   f  (e.g., a conductive connection that can comprise metal). The conductive jumper  420   f  is configured to electrically couple two electrical contacts  428   a ,  428   b  of the electronics unit  500  (e.g., a transmitter) in response to coupling the electronics unit  500  to the sensor module  134   j  and/or to the base  128 . 
     The conductive jumper  420   f  can be located at least partially between two electrical connections  426  (e.g., springs  306 ,  306   d ,  306   e  shown in  FIGS.  34 - 38   ). The conductive jumper  306   f  can include two springs  306   f  coupled by a conductive link  422   f . A first spring  306   f  of the jumper  420   f  can be coupled to a first contact  428   a , and a second spring  306   f  of the jumper  420   f  can be coupled to a second contact  428   b , which can complete an electrical circuit to enable the battery to provide electricity to the electronics unit  500 . The springs  306   f  can be leaf springs, coil springs, conical springs, and/or any other suitable type of spring. In some embodiments, the springs  306   f  are proximal protrusions that are coupled with the contacts  428   a ,  428   b.    
     As shown in  FIG.  66   , the conductive link  422   f  can be arched such that a sensor  138   b  (shown in  FIG.  34   ) passes under and/or through the arched portion of the conductive link  422   f . In several embodiments, the conductive link  422   f  is oriented within plus or minus 35 degrees of perpendicular to the sensor  138   b  such that the conductive link  422   f  crosses over the portion of the sensor  138   b  that is located inside the seal area (e.g., within the interior of the seal  192 ). 
       FIG.  67    illustrates a perspective view of portions of a sensor module  134   k  that is similar to the sensor module  134   j  shown in  FIGS.  65  and  66   .  FIG.  68    illustrates a top view of the sensor module  134   k  shown in  FIG.  67   . 
     Referring now to  FIGS.  67  and  68   , the sensor module  134   k  includes a different type of conductive jumper  420   g , which includes two helical springs  306   g  conductively coupled by a conductive link  422   g . The conductive link  422   g  is configured to cross over or under the sensor  138   b  (shown in  FIG.  34   ). As shown in  FIGS.  67  and  68   , the springs  306   g  are conical springs, however, some embodiments do not use conical springs. The springs  306   g  are configured to electrically couple two electrical contacts  428   a ,  428   b  of the electronics unit  500  to start the flow the electricity within the electronics unit  500 . Thus, the conductive jumper  420   g  can “activate” the electronics unit  500 . The conductive jumper  420   g  can be used with any of the sensor modules described herein. 
       FIGS.  69  and  70    illustrate perspective views of an electronics unit  500  just before the electronics unit  500  is coupled to a base  128 . As shown in  FIG.  70   , the electronics unit  500  can have two electrical contacts  428   a ,  428   b  configured to be electrically coupled to a conductive jumper  420   f  (shown in  FIGS.  65  and  66   ),  420   g  (shown in  FIGS.  67  and  68   ). The electronics unit  500  can also have two electrical contacts  428   c ,  428   d  configured to be electrically coupled to the springs  306 ,  306   d ,  306   e  (shown in  FIGS.  34 - 38   ) and/or to any other type of electrical connection  426  between the sensor  138  (shown in  FIG.  39   ) and the electronics unit  500 . 
     Coupling the electronics unit  500  to the sensor module  134   k  and/or to the base  128  can electrically and/or mechanically couple the electrical contacts  428   a ,  428   b  to the conductive jumper  420   f  (shown in  FIG.  65   ),  420   g  (shown in  FIG.  67   ). 
     Coupling the electronics unit  500  to the sensor module  134   k  and/or to the base  128  can electrically and/or mechanically couple the electrical contacts  428   c ,  428   d  to the springs  306 ,  306   d ,  306   e  (shown in  FIGS.  34 - 38   ) and/or to any other type of electrical connection  426  (e.g., as shown in  FIG.  67   ) between the sensor  138  (shown in  FIG.  39   ) and the electronics unit  500 . 
     Any of the features described in the context of  FIGS.  65 - 70    can be applicable to all aspects and embodiments identified herein. For example, the embodiments described in the context of  FIGS.  65 - 70    can be combined with the embodiments described in the context of  FIGS.  1 - 64   . Moreover, any of the features of an embodiment is independently combinable, partly or wholly with other embodiments described herein in any way, e.g., one, two, or three or more embodiments may be combinable in whole or in part. Further, any of the features of an embodiment may be made optional to other aspects or embodiments. Any aspect or embodiment of a method can be performed by a system or apparatus of another aspect or embodiment, and any aspect or embodiment of a system can be configured to perform a method of another aspect or embodiment. 
     Needle Angle and Off Set 
       FIG.  43    shows a front view of a “C-shaped” needle  156 .  FIG.  42    illustrates a bottom view of the C-shaped needle  156 . The needle  156  includes a channel  330 . A section  138   a  (shown in  FIG.  34   ) of the glucose sensor  138  (labeled in  FIG.  7   ) that is configured for subcutaneous sensing can be placed in the channel  330  (as shown in  FIG.  40   ). 
     The needle  156  can guide the sensor  138  into the skin of the host. A distal portion of the sensor  138  can be located in the channel  330  of the needle  156 . Sometimes, a distal end of the sensor  138  sticks out of the needle  156  and gets caught on tissue of the host as the sensor  138  and needle  156  are inserted into the host. As a result, the sensor  138  may buckle and fail to be inserted deeply enough into the subcutaneous tissue. In other words, in some embodiments, the sensor wire must be placed within the channel  330  of the C-shaped needle  156  to be guided into the tissue and must be retained in the channel  330  during deployment. 
     The risk of the sensor  138  sticking out of the channel  330  (and thereby failing to be property inserted into the host) can be greatly diminished by placing the sensor  138  in the channel  330  of the needle  156  with a particular angle  338  (shown in  FIG.  41   ) and offset  336  (shown in  FIG.  40   . Position B  334  in  FIG.  42    illustrates a sensor sticking out of the channel  330 . 
     The angle  338  and offset  336  cause elastic deformation of the sensor  138  to create a force that pushes the sensor  138  to the bottom of the channel  300  (as shown by position A  332  in  FIG.  42   ) while avoiding potentially detrimental effects of improper angles  338  and offset  336 . The angle  338  and offset  336  can also cause plastic deformation of the sensor  138  to help shape the sensor  138  in a way that minimizes the risk of the sensor  138  being dislodged from the channel  330  during insertion into the skin. 
     In several embodiments, the angle  338  and offset  336  shape portions of the sensor  138  for optimal insertion performance. For example, the angle  338  can bend the sensor  138  prior to placing portions of the sensor  138  in the channel  330  of the needle  156 . 
     As illustrated in  FIG.  39   , a portion of the glucose sensor  138   b  (also labeled in  FIG.  34   ) can be placed in a distally facing channel  342  (which, in some embodiments, is a tunnel). This channel  342  can help orient the glucose sensor  138   b  towards the channel  330  of the needle  156  (shown in  FIG.  43   ). 
     As illustrated in  FIG.  41   , the glucose sensor  138  can include an angle  338  between a portion of the glucose sensor  138  that is coupled to the sensor module housing  312  (shown in  FIG.  34   ) and a portion of the glucose sensor that is configured to be inserted into the host. In some embodiments, this angle  338  can be formed prior to coupling the sensor  138  to the sensor module house  312  (shown in  FIG.  34   ) and/or prior to placing a portion of the sensor  138  in the channel  330  of the needle  156  (shown in  FIG.  43   ). 
     Referring now to  FIG.  41   , an angle  338  that is less than 110 degrees can result in deployment failures (e.g., with an offset of 0.06 inches plus 0.06 inches and/or minus 0.03 inches). In some embodiments, an angle  338  that is less than 125 degrees can result in deployment failures (e.g., with an offset of 0.06 inches plus 0.06 inches and/or minus 0.03 inches). An angle  338  of 145 degrees (plus 5 degrees and/or minus 10 degrees) can reduce the probability of deployment failures. In some embodiments, the angle  338  is at least 120 degrees and/or less than 155 degrees. 
     In some embodiments, a manufacturing method includes bending the sensor  138  prior to placing portions of the sensor  138  in the channel  330  of the needle  156 . In this manufacturing method, an angle is measured from a central axis of a portion of the glucose sensor  138  that is coupled to the sensor module housing  312  (shown in  FIG.  34   ) and a portion of the glucose sensor that is configured to be inserted into the needle. According to this angle measurement, an angle that is greater than 70 degrees can result in deployment failures (e.g., with an offset of 0.06 inches plus 0.06 inches and/or minus 0.03 inches). In some embodiments, an angle that is greater than 55 degrees can result in deployment failures (e.g., with an offset of 0.06 inches plus 0.06 inches and/or minus 0.03 inches). An angle of 35 degrees (plus 10 degrees and/or minus 5 degrees) can reduce the probability of deployment failures. In some embodiments, the angle is at least 25 degrees and/or less than 60 degrees. 
     An offset  336  (shown in  FIG.  40   ) that is too large can result in the sensor  138  not being reliably held in the channel  330  (shown in  FIG.  42   ). In other words, a large offset  336  can result in the sensor  138  being located in position B  334  rather than securely in position A  332 . An offset  336  that is too small can place too much stress on the sensor  138 , which can break the sensor  138 . In light of these factors, in several embodiments, the offset  336  is at least 0.02 inches, at least 0.04 inches, less than 0.08 inches, and/or less than 0.13 inches. In some embodiments, the offset  336  is equal to or greater than 0.06 inches and/or less than or equal to 0.10 inches. The offset  336  is measured as shown in  FIG.  40    from the root of the needle  156 . 
     In some embodiments, at least a portion of the bend of the sensor  138  can include a strain relief. For example, the bend of the sensor  138  can be encapsulated in a polymeric tube or an elastomeric tube to provide strain relief for the sensor  138 . In some instances, the entire bend of the sensor  138  can be encapsulated in a polymeric tube or an elastomeric tube. In some embodiments, the tube is composed of a soft polymer. The polymeric tube or elastomeric tube can encapsulate the sensor  138  by a heat shrink process. In some embodiments, a silicone gel may be applied to the sensor at or near channel  342  (shown in  FIG.  39   ), or along at least a portion of the underside of proximal protrusion  320   d  (shown in  FIG.  35   ). 
     The needle channel width  344  (shown in  FIG.  42   ) can be 0.012 inches. In some embodiments, the width  344  is equal to or greater than 0.010 inches and/or less than or equal to 0.015 inches. The width  344  of the channel  330  is measured at the narrowest span in which the glucose sensor  138  could be located. 
     Referring now to  FIG.  40   , a funnel  182  in the base  128  can help guide the needle  156  and/or the glucose sensor  138  into the hole  180 . The funnel  182  and the hole  180  can help secure the sensor  138  in the C-shaped needle  156  during storage and deployment. For example, the hole  180  can be so small that there is not extra room (within the hole  180 ) for the sensor  138  to exit the channel  330  (shown in  FIG.  42   ) of the needle  156 . 
     Another role of the funnel  182  and hole  180  is to support the needle  156  and/or the sensor  138  against buckling forces during insertion of the needle  156  and/or the sensor  138  into the host. 
     The funnel  182  and the hole  180  also protect against inadvertent needle-stick injuries (because they are too small to enable, for example, a finger to reach the needle  156  prior to needle deployment). 
     The sensor module  134  is unable to pass through the funnel  182  and hole  180  (e.g., due to the geometries of the sensor module  134  and the funnel  182 ). Preventing the sensor module  134  from passing through the base  128  ensures the sensor module  134  is removed from the host&#39;s body when the base  128  is detached from the host. The angle  338  can prevent all of the sensor  138  from passing through the hole  180  to ensure the sensor  138  is removed from the host&#39;s body when the base  128  is detached from the host. 
     Any of the features described in the context of  FIGS.  39 - 43    can be applicable to all aspects and embodiments identified herein. For example, the embodiments described in the context of  FIGS.  39 - 43    can be combined with the embodiments described in the context of  FIGS.  1 - 38  and  44 - 70   . Moreover, any of the features of an embodiment is independently combinable, partly or wholly with other embodiments described herein in any way, e.g., one, two, or three or more embodiments may be combinable in whole or in part. Further, any of the features of an embodiment may be made optional to other aspects or embodiments. Any aspect or embodiment of a method can be performed by a system or apparatus of another aspect or embodiment, and any aspect or embodiment of a system can be configured to perform a method of another aspect or embodiment. 
     Needle-Free 
     Some embodiments use a needle to help insert a glucose sensor into subcutaneous tissue. Some people, however, are fearful of needles. In addition, needle disposal can require using a sharps container, which may not be readily available. 
     Many embodiments do not use a needle to insert the sensor, which can help people feel more comfortable inserting the sensor and can eliminate the need to use a sharps container to dispose of the applicator or portions thereof. 
     U.S. Patent Publication No. US-2011-0077490-A1, U.S. Patent Publication No. US-2014-0107450-A1, and U.S. Patent Publication No. US-2014-0213866-A1 describe several needle-free embodiments. The entire contents of U.S. Patent Publication No. US-2011-0077490-A1, U.S. Patent Publication No. US-2014-0107450-A1, and U.S. Patent Publication No. US-2014-0213866-A1 are incorporated by reference herein. 
     Any of the embodiments described herein can be used with or without a needle. For example, the embodiments described in the context of  FIGS.  1 - 50    can be used with or without a needle. For example, the embodiment shown in  FIG.  7    can be used in a very similar way without the needle  156 . In this needle-free embodiment, moving the first portion  150  distally drives a distal portion of the glucose sensor  138  into the skin (without the use of a needle  156 ). In needle-free embodiments, the sensor  138  can have sufficient buckling resistance such that (when supported by the hole  180 ) the sensor  138  does not buckle. Sharpening a distal tip of the sensor  138  can also facilitate needle-free insertion into the host. 
       FIG.  56    illustrates an embodiment very similar to the embodiment shown in  FIG.  7    except that the embodiment of  FIG.  56    does not include a needle. The telescoping assembly  132   b  pushes the sensor  138  (which can be any type of analyte sensor) into the body of the host. The embodiment shown in  FIG.  56    does not include a needle hub  162 , a spring  234 , or a needle retraction mechanism  158  (as shown in  FIG.  7   ) but can include any of the items and features described in the context of other embodiments herein. 
       FIG.  57    illustrates the first portion  150  moving distally relative to the second portion  152  of the telescoping assembly  132   b  to move the sensor module  134  and the sensor  138  towards the base  128  in preparation to couple the sensor module  134  and the sensor  138  to the base  128 . 
       FIG.  58    illustrates the first portion  150  in a distal ending position relative to the second portion  152 . The sensor module  134  and the sensor  138  are coupled to the base  128 . The base  128  is no longer coupled to the telescoping assembly  132   b  such that the telescoping assembly  132   b  can be discarded while leaving the adhesive  126  coupled to the skin of the host (as described in the context of  FIGS.  4 - 6   ). 
     The embodiment illustrated in  FIGS.  56 - 58    can be integrated into the applicator system  104  shown in  FIGS.  2  and  3   . 
     The items and features described in the context of  FIGS.  12 A- 50    can also be used with the embodiment illustrated in  FIGS.  56 - 58   . Items and features are described in the context of certain embodiments to reduce redundancy. The items and features shown in all the drawings, however, can be combined. The embodiments described herein have been designed to illustrate the interchangeability of the items and features described herein. 
       FIGS.  44  and  45    illustrate another embodiment of a telescoping assembly  132   g . This embodiment includes a first portion  150   g  that moves distally relative to a second portion  152   g  to push a glucose sensor  138   g  through a hole in a base  128   g  and into a host. 
     The first portion  150   g  (e.g., a pusher) of the telescoping assembly  132   g  can include a distal protrusion  352  that supports a substantially horizontal section of the glucose sensor  138   g  (e.g., as the glucose sensor  138   g  protrudes out from the sensor module  134   g ). The end of the distal protrusion  352  can include a groove  354  in which at least a portion of the glucose sensor  138   g  is located. The groove  354  can help retain the glucose sensor  138   g . The distal protrusion  352  can provide axial support to the glucose sensor  138   g  (e.g., to push the glucose sensor  138   g  distally into the tissue of the host). 
     The base  128   g  can include a funnel  182   g  that faces proximally to help guide a distal end of the glucose sensor  138   g  into a hole  180   g  in the base  128   g . The hole  180   g  can radially support the sensor  138   g  as the sensor  138   g  is inserted into the tissue of the host. 
     When the first portion  150   g  of the telescoping assembly  132   g  is in the proximal starting position, the distal end of the glucose sensor  138   g  can be located in the hole  180   g  to help guide the glucose sensor  138   g  in the proper distal direction. 
     The hole  180   g  can exit a convex distal protrusion  174   g  in the base  128   g . The convex distal protrusion  174   g  can help tension the skin prior to sensor insertion. As described more fully in other embodiments, the base  128   g  can rest against the skin of the host as the sensor module  134   g  moves distally towards the base  128   g  and then is coupled to the base  128   g.    
     The telescoping assembly  132   g  (e.g., an applicator) does not include a needle. As a result, there is no sharp in the applicator, which eliminates any need for post-use sharp protection. This design trait precludes a need for a retraction spring or needle hub. The distal end of the sensor wire  138   g  can be sharpened to a point to mitigate a need for an insertion needle. 
     The telescoping assembly  132   g  (e.g., an applicator) can include the first portion  150   g  and the second portion  152   g . The base  128   g  can be coupled to a distal end of the first portion  150   g . The glucose sensor  138   g  and the sensor module  134   g  can be coupled to a distal end of the first portion  150   g  such that he applicator does not require a spring, needle, or needle hub; the first portion  150   g  is secured in a proximal starting position by an interference between the first portion  150   g  and the second portion  152   g  of the telescoping assembly  132   g ; and/or applying a distal force that is greater than a breakaway threshold of the interference causes the first portion  150   g  to move distally relative to the second portion  152   g  (e.g., until the sensor  138   g  is inserted into the tissue and the sensor module  134   g  is coupled to the base  128   g ). 
       FIGS.  46  and  47    illustrate a similar needle-free embodiment. This embodiment does not use the distal protrusion  352  shown in  FIG.  45   . Instead, the sensor module  134   h  includes a distally oriented channel  358  that directs the sensor  138   h  distally such that the glucose sensor  138   h  includes a bend that is at least 45 degrees and/or less than 135 degrees. A channel cover  362  secures the glucose sensor  138   h  in the distally oriented channel  358 . 
     The embodiments illustrated in  FIGS.  44 - 47    can be integrated into the applicator system  104  shown in  FIGS.  2  and  3   . Referring now to  FIG.  2   , the electronics unit  500  (e.g., a transmitter having a battery) can be detachably coupled to the sterile barrier shell  120 . The rest of the applicator system  104  can be sterilized, and then the electronics unit  500  can be coupled to the sterile barrier shell  120  (such that the electronics unit  500  is not sterilized with the rest of the applicator system  104 ). 
     The items and features described in the context of  FIGS.  12 A- 43  and  48 - 70    can also be used with the embodiments illustrated in  FIGS.  44 - 47   . Items and features are described in the context of certain embodiments to reduce redundancy. The items and features shown in all the drawings, however, can be combined. The embodiments described herein have been designed to illustrate the interchangeability of the items and features described herein. 
     Any of the features described in the context of  FIGS.  44 - 47    can be applicable to all aspects and embodiments identified herein. For example, the embodiments described in the context of  FIGS.  44 - 47    can be combined with the embodiments described in the context of  FIGS.  1 - 43  and  48 - 70   . Moreover, any of the features of an embodiment is independently combinable, partly or wholly with other embodiments described herein in any way, e.g., one, two, or three or more embodiments may be combinable in whole or in part. Further, any of the features of an embodiment may be made optional to other aspects or embodiments. Any aspect or embodiment of a method can be performed by a system or apparatus of another aspect or embodiment, and any aspect or embodiment of a system can be configured to perform a method of another aspect or embodiment. 
     In some embodiments, the sensor  138  can be deployed (e.g., into the skin of the host) in response to coupling the electronics unit  500  (e.g., a transmitter) to the base  128 . The sensor  138  can be any type of analyte sensor (e.g., a glucose sensor). 
     Premature deployment of the sensor  138  can cause insertion of the sensor  138  into the wrong person and/or insufficient sensor insertion depth. Premature deployment can also damage the sensor  138 , which in some embodiments, can be fragile. Thus, there is a need to reduce the likelihood of premature sensor deployment. 
     One way to reduce the likelihood of premature sensor deployment is for the system to include an initial resistance (e.g., to coupling the electronics unit  500  to the base  128 ). The initial resistance can necessitate a force buildup prior to overcoming the initial resistance. When the initial resistance is overcome, the sensor  138  is typically deployed faster than would be the case without an initial resistance (e.g., due to the force buildup, which can be at least 0.5 pounds, 1 pound, and/or less than 5 pounds). This fast deployment can reduce pain associated with the sensor insertion process. 
     In some embodiments, the resistance to coupling the electronics unit  500  to the base  128  after overcoming the initial resistance is less than 10 percent of the initial resistance, less than 40 percent of the initial resistance, and/or at least 5 percent of the initial resistance. Having a low resistance to coupling the electronics unit  500  to the base  128  after overcoming the initial resistance can enable fast sensor insertion, which can reduce the pain associated with the sensor insertion process. 
       FIGS.  56 - 58    illustrate the first portion  150  deploying the sensor  138  into the skin of the host. In some embodiments, the first portion  150  is replaced with the electronics unit  500  shown in  FIG.  4    such that coupling the electronics unit  500  to the base  128  pushes the sensor  138  into the skin of the host. Referring now to  FIGS.  4  and  56 - 58   , the protrusion  240  (as explained in other embodiments) can be a portion of the electronics unit  500  such that moving the electronics unit distally relative to the second portion  152  and/or coupling the electronics unit  500  to the base  128  requires overcoming the initial resistance of the protrusion  240 . 
     In some embodiments configured such that the sensor  138  is deployed (e.g., into the skin of the host) in response to coupling the electronics unit  500  to the base  128 , a telescoping assembly  132   b  is not used. Instead, features of the base  128  provide the initial resistance to coupling the electronics unit  500  to the base  128 . Although the locking feature  230  in  FIG.  33    is used for different purposes in some other embodiments, the locking feature  230  of the base  128  can couple with a corresponding feature of the electronics unit  500 . This coupling can require overcoming an initial resistance. 
     Any of the features and embodiments described in the context of  FIGS.  1 - 70    can be applicable to all aspects and embodiments in which the sensor  138  is deployed (e.g., into the skin of the host) in response to coupling the electronics unit  500  (e.g., a transmitter) to the base  128 . 
     Vertical Locking 
     After a telescoping assembly (e.g., an applicator) has been used to insert a glucose sensor, the needle used to insert the glucose sensor could inadvertently penetrate another person. To guard against this risk, the telescoping assembly can protect people from subsequent needle-stick injuries by preventing the first portion of the telescoping assembly from moving distally relative to the second portion after the sensor has been inserted into the host. 
       FIG.  48    illustrates a perspective, cross-sectional view of a telescoping assembly  132   i  that includes a first portion  150   i  and a second portion  152   i . Referring now to  FIGS.  48 - 50   , the first portion  150   i  is configured to telescope distally relative to the second portion  152   i . The second portion  152   i  of the telescoping assembly  132   i  can include a proximal protrusion  364  that can slide past a lock-out feature  366  of the first portion  150   i  of the telescoping assembly  132   i  as the first portion  150   i  is moved distally. 
     The proximal protrusion  364  can be biased such that elastic deformation of the proximal protrusion  364  creates a force configured to press the proximal protrusion  364  into the bottom of the lock-out feature  366  once the proximal protrusion  364  engages the lock-out feature  366 . 
     The proximal protrusion  364  does not catch on the lock-out feature  366  as the first portion  150   i  moves distally a first time. Once the first portion  150   i  is in a distal ending position, a spring can push the first portion  150   i  to a second proximal position. Rather than returning to the starting proximal position, the proximal protrusion  364  catches on the lock-out feature  366  (due to the bias of the proximal protrusion  364  and the distally facing notch  368  of the lock-out feature  366 ). 
     Once a proximal end of the proximal protrusion  364  is captured in the lock-out feature  366 , the rigidity of the proximal protrusion  364  prevents the first portion  150   i  of the telescoping assembly  132   i  from moving distally a second time. 
     As the first portion  150   i  moves distally relative to the second portion  152   i , a ramp  370  of the first portion  150   i  pushes the proximal protrusion  364  outward (towards the lock-out feature  366 ). The proximal protrusion  364  can be located between two distal protrusions  372  of the first portion  150   i . The distal protrusions  372  can guide the proximal protrusion  364  along the ramp  370 . 
     As a portion of the proximal protrusion  364  slides along the ramp  370  (as the first portion  150   i  moves distally), the ramp bends the proximal protrusion  364  until a portion of the proximal protrusion  364  that was previously between the two distal protrusions  372  is no longer between the distal protrusions  372 . Once the portion of the proximal protrusion  364  is no longer between the two distal protrusions  372 , the proximal protrusion  364  is in a state to catch on the notch  368 . The notch  368  can be part of the distal protrusions  372 . 
     The second portion  152   i  of the telescoping assembly  132   i  can include a proximal protrusion  364 , which can be oriented at an angle between zero and 45 degrees relative to a central axis). The first portion  150   i  of the telescoping assembly  132   i  can include features that cause the proximal protrusion  364  to follow a first path as the first portion  150   i  moves distally and then to follow a second path as the first portion  150   i  moves proximally. The second path includes a locking feature  366  that prevents the first portion  150   i  from moving distally a second time. 
     The first portion  150   i  can include a ramp  370  that guides the proximal protrusion  364  along the first path. A distal protrusion (e.g., the ramp  370 ) of the first portion  150   i  can bias the proximal protrusion  364  to cause the proximal protrusion  364  to enter the second path as the first portion  150   i  moves proximally. The proximal protrusion  364  can be a flex arm. The lock  366  can comprise a distally facing notch  368  that catches on a proximal end of the proximal protrusion  364 . 
     As shown in  FIGS.  48  and  50   , the telescoping assembly  132   i  can include a sensor module  134   i . The sensor module  134   i  can be any of the sensor modules described herein. 
     Any of the features described in the context of  FIGS.  48 - 50    can be applicable to all aspects and embodiments identified herein. For example, the embodiments described in the context of  FIGS.  48 - 50    can be combined with the embodiments described in the context of  FIGS.  1 - 47  and  51 - 70   . Moreover, any of the features of an embodiment is independently combinable, partly or wholly with other embodiments described herein in any way, e.g., one, two, or three or more embodiments may be combinable in whole or in part. Further, any of the features of an embodiment may be made optional to other aspects or embodiments. Any aspect or embodiment of a method can be performed by a system or apparatus of another aspect or embodiment, and any aspect or embodiment of a system can be configured to perform a method of another aspect or embodiment. 
     Dual-Spring Assembly 
     Partial sensor insertion can lead to suboptimal sensing. In some cases, partial sensor insertion can create a needle-stick hazard (due to the needle not retracting into a protective housing). Thus, there is a need for systems that ensure full sensor insertion. 
     The embodiment illustrated in  FIGS.  61 - 64    dramatically reduces the odds of partial sensor insertion by precluding sensor insertion until sufficient potential energy is stored in the system. The potential energy is stored in a first spring  402 . 
     The system includes many items from the embodiment illustrated in  FIG.  7    (e.g., the base  128  and the sensor module  134 ). The system includes an optional needle  156  and needle hub  162 . The embodiment illustrated in  FIGS.  61 - 64    can also be configured to be needle-free by removing the needle  156 , the second spring  234 , the needle hub  162 , and the needle retraction mechanism  158 . 
     The telescoping assembly  132   k  has three portions  150   k ,  152   k ,  392 . Moving the third portion  392  distally relative to the second portion  152   k  stores energy in the first spring  402  (by compressing the first spring  402 ). Once the first portion  150   k  is unlocked from the second portion  152   k , the energy stored in the compressed first spring  402  is used to push the first portion  150   k  distally relative to the second portion  152   k  to drive the sensor  138  (shown in  FIG.  7   ) into the skin of the host. 
     To ensure the first portion  150   k  does not move distally relative to the second portion  152   k  until the first spring  402  is sufficiently compressed (and thus has enough stored energy), the first portion  150   k  is locked to the second portion  152   k . Once the first spring  402  is sufficiently compressed (and thus has enough stored energy), the system unlocks the first portion  150   k  from the second portion  152   k  to enable the stored energy to move the sensor  138  (and in some embodiments the needle  156 ) into the skin of the host. 
     The telescoping assembly  132   k  can lock the third portion  392  to the second portion  152   k  in response to the third portion  392  reaching a sufficiently distal position relative to the second portion  152   k . A protrusion  408  can couple with a hole  410  to lock the third portion  392  to the second portion  152   k.    
     Some embodiments do not include locking protrusion  408  and do not lock the third portion  392  to the second portion  152   k  in response to the third portion  392  reaching a sufficiently distal position relative to the second portion  152   k.    
     In several embodiments, sufficiently distal positions are at least 3 millimeters, at least 5 millimeters, and/or less than 30 millimeters distal relative to the proximal starting position. 
     The telescoping assembly  132   k  can lock the first portion  150   k  to the second portion  152   k  in response to the first portion  150   k  reaching a sufficiently distal position relative to the second portion  152   k . A protrusion  412  (e.g., a distal protrusion) can couple with a hole  414  (e.g., in a surface that is within plus or minus 30 degrees of perpendicular to the central axis of the telescoping assembly  132   k ) to lock the first portion  150   k  to the second portion  152   k.    
     Some embodiments include a needle  156  to help insert a sensor into skin of a host. In embodiments that include a needle  156 , the telescoping assembly  132   k  can include the needle retraction mechanism  158  described in the context of  FIG.  7   . Moving the first portion  150   k  to a sufficiently distal position relative to the second portion  152   k  can trigger the needle retraction mechanism  158  (e.g., can release a latch) to enable a second spring  234  to retract the needle  156 . 
       FIG.  61    illustrates a system for applying an on-skin sensor assembly  600  (shown in  FIGS.  4 - 6   ) to a skin of a host. The system comprises a telescoping assembly  132   k  having a first portion  150   k  configured to move distally relative to a second portion  152   k  from a proximal starting position (e.g., the position shown in  FIG.  61   ) to a distal position (e.g., the position shown in  FIG.  64   ) along a path; a sensor  138  (shown in  FIG.  64   ) coupled to the first portion  150   k ; and a base  128  comprising adhesive  126  configured to couple the sensor  138  to the skin. The telescoping assembly  132   k  can further comprise a third portion  392  configured to move distally relative to the second portion  152   k.    
     In some embodiments, the first portion  150   k  is located inside of the second portion  152   k  such that the second portion  152   k  wraps around the first portion  150   k  in a cross section taken perpendicularly to the central axis of the telescoping assembly  132   k.    
     In some embodiments, a first spring  402  is positioned between the third portion  392  and the second portion  152   k  such that moving the third portion  392  distally relative to the second portion  152   k  compresses the first spring  402 . The first spring  402  can be a metal helical spring and/or a metal conical spring. In several embodiments, the first spring  402  is a feature molded as part of the third portion  392 , as part of the second portion  152   k , or as part of the first portion  150   k . The first spring  402  can be molded plastic. 
     The telescoping assembly  132   k  can be configured such that the first spring  402  is not compressed in the proximal starting position and/or not compressed during storage. In several embodiments, the telescoping assembly  132   k  can be configured such that the first spring  402  is not compressed more than 15 percent in the proximal starting position and/or during storage (e.g., to avoid detrimental spring relaxation and/or creep of other components such as at least one of the third portion  392 , the second portion  152   k , and the first portion  150   k ). 
     Some embodiments that include a needle  156  do not include a needle hub  162 . In these embodiments, the second spring  234  can be located between the second portion  152   k  and the first portion  150   k  such that moving the first portion  150   k  distally relative to the second portion  152   k  compresses the second spring  234  to enable the second spring  234  to push the first portion  150   k  proximally relative to the second portion  152   k  to retract the needle  156  (e.g., after sensor insertion). 
     In several embodiments, the second spring  234  is compressed while the telescoping assembly  132   k  is in the proximal starting position. For example, the second spring  234  can be compressed at the factory while the telescoping assembly  132   k  is being assembled such that when the user receives the telescoping assembly  132   k , the second spring  234  is already compressed (e.g., compressed enough to retract the needle  156 ). 
     The second spring  234  can have any of the attributes and features associated with the spring  234  described in the context of other embodiments herein (e.g., in the context of the embodiment of  FIG.  7   ). 
     In some embodiments, the movement of the sensor module  134  (e.g., an analyte sensor module) and the sensor  138  (e.g., an analyte sensor) relative to the base  128  can be as described in the context of other embodiments (e.g., as shown by the progression illustrated by  FIGS.  7 - 11   ). 
     In the proximal starting position of the telescoping assembly  132   k , the first portion  150   k  can be locked to the second portion  152   k . The system can be configured such that moving the third portion  392  distally relative to the second portion  152   k  unlocks the first portion  150   k  from the second portion  152   k.    
     In several embodiments, a first proximal protrusion  394  having a first hook  396  passes through a first hole  398  in the second portion  152   k  to lock the first portion  150   k  to the second portion  152   k . The third portion  392  can comprise a first distal protrusion  404 . The system can be configured such that moving the third portion  392  distally relative to the second portion  152   k  engages a ramp  406  to bend the first proximal protrusion  394  to unlock the first portion  150   k  from the second portion  152   k.    
     In some embodiments, the sensor  138  is located within the second portion  152   k  while the base  128  protrudes from the distal end of the system such that the system is configured to couple the sensor  138  to the base  128  by moving the first portion  150   k  distally relative to the second portion  152   k.    
     In several embodiments, a sensor module  134  is coupled to a distal portion of the first portion  150   k  such that moving the first portion  150   k  to the distal position couples the sensor module  134  to the base  128 . This coupling can be as described in the context of other embodiments herein. The sensor  138  can be coupled to the sensor module  134  while the first portion  150   k  is located in the proximal starting position. 
     The system can be configured such that the third portion  392  moves distally relative to the second portion  152   k  before the first spring  402  moves the first portion  150   k  distally relative to the second portion  152   k . The system can be configured such that moving the third portion  392  distally relative to the second portion  152   k  unlocks the first portion  150   k  from the second portion  150   k  and locks the third portion  392  to the second portion  152   k.    
     A first protrusion  408  couples with a hole  410  of at least one of the second portion  152   k  and the third portion  392  to lock the third portion  392  to the second portion  152   k.    
     In some embodiments, the system comprises a second protrusion  412  that couples with a hole  414  of at least one of the first portion  150   k  and the second portion  152   k  to lock the first portion  150   k  to the second portion  152   k  in response to moving the first portion  150   k  distally relative to the second portion  152   k.    
     In several embodiments, a first spring  402  is positioned between the third portion  392  and the second portion  152   k  such that moving the third portion  392  distally relative to the second portion  152   k  compresses the first spring  402  and unlocks the first portion  150   k  from the second portion  152   k , which enables the compressed first spring  402  to push the first portion  150   k  distally relative to the second portion  152   k , which pushes at least a portion of the sensor  138  out of the distal end of the system and triggers a needle retraction mechanism  158  to enable a second spring  234  to retract a needle  156 . 
     In yet another aspect, disclosed herein is a dual spring-based sensor insertion device having a pre-connected sensor assembly (i.e. an analyte sensor electrically coupled to at least one electrical contact before sensor deployment). Such a sensor insertion device provides convenient and reliable insertion of a sensor into a user&#39;s skin by a needle as well as reliable retraction of a needle after the sensor is inserted, which are features that provide convenience to users as well as predictability and reliability of the insertion mechanism. The reliability and convenience of a dual spring based sensor insertion device having an automatic insertion and automatic retraction provide is a significant advancement in the field of sensor insertion devices. Furthermore, such a device can provide both safety and shelf stability. 
     In several embodiments, the insertion device can include a first spring and a second spring. In such embodiments, either or both of the first spring and the second spring can be integrally formed with portions of a telescoping assembly, such as the first portion and the second portion of a telescoping assembly. In several embodiments, either or both of the first spring and the second spring can be formed separately from and operatively coupled to portions of the telescoping assembly. For example, in some embodiments, the insertion spring can be integrally formed with a portion of the telescoping assembly while the retraction spring is a separate part which is operatively coupled to a portion of the telescoping assembly. 
     In some embodiments, rather than being configured to undergo compression during energization, either or both of the first spring and the second spring can be configured to undergo tensioning during energization. In these embodiments, the couplings between the springs and the portions of the telescoping assembly, as well as the couplings between the moving portions of the assembly (for example in the resting state, and during activation, deployment, and retraction) can be adjusted to drive and/or facilitate the desired actions and reactions within the system. For example, in an embodiment employing a tensioned retraction spring to drive the insertion process, the retraction spring can be coupled to or integrally formed with the second portion of the telescoping assembly. In such an embodiment, the retraction spring can be pre-tensioned in the resting state. In other such embodiments, the retraction spring can be untensioned in the resting state, and tensioned during the sensor insertion process. 
     In several embodiments, either or both of the first spring and the second spring can be substantially unenergized and/or unstressed when the system is in a resting state. In several embodiments, either or both of the first spring and the second spring can be energized and/or stressed when the system is in a resting state. As used herein, the term “energized” means that enough potential energy is stored in the spring to perform the desired actions and reactions within the system. In some embodiments, the first spring can be partly energized in the resting state, such that the user can supply a lesser amount of force to fully energize the first spring. In some embodiments, the second spring can be partly energized in the resting state, such that the energy stored in the first spring (either in the resting state or after energization by a user) can provide force to energize the second spring. In some embodiments, the energy stored in the first spring can provide sufficient force to energize the second spring to at least retract the needle from the skin. In some embodiments, either or both of the first spring and the second spring can be compressed or tensioned by 50% or more, 60% or more, 70% or more, 80% or more, 90% or more, or 100% in the resting state. In other embodiments, either or both of the first spring and the second spring can be compressed or tensioned by 50% or less, 40% or less, 30% or less, 20% or less, 15% or less, 10% or less, 5% or less, or 0% in the resting state. 
     In embodiments in which both the first spring and the second spring are substantially unenergized in the resting state, they can be stressed by the same amounts, similar amounts, or entirely different amounts. In embodiments in which both the first spring and the second spring are effectively energized in the resting state, they can be stressed by the same amounts, similar amounts, or entirely different amounts. In embodiments in which the second spring is substantially unenergized in the resting state, the first spring can be configured to store enough energy to drive both the desired movement in the system (e.g., the movement of the first portion in a distal direction), as well as the energization of the second spring. 
     With reference now to  FIGS.  71 - 75   , another embodiment of a system  104   m  for applying an on-skin sensor assembly to skin of a host is illustrated. The embodiment illustrated in  FIGS.  71 - 75    may reduce the potential of incomplete sensor insertion by precluding sensor insertion until sufficient potential energy is stored in the system. The potential energy for inserting the sensor can be stored in an actuator, such as a first spring  402   m . The embodiment may provide other advantages such as controlled speed, controlled force, and improved user experience. 
     The system  104   m  may include many features from the embodiment illustrated in  FIG.  7    (e.g., the needle  156 , the base  128  and the sensor module  134 ). The system  104   m  may include alternative elements, such as, but not limited to, a needle hub  162   m , a second spring  234   m , and a needle retraction mechanism  158   m . The embodiment illustrated in  FIGS.  71 - 75    can also be configured to be needle-free by removing the needle  156 , the second spring  234   m , the needle hub  162   m , and the needle retraction mechanism  158   m . In such embodiments, the sensor may be a self-insertable sensor. 
     The system  104   m  may include many features that are similar to those of the embodiment illustrated in  FIGS.  61 - 64    (e.g., a telescoping assembly  132   m ) including a first portion  150   m , a second portion  152   m , and a third portion  392   m ; with locking features  396   m  and  398   m  configured to releasably lock the first portion  150   m  to the second portion  152   m  until the third portion  392   m  has reached a sufficiently distal position relative to the second portion  152   m  to compress the first spring  402   m  and store enough energy in the spring  402   m  to drive insertion of the sensor  138  (and in some embodiments the needle  156 ) into the skin of a host; locking features  408   m  and  410   m  configured to lock the third portion  392   m  to the second portion  152   m  (e.g., to prevent proximal movement of the third portion  392   m  relative to the second portion  152   m ) in response to the third portion  392   m  reaching a sufficiently distal position relative to the second portion  152   m ; unlocking features  404   m  and  406   m  configured to unlock the locking features  396   m  and  398   m  at least after the third portion  392   m  is locked to the second portion  152   m  and/or the first spring  402   m  is sufficiently compressed; locking features  412   m  and  414   m  configured to lock the first portion  150   m  to the second portion  152   m  in response to the first portion  150   m  reaching a sufficiently distal position relative to the second portion  152   m  to drive the sensor  138  (and in some embodiments the needle  156 ) into the skin of the host; and a needle retraction mechanism  158   m  configured to unlock the needle hub  162   m  from the first portion  150   m  (e.g., to allow proximal movement of the needle hub  162   m  with respect to the first portion  150   m ) at least once the needle hub  162   m  has reached a sufficiently distal position and thereby enable a second spring  234   m  to retract the needle  156 ). 
       FIG.  71    illustrates a cross-sectional perspective view of the applicator system  104   m  in a resting state (e.g., as provided to the consumer, before activation by the user and deployment of the applicator system). As illustrated in the figure, the first spring  402   m  can be neither in tension nor compression, such that the first spring is substantially unenergized. In some embodiments, the first spring  402   m  can be slightly in tension or slightly in compression (e.g., neither tensioned nor compressed by more than 15 percent) in a resting state, such that the first spring is substantially or mostly unenergized in the resting state. In some embodiments, the first spring can be effectively unenergized, e.g. can be minimally energized but not to an extent that would create any type of chain reaction in the system, in a resting state. 
     In the embodiment illustrated in  FIGS.  71 - 75   , the first spring  402   m  is integrally formed as part of the third portion  392   m . In some embodiments, the first spring  402   m  can be integrally formed as part of other components of the system  104   m , such as, but not limited to, the first portion  150   m , second portion  152   m , etc. An integrally formed spring such as the one illustrated in  FIGS.  71 - 75    offers advantages including the reduction in the number of parts in a system as well as the reduction in the amount of assembly processes. The first spring  402   m  can be molded plastic. As illustrated in  FIG.  71   , in the resting state, the second spring  234   m  is also substantially unenergized (e.g., neither tensioned nor compressed by more than 15 percent). The second spring  234   m  is integrally formed as part of the needle hub  162   m . In some embodiments, the second spring  234   m  can be integrally formed as part of other components of the system  104   m , such as, but not limited to, the first portion  150   m , second portion  152   m , base  128 , etc. The second spring  234   m  can be molded plastic. Such a configuration can simplify manufacture and assembly of the system  104   m , while avoiding detrimental relaxation and/or creep of the first spring  402   m , the second spring  234   m , or other components of the system  104   m  during storage and/or before deployment. It is also contemplated that in other embodiments, the first spring  402   m  and/or the second spring  234   m  can comprise metal. 
     In some embodiments, first spring  402   m  and/or second spring  234   m  can comprise a molded plastic, such as, but not limited to: polycarbonate (PC), acrylonitrile butadiene styrene (ABS), PC/ABS blend, Nylon, polyethylene (PE), polypropylene (PP), and Acetal. In some embodiments, first spring  402   m  and/or second spring  234   m  have a spring constant less than 10 lb/inch. 
     Applicator system  104  may be energized by moving one component relative to another. For example, moving the third portion  392   m  distally relative to the second portion  152   m , when the second portion  152   m  is placed against the skin of a host or another surface can store energy in the first spring  402   m  as it compresses against first portion  150   m . The third portion  392   m  may be moved distally until the locking features  408   m  and  410   m  (see  FIG.  73   ) engage together. In some embodiments, the third portion  392   m  may be moved further distally until unlocking features  404   m  engages locking feature  396   m . Unlocking feature  404   m  may engage and release locking feature  396   m  and allow first portion  150   m  to move distally. In some embodiments, locking features  408   m  and  410   m  couple together before locking feature  396   m  is disengaged from locking feature  398   m . In other embodiments, unlocking feature  404   m  engages locking feature  396   m  and causes locking feature  396   m  to disengage from locking feature  398   m , locking features  408   m  and  410   m  may couple together. In some embodiments, locking feature  408   m  is a protrusion featuring a hook portion, locking feature  410   m  is a hole featuring an angled surface, unlocking feature  404   m  is a distal protrusion featuring an angled surface, locking feature  396   m  is a hook featuring a ramp  406   m , and locking feature  398   m  is an aperture. The sensor module  134  remains in a proximal starting position while the first spring  402   m  is being energized. 
       FIG.  72    illustrates a cross-sectional perspective view of the applicator system  104   m , with the first spring  402   m  compressed and with the unlocking features  404   m  and  406   m  engaged so as to unlock the first portion  150   m  from the second portion  152   m . Until the first portion  150   m  is unlocked from the second portion  152   m , the sensor module  134  remains at its proximal starting position, and the second spring  234   m  remains substantially unenergized.  FIG.  73    illustrates a rotated cross-sectional perspective view of the applicator system  104   m , and shows the locking features  408   m  and  410   m  engaged to prevent proximal movement of the third portion  392   m  with respect to the second portion  152   m . In some embodiments, as illustrated in  FIG.  73   , the system can include a secondary locking feature  409   m  which is configured to cooperate with the opening  410   m  to prevent the third portion  392  from falling off or otherwise separating from the remainder of the system  104   m  prior to deployment. 
       FIG.  74    illustrates a cross-sectional perspective view of the applicator system  104   m , with the system  104   m  having been activated by the disengagement of the first portion  150   m  with respect to the second portion  152   m . As can be seen in  FIG.  74   , once the first portion  150   m  and the second portion  152   m  are disengaged or released, the potential energy stored in the first spring  402   m  drives the first portion  150   m  in a distal direction along with the needle hub  162   m  and the sensor module  134 . This movement compresses the second spring  234   m  and deploys the needle  156  and the sensor module  134  distally to a distal insertion position in which the sensor module  134  is coupled to the base  128  and the needle  156  extends distally of the base  128 . Once the needle  156  and the sensor module  134  reach the distal insertion position, the locking features  412   m ,  414   m  (see  FIG.  73   ) engage to prevent proximal movement of the first portion  150   m  with respect to the second portion  152   m , and the unlocking features of the needle retraction mechanism  158   m  (e.g., the proximal protrusions  170   m , the release feature  160   m , and the latch  236   m  comprising ends  164   m  of the release feature  160   m  and overhangs  166   m  of the first portion  150   m ) cooperate to release the latch  236   m . Optionally, the user may hear a click after the second spring  243   m  is activated, which may indicate to the user that the cap is locked in place. 
     Once the latch  236   m  is released, the potential energy stored in the compressed second spring  234   m  drives the needle hub  162   m  back in a proximal direction, while the first portion  150   m  remains in a distal deployed position along with the sensor module  134 . The potential energy stored can be between 0.25 pounds to 4 pounds. In preferred embodiments, the potential energy stored is between about 1 to 2 pounds.  FIG.  75    illustrates a cross-sectional perspective view of the applicator system  104   m  with the sensor module  134  in a distal deployed position, coupled to the base  128 , and with the needle hub  162   m  retracted to a proximal retracted position. 
     Systems configured in accordance with embodiments may provide an inherently safe and shelf stable system for insertion of a sensor. An unloaded (i.e. substantially uncompressed and substantially unactivated) spring may not fire prematurely. Indeed, such a system is largely incapable of unintentional firing without direct interaction from a user since the first spring and/or second spring are substantially un-energized on the shelf. Moreover, it is contemplated that a system having a substantially uncompressed spring prior to activation possesses shelf stability since elements of the system are not exposed to a force or phase change over time (such as creep, environment, defects from time dependent load conditions, etc.) as compared to pre-energized insertion devices. Substantially uncompressed first and second springs can provide a system where the substantially unenergized first spring  404   m  is configured to load energy sufficient to drive a sensor from a proximal position to a distal position and also to transfer energy to the second spring  234   m  to drive a needle to a fully retracted position. 
     Other embodiments can also be configured to achieve these benefits. For example,  FIGS.  76 - 79    illustrate another embodiment of a system  104   n  for applying an on-skin sensor assembly to skin of a host. The system  104   n  includes many features that are similar to those of the embodiment illustrated in  FIGS.  71 - 75    (e.g., a telescoping assembly  132   n  including a first portion  150   n , a second portion  152   n , and a third portion  392   n ; a needle hub  162   n ; a first spring  402   n ; and a second spring  234   n ). In the embodiment illustrated in  FIGS.  76 - 79   , the first spring  402   n  is formed separately from and operatively coupled to the third portion  392   n . The second spring  234   n  is formed separately from and operatively coupled to the needle hub  162   n . The first spring and/or the second spring can each comprise a helical spring having a circular cross section. In some embodiments, the first spring and/or the second spring can each comprise a helical spring having a square or non-circular cross section. The first spring and/or the second spring can comprise metal, such as, but not limited to, stainless steel, steel, or other types of metals. Alternatively, in some embodiments, one or both of the first spring and the second spring can be integrally formed with a portion of the applicator assembly. For example and without limitation, in some embodiments the first spring can be integrally formed with the first portion. In some embodiments, the second spring can be integrally formed with the needle hub. In several embodiments, the first spring and/or the second spring can be molded plastic, such as, but not limited to, PC or ABS. 
       FIG.  76    illustrates a cross-sectional side view of the system  104   n  in a resting state, in which both the first spring  402   n  and the second spring  234   n  are unstressed and unenergized. In the resting state, the first portion  150   n  can be fixed with respect to the second portion  152   n , at least in an axial direction, whereas the third portion  392   n  is movable in at least a distal direction with respect to the first portion  150   n . The first portion  150   n  and the second portion  152   n  can be fixed with respect to one another in any suitable fashion, for example by cooperating releasable locking features (e.g., the locking features as described in  FIGS.  71 - 75   , or similar features) coupled to or forming part of the first portion  150   n  and the second portion  152   n . The system  104   n  includes an on-skin component  134   n  which is releasably coupled to the needle hub  162   n . The on-skin component can comprise a sensor module, such as the sensor module  134  described in connection with  FIG.  3   , or a combined sensor module and base assembly, or an integrated sensor module/base/transmitter assembly, or any other component which is desirably applied to the skin of a host, whether directly or indirectly, for example via an adhesive patch. 
     In the resting state illustrated in  FIG.  76   , the on-skin component  134   n  is disposed at a proximal starting position, between the proximal and distal ends of the system  104   n . The distal end of the needle  156  may also be disposed between the proximal and distal ends of the system  104   n . In the resting state, the distal end of the first spring  402   n  abuts a proximally-facing surface of the first portion  150   n . The application of force against the proximally-facing surface of the third portion  392   n  causes the third portion  392   n  to move distally with respect to the first portion  150   n , compressing and thus energizing the first spring  402   n . In some embodiments, this process may be similar to the spring energization process described in connection with  FIG.  71   . 
       FIG.  77    illustrates a cross-sectional side view of the applicator system of  FIG.  76   , with the first spring  402   n  energized. When the third portion  392   n  has been moved sufficiently distally to energize the first spring  402   n , the third portion  392   n  becomes fixed, at least in an axial direction, with respect to the second portion  152   n . At or about the same time (e.g. simultaneously or subsequently), the first portion  150   n  becomes movable in at least a distal direction with respect to the second portion  152   n . The third portion  392   n  and the second portion  150   n  can be fixed with respect to one another in any suitable fashion, for example by cooperating locking features (e.g., the locking features described in  FIGS.  71 - 75   , or similar features) coupled to or forming part of the third portion  392   n  and the second portion  152   n , which are configured to engage with one another once the third portion  392   n  has reached a sufficiently distal position. The first portion  150   n  and the second portion  152   n  can be rendered movable with respect to one another by structure(s) (not shown in  FIGS.  76 - 79   ) configured to release the locking features which coupled them together in the resting configuration illustrated in  FIG.  76   . The first portion  150   n  includes overhangs (sometimes referred to as detents, undercuts, and/or needle hub engagement features)  166   n  which cooperate with release feature  160   n  of the needle hub  162   n  to fix the needle hub  162   n  with respect to the first portion  150   n , both while the system is in a resting state and during energization of the spring  392   n.    
       FIG.  78    illustrates a cross-sectional side view of the system  104   n , with the first portion  150   n  and the second portion  152   n  unlocked, activating the first spring  402   n  and allowing the energy stored therein to drive the first portion  150   n  in a distal direction. The movement of the first portion  150   n  also urges the needle hub  162   n  (as well as the on-skin component  134   n  which is coupled to the needle hub  162   n ) in a distal direction, compressing the second spring  234   n  against a proximally-facing surface of the second portion  152   n , coupling the on-skin component  134   n  to the base  128   n , and driving the needle  156  into the distal insertion position illustrated in  FIG.  78   . When the needle hub  162   n  has reached a sufficiently distal position to achieve these functions, the ends of the release feature  160   n  contact ramps  170   n  of the second portion  152   n  which cause the release feature  160   n  to compress inward (towards the central axis of the system  104   n ), disengaging the ends of the release feature  160   n  from the overhangs  166   n . In some embodiments, this process may be similar to the spring compression process described in connection with  FIG.  74   . In some embodiments, ramps  170   n  are proximally facing ramps. In other embodiments, ramps  170   n  are distally facing ramps (not shown). In some embodiments, the release feature or features can be configured to be compressed inward (or otherwise released) by relative rotational movement of certain components of the system, such as, for example, by twisting or other rotational movement of the first portion with respect to the second portion. In some embodiments, the release feature or features can extend in a direction normal to the axis of the system, and/or can extend circumferentially about the axis of the system, instead of (or in addition to) extending generally parallel to the axis of the system as illustrated in  FIG.  78   . 
       FIG.  79    illustrates a cross-sectional side view of the system  104   n , with the needle hub  162   n  released from engagement with the overhangs  166 , activating the second spring  234   n  and allowing the energy stored therein to drive the needle hub  162   n  in a proximal direction. As the needle hub  162   n  retracts to a proximal position, the on-skin component  134   n  decouples from the needle hub  162   n  to remain in a deployed position, coupled to the base  128   n.    
       FIGS.  80 - 85    illustrate another embodiment of a system  104   p  for applying an on-skin sensor assembly to skin of a host. A sensor insertion system such as the one illustrated in  FIGS.  80 - 85    may provide enhanced predictability in spring displacement of the second energized spring  234   p  because the second spring  234   p  is already compressed. Such a configuration can aid in properly ensuring the needle is retracted at a sufficient distance from the skin. In some embodiments, a system incorporating a pre-energized retraction spring can provide effective and reliable insertion and retraction while requiring a lesser amount of user-supplied force than, for example, a system in which both the insertion and retraction springs are substantially unenergized prior to deployment, making such a configuration more convenient for at least some users. Further, in some embodiments, a system incorporating one or more metal springs can provide effective and reliable insertion and retraction while requiring a lesser amount of force than a system in which both the insertion and retraction springs comprise plastic. The system  104   p  includes many features that are similar to those of the embodiment illustrated in  FIGS.  76 - 79    (e.g., a telescoping assembly  132   p  including a first portion  150   p , a second portion  152   p , and a third portion  392   p ; a needle hub  162   p ; a first spring  402   p ; a second spring  234   p ; an on-skin component  134   n , and a base  128   p ). In the embodiment illustrated in  FIGS.  80 - 85   , the first spring  402   p  is formed separately from and operatively coupled to the third portion  392   p . The second spring  234   p  is formed separately from and operatively coupled to the needle hub  162   p . The first spring and/or the second spring can comprise metal. Alternatively, in some embodiments, one or both of the first spring and the second spring can be integrally formed with a portion of the applicator assembly. For example and without limitation, in some embodiments the first spring can be integrally formed with the first portion. In some embodiments, the second spring can be integrally formed with the needle hub. In several embodiments, the first spring and/or the second spring can be molded plastic. 
       FIG.  80    illustrates a cross-sectional side view of the system  104   p  in a resting state, in which the first spring  402   p  is substantially unstressed and unenergized, but in which the second spring  234   n  is pre-energized (e.g., compressed). In the resting state illustrated in  FIG.  80   , the first portion  150   p  is locked to the second portion  152   p  so as to prevent proximal or distal movement of the first portion  150   p  with respect to the second portion  152   p . The first portion  150   p  and the second portion  152   n  can be locked together in any suitable fashion, for example by cooperating releasable locking features  396   p  and  398   p  (see  FIGS.  84  and  85   ) coupled to or forming part of the first portion  150   p  and the second portion  152   p . The needle hub  162   n  is also releasably fixed to the first portion  150   p . The needle hub  162   n  can be fixed to the first portion  150   p  in any suitable fashion, for example by features of the first portion  150   p  configured to engage or compress release feature (sometimes referred to as needle hub resistance features)  160   p  of the needle hub  162   p.    
     In the resting state illustrated in  FIG.  80   , the on-skin component  134   p  is disposed at a proximal starting position, such that the distal end of the needle  156  is disposed between the proximal and distal ends of the system  104   p . In the resting state, the distal end of the first spring  402   p  abuts a proximally-facing surface of the first portion  150   p . The application of force against the proximally-facing surface of the third portion  392   p  causes the third portion  392   p  to move distally with respect to the first portion  150   p , compressing and thus energizing the first spring  402   p . In some embodiments, this process may be similar to the spring energization process described in connection with  FIG.  76   . 
       FIG.  81    illustrates a cross-sectional side view of the system  104   p  of  FIG.  80   , after the third portion  392   n  has been moved to a sufficiently distally position to energize the first spring  402   p  and optionally lock the third portion  392   p  to the second portion  152   p . The third portion  392   n  and the second portion  150   n  can lock together in any suitable fashion, for example by cooperating locking features (e.g., the locking features described in  FIGS.  76 - 79   , or similar features) coupled to or forming part of the third portion  392   p  and the second portion  152   p . At or about the same time as the third portion  392   p  locks to the second portion  152   p  (e.g. simultaneously or subsequently), the unlocking features  404   p  and  406   p  (see  FIGS.  84  and  85   ) cooperate to release the lock between the first portion  150   p  and the second portion  152   p.    
       FIG.  82    illustrates a cross-sectional side view of the system  104   p , with the first spring  402   p  activated to drive the first portion  150   p  in a distal direction. The movement of the first portion  150   p  also urges the needle hub  162   p  (as well as the on-skin component  134   p  which is coupled to the needle hub  162   p ) in a distal direction, coupling the on-skin component  134   p  to the base  128   p , and also driving the needle  156  in a distal direction, past a distal end of the system  104   p . At or about the time the needle hub  162   p  reaches the distal insertion position illustrated in  FIG.  82    (e.g., immediately before, simultaneously, or subsequently), the ends of the release feature  160   p  contact ramps  170   p  of the second portion  152   p , causing the release feature  160   p  to compress inward (towards the central axis of the system  104   p ), unlocking the needle hub  162   p from the first portion  150   p  and releasing or activating the second spring  234   p . In some embodiments, ramps  170   p  are proximally facing ramps. In other embodiments, ramps  170   p  are distally facing ramps (not shown). Activation of the second spring  234   p  urges the needle hub  162   p  in a proximal direction. 
       FIG.  83    illustrates a cross-sectional side view of the system  104   p , with the needle hub  162   p  unlocked from the first portion  150   p  and retracted to a proximal position. As the needle hub  162   p  retracts to a proximal position, the on-skin component  134   p  decouples from the needle hub  162   p  to remain in a deployed position, coupled to the base  128   p.    
       FIG.  84    illustrates a perspective view of the system  104   p  in a resting state, with the first portion  150   p  and the third portion  392   p  shown in cross section to better illustrate certain portions of the system  104   p , such as the locking features  396   p ,  398   p  and the unlocking features  404   p ,  406   p .  FIG.  85    illustrates another perspective view of the system  104   p , also with the first portion  150   p  and the third portion  392   p  shown in cross section, with the first spring  402   p  energized but not yet activated. 
       FIGS.  86 - 88    illustrate another embodiment of a system  104   q  for applying an on-skin sensor assembly to skin of a host, wherein the insertion spring is pre-compressed and the retraction spring is substantially uncompressed. Such a system may allow a user to activate the insertion and retraction of a needle with fewer steps. It is contemplated that advantages may include a relatively smaller applicator size and more predictable spring displacement of the first spring because the first spring is already compressed, thereby aiding in ensuring proper needle insertion into the skin of a user. In some embodiments, a system incorporating a pre-energized insertion spring can provide effective and reliable insertion and retraction while requiring a lesser amount of user-supplied force than, for example, a system in which both the insertion and retraction springs are substantially unenergized prior to deployment, making such a configuration more convenient for at least some users. The system  104   q  includes many items that are similar to those of the embodiment illustrated in  FIGS.  76 - 79    (e.g., a telescoping assembly  132   q  including a first portion  150   q , a second portion  152   q , and a third portion  392   q ; a needle hub  162   q ; a first spring  402   q ; a second spring  234   q ; an on-skin component  134   q , and a base  128   q ). In the system  104   q , the first spring  402   q  is formed separately from and operatively coupled to the third portion  392   q . The second spring  234   q  is formed separately from and operatively coupled to the needle hub  162   q . The first spring and/or the second spring can comprise metal. Alternatively, in some embodiments, one or both of the first spring and the second spring can be integrally formed with a portion of the applicator assembly. 
       FIG.  86    illustrates a cross-sectional side view of the system  104   q  in a resting state, in which the first spring  402   q  is already energized but in which the second spring  234   q  is substantially unenergized (e.g. mostly uncompressed or unstressed; can be partially energized). In the resting state illustrated in  FIG.  86   , the first portion  150   q  is locked to the second portion  152   q  so as to prevent proximal or distal movement of the first portion  150   q  with respect to the second portion  152   q . The first portion  150   q  and the second portion  152   q  can be locked together in any suitable fashion, for example by cooperating releasable locking features (e.g., the locking features described in  FIGS.  76 - 79    or other suitable locking features) coupled to or forming part of the first portion  150   q  and the second portion  152   q . The needle hub  162   q  is also releasably locked to the first portion  150   q . The needle hub  162   q  can be locked to the first portion  150   q  in any suitable fashion, for example by features of the first portion  150   q  configured to engage or compress release feature  160   q  of the needle hub  162   q . The third portion  392   q  and the second portion  152   q  are also locked together, so as to prevent relative movement of the third portion  392   p  and the second portion  152   q  in the axial direction. The third portion  392   q  and the second portion  152   q  can be locked together in any suitable fashion, for example by cooperating locking features (not shown in  FIGS.  86 - 89   ), which may be coupled to or form part of the third portion  392   q  and the second portion  152   q . In the resting state illustrated in  FIG.  80   , the on-skin component  134   q  is disposed at a proximal starting position, such that the distal end of the needle  156  is disposed between the proximal and distal ends of the system  104   q.    
     To trigger deployment of the system  104   q , the locking features coupling the first portion  150   q  to the second portion  152   q  can be unlocked, decoupling these two portions and thereby releasing or activating the first spring  402   q . The locking features can be unlocked by a user-activated trigger mechanism, such as, for example, a button disposed on or in a top or side surface of the system  104   q , or a twist-release feature configured to disengage the locking features when the third portion  392   q  is rotated about the axis of the system, relative to the first portion  150   q  and/or the second portion  152   q . Some examples of triggering mechanisms are described in connection with  FIGS.  92 - 104   . 
       FIG.  87    illustrates a cross-sectional side view of the system  104   q , after the first portion  150   q  and the second portion  152   q  have been unlocked. As can be seen in  FIG.  87   , the first spring  402   q  drives the first portion  150   q  in a distal direction as the first spring  402   q  expands. The movement of the first portion  150   q  also urges the needle hub  162   q  (as well as the on-skin component  134   q  which is coupled to the needle hub  162   q ) in a distal direction, coupling the on-skin component  134   q  to the base  128   q , compressing the second spring  234   q , and driving the needle  156  in a distal direction past a distal end of the system  104   q . At or about the time the needle hub  162   q  reaches the distal insertion position illustrated in  FIG.  87    (e.g., immediately before, simultaneously, or subsequently), the ends of the release feature  160   q  contact interference features  170   q  of the second portion  152   q , causing the release feature  160   q  to compress inward (towards the central axis of the system  104   q ), unlocking the needle hub  162   q  from the first portion  150   q  and activating the now-energized second spring  234   q . In some embodiments, interference features  170   q  are proximally facing interference features. In other embodiments, interference features  170   q  are distally facing interference features (not shown). 
     Activation of the second spring  234   q  by the user or mechanisms urges the needle hub  162   q  in a proximal direction, while the on-skin component  134   q , having been coupled to the base  128   q , remains in a deployed distal position.  FIG.  88    illustrates a cross-sectional side view of the system  104   q , with the on-skin component  134   q  in a deployed position and the needle hub  162   q  retracted to a proximal position. 
       FIGS.  89 - 91    illustrate another embodiment of a system  104   r  for applying an on-skin sensor assembly to skin of a host. It is contemplated that the system  104   r  as illustrated with reference to  FIGS.  89 - 91    provides for predictable spring displacement of the first spring  402   r  because it is compressed, thereby aiding in proper needle insertion into the skin of the user. Moreover, it is contemplated that the compressed second spring  234   r  provides predictable spring displacement and aids in properly ensuring that the needle is properly retracted from the skin of the user. In some embodiments, a system incorporating pre-energized insertion and retraction springs can provide effective and reliable insertion and retraction while requiring a lesser amount of user-supplied force than, for example, a system in which one or both of the insertion and retraction springs are substantially unenergized prior to deployment, making such a configuration more convenient for at least some users. The system  104   r  includes many items that are similar to those of the embodiment illustrated in  FIGS.  76 - 79    (e.g., a telescoping assembly  132   r  including a first portion  150   r , a second portion  152   r , and a third portion  392   r ; a needle hub  162   r ; a first spring  402   r ; a second spring  234   r ; an on-skin component  134   r , and a base  128   r ). As illustrated in  FIGS.  89 - 91   , both the first spring  402   r  and the second spring  234   r  are pre-compressed. In the system  104   r , the first spring  402   r  is formed separately from and operatively coupled to the third portion  392   r . The second spring  234   r  is formed separately from and operatively coupled to the needle hub  162   r . The first spring and/or the second spring can comprise metal. Alternatively, in some embodiments, one or both of the first spring and the second spring can be integrally formed with a portion of the applicator assembly. 
       FIG.  89    illustrates a cross-sectional side view of the system  104   r  in a resting state, in which both the first spring  402   r  and the second spring  234   r  are pre-energized (e.g., compressed sufficiently to drive the needle insertion and retraction processes). In the resting state illustrated in  FIG.  89   , the first portion  150   r  is locked to the second portion  152   r  so as to prevent proximal or distal movement of the first portion  150   r  with respect to the second portion  152   r . The first portion  150   r  and the second portion  152   r  can be locked together in any suitable fashion, for example by cooperating releasable locking features (e.g., the locking features described in connection with  FIGS.  80 - 83   , or other suitable locking features) coupled to or forming part of the first portion  150   r  and the second portion  152   r . The needle hub  162   r  is also releasably locked to the first portion  150   r . The needle hub  162   r  can be locked to the first portion  150   r  in any suitable fashion, for example by features of the first portion  150   r  configured to engage or compress release feature  160   r  of the needle hub  162   r . The third portion  392   r  and the second portion  152   r  are also locked together, so as to prevent relative movement of the third portion  392   p  and the second portion  152   r  in at least the axial direction. The third portion  392   r  and the second portion  152   r  can be locked together in any suitable fashion, for example by cooperating locking features (not shown in  FIGS.  89 - 91   ), which may be coupled to or form part of the third portion  392   r  and the second portion  152   r . In the resting state illustrated in  FIG.  89   , the on-skin component  134   r  is disposed at a proximal starting position, such that the distal end of the needle  156  is disposed between the proximal and distal ends of the system  104   r.    
     To trigger deployment of the system  104   r , the locking features coupling the first portion  150   r  to the second portion  152   r  can be unlocked, decoupling these two portions and thereby releasing or activating the first spring  402   r .  FIG.  90    illustrates a cross-sectional side view of the system  104   r , after the first portion  150   r  and the second portion  152   r  have been unlocked. As can be seen in  FIG.  90   , the first spring  402   r  drives the first portion  150   r  in a distal direction as the first spring  402   r  expands or decompresses. The movement of the first portion  150   r  also urges the needle hub  162   r  (as well as the on-skin component  134   r  which is coupled to the needle hub  162   r ) in a distal direction until the on-skin component  134   r  is coupled to the base  128   r , and until the needle  156  reaches a distal insertion position beyond a distal end of the system  104   r . At or about the time the needle hub  162   r  reaches the distal insertion position illustrated in  FIG.  87    (e.g., immediately before, simultaneously, or subsequently), the ends of the release feature  160   r  contact corresponding interference features  170   r  of the second portion  152   r , causing the release feature  160   r  to compress inward (towards the central axis of the system  104   r ), unlocking the needle hub  162   r  from the first portion  150   r  and releasing or activating the second spring  234   r.    
     Activation of the second spring  234   r  drives the needle hub  162   r  in a proximal direction, while the on-skin component  134   r , having been coupled to the base  128   r , remains in a deployed distal position.  FIG.  91    illustrates a cross-sectional side view of the system  104   r , with the on-skin component  134   r  in a deployed position and the needle hub  162   r  retracted to a proximal position. From this configuration, the system  104   r  can be removed and separated from the deployed on-skin component  134   r  and the base  128   r.    
       FIGS.  92 - 100    illustrate yet another embodiment of a system  104   s  for applying an on-skin sensor assembly to skin of a host comprising a safety feature to prevent accidental firing of the sensor insertion device. The system  104   s  includes many items that are similar to those of the embodiment illustrated in  FIGS.  76 - 79    (e.g., a telescoping assembly  132   s  including a first portion  150   s , a second portion  152   s , and a third portion  392   s ; a needle hub  162   s ; a first spring  402   s ; a second spring  234   s ; an on-skin component  134   s , and a base  128   s ). In the system  104   s , the first spring  402   s  may be formed separately from and operatively coupled to the third portion  392   s . The second spring  234   s  may be formed separately from and operatively coupled to the needle hub  162   s . The first spring and/or the second spring can comprise metal. Alternatively, in some embodiments, either or both of the first spring and the second spring can be integrally formed with a portion of the applicator assembly. 
       FIG.  92    illustrates a side view of the system  104   s  in a resting state, in which the first spring  402   s  is unstressed and unenergized, but in which the second spring  234   s  is already energized (e.g., compressed). The system  104   s  includes a cocking mechanism  702  by which the first spring  402   s  can be energized (e.g. compressed) without automatically triggering deployment of the first portion  150   s  or activation of the first spring  402   s . The system  104   s  also includes a trigger button  720  configured to activate the first spring  402   s  after the system is cocked.  FIG.  93    illustrates a side view of the applicator system  104   s , after being cocked but before being triggered. 
       FIG.  94    illustrates a cross-sectional perspective view of the system  104   s  in a resting state, showing the first spring  402   s  substantially uncompressed. The cocking mechanism  702  includes a pair of proximally-extending lever arms  704 , each with a radially-extending angled tab  706 . In some embodiments, the lever arms  704  can be integrally formed with the second portion  152   s , as shown in  FIG.  94   , while in other embodiments, the lever arms  704  can be separate from and operatively coupled to the second portion  152   s . In the resting state illustrated in  FIG.  94   , the angled tabs  706  extend through distal apertures  708  in the third portion  392   s  so as to prevent proximal movement of the third portion  392   s  with respect to the second portion  152   s . The angled tabs  706  are also configured to inhibit distal movement of the third portion  392   s  with respect to the second portion  152   s , unless and until a sufficient amount of force is applied to the third portion  392   s  to deflect the angled tabs  706  and the lever arms  704  inward, as illustrated in  FIG.  95   . 
     As sufficient force is applied to the third portion  392   s  in a distal direction (e.g. by the hand or thumb of a user), the angled tabs  706  deflect inward and release from engagement with the distal apertures  708 , allowing the third portion  392   s  to move distally with respect to the second portion  152   s . This may allow the user to compress and energize the first spring  402   s . When the third portion  392   s  has reached a sufficiently distal position to compress the first spring  402   s  enough to drive the sensor into the skin of a host, the angled tabs  706  engage with proximal apertures  710  of the third portion  392   s  to lock the position of the third portion  392   s  with respect to the second portion  152   s , as illustrated in  FIG.  96   . The angled tabs  706  may be configured to generate a “click” sound when engaged to proximal apertures  710  so as to prevent proximal movement of the third portion  392   s  with respect to the second portion  152   s , so that a user can feel and/or hear when these parts are engaged. In the configuration illustrated in  FIG.  96   , the system  104   s  is energized in which the third portion  392  is in a cocked position. The system  104   s  is ready to deploy the sensor, but does not deploy until further action is taken by the user. 
       FIG.  97    illustrates a cross-sectional side view of the system  104   s , in a cocked but untriggered state. In this state, the first portion  150   s  is locked to the second portion  152   s  so as to prevent proximal or distal movement of the first portion  150   s  with respect to the second portion  152   s . The first portion  150   s  and the second portion  152   s  can be locked together in any suitable fashion, for example by cooperating releasable locking features  396   s  and  398   s  operatively coupled to or forming part of the first portion  150   s  and the second portion  152   s . The trigger button  720  includes a distally-extending protrusion  722  which, once depressed to a sufficiently distal position by a user, is configured to cooperate with an unlocking feature  406   s  of the locking feature  396   s  to decouple the first portion  150   s  from the second portion  152   s . The trigger button  720  can be operatively coupled to the third portion  392   s , as illustrated in  FIGS.  92 - 100   , or can be integrally formed with the third portion, for example as a lever arm formed within a proximal or side surface of the third portion. In some embodiments, the trigger button can be disposed at the top of the system (such that the application of force in the distal direction triggers the system to activate), or at a side of the system (such that the application of force in a radially inward direction, normal to the direction of needle deployment, triggers system to activate). 
       FIG.  98    illustrates a cross-sectional side view of the energized system  104   s  as the trigger button  720  has been depressed sufficiently to cause the protrusion  722  to flex the locking feature  396   s  radially inward, disengaging it from the opening  398   s  and unlocking the first portion  150   s  from the second portion  152   s . Depressing the trigger button  720  thus activates the first spring  402 , pushing the first portion  150   s  and the needle hub  162   s , along with the on-skin component  134   s  which is coupled thereto, in a distal direction until the on-skin component is coupled to the base  128   s , as illustrated in  FIG.  99   . At or about the time the needle hub  162   s  reaches the distal insertion position illustrated in  FIG.  99    (e.g., immediately before, simultaneously, or subsequently), corresponding release features of the needle hub  162   s  and the first portion  150   s  can engage (via, for example, the release features described in any of  FIGS.  76 - 91   , or any other suitable release features), releasing the needle hub  162   s  from the first portion  150   s  and releasing or activating the second spring  234   s . Activation of the second spring  234   s  urges the needle hub  162   s  in a proximal direction. 
       FIG.  100    illustrates a cross-sectional side view of the applicator system of  FIG.  92   , with the on-skin component  134   s  in a deployed position and the needle hub  162   s  retracted to a proximal position. As the needle hub  162   s  retracts to a proximal position, the on-skin component  134   s  decouples from the needle hub  162   s  to remain in a deployed position, coupled to the base  128   s . From this configuration, the remainder of the system  104   s  can be removed and separated from the deployed on-skin component  134   s  and the base  128   s.    
     Any of the features described in the context of any of  FIGS.  61 - 99    can be applicable to all aspects and embodiments identified herein. For example, the embodiments described in the context of  FIGS.  61 - 64    can be combined with the embodiments described in the context of  FIGS.  1 - 60  and  65 - 70   . As another example, any of the embodiments described in the context of  FIGS.  92 - 109    can be combined with any of the embodiments described in the context of  FIGS.  1 - 60  and  65 - 91  and  110 - 143   . Moreover, any of the features of an embodiment is independently combinable, partly or wholly with other embodiments described herein in any way, e.g., one, two, or three or more embodiments may be combinable in whole or in part. Further, any of the features of an embodiment may be made optional to other aspects or embodiments. Any aspect or embodiment of a method can be performed by a system or apparatus of another aspect or embodiment, and any aspect or embodiment of a system can be configured to perform a method of another aspect or embodiment. 
     Trigger Mechanisms and Safety Locks 
     In some embodiments, the application of enough force to sufficiently energize the first spring to drive insertion of the sensor can also serve to activate the first spring. In other embodiments, the energizing of the first spring can be decoupled from the activation of the first spring, requiring separate actions on the part of the user to energize (e.g. compress) the first spring and to trigger deployment of the system. 
     For example, the embodiment illustrated in  FIGS.  92 - 100    includes a trigger mechanism in the context of a user-energized actuator. In such an embodiment, the user first cocks the system  104   s  to energize the first spring  402   s , and then, in a separate action, triggers the activation of the first spring  402   s  using the trigger button  720 . The locking feature is easy to release by the user and when combined with a trigger mechanism, allows for single handed use. 
     In some embodiments, the actuator or insertion spring is already energized when the system is in a resting state. In these embodiments, a trigger mechanism, such as the trigger mechanism described in the context of  FIGS.  92 - 100   , can be used to activate the already-energized insertion spring without any action by the user to energize the spring. 
       FIG.  101    illustrates a side view of one such applicator system  104   t , with a side trigger button  730 . The system  104   t  can be configured substantially similar to the system  104   q  or the system  104   r  illustrated within the context of  FIGS.  86 - 88  and  89 - 91   , respectively, with like reference numerals indicating like parts. As can be seen in  FIG.  101   , the trigger button  730  is operatively coupled to the third member  392   t.    
       FIG.  102    illustrates another side view of the system  104   t , with the first portion  150   t  and the third portion  392   t  shown in cross-section to illustrate the trigger mechanism. As can be seen in  FIG.  102   , the trigger button  730  includes a protrusion  732  that extends radially inward, toward a central axis of the system  104   t . The protrusion  732  is radially aligned with the locking feature  396   t  of the first portion  150   t . When a user exerts a sideways (e.g., radially inward) force on the trigger button  730 , the protrusion  732  urges the locking feature  396   t  radially inward, releasing it from engagement with the ledge feature  398   t  (which may be configured similarly to, for example, the ledge locking feature  398   p  illustrated in  FIGS.  84  and  85   ) in the second portion  152   t  and activating the first spring  402   t . In other embodiments, the locking features  396 ,  398  can comprise cooperating structure of a key/keyway mechanism which is configured to release when the features  396 ,  398  are brought into a certain orientation with respect to one another (e.g., using a radially applied force, an axially applied force, a twisting movement or rotational force, or other type of activation). 
       FIG.  103    illustrates a side view of another applicator system  104   u , with an integrated side trigger button  730 . The system  104   u  can be configured substantially similar to the system  104   q  or the system  104   r  illustrated within the context of  FIGS.  86 - 88  and  89 - 91   , respectively, with like reference numerals indicating like parts. As can be seen in  FIG.  103   , the trigger button  740  is a distally-extending lever arm integrally formed in the third member  392   u .  FIG.  104    illustrates another side view of the system  104   u , with the first portion  150   u  and the third portion  392   u  shown in cross-section to better illustrate the trigger mechanism. As can be seen in  FIG.  104   , the trigger button  740  is radially aligned with a radially-extending tab  742  of the first portion  150   u . The tab  742  is connected to the locking feature  396   u  via an elongated member  394   u , which acts as a lever arm. In some embodiments, tab  742  locking feature  396   u , and elongated member  394   u  are integrally formed together. When a user exerts a sideways (e.g., radially inward) force on the trigger button  740 , the button  740  pushes the tab  742  radially inward, releasing the locking feature  396   u  from engagement with the locking feature  398   u  in the second portion  152   u  and activating the first spring  402   u.    
     Trigger mechanisms such as those described in the context of any of  FIGS.  92 - 104    can be used in embodiments comprising pre-energized actuators or insertion springs, as well as in embodiments comprising user-energized actuators or insertion springs. 
     In several embodiments, a sensor inserter system can include a safety mechanism configured to prevent premature energizing and/or actuation of the insertion spring.  FIGS.  105 - 109    illustrate one such system  104   v , which incorporates a safety lock mechanism  750 .  FIG.  105    illustrates a perspective view of the system  104   v . The system  104   v  can be configured substantially similar to any of the systems  104   m ,  104   n ,  104   p  illustrated within the context of  FIGS.  71 - 87   , with like reference numerals indicating like parts. The safety lock mechanism  750  includes a release button  760 , which can be integrally formed with the third portion  392   v  as shown in  FIG.  105    (similar to the trigger button  740  described in connection with  FIGS.  103 - 104   ), or which can be operatively coupled to the third portion  392   v . In the system  104   v , the release button  760  comprises a lever arm which is integrally formed in a side of the third portion  392   v , although other configurations (e.g. a top button) are also contemplated. The release button can be configured to protrude radially from a side or a top of the third portion, or can be configured with an outer surface which is flush with the surrounding surface of the third portion  392   v.    
       FIG.  106    illustrates a cross-sectional perspective view of a portion of the system  104   v , with the safety mechanism  750  in a locked configuration and the first spring  402   v  unenergized. The safety mechanism  750  includes a proximally-extending locking tab  752  of the second portion and an inwardly-extending overhang (or undercut)  754  of the third portion  392   v . In the locked configuration illustrated in  FIG.  106   , the tab  752  is flexed radially outward and its proximal end is constrained by the overhang  754 , preventing distal movement of the third portion  392   v  with respect to the second portion  152   v  and thus preventing energization of the spring  392   v . The release button  760  includes a protrusion  758  which extends inwardly, in radial alignment with a portion of the tab  752 . A lateral (e.g. radially inward) force applied to the release button  760  pushes the tab  752  radially inward, sliding the proximal end of the tab  752  against an angled surface  756  of the overhang  754  and out of engagement with the overhang  754 , so that the tab  752  can release to an unstressed configuration as shown in  FIG.  108   . Once the tab  752  is released, the third portion  392   v  can be moved distally with respect to the second portion  152   v , for example to energize the first spring  402   v . In some embodiments, as illustrated in  FIG.  109   , the tab  752  can be configured to prevent further distal movement of the third portion  392   v  beyond a desired threshold, for example by abutting a distally-facing surface  762  of the third portion  392   v.    
     Although the safety lock mechanism  750  is illustrated in the context of a system configured to be energized by a user, in some embodiments, a pre-energized system can also employ a safety lock mechanism, for example to prevent premature triggering or activation of an already energized spring. 
     In some embodiments, the locking and unlocking (and/or coupling and decoupling) of the components of a sensor inserter assembly can follow this order: The sensor inserter assembly begins in a resting state in which the third portion  392  is locked with respect to the first portion  150 , the first portion  150  is locked with respect to the second portion  152 , and the sensor module  134  is coupled to the first portion  150  (optionally via the needle hub  162 ). Before energizing or triggering of the insertion spring  402 , the third portion  392  is unlocked with respect to the first portion  150  and/or the second portion  150 . The insertion spring  402 , if not already energized, is then energized by distal movement of the third portion  392 . Then, the third portion  392  is locked with respect to the second portion  152 . The first portion  150  is then released from the second portion  152  to activate the insertion spring  402 . As the insertion spring  402  deploys, the sensor module  134  couples to the base  128 . Then the first portion  150  (and/or the needle hub  162 ) releases the sensor module  134 , and the second portion  152  releases the base  128 . In several embodiments, the locations of the various locking and unlocking (and/or coupling and decoupling) structures along the axis of the assembly are optimized to ensure this order is the only order that is possible. (Some embodiments use different locking and unlocking orders of operation.) 
     Systems such as those illustrated in  FIGS.  92 - 109    provide reliable trigger mechanisms to release an insertion spring when the insertion spring is in a loaded condition. It is contemplated such systems provide several advantages to the user including ease in firing, single handed firing (by allowing the user to hold onto the sides of the insertion device and fire the insertion device using the same hand). It is contemplated that a system comprising a top trigger can provide a smaller width profile than a system having a side button while requiring less user dexterity. 
     Release After Deployment 
     In several embodiments, a sensor inserter system is configured to move an on-skin component (such as, for example, a sensor module  134 , a sensor assembly (for example comprising a sensor, electrical contacts, and optionally a sealing structure), a combination sensor module and base, an integrated sensor module and transmitter, an integrated sensor module and transmitter and base, or any other component or combination of components which is desirably attached to the skin of a host) from a proximal starting position within the sensor inserter system to a distal deployed position in which it can attach to the skin of a host, while at the same time inserting a sensor (which may form part of the on-skin component) into the skin of the host. In some embodiments, the sensor is coupled to electrical contacts of the on-skin component during the deployment and/or insertion process. In other embodiments, the on-skin component is pre-connected, that is to say, the sensor is coupled to electrical contacts of the on-skin component before the deployment and/or insertion processes begin. The sensor assembly can be pre-connected, for example, during manufacture or assembly of the system. 
     Thus, in several embodiments, a sensor inserter system can be configured to releasably secure the on-skin component in its proximal starting position, at least before or until deployment of the inserter system, and can also be configured to release the on-skin component in a distal position after the inserter system is deployed. In some embodiments, the system can be configured to couple the on-skin component to a base and/or to an adhesive patch during the deployment process, either as the on-skin component is moved from the proximal starting position to the distal deployed position or once it reaches the distal deployed position. In some embodiments, the system can be configured to separate from (or become separable from) the on-skin component, base, and/or adhesive patch after the on-skin component is deployed in the distal position and the needle hub (if any) is retracted. 
     In embodiments, various mechanical interlocks (e.g., snap fits, friction fits, interference features, elastomeric grips) and/or adhesives can be used to couple the on-skin component to the sensor inserter system and releasably secure it in a proximal starting position, and/or to couple the on-skin component (and base, if any) to the adhesive patch once the on-skin component is deployed. In addition, various mechanical features (e.g. snap fits, friction fits, interference features, elastomeric grips, pushers, stripper plates, frangible members) and/or adhesives can be used to decouple the on-skin component from the sensor inserter system once it reaches the distal deployed position. Further, various mechanical features, (e.g. snap fits, friction fits, interference features, elastomeric grips, pushers, stripper plates, frangible members) and/or adhesives can be used to separate, unlock, or otherwise render separable the on-skin component, base, and/or adhesive patch from the remainder of the system after the on-skin component is deployed in the distal position and the needle hub (if any) is retracted. 
     With reference now to  FIGS.  110 - 119   , a sensor inserter system  104   w  according to some embodiments is illustrated. The system  104   w  can be configured substantially similar to the system  104   v  illustrated within the context of  FIGS.  105 - 109    and system  104   m  illustrated within the context of  FIGS.  71 - 75   , with like reference numerals indicating like parts. The system  104   w  includes, for example, a telescoping assembly  132   w  including a first portion  150   w , a second portion  152   w , and a third portion  392   w ; a safety mechanism  750 , a needle hub  162   w ; a first spring  402   w ; a second spring  234   w ; an on-skin component  134   w , and a base  128   w.    
       FIG.  110    illustrates a cross-sectional perspective view of the system  104   w  in a resting and locked state, with the on-skin component  134   w  secured in a proximal starting position. In this state, as well as in the unlocked state illustrated in  FIG.  111    and the energized state illustrated in  FIG.  112   , the on-skin component  134   w  is secured in the proximal starting position by a securement member  800 . As can be seen in  FIG.  110   , the system  104   w  includes a secondary locking feature  409   w , configured as a ledge extending from the distal end of the locking protrusion  408   w , which is configured to cooperate with an opening  410   w  to prevent the third portion  392  from moving in a proximal direction with respect to the second portion  152   w  prior to deployment. In the embodiment illustrated in  FIGS.  110 - 119   , the securement member  800  is integrally formed with the needle hub  162   w . In other embodiments, the securement member can be integrally formed with the first portion  150   w . In still other embodiments, the securement member can be separately formed from and operatively coupled to the needle hub  162   w  and/or to the first portion  150   w . The securement member  800  extends substantially parallel to the needle  158 . In the embodiment illustrated in  FIGS.  110 - 119   , the securement member  800  comprises a pair of distally-extending legs  802  (see  FIGS.  115  and  116   ). Some embodiments can, however, include only one distally-extending leg  802 , while others can include three, four, or more legs  802 . In embodiments comprising only one leg  802 , the leg can be configured to adhere or otherwise couple to a center region or a perimeter of the on-skin component. In embodiments comprising more than one leg  802 , the legs can be configured to adhere or otherwise couple to the on-skin component symmetrically or asymmetrically about a center of the on-skin component. The legs  802  can have an ovoid cross section, or can have any other suitable cross section, including circular, square, triangular, curvilinear, L-shaped, O-shaped, U-shaped, V-shaped, X-shaped, or any other regular or irregular shape or combination of shapes. In embodiments, the securement member  800  can comprise legs, columns, protrusions, and/or elongate members, or can have any other suitable configuration for holding the on-skin component in the proximal starting position. 
       FIG.  113    illustrates a cross-sectional perspective view of the system  104   w , in an activated state, with the insertion spring  402   w  activated, the retraction spring  234   w  energized, and the needle hub  162   w  and the securement member  800  moved to a distal deployed position. The on-skin component  134   w , being coupled to the securement member  800  until this stage, has also been moved to a distal deployed position. When the on-skin component  134   w  reaches the distal deployed position, it is coupled to the base  128   w.    
       FIG.  114    illustrates a cross-sectional perspective view of the system  104   w  after the on-skin component has been coupled to the base  128   w  and the needle hub  162   w  (along with the securement member  800 ) has been retracted to a proximal position. After the on-skin component  134   w  is coupled to the base  128   w , a resistance member  804  facilitates decoupling of the on-skin component  134   w  from the securement member  800  by resisting unwanted proximal movement of the on-skin component  134   w  away from the base  128   w . Generally, the resistance member  804  can be a backstop or backing structure configured to inhibit or prevent, or otherwise resist any tendency of the on-skin component  134   w  to move in a proximal direction as the securement member  800 , which is releasably coupled to the on-skin component  134   w , moves in a proximal direction. Because the first portion  150   w  is fixed to the second portion  152   w  at this stage, and the needle hub  162   w  is released from the first portion  150   w , the needle hub  162   w  can retract in a proximal direction while the first portion  150   w  (and the resistance member  804 ) remains planted in a distal position, inhibiting proximal movement of the on-skin component  134   w . The energy stored in the retraction spring  234   w  is sufficient to overcome a retention force and decouple the on-skin component  134   w  from the securement member  800  and urge the needle hub  162  in a proximal direction. The potential energy stored can be between 0.25 pounds to 4 pounds. In preferred embodiments, the potential energy stored is between about 1 to 2 pounds. 
     In some embodiments, a sensor inserter system can be configured such that the on-skin component couples with the base at approximately the same time the retraction mechanism is activated. In some embodiments, a sensor inserter system can be configured such that the on-skin component couples with the base before the retraction mechanism is activated, before the second spring is activated, or otherwise before the second spring begins retracting the needle hub in a proximal direction away from the deployed position. In some embodiments, a sensor inserter system can be configured such that the second spring is activated at least 0.05 seconds, at least 0.1 seconds, at least 0.2 seconds, at least 0.3 seconds, at least 0.4 seconds, at least 0.5 seconds, at least 0.6 seconds, at least 0.7 seconds, at least 0.8 seconds, at least 0.8 seconds, at least 1 second, or longer than 1 second after the on-skin component couples with the base. In other embodiments, a sensor inserter system can be configured such that the second spring is activated at most 0.05 seconds, at most 0.1 seconds, at most 0.2 seconds, at most 0.3 seconds, at most 0.4 seconds, at most 0.5 seconds, at most 0.6 seconds, at most 0.7 seconds, at most 0.8 seconds, at most 0.8 seconds, or at most 1 second after the on-skin component couples with the base. 
     The on-skin component  134   w  is now coupled with the base  128   w . The base  128   w  (and adhesive patch) is initially coupled to the second portion  152   w  by a latch or flex arm  220   w  coupled to an undercut or locking feature  230   w  (similarly shown in  FIGS.  18 - 19   ). When the first portion  150   w  reaches its most distal position during insertion of the sensor  138 , a delatching feature of the first portion  150   w  pushes the latch of the second portion  152   w  out of the undercut. This decouples the base  128   w  from the second portion  152   w , and thus allows the user to take the remainder of the system  104   w  off the skin, leaving only the adhesive patch, the base  128   w , and the on-skin component  134   w  on the skin. 
     In the embodiment illustrated in  FIGS.  110 - 119   , the resistance member  804  is integrally formed with the first portion  150   w . In other embodiments, the resistance member can be integrally formed with the second portion  152   w . In still other embodiments, the resistance member can be separately formed from and operatively coupled to the first portion  150   w  and/or to the second portion  152   w . In the embodiment illustrated in  FIGS.  110 - 119   , the resistance member  804  comprises a distally-facing surface of the first portion  150   w.    
     The system  104   w  can be configured to couple the on-skin component  134   w  to the base  128   w  via an adhesive  806 .  FIG.  115    illustrates a perspective view of the needle hub  162   w , shown securing the on-skin component  134   w  during deployment, with the base  128   w  removed to illustrate the adhesive  806  disposed on a distally-facing surface of the on-skin component  134   w . The adhesive  806  can be configured to couple the on-skin component  134   w  to the base  128   w  on contact. Alternatively or in addition to the adhesive  806 , some embodiments can include an adhesive disposed on a proximally-facing surface of the base, so as to couple the on-skin component to the base upon contact. In some embodiments, the adhesive can be a pressure-sensitive adhesive. In some embodiments, the securement member can be configured to couple the on-skin component to the needle hub only along a plane extending normal to the axial direction of the system. In addition or in the alternative, the securement can be configured to couple the on-skin component in a lateral or radial direction. 
       FIG.  116    illustrates another perspective view of the needle hub  162   w , shown decoupled from the on-skin component  134   w , with the base  128   w  removed to illustrate the adhesive  808  disposed on the distally-facing surfaces of the securement member  800 . The adhesive  808  can be configured to couple the on-skin component  134   w  to the securement member  800  while in the proximal starting position and during movement of the on-skin component  134   w  in the proximal direction, and to allow the release of the on-skin component  134   w  from the securement member  800  after the on-skin component  134   w  is coupled to the base  128   w . Alternatively or in addition to the adhesive  808 , some embodiments can include an adhesive disposed on a proximally-facing surface of the on-skin component  134   w . In some embodiments, the adhesive can be a pressure-sensitive adhesive. In some embodiments, the adhesive  808  can have a smaller surface area and/or a lower adhesion strength than the adhesive  806 , such that the adhesion strength of the adhesive  806  which couples the on-skin component to the base outweighs the adhesion strength of the adhesive  808  which couples the on-skin component to the securement member. In other embodiments, the adhesion strength of the adhesive  808  can be the same or greater than the adhesion strength of the adhesive  806 . In these embodiments, a resistance member can be employed to facilitate the decoupling of the on-skin component  134   w  from the securement member  800  after deployment. 
       FIG.  117    illustrates a perspective view of a portion of the system  104   w , illustrating the resistance member  804 . The resistance member  804  is configured to rest above and/or contact a proximally-facing surface of the on-skin component  134   w , at least when the on-skin component  134   w  is in a distal deployed position. The resistance member  804  can serve to inhibit proximal movement of the on-skin component  134   w  as the needle hub  162   w  and securement member  800  retract in a proximal direction. The resistance member  804  can function in a manner similar to a stripper plate in punch and die manufacturing or injection molding processes. 
     In embodiments, the resistance member can have any configuration suitable for resisting decoupling of the on-skin component from the base. In the embodiment illustrated in  FIGS.  110 - 119   , the resistance member  804  has a curvilinear cross section, and extends through an arc of roughly 300 degrees about the perimeter of the on-skin component  134   w . In some embodiments, the resistance member  804  can extend through an arc of roughly 30 degrees, 60 degrees, 90 degrees, 120 degrees, 150 degrees, 180 degrees, 210 degrees, 240 degrees, 270 degrees, 300 degrees, or 330 degrees about the perimeter of the on-skin component  134   w , or through an arc greater than, less than, or within a range defined by any of these numbers. In some embodiments, the resistance member  804  can extend continuously or discontinuously about the perimeter of the on-skin component. In some embodiments, the resistance member  804  can extend about the entire perimeter of the on-skin component. In some embodiments, the resistance member  804  can comprise one or more contact points or surfaces that hold the on-skin component  134   w  in the distal position as the securement feature  800  moves in an opposite (e.g., proximal) direction. 
     In other embodiments, the resistance member  804  can comprise multiple discrete members (e.g., legs) configured to contact multiple locations about the perimeter of the on-skin component  134   w . For example, in some embodiments, the resistance member  804  can include at least two legs disposed apart from one another about a center point of the on-skin component. In some embodiments, the resistance member  804  can include two legs disposed roughly 180 degrees about a center point of the on-skin component. In some embodiments, the resistance member  804  can include three legs disposed roughly 120 degrees about a center point of the on-skin component. In such an embodiment, the legs can be arranged symmetrically about the on-skin component (e.g. with radial symmetry, or reflectional/bilateral symmetry). 
       FIG.  117    also illustrates locator features  810  which can be formed in, or integrally coupled to, the first portion  150   w  and/or the resistance member  804 . The locator features  810  can comprise distally-extending tabs of the first portion  150   w  and/or of the resistance member  804 . The locator features  810  can be configured to align with corresponding indentations  812  in the on-skin component (see  FIG.  119   ) so as to ensure proper positioning of the sensor module  134   w  with respect to the first portion  150   w  and/or the resistance member  804  during assembly. 
       FIG.  118    illustrates a perspective view of the sensor module  134   w , before being coupled to the base  128   w  by contacting the adhesive  806 . The base  128   w  itself is coupled (for example by an adhesive) to a proximal surface of an adhesive patch  900 .  FIG.  119    illustrates a perspective view of the sensor module  134   w  after being coupled to the base  128   w  on the adhesive patch  900 . 
     In embodiments, providing a resistance member can facilitate a reliable transfer of the on-skin component to the base, by creating a counterforce against the securement member as the needle hub retracts in the proximal direction. The counterforce allows the securement member to separate from the on-skin component while inhibiting or preventing the disengagement of the on-skin component from the base (if any) and/or from the adhesive patch. In embodiments, the retraction spring can be configured to store and provide sufficient energy to both retract the needle and decouple the on-skin component from the needle hub. The combination of the resistance member and securement member can also be configured to provide positional control of the on-skin component from a secured configuration (e.g., in the proximal starting position and during movement of the on-skin component toward the distal deployed position) to a released configuration (when the on-skin component reaches the distal deployed position and/or couples to the base and/or adhesive patch). 
     It is contemplated that providing a base which begins in the distal deployed position when the system is in a resting or stored state can serve to protect the needle (and the user) before the system is deployed. For example, a base which is coupled to a distal end of the system in a resting or pre-deployment state can prevent a user from reaching into the distal end of the system and pricking him or herself. This configuration can thus potentially reduce needle stick hazards. In addition, a base which is coupled to a distal end of the system in a resting or pre-deployment state facilitate the setting of the adhesive patch on the skin before deployment. For example, with such a configuration, the user can use the body of the sensor inserter system to assist in applying a force in a distal direction to adhere the adhesive patch to the skin. In addition, the base can provide structural support to guide the needle into the skin during deployment. 
       FIGS.  120 - 122    illustrate another configuration for coupling an on-skin component to a base, in accordance with several embodiments.  FIG.  120    shows a side view of an on-skin component  134   x  and a base  128   x , prior to coupling of the on-skin component  134   x  to the base  128   x . The on-skin component  134   x  includes a distally-extending sensor  138 , and the base  128   x  is coupled to an adhesive patch  900 .  FIG.  121    illustrates a perspective view of these same components. The base  128   x  comprises a flexible elastomeric member with a proximally-extending ridge  814  extending about a proximally-facing surface  816 . The base  128   x  can have a shape configured to correspond to a shape of the on-skin component  134   x . In a relaxed state, as illustrated in  FIGS.  120  and  121   , and before making contact with the on-skin component  134   x , the base  128   x  has a deformed, somewhat convex curvature. The base  128   x  and the adhesive patch  900  can be coupled to the other components of a sensor inserter assembly in this configuration. During deployment, as the on-skin component  134   x  begins to contact the base  128   x , the proximally-facing surface  816  flexes up to meet the distal surface of the on-skin component  134   x , causing the ridge  814  to grip securely about the perimeter of the on-skin component  134   x , as illustrated in  FIG.  122   . 
       FIG.  123    illustrates a perspective view of a portion of another inserter system  104   y , according to some embodiments. The system  104   y  can be configured substantially similar to any of the systems  104  illustrated herein, with like reference numerals indicating like parts. The inserter system  104   y  includes an on-skin component  134   y  which includes a combination sensor module and base. In embodiments, the combination sensor module and base can be integrally formed with one another, as illustrated in  FIG.  123   , or operatively coupled to one another. The system  104   y  also includes a securement member  800   y  which is configured to releasably secure the on-skin component  134   y  in a proximal starting position, at least until the system  104   y  is activated. The securement member  800   y  is integrally formed with the needle hub  162   y , and includes three proximally-extending legs  802   y  configured to releasably couple to (e.g. via adhesive  808 ) various locations on the proximal surface of the on-skin component  134   y . It is contemplated that the addition of a third (or further) leg  802   y  can help to balance the sensor module and prevent it from canting to one side or another during deployment and/or retraction. The system  104   y  also includes a resistance member  804 . The resistance member  804  may be integrally molded with first portion  150   y.    
       FIG.  124    illustrates another perspective view of the on-skin component  134   y  and the needle hub  162   y , with the remainder of the system  104   y  removed to illustrate the configuration of the securement member  800   y .  FIG.  125    illustrates a perspective view of a portion of the applicator system shown in  FIG.  123   , with the on-skin component  134   y  in a released configuration and separated from the needle hub  162   y  and with two of the legs  802   y  removed for purposes of illustration. The resistance member  804   y  can be configured to encompass or at least partially encompass the sensor module portion of the on-skin component  134   y . The resistance member  804   y  can comprise one or more elongate members, columns, legs, and/or protrusions, or can have any other suitable configuration for facilitating the release of the on-skin component from the needle hub  162   y . The resistance member  804   y  (or any portion thereof) can have a curvilinear cross section, as illustrated in  FIG.  125   , or can have any other suitable cross section, including circular, square, triangular, ovoid, L-shaped, O-shaped, U-shaped, V-shaped, X-shaped, or any other regular or irregular shape or combination of shapes. 
     As shown in  FIG.  125   , the system  104   y  can include an adhesive patch  818  disposed on the distally-facing surface of the on-skin component  134   y . The adhesive patch  818  can be configured to couple the on-skin component  134   y  to the skin on contact. In some embodiments, the adhesive patch can be a pressure-sensitive adhesive. In some embodiments, the adhesive patch  818  is a double sided adhesive, in which an adhesive is disposed on both the proximally facing surface of the adhesive patch  818  and the distally facing surface of the adhesive patch  818 . The proximally facing adhesive can be configured to couple with the distal end of the on-skin component  134   y , and the distally facing adhesive can be configured to couple with the skin. In other embodiments, the proximally facing surface of the adhesive patch  818  is configured to couple with the distally facing surface of the on-skin component by a coupling process such as, but not limited to, heat staking, fastening, welding, or bonding. In some embodiments, the adhesive patch  818  can be covered by a removable liner prior to deployment. In other embodiments, the adhesive patch  818  can be exposed (e.g., uncovered) within the system prior to deployment. 
     Alternatively, in some embodiments the adhesive patch  818  can be releasably secured to the distal end of the system before deployment, with an adhesive disposed on a proximally-facing surface of the adhesive patch  818 , so as to couple the on-skin component to the adhesive patch  818  upon contact as part of the sensor insertion process. In addition, in such an embodiment, the adhesive patch  818  can include an adhesive disposed on a distally-facing surface of the adhesive patch  818  to couple the on-skin component to the skin. 
     Such a configuration can include fewer components to be coupled and decoupled during the deployment and insertion process, which can increase reliability of systems configured in accordance with embodiments. For example, systems configured in accordance with embodiments can reduce the chance of improper transfer of system components to the skin. In addition, it is contemplated that embodiments comprising an adhesive patch disposed within the system in a resting state (as opposed to an adhesive patch disposed at a distal end of the system in the resting state) can allow for the system to be more easily re-positioned on the skin as many times as desired before being adhered to the skin. 
       FIGS.  126 - 128    illustrate another configuration for releasably securing an on-skin component in a proximal position, in accordance with several embodiments.  FIG.  126    illustrates a perspective view of a portion of a securement member  800   z  shown secured to an on-skin component  134   z  comprising a sensor module. The securement member  800   z  can include at least one leg  802   z . As shown in the figure, the securement member  800   z  includes two proximally-extending legs  802   z . The on-skin component  134   z  includes two elastomeric grips  824  extending laterally from the sensor module. The grips  824  are sized and shaped to cooperate with laterally-facing surfaces of the legs  802   z  to releasably secure the on-skin component  134   z  in a proximal position. In the embodiment illustrated in  FIGS.  126 - 128   , the grips  824  are integrally formed with the sensor module, and have a bracket-shaped cross section, as viewed in a plane extending normal to the axial direction. In embodiments, the securement member  800   z  and the grips  824  can have any suitable cooperating configuration to allow the on-skin component  134   z  to releasably couple the securement member  800   z  to the grips  824 , for example via a friction fit, interference fit, or corresponding undercut engagement features. Some embodiments can additionally employ an adhesive disposed between the securement member  800   z  and the on-skin component  134   z , to provide additional securement of the on-skin component  134   z.    
       FIG.  127    illustrates a perspective view of a portion of the securement member  800   z , with the sensor module of the on-skin component  134   z  shown in cross section to illustrate the configuration of the grips  824 .  FIG.  127    also shows a decoupling feature  804   z  configured to resist proximal movement of the on-skin component  134   z  after deployment of the on-skin component  134   z  to the distal deployed position, for example during retraction of the needle hub  162   z . The decoupling feature  804   z  can be fixed with respect to the remainder of the sensor inserter system as the needle hub  162   z  retracts in a proximal direction, providing enough resistance to overcome the friction fit (and adhesive, if any) between the securement member  800   z  and the grips  824  to release the securement member  800   z  from the grips  824 .  FIG.  128    illustrates a perspective view of the on-skin component  134   z , after decoupling of the on-skin component  134   z  from the securing member  800   z.    
       FIGS.  129 - 131    illustrate still another configuration for releasably securing an on-skin component, in accordance with several embodiments.  FIG.  129    illustrates a perspective view of a portion of a sensor inserter assembly  104   aa  with the second portion  150   aa  shown in cross section, and with a securing member  800   aa  shown securing an on-skin component  134   aa  in a proximal position. The on-skin component  134   aa  may comprise an integrally formed sensor module/base assembly. As shown in the figure, the securement member  800   aa  comprises an elastomeric cap which is coupled to a portion of the on-skin component  134   aa . As shown, the securement member  800   aa can be coupled to a protrusion (or neck)  826  formed in the on-skin component  134   aa .  FIG.  130    illustrates a perspective view of a portion of the assembly  104   aa  of  FIG.  129   , shown with a portion of the securing member  800   aa  cut away to better illustrate the configuration of the securing member  800   aa  and the protrusion  826 . The protrusion  826  can be configured to encircle, or at least partially encircle, the needle  158  when it extends in a proximal direction through the on-skin component  134   aa . The protrusion  826  can also be configured to secure the securement member  800   aa  to the on-skin component  134   aa . The securement member  800   aa  has an opening  828  which is sized and shaped to create a friction fit between the opening  828  and the needle  158 . In the configuration illustrated in  FIGS.  129  and  130   , with the needle  158  extending distally through the securement member  800   aa  and the protrusion  826 , the friction fit between the securement member  800   aa  and the needle  158  serves to resist at least distal movement of the on-skin component  134   aa  with respect to the needle  158 . 
     The embodiment illustrated in  FIGS.  129 - 131    may also include a resistance member  804   aa . The resistance member may be substantially similar to any resistance member described in  FIGS.  110 - 128   . The resistance member  804   aa  can include a distally-facing surface of the first portion  150   aa , and can have a similar configuration to the resistance member  804  described in the context of  FIG.  117   . The resistance member  804   aa  can provide enough resistance in a distal direction to allow the second spring and needle hub (not shown) to overcome the friction fit between the securement member  800   aa  and the needle  158 . It is contemplated that this would allow the needle  158  to retract away from the skin and at the same time allow the needle to decouple from the securement member  800   aa .  FIG.  131    illustrates a perspective view of a portion of the assembly  104   aa , after decoupling of the on-skin component  134   aa  from the needle  158 , shown with the protrusion  826  of the on-skin component  134   aa  and the securing member  800   aa  cut away for purposes of illustration. 
       FIGS.  132 - 133    illustrate another configuration for releasably securing an on-skin component in a proximal position, in accordance with several embodiments.  FIG.  132    illustrates a perspective view of a portion of a securement member  800   ab  shown secured to an on-skin component  134   ab  comprising a sensor module, with the second portion  150   ab  shown in cross section. The securement member  800   ab  may include at least one engagement feature. As shown in the figure, the at least one engagement feature can be two proximally-extending legs  802   ab . The on-skin component  134   ab  may include at least one receiving feature. As shown, the at least one receiving feature can be two elastomeric grips  824   ab  extending laterally from the on-skin component  134   ab . The grips  824   ab  are deformable and sized and shaped to receive the legs  802   ab via friction or interference fit and thereby releasably secure the on-skin component  134   ab  in a proximal position. In the embodiment illustrated in  FIGS.  132 - 133   , the legs  802   ab  of the securement member  800   ab  have a circular cross-section. The grips  824   ab  are integrally formed with the sensor module, and have an annular-shaped cross section, as viewed in a plane extending normal to the axial direction. The grips  824   ab  may each include an opening which can be configured to receive the legs  802   ab  via frictional engagement. In embodiments, the securement member  800   ab  and the grips  824   ab  can have any suitable cooperating configuration to releasably couple the securement member  800   ab  to the grips  824   ab . Some embodiments can additionally employ an adhesive disposed axially between the securement member  800   ab  and the on-skin component  134   ab , to provide additional securement of the on-skin component  134   ab  in the proximal starting position.  FIG.  133    illustrates a perspective view of the needle hub  162   ab  and the on-skin component  134   ab , after decoupling of the on-skin component  134   ab  from the needle hub  162   ab.    
       FIGS.  134 - 136    illustrate yet another configuration for releasably securing an on-skin component in a proximal position.  FIG.  134    illustrates an exploded perspective view of a portion of an assembly  134   ac , with a securement member  800   ac  configured to releasably couple an on-skin component  134   ac  to a needle hub  162   ac . The securement member  800   ac  may include at least one engagement feature. As shown in the figure, the securement member  800   ac  can include two proximally-extending legs  802   ac . The on-skin component  134   ac  includes two elastomeric grips  824   ac  extending laterally from the sensor module. The grips  824   ac  are sized and shaped to receive the legs  802   ac  in a snap fit to securely hold the on-skin component  134   ac  in a proximal position. In the embodiment illustrated in  FIGS.  134 - 136   , the legs  802   ac  of the securement member  800   ac  have a circular cross-section, with a recessed section  832  configured to receive the grips  824   ac . The grips  824   ac  can be integrally formed with the sensor module, each grip having a frangible link  830  coupling the grips  824   ac  to the sensor module. The grips  824   ac  have an annular-shaped cross section, as viewed in a plane extending normal to the axial direction, the grips being configured to receive the recessed sections  832  of the legs in a secure interlocking engagement. In embodiments, the securement member  800   ab  and the grips  824   ab  can have any suitable cooperating configuration to securely couple the securement member  800   ac  to the grips  824   ac  and prevent slippage of the grips along the legs  802   ac  as the needle hub  162   ac  deploys and as it retracts after deployment. Some embodiments can additionally employ an adhesive disposed axially between the securement member  800   ac  and the on-skin component  134   ab , to provide additional securement of the on-skin component  134   ac  in the proximal starting position and during deployment. 
       FIG.  135    illustrates a perspective view of a portion of the system  104   ac , with the securement member  800   ac  securely coupled to the on-skin component  134   ac . Some embodiments can additionally employ an adhesive disposed axially between the securement member  800   ac  and the on-skin component  134   ac , to provide additional securement of the on-skin component  134   ac  in the proximal starting position. The frangible links  830  are configured to shear or otherwise detach upon application of a minimum threshold of force, as the needle hub  162  retracts in a proximal direction after deployment, separating the grips  824   ac  from the remainder of the on-skin component  134   ac  and leaving the on-skin component  824   ac  in the deployed distal position.  FIG.  136    illustrates a perspective view of a portion of the system  104   ac , with the frangible links  830  broken and the securement member  800   ac  decoupled from the on-skin component  134   ac . In some embodiments, a resistance member can also be employed to prevent proximal movement of the on-skin component  134   ac  as the needle hub  162   ac  retracts, facilitating the breakage of the frangible links  830 . 
     Frangible couplings can also be employed between an on-skin component and the second portion of a sensor inserter system to releasably secure the on-skin component in a proximal starting position prior to deployment. For example,  FIGS.  137 - 140    illustrate various perspective views of a sensor inserter system  104   ad  with an on-skin component  134   ad  releasably secured in a proximal position within the system  104   ad . The on-skin component  134   ad  can include a combination sensor module and base disposed on an adhesive patch  900   ad . To facilitate in releasably securing the on-skin component  134   ad  to the second portion  152   ad , the second portion  152   ad  can include at least one distally-extending protrusion  834 . As shown in the figure, the second portion  152   ad  includes four distally-extending protrusions  834  configured to securely couple with corresponding sockets  836  formed in or otherwise extending from the adhesive patch  900   ad . The sockets  836  are connected to the adhesive patch  900   ad  via frangible links  838 , which can also be integrally formed in the adhesive patch  900   ad . In the resting state illustrated in  FIG.  137   , the adhesive patch  900   ad  is secured in a proximal position by the coupling of the sockets  836  to the posts  838 . As the system  104   ad  is deployed and a force is applied to the on-skin component  134   ad  in a distal direction, the frangible links  838  detach, allowing the adhesive patch  900   ad  (and the on-skin component  134   ad  which is already coupled thereto) to move to the distal deployed position.  FIG.  138    illustrates a perspective view of the sensor inserter system  104   ad , with the frangible links  838  detached and the adhesive patch  900   ad  released from securement.  FIGS.  139  and  140    illustrate perspective views of the adhesive patch  900   ad  and the on-skin component  134   ad , with the frangible links  838  in intact and detached configurations, respectively. Once the frangible links  838  are detached and the on-skin component  134   ad  (along with the patch  900   ad ) is deployed in the distal position, the remainder of the system  104   ad  can easily be lifted off the skin of the host and removed. 
       FIG.  141    illustrates another configuration for releasably securing a base and adhesive patch to a sensor inserter assembly.  FIG.  141    illustrates a cross-sectional perspective view of a portion of a system  104   ae , with the first portion  150   ae , the second portion  152   ae , and the third portion  392   ae  shown in cross section. The system  104   ae  includes an on-skin component  134   ae  which is releasably secured in a proximal starting position. The system  104   ae  also includes a base  128   ae  coupled to an adhesive patch  900   ae . The base  128   ae  and the adhesive patch  900   ae  are disposed in a distal position, at a distal end of the system  104   ae . The base  128   ae  is coupled to the system  104   ae  via a plurality of ribs  840  extending radially inward from the second portion  152   ae . The ribs  840  can be sized and shaped to grip the edges of the base  128   ae  with a friction/interference fit. The friction/interference fit between the ribs  840  and the base  128   ae  can be configured to be strong enough to securely couple the base  128   ae  to the system  104   ae  during storage and prior to deployment, but weak enough that the adhesive coupling between the adhesive patch  900   ae  and the skin of the host overcomes the strength of the friction fit. Thus, once the adhesive patch  900   ae  is adhered to the skin of the host, the second portion  152   ae  can be lifted off the base  128   ae  and the sensor system  104   ae  can be removed without pulling the base  128  in a proximal direction. In some embodiments, the base  128   ae  may comprise an elastomeric material. Further, in some embodiments, the base  128   ae  may have a hardness value less than a hardness value of the on-skin component  134   ae . In other embodiments, the base  128   ae  may have a hardness value more than a hardness value of the on-skin component  134   ae.    
       FIGS.  142  and  143    illustrate yet another configuration for releasably securing an adhesive patch, optionally including a base, to a sensor inserter system.  FIG.  142    shows a sensor inserter system  104   af  with an adhesive patch  900   af  coupled to the second portion  152   af  of the system  104   af .  FIG.  143    shows the system  104   af  with the patch  900   af  separated from the second portion  152   af . As shown in  FIG.  143   , the second portion  152   af  includes a plurality of adhesive dots  842  disposed on a distally-facing surface or edge of the second portion  152   af . The adhesive dots  842  can be configured to be strong enough to securely couple the adhesive patch  900   af  (and base, if any) to the system  104   af  during storage and prior to deployment, but weak enough that the adhesive coupling between the adhesive patch  900   af  and the skin of the host overcomes the strength of the adhesive dots  842 . Thus, once the adhesive patch  900   af  is adhered to the skin of the host, the second portion  152   af  can be lifted off the applicator patch  900   af  (and base, if any) and the sensor system  104   af  can be removed without pulling the adhesive patch  900   af  (or base, if any) in a proximal direction. Alternatively or in addition to the adhesive dots  842 , some embodiments can include an adhesive disposed on a proximally-facing surface of the adhesive patch  900   af . In some embodiments, the adhesive can be a pressure-sensitive adhesive. 
     Interpretation 
     For ease of explanation and illustration, in some instances the detailed description describes exemplary systems and methods in terms of a continuous glucose monitoring environment; however it should be understood that the scope of the invention is not limited to that particular environment, and that one skilled in the art will appreciate that the systems and methods described herein can be embodied in various forms. Accordingly any structural and/or functional details disclosed herein are not to be interpreted as limiting the systems and methods, but rather are provided as attributes of a representative embodiment and/or arrangement for teaching one skilled in the art one or more ways to implement the systems and methods, which may be advantageous in other contexts. 
     For example, and without limitation, described monitoring systems and methods may include sensors that measure the concentration of one or more analytes (for instance glucose, lactate, potassium, pH, cholesterol, isoprene, and/or hemoglobin) and/or other blood or bodily fluid constituents of or relevant to a host and/or another party. 
     By way of example, and without limitation, monitoring system and method embodiments described herein may include finger-stick blood sampling, blood analyte test strips, non-invasive sensors, wearable monitors (e.g. smart bracelets, smart watches, smart rings, smart necklaces or pendants, workout monitors, fitness monitors, health and/or medical monitors, clip-on monitors, and the like), adhesive sensors, smart textiles and/or clothing incorporating sensors, shoe inserts and/or insoles that include sensors, transdermal (i.e. transcutaneous) sensors, and/or swallowed, inhaled or implantable sensors. 
     In some embodiments, and without limitation, monitoring systems and methods may comprise other sensors instead of or in additional to the sensors described herein, such as inertial measurement units including accelerometers, gyroscopes, magnetometers and/or barometers; motion, altitude, position, and/or location sensors; biometric sensors; optical sensors including for instance optical heart rate monitors, photoplethysmogram (PPG)/pulse oximeters, fluorescence monitors, and cameras; wearable electrodes; electrocardiogram (EKG or ECG), electroencephalography (EEG), and/or electromyography (EMG) sensors; chemical sensors; flexible sensors for instance for measuring stretch, displacement, pressure, weight, or impact; galvanometric sensors, capacitive sensors, electric field sensors, temperature/thermal sensors, microphones, vibration sensors, ultrasound sensors, piezoelectric/piezoresistive sensors, and/or transducers for measuring information of or relevant to a host and/or another party. 
     None of the steps described herein is essential or indispensable. Any of the steps can be adjusted or modified. Other or additional steps can be used. Any portion of any of the steps, processes, structures, and/or devices disclosed or illustrated in one embodiment, flowchart, or example in this specification can be combined or used with or instead of any other portion of any of the steps, processes, structures, and/or devices disclosed or illustrated in a different embodiment, flowchart, or example. The embodiments and examples provided herein are not intended to be discrete and separate from each other. 
     The section headings and subheadings provided herein are nonlimiting. The section headings and subheadings do not represent or limit the full scope of the embodiments described in the sections to which the headings and subheadings pertain. For example, a section titled “Topic 1” may include embodiments that do not pertain to Topic 1 and embodiments described in other sections may apply to and be combined with embodiments described within the “Topic 1” section. 
     Some of the devices, systems, embodiments, and processes use computers. Each of the routines, processes, methods, and algorithms described in the preceding sections may be embodied in, and fully or partially automated by, code modules executed by one or more computers, computer processors, or machines configured to execute computer instructions. The code modules may be stored on any type of non-transitory computer-readable storage medium or tangible computer storage device, such as hard drives, solid state memory, flash memory, optical disc, and/or the like. The processes and algorithms may be implemented partially or wholly in application-specific circuitry. The results of the disclosed processes and process steps may be stored, persistently or otherwise, in any type of non-transitory computer storage such as, for example, volatile or non-volatile storage. 
     Any of the features of each embodiment is applicable to all aspects and embodiments identified herein. Moreover, any of the features of an embodiment is independently combinable, partly or wholly with other embodiments described herein in any way, e.g., one, two, or three or more embodiments may be combinable in whole or in part. Further, any of the features of an embodiment may be made optional to other aspects or embodiments. Any aspect or embodiment of a method can be performed by a system or apparatus of another aspect or embodiment, and any aspect or embodiment of a system can be configured to perform a method of another aspect or embodiment. 
     The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and subcombinations are intended to fall within the scope of this disclosure. In addition, certain method, event, state, or process blocks may be omitted in some implementations. The methods, steps, and processes described herein are also not limited to any particular sequence, and the blocks, steps, or states relating thereto can be performed in other sequences that are appropriate. For example, described tasks or events may be performed in an order other than the order specifically disclosed. Multiple steps may be combined in a single block or state. The example tasks or events may be performed in serial, in parallel, or in some other manner. Tasks or events may be added to or removed from the disclosed example embodiments. The example systems and components described herein may be configured differently than described. For example, elements may be added to, removed from, or rearranged compared to the disclosed example embodiments. 
     Conditional language used herein, such as, among others, “can,” “could,” “might,” “may,” “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular embodiment. The terms “comprising,” “including,” “having,” and the like are synonymous and are used inclusively, in an open-ended fashion, and do not exclude additional elements, features, acts, operations and so forth. Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list. Conjunctive language such as the phrase “at least one of X, Y, and Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to convey that an item, term, etc. may be either X, Y, or Z. Thus, such conjunctive language is not generally intended to imply that certain embodiments require at least one of X, at least one of Y, and at least one of Z to be present. 
     The term “and/or” means that “and” applies to some embodiments and “or” applies to some embodiments. Thus, A, B, and/or C can be replaced with A, B, and C written in one sentence and A, B, or C written in another sentence. A, B, and/or C means that some embodiments can include A and B, some embodiments can include A and C, some embodiments can include B and C, some embodiments can only include A, some embodiments can include only B, some embodiments can include only C, and some embodiments can include A, B, and C. The term “and/or” is used to avoid unnecessary redundancy. 
     All references cited herein are incorporated herein by reference in their entirety. To the extent publications and patents or patent applications incorporated by reference contradict the disclosure contained in the specification, the specification is intended to supersede and/or take precedence over any such contradictory material. 
     Unless otherwise defined, all terms (including technical and scientific terms) are to be given their ordinary and customary meaning to a person of ordinary skill in the art, and are not to be limited to a special or customized meaning unless expressly so defined herein. It should be noted that the use of particular terminology when describing certain features or aspects of the disclosure should not be taken to imply that the terminology is being re-defined herein to be restricted to include any specific characteristics of the features or aspects of the disclosure with which that terminology is associated. Terms and phrases used in this application, and variations thereof, especially in the appended claims, unless otherwise expressly stated, should be construed as open ended as opposed to limiting. 
     While certain example embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions disclosed herein. Thus, nothing in the foregoing description is intended to imply that any particular feature, characteristic, step, module, or block is necessary or indispensable. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions, and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions disclosed herein.