Patent Publication Number: US-2022226573-A1

Title: Infusion device

Description:
TECHNICAL FIELD 
     This disclosure relates generally to systems for the insertion of medical devices. 
     BACKGROUND 
     Certain diseases or conditions may be treated, according to modern medical techniques, by delivering a medication fluid or other substance to the body of a patient, either in a continuous manner or at particular times or time intervals within an overall time period. For example, diabetes is often treated by delivering defined amounts of insulin to the patient at appropriate times. Some modern systems employ programmable fluid infusion devices (e.g., insulin pumps) to deliver controlled amounts of insulin to a patient. Some modes of providing insulin therapy to a user include delivery of insulin through manually operated syringes and insulin pens. Some other modes employ programmable fluid infusion devices (e.g., insulin pumps) to deliver controlled amounts of insulin to a user. Moreover, in certain instances, it may be desirable for a user to receive feedback from a physiological characteristic monitor, such as a glucose monitor. In these instances, the physiological characteristic monitor and the infusion set are often separately coupled to the user&#39;s anatomy at different insertion sites. 
     BRIEF SUMMARY 
     The disclosure generally relates to an infusion device configured to implant a cannula and a sensor within a user. The infusion device may be configured to be worn by the user. The infusion device may include a first unit configured to contact the skin of the user and a second unit configured to engage the first unit. The second unit includes a first insertion needle configured to implant the cannula and a second insertion needle to implant the sensor when the second unit engages the first unit. The infusion device is configured to fluidly connect the cannula with a fluid reservoir within the first unit and electrically connect the sensor with processing circuitry within the first unit engages the second unit. The first unit may be configured to substantially limit motion of the second unit relative to the first unit when the second unit engages the first unit. 
     The infusion device may be configured to withdraw the first and second insertion needles when the second unit is engaged with the first unit. The infusion device is configured such that the cannula and the sensor remain implanted when the insertion needles are withdrawn and the second unit engages the first unit. The second unit may be configured such that the cannula and the sensor withdraw from the user when the user disengages the second unit from the first unit. The infusion device is configured such that the second unit may be replaced as the first unit remains proximate the user. 
     In an example, an infusion device comprises: a first unit defining a first housing, wherein the first housing defines a first channel extending through the first housing, a second channel extending through the first housing, and a fluid access, wherein the first unit includes processing circuitry and a fluid reservoir in fluid communication with the fluid access; and a second unit defining an second housing configured to engage the first housing, the second unit comprising: a cannula having a first end and a second end; and a sensor; a first insertion needle releasably carrying the cannula; and a second insertion needle releasably carrying the sensor, wherein: the first insertion needle is configured to insert the first end of the cannula through the first channel when the second housing engages the first housing, the second insertion needle is configured to insert the sensor through the second channel when the second housing engages the first housing, the second end of the cannula is configured to insert through the fluid access when the second housing engages the first housing, and the infusion device is configured to electrically connect the sensor and the processing circuitry when the second housing engages the first housing. 
     In an example, an infusion device comprises: a first unit defining a first housing, wherein the first housing defines a first channel extending through the first housing, a second channel extending through the first housing, and a fluid access, and wherein the first unit includes processing circuitry, a fluid reservoir, and a fluid pump in fluid communication with the fluid access; and a second unit defining an second housing configured to engage the first housing, the second unit comprising: a cannula having a first end and a second end; and a sensor; a first insertion needle releasably carrying the cannula and configured to extend through the first channel; and a second insertion needle releasably carrying the sensor and configured to extend through the second channel, wherein: the first housing is configured to substantially secure the second housing from movement relative to the first housing when the second housing engages the first housing, the first insertion needle is configured to extend a portion of the cannula including the first end through the first channel when the first insertion needle extends through the first channel, the second insertion needle is configured to extend a portion of the sensor through the second channel when the second insertion needle extends through the second channel, the second end of the cannula is configured to insert through the fluid access when the second housing engages the first housing, wherein the cannula is configured to establish fluid communication from the first end of the cannula to the fluid reservoir when the second end of the cannula inserts through the fluid access, and the infusion device is configured to electrically connect the sensor and the processing circuitry when the second housing engages the first housing. 
     In an example, a technique includes: engaging a first housing defined by a first unit and a second housing defined by a second unit, wherein the first unit includes processing circuitry and a fluid reservoir, and wherein the second unit includes a first insertion needle, a second insertion needle, a cannula, and a sensor; inserting, when the second housing engages the first housing, a first end of the cannula through a first channel defined by the first housing using the first insertion needle; inserting, when the second housing engages the first housing, the sensor through a second channel defined by the first housing using the second insertion needle; inserting, when the second housing engages the first housing, a second end of the cannula into a fluid access defined by the first housing; and electrically connecting, when the second housing engages the first housing, the sensor and the processing circuitry. 
     The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an infusion device attached to the body of a user. 
         FIG. 2  is a simplified block diagram representation of an infusion device. 
         FIG. 3  is a schematic side view of an infusion device attached to the body of a user. 
         FIG. 4A  is a schematic of a first unit and a second unit. 
         FIG. 4B  is a schematic of the first unit and the second unit of  FIG. 4A  in another configuration. 
         FIG. 4C  is a schematic of the first unit and the second unit of  FIG. 4A  and  FIG. 4B  in a different configuration. 
         FIG. 5  is a schematic of second unit aligning with a first unit. 
         FIG. 6A  is a first view of an inserter and a second unit. 
         FIG. 6B  is a second view of the inserter and second unit of  FIG. 6A . 
         FIG. 7  is a schematic of a second unit. 
         FIG. 8A  is a schematic of an inserter in a first configuration. 
         FIG. 8B  is a schematic of the inserter of  FIG. 8A  in a second configuration. 
         FIG. 8C  is a schematic of the inserter of  FIG. 8A  and  FIG. 8B  in a third configuration. 
         FIG. 8D  is a schematic of the inserter of  FIG. 8A ,  FIG. 8B , and  FIG. 8C  in a fourth configuration. 
         FIG. 9  illustrates an example technique of using an infusion device. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure describes an infusion device configured to implant a cannula and a sensor within a user. The infusion device may be generally related to a fluid infusion device configured to provide a therapeutic fluid to a user and monitor a physiological characteristic of the user. For example, the cannula may be a fluid delivery cannula configured to deliver a fluid (e.g., insulin) to the user. The sensor may be an analyte sensor (e.g., a glucose sensor) configured to detect a physiological characteristic of the user (e.g., a glucose level). The infusion device may be configured to implant the cannula and the sensor in the user substantially concurrently but the examples are not limited to substantially concurrent implantation of the cannula and sensor. In examples, the infusion device is a portable system configured to be worn by the user. 
     The infusion device includes a first unit configured to engage a second unit. The first unit and the second unit may be substantially separate units. For example, the first unit may include one or more components mechanically supported and/or housed substantially within a first housing and the second unit may include one or more components mechanically supported and/or housed substantially within a second housing. The second unit (e.g., the second housing) may be configured to engage the first unit (e.g., the first housing) such that the components of the first unit and second unit cooperatively operate for therapeutic benefit to a user. 
     In examples, the infusion device is configured such that the second unit may be engaged with (e.g., installed on) and/or disengaged from (e.g., separated) from the first unit through actions of the user. For example, the user may install the second unit on the first unit to initiate use of the cannula, sensor, and other components of the infusion device. The user may separate the second unit and the first unit subsequent to the use. In examples, the first unit is configured to position on the user (e.g., on the skin of the user), and the second unit may be installed and removed as the first unit is positioned on the user. The second unit may be substantially replaceable, such that a particular second unit initially installed on the first unit may be removed by the user and replaced with another similar and/or largely identical second unit. In some examples, the infusion device is configured such that second unit may be replaced as the first unit remains positioned on the user. 
     The first unit may be configured to limit movement of the second unit relative to the first unit when the second unit engages the first unit. In examples, the second housing of the second unit is configured to engage (e.g., mechanically engage) the first housing of the first unit to limit movement of the second housing relative to the first housing. The second housing may be configured to engage the first housing such that components of the first unit and components of the second unit cooperatively operate to provide a therapeutic fluid (e.g., insulin) to the user using the cannula and monitor a physiological characteristic of the user (e.g., a glucose level) using the sensor. 
     The infusion device may be configured to implant the cannula and the sensor within the user. In examples, the infusion device is configured to implant the cannula and the sensor during the installation of the second unit on the first unit. The second unit may include a first insertion needle configured to implant the cannula and a second insertion needle configured to implant the sensor. The first insertion needle and the second insertion needle may releasably carry the cannula and the sensor respectively. The infusion device may be configured to cause the implantation of the cannula and the sensor within the user as the second unit engages or is caused to engage the first unit. For example, the infusion device may be configured such that, as the second unit engages the first unit (e.g., by the user), the first insertion needle and the second insertion needle extend from the infusion device to cause the implantation. 
     In examples, the cannula is an insulin cannula and the sensor is a glucose sensor. Including both the glucose sensor and the insulin cannula within the infusion device (e.g., in the first unit) may allows for a “closed loop” system whereby insulin delivery to the user may be adjusted in real-time based on readings from the glucose sensor. Such real-time adjustment may assist in maintaining the user&#39;s glucose levels. Further, this may reduce a need for the user to carry separate insulin infusion and glucose monitoring devices. Including both an insulin cannula and glucose sensor may provide for an infusion device that is more convenient for a user to carry, and may reduce/and/or limit the number of insertion sites required. 
     In examples, the infusion device is configured to implant the cannula and the sensor when the first unit is positioned on the user (e.g., on the skin of the user). The first insertion needle and second insertion needle may be configured to extend from the second unit and pass through the first unit to cause the implantation as the second unit engages the first unit, such that distal ends of the first insertion needle and the second insertion needle pierce the skin of the user. In examples, the infusion device is configured such the first housing of the first unit is between the second housing of the second unit and the user when the first insertion needle and the second insertion needle cause the implantation. The infusion device may be configured such that, when the first unit is positioned on the user, movement of the second unit in a first direction toward the first unit causes the first insertion needle and the second insertion needle to extend through the first unit to implant the cannula and the sensor. The movement in the first direction (e.g., toward the user) may cause the second housing of the second unit to engage the first housing of the first unit, such that the first housing limits and/or substantially prevents movement of the second housing relative to the first housing. 
     In examples, the infusion device is configured such that the first insertion needle and the second insertion needle pierce the skin of the user substantially concurrently. The infusion device may be configured to cause the first insertion needle and the second insertion needle to pierce the skin of the user substantially concurrently as the second unit is caused to engage the first unit (e.g., caused by the user). The infusion device may cause the first insertion needle and the second insertion needle to insert substantially concurrently in order to, for example, limit discomfort to the user that might be caused by insertions separated by a user-discernable chronological time increment. In other examples, the infusion device may be configured to cause the first insertion needle and the second insertion needle to insert at substantially different chronological time increments. 
     The infusion device may be configured such that the first insertion needle and/or the second insertion needle may be withdrawn in a second direction (e.g., away from the user) substantially opposite the first direction as the second housing remains engaged with the first housing, such that the first insertion needle and/or the second insertion needle may be withdrawn from the user as the second unit remains positioned on the first unit. The first insertion needle may be configured to release the cannula as the first insertion needle is withdrawn in the second direction. The second insertion needle may be configured to release the sensor as the second insertion needle is withdrawn in the second direction. 
     Hence, the infusion device may be configured such that the user may cause the implantation of the cannula and the sensor by causing the second unit to engage the first unit. The infusion device may be configured such that the user may cause the withdrawal of the first insertion needle and/or the second insertion needle following the implantations. In examples, the infusion device is configured such that the first insertion needle and/or the second insertion needle are substantially separable from the first unit and the second unit when the first insertion needle and/or the second insertion needle are withdrawn in the second direction (e.g., away from the user), such that user may continue to utilize the functions of the first unit and the second unit without the continued presence of the first insertion needle and/or the second insertion needle. 
     In examples, the first unit includes a fluid reservoir configured to hold a fluid and processing circuitry configured to communicate with a sensor. The second unit may include the cannula and the sensor. The infusion device may be configured such that, when the second housing engages the first housing, the cannula of the second unit is placed in fluid communication with the fluid reservoir of the first unit to allow the infusion device to supply the therapeutic fluid. The infusion device may be configured such that, when the second housing engages the first housing, the processing circuitry is placed in electrical communication with the sensor, such that the sensor may communicate with the processing circuitry. In examples, the infusion device includes a fluid pump (e.g., an insulin pump) configured to deliver the therapeutic fluid from the fluid reservoir to the cannula, and the processing circuitry is configured to control an operation of the fluid pump (e.g., to commence or cease pumping) based on a signal from the sensor. 
     The second unit is configured to support the cannula such that cannula establishes fluid communication with the fluid reservoir of the first unit when the second housing is engaged with the first housing. In examples, the cannula includes a lumen configured to establish fluid communication between a first end of the cannula (“cannula first end”) and a second end of the cannula (“cannula second end”) opposite the cannula first end. In examples, the second unit mechanically supports a portion of the cannula including the cannula second end such that, when the second housing engages the first housing, the cannula second end inserts into the fluid access of the first unit. The first insertion needle may be configured to implant the cannula first end in the user as the second housing engages the first housing. Thus, the second unit may be configured such that the cannula establishes fluid communication between the fluid reservoir of the first unit and the cannula first end implanted in the user when the second housing engages the first housing. In examples, the cannula second end is configured to pierce a septum providing fluid isolation between the fluid reservoir and the first housing of the first unit to establish the fluid communication. 
     The infusion device is configured to establish communication between the sensor of the second unit and the processing circuitry of the first unit when the second housing engages the first housing. In examples, the first unit includes a first connector in electrical communication with the processing circuitry and the second unit includes a second connector in electrical communication with the sensor. The infusion device is configured such that engagement of the second housing with the first housing causes the second connector to establish an electrical connection with the first connector, such that the sensor (in the second unit) may provide a signal to the processing circuitry (in the first unit) indicative of a physiological characteristic of the user (e.g., a glucose level). The infusion device may be configured such that when the second end of the cannula inserts into the fluid access (e.g., when the second unit engages the first unit), the second connector establishes the electrical connection with the first connector. In examples, the second unit is configured to mechanically support the second electrical connector and the cannula such that engagement of the second unit and the first unit causes the second end of the cannula to insert into the fluid access and the second connector to establish the electrical connection with the first connector. 
     The first unit may be configured such that the first insertion needle and/or the second insertion needle are translatable relative to the first unit. The first insertion needle may be translatable through the first unit in a distal direction (“first needle distal direction”) and in a proximal direction (“first needle proximal direction) opposite the first needle distal direction. The first needle may be configured to translate in the first needle distal direction to pierce the skin of the user, and to translate in the first needle proximal direction to withdraw from the skin of the user. 
     In examples, the second unit is configured such that the first insertion needle and/or the second insertion needle is translatable relative to the second unit. For example, the second unit may be configured such that the first insertion needle and/or second insertion needle translates relative to the second unit in order to withdraw the first insertion needle and/or the second insertion needle from the user as the second housing remains engaged with the first housing. In examples, the infusion device is configured such that the first insertion needle may extend and/or translate through the first unit concurrent with extending through the second unit. 
     The infusion device may be configured to substantially the second unit and the first unit such that the first insertion needle may pass through the second unit and the first unit when the engagement of the second housing and the first housing causes the second end of the cannula to insert into the fluid access. The infusion device may be configured to substantially the second unit and the first unit such that the first insertion needle may pass through the second unit and the first unit when the engagement of the second housing and the first housing causes the second connector to establish the electrical connection with the first connector. 
     In examples, the infusion device includes an inserter configured to cause the second housing of the second unit to engage the first housing of the first unit. The inserter may be configured to hold the second unit is a disengaged position wherein the second unit is displaced from the first unit, and configured to cause the second unit to move to an engaged position wherein the second housing engages the first housing. In examples, the inserter is configured to mate with the first unit when the inserter holds the second unit in the disengaged position. The inserter may be configured to substantially align the second unit and the first unit, such that when the inserter causes the engagement of the second housing and the first housing, the components of the first unit and second unit may cooperatively operate to provide a therapeutic fluid (e.g., insulin) to the user using the cannula and monitor a physiological characteristic of the user (e.g., a glucose level) using the sensor. 
     In examples, the inserter is configured such that a user may cause the inserter to move the second unit from the disengaged position to the engaged position. In examples, the inserter is configured to cause the first insertion needle to implant the cannula and the second insertion needle to implant the sensor into the user when the inserter moves the second unit from the disengaged position to the engaged position. The inserter may include a user input device configured to cause the inserter to move the second unit from the disengaged position to the engaged position, such that the user may control the implantation of the cannula and the sensor. The user input device may be, for example, a button on the inserter, a wireless communication device, or some other user-controlled activation device. In some examples, the inserter is configured such that a force imparted by the user on the inserter (e.g., a force toward the skin of the user) causes the inserter to move the second device from the disengaged position to the engaged position. The inserter may be configured such that, when the inserter moves the second device from the disengaged position to the engaged position, the first insertion needle moves in the first distal direction through the first housing and the second insertion needle moves in the second distal direction through the first housing to pierce the skin of the user and implant the cannula and the sensor respectively. 
     The inserter may be configured to align the second electrical connector of the second unit with the first electrical connector of the first unit when the inserter mates with the first unit, such that the second electrical connector establishes the electrical connection with the first electrical connector when the inserter moves the second unit from the disengaged position to the engaged position. In examples, the inserter is configured to align the second end of the cannula of the second unit with the fluid access of the first unit when the inserter mates with the first unit, such that the second end of the cannula inserts in the fluid access when the inserter moves the second unit from the disengaged position to the engaged position. The inserter may be configured to align the second unit first channel with the first unit first channel when the inserter mates with the first unit, such that the first insertion needle may extend through the second unit first channel and the first unit first channel when the inserter moves the second unit to the engaged position. The inserter may be configured to align the second unit second channel with the first unit second channel when the inserter mates with the first unit, such that the second insertion needle may extend through the second unit second channel and the first unit second channel when the inserter moves the second unit to the engaged position. 
     The inserter may be configured to withdraw the first insertion needle and/or the second insertion needle from the user when the cannula and the sensor are implanted. The inserter may withdraw the first insertion needle and/or the second insertion needle as the second housing remains engaged with the first housing. In examples, the inserter is configured to cause an initial insertion of the first insertion needle and the second insertion needle followed by a subsequent withdrawal of the first insertion needle and the second insertion needle in response to a single actuation of the inserter by the user. The inserter may be configured to initially cause the first insertion needle and/or the second insertion needle to extend in a distal direction by a certain amount to implant the cannula and the sensor, then subsequently cause the first insertion needle and/or the second insertion needle to withdraw in a proximal direction to withdraw from the skin of the user. The inserter may be configured to withdraw the first insertion needle and the second insertion from the first unit and the second unit as the first housing remains engaged with the second housing and the cannula and the sensor remain implanted in the user. 
     The infusion device may be configured to be positioned proximate to the skin of the user. In examples, the first housing is configured to contact the skin of the user. The infusion device may be configured to substantially secure its location on the user in order to, for example, allow mobility to the user as the infusion device administers and monitors therapies delivered to the user. For example, the infusion device may be configured to allow a degree of user mobility as the infusion device delivers insulin to the user through the cannula and monitors a glucose level of the user using the sensor. The infusion device may be substantially secured to the user using any suitable arrangement. In some examples, the first housing and/or second housing of the infusion device includes an adhesive element configured to removably secure the infusion device to the skin of the user. The infusion device may be utilized to administer a variety of medications to a user such as, but not limited to, disease treatments, drugs to treat pulmonary hypertension, iron chelation drugs, pain medications, anti-cancer treatments, medications, vitamins, hormones, and the like. 
       FIG. 1  is a schematic view of portions of an infusion device  100  contacting a body  101  of a user. In  FIG. 1 , infusion device  100  is implemented as a fluid infusion device configured to provide a therapeutic fluid to the user and monitor a physiological characteristic of the user. Infusion device  100  includes a cannula  102  and a sensor  104 . Cannula  102  is implanted in body  101  and configured to deliver a fluid (e.g., insulin) to the body  101 . Sensor  104  (e.g., a glucose sensor) is implanted in body  101  and configured to detect a physiological characteristic of the user (e.g., a glucose level). 
     Infusion device  100  includes a first unit  106  and a second unit  108 . First unit  106  includes a first housing  110  mechanically supporting one or more components of first unit  106 , such as fluid reservoir  112  and processing circuitry  114 . Second unit  108  includes second housing  116  mechanically supporting one or more components of second unit  104 , such as cannula  102  and sensor  104 . Second unit  108  engages (e.g., mechanically engages) first unit  106  such that components of second unit  108  and components of first unit  106  cooperatively operate to provide a therapeutic fluid (e.g., insulin) to the user using cannula  102  and monitor a physiological characteristic of the user (e.g., a glucose level) using sensor  104 . Portions of first unit  106  and second unit  108  are depicted as hidden using dashed lines in  FIG. 1 . 
     Infusion device  100  may be configured such that first unit  106  and second unit  108  define substantially separable units. Infusion device  100  may be configured such that second unit  108  may be engaged with (e.g., installed on) and/or disengaged from (e.g., separated) from first unit  106  through actions of the user, or by some other actions. For example, the user may install second unit  108  on first unit  106  prior to use. In examples, second unit  108  may be caused to engage first unit  106  using an inserter (not shown) configured to be operated by the user. The user may separate second unit  108  and first unit  106  subsequent to the use. In examples, first unit  106  is configured to position on the user (e.g., on skin  118  of body  101  of the user), and second unit  108  may be installed and removed as first unit  106  remains positioned on the user. Second unit  108  may be substantially replaceable, such that the particular second unit  108  depicted in  FIG. 1  may be removed by the user and substituted with a replacement second unit (not shown) configured similarly and/or largely identically to second unit  108 . 
     Infusion device  100  may be configured such that disengagement and separation of second unit  108  from first unit  106  withdraws cannula  102  and sensor  104  from body  101  of the user, allowing for disposal of cannula  102  and sensor  104  following use. Infusion device  100  may be configured such that a cannula and sensor of the replacement second unit implants within the body  101  when the replacement second unit is substituted for second unit  108 . As will be discussed, second unit  108  may include a first insertion needle (not shown) and a second insertion needle (not shown) configured to implant cannula  102  and sensor  104  in body  101  when second unit  108  engages first unit  106 . The first insertion needle and second insertion needle may be withdrawn and substantially displaced from second unit  108  and first unit  106  following the implantation to, for example, improve the comfort of the user when infusion device  100  is worn as a portable device. 
     First unit  106  may be configured to limit movement of second unit  108  when second unit  108  and first unit  106  are engaged. In examples, second housing  116  is configured to engage (e.g., mechanically engage) first housing  110  to limit movement of second housing  116  relative to first housing  110 . In examples, second housing  116  or first housing  110  defines a fixation device  120  configured to limit movement of second housing  116  relative to first housing  110  when second housing  116  engages first housing  110 . Fixation device  120  may be, for example, a mechanical structure configured to substantially secure second housing  116  in a position relative to first housing  110 , a magnetic or electromagnetic device configured to substantially secure second housing  116  in a position relative to first housing  110 , or some other device configured to limit movement of second housing  116  relative to first housing  110 . In some examples, fixation device  120  is configured to substantially secure second housing  116  relative to first housing  110  when second housing  116  is caused to engage first housing  110  (e.g., by the user). Fixation device  120  may be substantially affixed to either first housing  110  or second housing  116 . That is, when second unit  108  is disengaged and displaced from first unit  106 , fixation device  120  may be configured to remain affixed to either first unit  106  or second unit  108 . 
     In examples, fixation device  120  is configured to establish at least a first position wherein fixation device  120  limits movement of second housing  116  relative to first housing  110  and at least a second position wherein fixation device  120  allows substantially independent movement of second housing  116  relative to first housing  110  (e.g., such that second unit  108  may be removed from first unit  106 ). Infusion device  100  may include a input device  122  configured to cause fixation device  120  to transition from the first position to the second position, or vice-versa. Input device  122  may be configured to be actuated by the user as needed to, for example, separate second unit  108  from first unit  106  when the unit desires to replace second unit  108 . In examples, input device  122  is a manually operated button on first unit  106  and/or second unit  108 , a circuitry configured to receive a communication (e.g., a wireless communication) from a smart phone, tablet, or other device, or some other device configured for control by the user. 
     In examples, input device  122  is a multipurpose input device configured to prompt multiple operations of infusion device  100 . For example, input device  122  may be configured to cause one or more of the following functions, without limitation: waking up a processor and/or electronics of infusion device  100 ; configuring one or more settings of infusion device  100 ; initiating delivery of medication fluid; initiating a fluid priming operation; disabling alerts or alarms generated by infusion device  100 ; and the like. In lieu of a button, input device  122  can employ a slider mechanism, a pin, a lever, a switch, a touch-sensitive element, or the like. Input device  108  may be configured to receive a communication from a device remote from first housing  110  and/or second housing  116  (e.g., a wireless communication) to initiate to cause infusion device  100  to perform one or more of the described functions, or other functions. Infusion device  100  may include more than one input device  122  (e.g., more than one button) to initiate the various functions described. 
     Second housing  116  is configured to engage first housing  110  (e.g., via fixation device  120 ) such that components of first unit  106  and components of second unit  108  cooperatively operate to provide a therapeutic fluid (e.g., insulin) to the user using cannula  102  and monitor a physiological characteristic of the user (e.g., a glucose level) using sensor  104 . In examples, first unit  106  includes fluid reservoir  112  configured to hold a fluid and processing circuitry  114  configured to communicate with a sensor. Second unit  108  may include cannula  102  and sensor  104 . Infusion device  100  may be configured such that, when second housing  116  is engaged with first housing  110  (e.g., when second unit  108  is installed on first unit  106 ), cannula  102  of second unit  108  is placed in fluid communication with fluid reservoir  112  of first unit  106  to supply the therapeutic fluid. Infusion device  100  may be configured such that, when second housing  116  is engaged with first housing  110 , processing circuitry  114  is placed in electrical communication with sensor  104 , such that sensor  104  may communicate with processing circuitry  114 . In examples, infusion device  100  includes a fluid pump  127  configured to deliver fluid from fluid reservoir  112  to cannula  102 , and processing circuitry  114  is configured to control an operation of the fluid pump  127  (e.g., to commence or cease pumping) based on a signal from sensor  104 . Thus, infusion device  100  is configured to cause the components of first unit  106  and the components of second unit  108  to cooperatively operate for therapeutic benefit to the user when second unit  108  is engaged with (e.g., installed on) first unit  106 . 
     In examples, sensor  104  is configured to sense a physiological characteristic of a user. In some examples, sensor  104  is an glucose sensor. For example, sensor  104  may be an electrochemical sensor that includes the glucose oxidase enzyme. The glucose oxidase enzyme may enable sensor  104  to monitor glucose levels in a diabetic patient or user by affecting a reaction of glucose and oxygen. In some examples, sensor  104  includes a working electrode, a counter electrode, and a reference electrode. The working electrode may be coated with the glucose oxidase enzyme. The reference electrode may be configured to maintain a constant voltage to support a reaction at working electrode. The counter electrode may be configured to supply current to maintain the set potential on the working electrode. The working electrode, counter electrode and reference electrode may each be composed of a suitable biocompatible metal or metal alloy, such as copper, platinum, platinum-iridium, silver, gold, etc., and may be extruded. When glucose and oxygen diffuse to the glucose oxidase layer, hydrogen peroxide is formed. Hydrogen peroxide present at the working electrode metallization layer breaks down and generates electrons when a voltage is applied to the working electrode. In examples, sensor  104  is configured such that these electrons generates an electrical signal, which is transmitted by the working electrode and communicated to the processing circuitry of infusion device  100 . 
     In examples, cannula  102  is substantially affixed to second unit  108 , such that when second unit  108  is disengaged and displaced from first unit  106  (e.g., due to replacement of second unit  108  by a user), the separation of second unit  108  from first unit  106  causes cannula  102  to withdraw from body  101  of the user. Sensor  104  may be substantially affixed to second unit  108 , such that when second unit  108  is disengaged and displaced from first unit  106 , the separation of second unit  108  from first unit  106  causes sensor  104  to withdraw from body  101  of the user. Hence, infusion device  100  may be configured such that disengagement and separation of second unit  108  from first unit  106  withdraws cannula  102  and sensor  104  from body  101  of the user, allowing for disposal of cannula  102  and sensor  104  following use. 
     In examples, infusion device  100  is a portable device. Infusion device  100  may be a wearable device configured to be worn by the user. In examples, first housing  110  defines a base surface  111  configured to position proximate the skin  118  of the user. Base surface  111  may be configured to serve as the user-mounting structure of infusion device  100 . Infusion device  100  may include an adhesive element  113  configured to substantially affix first housing  110  to the body of the user. In examples, adhesive element  113  may be located on base surface  111  of the first housing  110  such that first housing  110  can be temporarily adhered to the skin  118  of the user. Adhesive element  113  may cover substantially all of base surface  111 , or may only partially cover base surface  111  if so desired. Adhesive element  113  may be, for example, a piece of double sided adhesive tape that is cut into the desired shape and size. In some examples, infusion device  100  is manufactured with an adhesive liner overlying adhesive element  113 , and the adhesive liner is peeled away to expose the sticky surface of adhesive element  113 . 
     In examples, base surface  111  defines a first hole  115  forming an opening through first housing  110 . First hole  115  may be defined to accommodate passage of a first insertion needle (not shown) and at least a portion of cannula  102  from a position within first housing  110  to a position outside of first housing  110 . First hole  115  may be configured to accommodate retraction of the first insertion needle from a position outside first housing  110  to a position within first housing  110  (e.g., during withdrawal of the first insertion needle). Base surface  111  may define at least a second hole  117  forming an opening through first housing  110 . Second hole  117  may be defined to accommodate passage of a second insertion needle (not shown) and at least a portion of sensor  104  from a position within first housing  110  to a position outside of first housing  110 . Second hole  117  may be configured to accommodate retraction of the second insertion needle from a position outside first housing  110  to a position within first housing  110  (e.g., during withdrawal of the second insertion needle). 
       FIG. 2  depicts an example simplified block diagram representation of infusion device  100  with second housing  116  engaged with first housing  110 . Infusion device  100  includes cannula  102 , sensor  104 , first unit  106  defining first housing  110 , second unit  108  defining second housing  116 , fluid reservoir  112 , and processing circuitry  114 . Infusion device  100  is configured to provide a fluid from fluid reservoir  112  to cannula  102  for the delivery of a fluid (e.g., insulin) to a user. Infusion device  100  is configured to monitor a physiological characteristic of the user using sensor  104 . 
     Infusion device  100  is configured to define a first flow path  125  configured to provide a fluid (e.g., insulin) from fluid reservoir  112  to cannula  102  when second housing  116  engages first housing  110 . Infusion device  100  may include fluid pump  127  (e.g., an insulin pump) configured to provide the fluid to cannula  102 . In examples, infusion device  100  is configured to define first flow path  125  from a discharge  129  of fluid pump  127  through a lumen  128  of the cannula  102 . In examples, infusion device  100  includes a first conduit  131  configured to define first flow path  125 . In examples, infusion device  100  includes an access septum  163  configured to provide a fluid connection between flow path  125  and cannula  102 . Infusion device  100  may be configured such that cannula  102  punctures through access septum  163  to place cannula  102  in fluid communication with flow path  125 . Infusion device  100  may be configured to define a second flow path  135  from fluid reservoir  112  to a suction  137  of fluid pump  127 . In examples, infusion device  100  includes a second conduit  139  configured to define second flow path  135 . Fluid pump  127  may include motor  141  configured to cause fluid pump  127  to create pressure to deliver fluid (e.g., via first flow path  125 ). In some examples. Fluid pump  127  includes a piston and motor  141  is configured to cause translation of the piston. The piston may be configured to cause motivation of a fluid (e.g., insulin) through flow path  125  when motor  141  causes the translation. In some examples, the piston may be configured to reside in a reservoir filled with the fluid. The motor  141  may be a brushless DC motor, a brushed DC motor, or some other type of motor. In examples, motor  141  is powered and/or controlled by processing circuitry  114 . 
     Infusion device  100  is configured to establish electrical connectivity between sensor  104  and processing circuitry  114  when second housing  116  engages first housing  110 . Infusion device  100  may include a first connector  132  in electrical communication with processing circuitry  114  (e.g., via communication link  134 ) and a second connector  136  in electrical communication with sensor  104  (e.g., via communication link  138 ). In examples, first unit  106  includes first connector  132  and second unit  108  includes second connector  136 . Infusion device  100  is configured to establish electrical connectivity between first connector  132  and second connector  136  when second housing  116  is engaged with first housing  110 , such that sensor  104  is electrically connected to processing circuitry  114 . 
     Infusion device  100  may include one or more of a processor device  143 ; a memory element  145  to store data, processor-readable program instructions, and the like; a battery  147  or other power source; and a sensor interface  149  configured to receive a signal from sensor  104 . In examples, sensor interface  149  is configured to provide a signal indicative of a physiological parameter (e.g., a glucose level) to processing circuitry  114 . In examples, sensor interface  104  is configured to measure a current from sensor  104 . Sensor  104  may include hardware enabling measurement of the current, such as a potentiometer. In examples, processor device  143 , sensor interface  104 , and/or other components of infusion device  100  may be configured to conduct electrochemical impedance (EIS) measurements to determine, for example, a glucose level. Processor device  143 , memory element  145 , battery  147 , and/or sensor interface  149  may be included on an electronics assembly  151 . In examples, sensor interface  149  is configured to establish electrical connectivity between conductors of sensor  104  and conductors of electronics assembly  151 . Electronics assembly  151  (or the components of electronics assembly  151 ) can be electrically coupled to other elements of infusion device  100  as needed to support the operation of infusion device  100 . The electrical connections to electronics assembly  151  can be direct or indirect if so desired. Moreover, one or more components of electronics assembly  151  may support wireless data communication in some embodiments. 
     In examples, processor device  143  includes processing circuitry  114 . In examples, processing circuitry  114  is configured to control an operation of fluid pump  127 . For example, processing circuitry  114  may be configured to cause fluid pump  127  to commence, continue, and/or cease transporting fluid from fluid reservoir  112  to cannula  102 . In examples, sensor  104  is configured to generate a signal indicative of a physiological characteristic of the user (e.g., a glucose level), and processing circuitry  114  is configured to determine the physiological characteristic using the indicative signal. In some examples, processing circuitry  114  is configured to control an operation of fluid pump  127  based on the indicative signal reported by the sensor  104 . 
     First housing  110  of first unit  106  and second housing  116  of second unit  108  are suitably shaped, sized, and configured to house or support fluid pump  127 , first conduit  131 , second conduit  139 , motor  141 , processor device  143 , memory element  145 , battery  147 , sensor interface  149 , electronics assembly  151 , and/or other components of infusion device  100  described. In examples, first unit  106  includes fluid reservoir  112 , processing circuitry  114 , fluid pump  127 , first conduit  131 , first connector  132 , communication link  134 , second conduit  139 , motor  141 , processor device  143 , memory element  145 , and/or battery  147 . In examples, second unit  108  includes cannula  102 , sensor  104 , second connector  136  and/or communication link  134 . 
       FIG. 3  is a schematic view of infusion device  100  including cannula  102 , sensor  104 , first unit  106  defining first housing  110 , second unit  108  defining second housing  116 , fluid reservoir  112 , processing circuitry  114 , fixation device  120 , input device  122 , and fluid pump  127 .  FIG. 3 . Depicts example configurations of selected components within first unit  106  and second unit  108  when second unit  108  is engaged with first unit  106 . First housing  110  and second housing  116  are illustrated as cross-sections with a cutting plane parallel to the page. 
     Second unit  108  mechanically supports cannula  102  such that cannula  102  substantially establishes fluid communication with fluid reservoir  112  when the second housing  116  is engaged with first housing  110 . Cannula  102  may include a first end  124  (“cannula first end  124 ”), a second end  126  (“cannula second end  126 ”) opposite cannula first end  124 , and a lumen  128  providing fluid communication cannula first end  124  and cannula second end  126 . Cannula first end  124  may be configured to implant into body  101  of the use (e.g., using a first insertion needle (not shown)). Second unit  108  mechanically supports a portion of cannula  102  including cannula second end  126  such that cannula second end  126  inserts into a fluid access  130  of first unit  106  when second housing  116  engages first housing  110 . Fluid access  130  is in fluid communication with fluid reservoir  112  (e.g., via first conduit  131 ). Thus, when cannula second end  126  inserts into fluid access  130 , cannula  102  establishes fluid communication between fluid reservoir  112  and cannula first end  124 . In examples, the portion of cannula  102  including cannula second end  126  is substantially affixed to second unit  108 . That is, when second unit  108  is disengaged and displaced from first unit  106 , cannula  102  may be configured to remain affixed to second unit  108 , such that the separation of first unit  106  from second unit  108  causes cannula first end  124  to withdraw from body  101  of the user. 
     Infusion device  100  is configured to establish communication between sensor  104  of second unit  108  and processing circuitry  114  of first unit  106  when second housing  116  engages first housing  110 . Some portion of sensor  104  may be configured to implant into body  101  of the use (e.g., using a second insertion needle (not shown)). In examples, first unit  106  includes first connector  132  in electrical communication with processing circuitry  114  (e.g., via communication link  134 ) and second unit  108  includes second connector  136  in electrical communication with sensor  104  (e.g., via communication link  138 ) to establish the communication between sensor  104  and processing circuitry  114 . Infusion device  100  may be configured such that engagement of second housing  116  with first housing  110  causes second connector  136  to establish an electrical connection with first connector  132 , such that sensor  104  may provide a signal to processing circuitry  114  indicative of a physiological characteristic of the user (e.g., a glucose level). 
     Second unit  108  may mechanically support second connector  136  and cannula  102  such that engagement of second housing  116  and first housing  110  causes cannula second end  126  to insert into fluid access  130  and second connector  136  to establish an electrical connection with first connector  132 . Infusion device  100  may be configured such that, when fixation device  120  limits movement of second housing  116  relative to first housing  110 , second cannula end  126  inserts into fluid access  130  and second connector  136  establishes the electrical connection with first connector  132 . In examples, infusion device  100  is configured such that the engagement of second unit  108  with first unit  106  causes infusion device  100  to establish the fluid communication between fluid reservoir  112  and cannula first end  124  and/or the electrical connection between sensor  104  and processing circuitry  114 . For example, second unit  108  may be caused to engage with first unit  110  by moving second unit  108  toward first unit  106  (e.g., in a distal direction D) until fixation device  120  acts to limit motion of second housing  116  relative to first housing  110 . Fixation device  120  may act to limit motion of second housing  116  relative to first housing  110  in the distal direction D, a proximal direction P, or in any other direction. 
     Infusion device  100  may be configured to cause the implantation of cannula  102  and sensor  104  in the body  101  of the user when second unit  108  engages or is caused to engage with first unit  106 . In examples, as will be discussed, second unit  108  includes a first insertion needle configured to cause the implantation of cannula first end  124  and a second insertion needle configured to cause the implantation of sensor  104  when second unit  108  engages or is caused to engage with first unit  106 . The first insertion needle and second insertion needle may be configured to move substantially independently of first unit  106  and second unit  108  in the distal direction D, such that the first insertion needle and the second insertion needle may be withdrawn as second housing  116  remains engaged with first housing  100  (e.g., via fixation device  120 ) and cannula first end  124  and sensor  104  remain implanted within body  101  of the user. 
     First unit  106  and second unit  108  may have any shape, size, and/or configuration sufficient to allow second housing  116  to engage first unit  116  such that reservoir  112  is in fluid communication with cannula  102  and sensor  104  is in electrical communication with processing circuitry  114 . In examples, first housing  110  defines a recess and some portion of second housing  116  may be configured to substantially insert into the recess when second housing  116  engages first housing  110 , although this is not required. First housing  110  and second housing  116  may be configured in any manner such that movement of first housing  110  is limited relative to second housing  116  when second housing  116  engages first housing  110 . First housing  110  and second housing  116  may have any shape, size, and/or configuration sufficient to support (e.g., mechanically support) the components of first unit  106  and second unit  108  respectively. 
       FIG. 4A  illustrates infusion device  100  with first unit  106  positioned proximate the skin  118  of a user and second unit  108  in a disengaged position, such that second unit  108  is displaced from first unit  106 . Second unit  108  includes first insertion needle  140  releasably engaged with cannula  102  and second insertion needle  142  releasably engaged with sensor  104 .  FIG. 4B  illustrates infusion device  100  with second unit  108  having moved in the distal direction D to engage first unit  106 . The engagement of second unit  108  with first unit  106  has caused first insertion needle  140  to implant cannula first end  124  and caused second insertion needle  142  to implant sensor  104  through skin  118  and into the user.  FIG. 4C  illustrates infusion device  100  with first insertion needle  140  and second insertion needle  142  withdrawn in the proximal direction P as second unit  108  remains engaged with first unit  106  and cannula first end  124  and sensor  104  remain implanted within the user. 
     First insertion needle  140  defines a distal end  144  (“first needle distal end  144 ”) and second insertion needle  142  defines a distal end  146  (“second needle distal end  146 ”) configured to pierce the skin  118  of the user. Infusion device  100  is configured such that, as discussed, first unit  106  may be proximate (e.g., in contact with) the skin  118  of the user when second unit  108  is displaced and substantially separated from first unit  106  (as depicted in  FIG. 4A ). Infusion unit  100  may be configured such that, when second unit  108  is moved in the distal direction D and engaged with first unit  106  (as depicted in  FIG. 4B ), the movement of second unit  108  toward first unit  106  causes first insertion needle  140  and second insertion needle  142  to extend through first unit  106  and pierce the skin  118  of the user, causing implantation of cannula  102  (e.g., cannula first end  124 ) and sensor  104 . When first unit  106  is engaged with second unit  108 , second unit  108  causes cannula second end  126  to insert into fluid access  130 , such that fluid reservoir  112  is in fluid communication with cannula first end  124  (e.g., via first conduit  131 ), and causes second connector  136  to electrically connect with first connector  132 , such that processing circuitry  114  is electrically connected to sensor  104 . Infusion device  100  may be configured such that first insertion needle  140  and second insertion needle  142  may be proximally withdrawn and separated from first unit  106  and second unit  108  as cannula  102  and sensor  104  remain implanted in the user and second unit  108  remains engaged with first unit  106  (as depicted in  FIG. 4C ). Infusion device  100  remains configured to provide a fluid (e.g., insulin) to the user using cannula  102  and monitor a physiological characteristic (e.g., a glucose level) of the user using sensor  104  when first insertion needle  140  and second insertion needle  142  are withdrawn and separated from first unit  106  and second unit  108 . 
     First unit  106  is configured such that first insertion needle  140  is translatable (e.g., in the distal direction D and/or proximal direction P) relative to first unit  106 . In examples, first unit  106  defines a first channel  148  (“first unit first channel  148 ”) extending through first housing  110 . In examples, first unit  106  includes a facing surface  150  (“first facing surface  150 ”) configured to substantially face second unit  108 , and first unit first channel  148  defines a passage through first housing  110  from first facing surface  150  to first hole  115 . In examples, second unit  108  defines a facing surface  152  (“second facing surface  152 ”), and first facing surface  150  is configured to substantially face second facing surface  152 . 
     First unit first channel  148  is configured to accommodate passage of first insertion needle  140  and a portion of cannula  102  when second unit  108  is displaced in the distal direction D towards first unit  106  (e.g., when second facing surface  152  is moved toward first facing surface  150 ). In some examples, first insertion needle  140  and the portion of cannula  102  are configured to extend from second unit  108  and through first unit first channel  148  when second unit  108  is displaced in the distal direction D towards first unit  106 . First insertion needle  140  and the portion of cannula  102  are configured to translate through first unit first channel  148  in the distal direction D relative to first unit  106 , such that first insertion needle  140  and cannula  102  may extend through first unit first channel  148  to pierce the skin  118  of the user as second unit  108  is displaced in the distal direction D to engage second unit  108 . 
     In examples, infusion device  100  includes a needle carrier  156  configured to cause first insertion needle  140  to translate through first unit first channel  148  in the distal direction D or in the proximal direction P. In examples, needle carrier  156  is configured to limit movement of first insertion needle  140  relative to needle carrier  156 , such that movement of needle carrier  156  (e.g., in the distal direction D or proximal direction P) causes corresponding movement of first insertion needle  140  through first unit first channel  148 . In examples, needle carrier  156  is configured to receive a force in the distal direction D and transmit at least some portion of the force in the distal direction D to first insertion needle  140  to cause, for example, first insertion needle  140  to move distally through first unit first channel  148 . Needle carrier  156  may be configured to receive a force in the proximal direction P and transmit at least some portion of the force in the proximal direction P to first insertion needle  140  to cause, for example, first insertion needle  140  to move proximally through first unit first channel  148 . Needle carrier  156  may be configured to cause the movement of first insertion needle  140  in any appropriate manner. In examples, first insertion needle  140  is substantially affixed to needle carrier  156  using a fastener, a clamp, an interference fit with needle carrier  156 , or some other appropriate arrangement sufficient to allow needle carrier  156  to transmit a force to first insertion needle  140 . 
     First insertion needle  140  may translate relative to first housing  110  and/or the second housing  116  in a first needle distal direction and/or in a first needle proximal direction. In examples, the first needle distal direction is a direction which would cause first needle distal end  144  to move away from first housing  110  while some portion of first insertion needle  140  is within first unit first channel  148 . The first needle proximal direction is a direction which would cause first needle distal end  144  to move toward first housing  110  while some portion of first insertion needle  140  is within first unit first channel  148 . First insertion needle  140  may translate in the first needle distal direction or the first needle proximal direction regardless of whether the portion of first insertion needle  140  is within first unit first channel  148 , provided that the first needle distal direction and/or the first needle proximal direction would cause motion of the first needle distal end  144  as described if the portion of first insertion needle  144  were in first unit first channel  148 . 
     First unit  106  is configured such that second insertion needle  142  is translatable (e.g., in the distal direction D and/or proximal direction P) relative to first unit  106 . In examples, first unit  106  defines a second channel  154  (“first unit second channel  154 ”) extending through first housing  110 . In examples, first unit second channel  154  defines a passage through first housing  110  from first facing surface  150  to second hole  117 . 
     First unit second channel  154  is configured to accommodate passage of second insertion needle  142  and at least a portion of sensor  104  when second unit  108  is displaced in the distal direction D towards first unit  106  (e.g., when second facing surface  152  is moved toward first facing surface  150 ). In some examples, second insertion needle  142  and sensor  104  are configured to extend from second unit  108  and through first unit second channel  154  when second unit  108  is displaced in the distal direction D towards first unit  106 . Second insertion needle  142  and sensor  104  are configured to translate through first unit second channel  154  in the distal direction D relative to first unit  106 , such that second insertion needle  142  and sensor  104  may extend through first unit second channel  106  to pierce the skin  118  of the user as second unit  108  is displaced in the distal direction D to engage second unit  108 . 
     In examples, needle carrier  156  is configured to cause second insertion needle  142  to translate through first unit second channel  154  in the distal direction D or in the proximal direction P. In examples, needle carrier  156  is configured to limit movement of second insertion needle  142  relative to needle carrier  156 , such that movement of needle carrier  156  (e.g., in the distal direction D or proximal direction P) causes corresponding movement of second insertion needle  142  through first unit second channel  154 . In examples, needle carrier  156  is configured to receive a force in the distal direction D and transmit at least some portion of the force in the distal direction D to second insertion needle  142  to cause, for example, second insertion needle  142  to move distally through first unit second channel  154 . Needle carrier  156  may be configured to receive a force in the proximal direction P and transmit at least some portion of the force in the proximal direction P to second insertion needle  142  to cause, for example, second insertion needle  142  to move proximally through first unit second channel  154 . In examples, second insertion needle  142  is substantially affixed to needle carrier  156  using a fastener, a clamp, an interference fit with needle carrier  156 , or some other appropriate arrangement sufficient to allow needle carrier  156  to transmit a force to second insertion needle  142 . 
     Second insertion needle  142  may translate relative to first housing  110  and/or second housing  116  in a second needle distal direction and/or in a second needle proximal direction. In examples, the second needle distal direction is a direction which would cause second needle distal end  146  to move away from first housing  110  while some portion of second insertion needle  142  is within first unit second channel  154 . The second needle proximal direction is a direction which would cause second needle distal end  146  to move toward first housing  110  while some portion of second insertion needle  142  is within first unit second channel  154 . Second insertion needle  142  may translate in the second needle distal direction or the second needle proximal direction regardless of whether the portion of second insertion needle  142  is within first unit second channel  154 , provided that the second needle distal direction and/or the second needle proximal direction would cause motion of second needle distal end  146  as described if the portion of second insertion needle  142  were in first unit second channel  154 . The second needle distal direction may be similar to or substantially the same as the first needle distal direction. The second needle proximal direction may be similar to or substantially the same as the first needle proximal direction. 
     Infusion device  100  may be configured such that first insertion needle  140  and second insertion needle  142  pierce the skin  118  of the user substantially concurrently as second unit  108  moves in the distal direction D toward first unit  106  (e.g., as infusion device  100  transitions from the configuration of  FIG. 4A  to the configuration of  FIG. 4B ). Infusion device  100  may be configured such that first insertion needle  140  and second insertion needle  142  pierce the skin  118  substantially concurrently in order to, for example, limit discomfort to the user that might be caused by insertions separated by a user-discernable chronological time increment. In examples, needle carrier  156  is configured to cause substantially concurrent translation of first insertion needle  140  through first unit first channel  148  and second insertion needle  142  through first unit second channel  154 . Needle carrier  156  may be configured to substantially concurrently translate first insertion needle  140  and second insertion needle  142  to cause first needle distal end  144  and second needle distal end  146  to pierce the skin  118  substantially concurrently. 
     In some examples, needle carrier  156  is configured to impart a force in the distal direction D to second unit  108  to cause second unit  108  to move distally toward first unit  106 . In examples, needle carrier  156  is configured to receive a force in the distal direction D (e.g., from a inserter (not shown)) and transmit a portion of the force to second unit  108  to cause second unit  108  to move distally toward first unit  106 . Infusion device  100  may be configured such that, when needle carrier  156  receives the force in the distal direction D, needle carrier  156  transmits a portion of the force to second housing  116 , causing second unit  108  to contact first unit  106  (e.g., causing second facing surface  152  to contact first facing surface  150 ) and causing second housing  116  to engage first housing  110  (as depicted in  FIG. 4B ). In examples, needle carrier  156  is configured to impart the distal force on second unit  108  on a bearing surface  158  opposite the second facing surface  152  of second unit  108 . 
     In examples, second unit  108  is configured such that first insertion needle  140  is translatable (e.g., in the distal direction D and/or proximal direction P) relative to second unit  108  to permit, for example, withdrawal of first insertion needle  140  from second unit  108  as second unit  108  remains engaged with first unit  106  (as depicted in  FIG. 4C ). In examples, second unit  108  defines a first channel  160  (“second unit first channel  160 ”) extending through second housing  116 . In examples, second unit first channel  160  defines a passage through second housing  116  between second facing surface  152  and a bearing surface  158 . Second unit first channel  160  may be configured to accommodate passage of at least first insertion needle  140  when first insertion needle  140  is displaced in the proximal direction P away from second unit  108  (as depicted in  FIG. 4C ). In some examples, first insertion needle  140  and a portion of cannula  102  are configured to extend from second unit  108  through second unit first channel  160  when second unit  108  is displaced in the distal direction D towards first unit  106 . First insertion needle  140  may be configured to translate through second unit first channel  160  in the proximal direction P relative to second unit  108 , such that first insertion needle  140  may be withdrawn from second unit  108  following implantation of cannula  102 . 
     Needle carrier  156  may be configured to cause first insertion needle  140  to translate through second unit first channel  160  in the proximal direction P when needle carrier  156  translates in the proximal direction P away from second unit  108 . In examples, needle carrier  156  is configured to receive a force in the proximal direction P and transmit at least some portion of the force in the proximal direction P to first insertion needle  140  to cause, for example, first insertion needle  140  to move proximally through second unit first channel  160 . In examples, needle carrier  156  is configured to receive a force in the proximal direction P (e.g., from a inserter (not shown)) and transmit a portion of the force to first insertion needle  140  to cause first insertion needle  140  to displace from second unit  108  in the proximal direction P. 
     In examples, second unit  108  is configured such that second insertion needle  142  is translatable (e.g., in the distal direction D and/or proximal direction P) relative to second unit  108  to permit, for example, withdrawal of second insertion needle  142  from second unit  108  as second unit  108  remains engaged with first unit  106  (as depicted in  FIG. 4C ). In examples, second unit  108  defines a second channel  162  (“second unit second channel  162 ”) extending through second housing  116 . In examples, second unit second channel  162  defines a passage through second housing  116  between second facing surface  152  and a bearing surface  158 . Second unit second channel  162  may be configured to accommodate passage of at least second insertion needle  142  when second insertion needle  142  is displaced in the proximal direction P away from second unit  108  (as depicted in  FIG. 4C ). In some examples, second insertion needle  142  and a portion of sensor  104  are configured to extend from second unit  108  through second unit second channel  162  when second unit  108  is displaced in the distal direction D towards first unit  106 . Second insertion needle  142  may be configured to translate through second unit second channel  162  in the proximal direction P relative to second unit  108 , such that second insertion needle  142  may be withdrawn from second unit  108  following implantation of sensor  104 . 
     Needle carrier  156  may be configured to cause second insertion needle  142  to translate through second unit second channel  162  in the proximal direction P when needle carrier  156  translates in the proximal direction P away from second unit  108 . In examples, needle carrier  156  is configured to receive a force in the proximal direction P and transmit at least some portion of the force in the proximal direction P to second insertion needle  142  to cause, for example, second insertion needle  142  to move proximally through second unit second channel  162 . In examples, needle carrier  156  is configured to receive a force in the proximal direction P (e.g., from a inserter (not shown)) and transmit a portion of the force to second insertion needle  142  to cause second insertion needle  142  to displace from second unit  108  in the proximal direction P. 
     Infusion device  100  may be configured such that first insertion needle  140  and second insertion needle  142  withdraw from the skin  118  of the user substantially concurrently needle carrier  156  moves in the proximal direction P away from second unit  108  (e.g., as infusion device  100  transitions from the configuration of  FIG. 4B  to the configuration of  FIG. 4C ). Infusion device  100  may be configured such that first insertion needle  140  and second insertion needle  142  withdraw from skin  118  substantially concurrently in order to, for example, limit discomfort to the user that might be caused by withdrawals separated by a user-discernable chronological time increment. In examples, needle carrier  156  is configured to cause substantially concurrent translation of first insertion needle  140  through second unit first channel  160  and second insertion needle  142  through second unit second channel  162 . Needle carrier  156  may be configured to substantially concurrently translate first insertion needle  140  and second insertion needle  142  to cause first needle distal end  144  and second needle distal end  146  to withdraw from the skin  118  substantially concurrently. 
     In examples, infusion device  100  is configured to fluidly isolate fluid access  130  from portions of infusion device  100  in order to substantially prevent adverse impacts with may arise through contact with a fluid in fluid reservoir  112 . The portions of infusion device may include, for example without limitation, first connector  132 , second connector  136 , first facing surface  150 , second facing surface  152 , and/or other components of first unit  106  and/or second unit  108 . In examples, infusion device  100  includes an access septum  163  configured to fluidly isolate the portions of infusion device  100  and fluid access  130 . Infusion device  100  may be configured such that, as second housing  116  engages first housing  110 , cannula second end  126  punctures access septum  163  to place cannula second end  126  in fluid communication with fluid access  130 . Access septum  163  may be comprised of a self-sealing material, such that access septum  163  substantially closes around cannula second end  126  to substantially maintain a fluid isolation between the portions of infusion device  100  and fluid access  130 . 
     In examples, infusion device  100  is configured to fluidly isolate second unit first channel  160  from portions of infusion device  100  in order to substantially prevent adverse impacts with may arise through contact with a fluid flowing through cannula  102 . The portions of infusion device may include, for example without limitation, first connector  132 , second connector  136 , first facing surface  150 , second facing surface  152 , and/or other components of first unit  106  and/or second unit  108 . In examples, infusion device  100  includes a second septum  165  configured to fluidly isolate the portions of infusion device  100  and second unit first channel  160 . Infusion device  100  may be configured such that first insertion needle  140  extends through second septum  165  when first insertion needle  140  extends through second unit first channel  160 . Second septum  165  may be comprised of a self-sealing material, such that second septum  165  substantially closes around first insertion needle  140  to substantially maintain a fluid isolation between the portions of infusion device  100  and second unit first channel  160 . Second septum  165  may be configured to self-seal when first insertion needle  140  is withdrawn through second septum  165  (e.g., withdrawn in the proximal direction P) to maintain the fluid isolation between the portions of infusion device  100  and second unit first channel  160  when first insertion needle  140  is displaced from second unit  108 . 
       FIG. 5  illustrates an example infusion device configured to substantially align one or more components of first unit  106  and one or more components of second unit  108  such that infusion device configures to deliver a therapeutic fluid and monitor a physiological characteristic of a user when second unit  108  engages first unit  106 . For example, second housing  116  may define an axis A 1 , and second unit  108  may be configured to translate first insertion needle  140  and a cannula first end  124  over the axis A 1  when second unit  108  is translated toward first unit  106 . In examples, second unit  108  is configured such that the axis A 1  passes through first unit first channel  148  of first unit  106 , such that movement of second unit  108  in the distal direction D toward first unit  106  causes first insertion needle  140  and cannula first end  124  to extend through first unit first channel  148 . In examples, second housing  116  defines and axis A 2  and is configured to translate second insertion needle  142  and a sensor  104  over the axis A 2  when second unit  108  is translated toward first unit  106 . Second unit  108  may be configured such that the axis A 2  passes through first unit second channel  154  of first unit  106 , such that movement of second unit  108  in the distal direction D toward first unit  106  causes second insertion needle  142  and sensor  104  to extend through first unit second channel  154 . 
     In examples, second housing  116  defines the axis A 1  and the axis A 2  such that when the axis A 1  passes through first unit first channel  148 , the axis A 2  passes through first unit second channel  154 . Hence, second unit  108  may be configured to translate first insertion needle  140  and cannula first end  124  over axis A 1  through first unit first channel  148  and translate second insertion needle  142  and sensor  104  over axis A 2  through first unit second channel  154  when second unit  108  translates toward first unit  106  in the distal direction D. Second unit  108  may be configured to substantially align first insertion needle  140  and cannula first end  124  with first unit first channel  148  and substantially align second insertion needle  142  and sensor  104  with first unit second channel  154  when second unit  108  is in a disengaged position (e.g., displaced from first unit  106  ( FIG. 4A, 5 ), such that engagement of second housing  116  with first housing  110  causes first insertion needle  140  and cannula first end  124  to pass through first unit first channel  148  and causes second insertion needle  142  and sensor  104  to pass through first unit second channel  154 . 
     In examples, second housing  116  defines an axis A 3 . Second unit  108  may be configured to translate cannula second end  126  over the axis A 3  when second unit  108  is translated toward first unit  106 . In examples, second unit  108  is configured such that the axis A 3  passes through fluid access  130  of first unit  106 , such that movement of second unit  108  in the distal direction D toward first unit  106  causes cannula second end  126  to insert within fluid access  130  and establish fluid communication between fluid reservoir  112  and cannula first end  124  (e.g., via first conduit  131 ). Second housing  116  may define the axis A 3  such that, when the axis A 3  passes through fluid access  130 , the axis A 1  passes through first unit first channel  148  and/or the axis A 2  passes through first unit second channel  154 . Hence, second unit  108  may be configured to substantially align cannula second end  126  with fluid access  130 , substantially align first insertion needle  140  and cannula first end  124  with first unit first channel  148 , and/or substantially align second insertion needle  142  and sensor  104  with first unit second channel  154 . Second unit  108  may be configured such that, when second unit  108  is in a disengaged position (e.g., displaced from first unit  106  ( FIG. 4A, 5 ), engagement of second housing  116  with first housing  110  causes second cannula end  126  to insert into fluid access  130 , causes first insertion needle  140  and cannula first end  124  to pass through first unit first channel  148 , and/or causes second insertion needle  142  and sensor  104  to pass through first unit second channel  154 . 
     In examples, second housing  116  defines an axis A 4 . Second unit  108  may be configured to translate second connector  136  over the axis A 4  when second unit  108  is translated toward first unit  106 . In examples, second unit  108  is configured such that the axis A 4  passes through first connector  132  of first unit  106 , such that movement of second unit  108  in the distal direction D toward first unit  106  causes second connector  136  to contact first connector  132  and establish electrical communication between sensor  104  and processing circuitry  114 . Second housing  116  may define the axis A 4  such that, when the axis A 4  passes through first connector  132 , the axis A 1  passes through first unit first channel  148 , the axis A 2  passes through first unit second channel  154 , and/or the axis A 3  pass through fluid access  130 . Hence, second unit  108  may be configured to substantially align second connector  136  with first connector  132 , substantially align first insertion needle  140  and cannula first end  124  with first unit first channel  148 , substantially align second insertion needle  142  and sensor  104  with first unit second channel  154 , and/or substantially align cannula second end  126  with fluid access  130 . Second unit  108  may be configured such that, when second unit  108  is in a disengaged position (e.g., displaced from first unit  106  ( FIG. 4A, 5 ), engagement of second housing  116  with first housing  110  causes second connector  136  to contact first connector  132 , causes first insertion needle  140  and cannula first end  124  to pass through first unit first channel  148 , causes second insertion needle  142  and sensor  104  to pass through first unit second channel  154 , and/or causes cannula second end  126  to insert into fluid access  130 . 
       FIG. 6A  and  FIG. 6B  illustrate an inserter  170  configured to cause second housing  116  of second unit  108  to engage first housing  110  of first unit  106 . Inserter  170  is configured to hold second unit  108  is a disengaged position wherein second unit  108  is displaced from first unit  106  (e.g.,  FIG. 4A ). Inserter  170  is configured to cause second unit  108  to move to an engaged position wherein second housing  116  engages first housing  110  (e.g.,  FIG. 4B ), to cause implantation of cannula  102  and sensor  104  using first insertion needle  140  and second insertion needle  142  respectively. Inserter  170  may be configured to withdraw first insertion needle  140  and second insertion needle  142  (e.g.,  FIG. 4C ) following the implantation of cannula  102  and sensor  104 .  FIG. 6A  and  FIG. 6B  illustrate an example inserter  170  with reference to the x-y-z axes shown. The z axis proceeds out of the page in  FIG. 6A  and the y axis proceeds into the page in  FIG. 6B . Inserter  170  is depicted holding second unit  108  in the disengaged position in  FIG. 6A  and  FIG. 6B , with second unit  108  hidden and illustrated in dashed lines in  FIG. 6A . 
     Inserter  170  may be configured to allow a user to cause inserter  170  to engage second housing  116  and first housing  110  to cause the implantation of cannula  102  and sensor  104 . In examples, inserter  170  includes a plunger  172  configured for manipulation by the user to cause the implantation. Plunger  172  may be engaged with a serter member  174  configured align second unit  108  and first unit  106 , such that when inserter  170  causes the engagement of second housing  116  and the first housing  110 , the components of first unit  106  and second unit  108  cooperatively operate to provide a therapeutic fluid (e.g., insulin) to the user using cannula  102  and monitor a physiological characteristic of the user (e.g., a glucose level) using sensor  104 . In examples, inserter  170  is configured such that a user may cause plunger  172  to translate in the distal direction D relative to serter member  174  to commence the implantation of cannula  102  and sensor  104 . 
     Inserter  170  may be configured to mate with first unit  106  when inserter  170  holds second unit  108  in the disengaged position. In examples, inserter  170  (e.g., serter member  174 ) includes one or more alignment members  176  including member  175  and member  177  configured to mate with first unit  106 . Alignment members  176  may be configured such that, when inserter  170  mates with first unit  106 , inserter  170  causes axis A 1  ( FIG. 5 ) to pass through first insertion needle  140  and first unit first channel  148 , axis A 2  to pass through second insertion needle  142  and first unit second channel  154 , axis A 3  to pass through cannula second end  126  and fluid access  130 , and/or axis A 4  to pass through second connector  136  and first connector  132 . 
       FIG. 7  illustrates an example first unit  106  including first housing  110 , first unit first channel  148 , first unit second channel  154 , fluid access  130 , and first connector  132 . The x-y-z axis of  FIGS. 6A, 6B  is included for reference, with the y axis proceeding out of the page. In examples, first housing  110  defines one or more aligning members  178 , including member  179  and member  180 . In examples, first housing  110  defines aligning members  178  around an outer perimeter  182  of first housing  110 . In examples, insertor  170  is configured to mate with first unit  106  by causing alignment members  176  of insertor  170  to engage aligning members  178  of first unit  106 . For example, one or more of alignment members  176  may define a protrusion configured to insert into a recess defined by one or more of aligning members  178 . In examples, one or more of alignment members  176  may define a recess configured to insert into a protrusion of one or more of aligning members  178 . 
     Insertor  170  may be configured to translate relative to first unit  106  (e.g., in the distal direction D) to cause the engagement between alignment members  176  and aligning members  178 . For example, insertor  170  may be configured to be translated in the distal direction D such that insertor  170  (e.g., serter member  174 ) substantially surrounds outer perimeter  182  of first unit  106  when alignment members  176  engage aligning members  178 . Insertor  170  may be configured such that, when alignment members  176  engage aligning members  178 , inserter  170  causes axis A 1  ( FIG. 5 ) to pass through first insertion needle  140  and first unit first channel  148 , axis A 2  to pass through second insertion needle  142  and first unit second channel  154 , axis A 3  to pass through cannula second end  126  and fluid access  130 , and/or axis A 4  to pass through second connector  136  and first connector  132 . Hence, insertor  170  may be configured to substantially align second unit  108  and first unit  106  such that when inserter  170  causes the engagement of second housing  116  and the first housing  110 , the components of first unit  106  and second unit  108  cooperatively operate to provide a therapeutic fluid (e.g., insulin) to the user using cannula  102  and monitor a physiological characteristic of the user (e.g., a glucose level) using sensor  104 . 
       FIG. 8A  illustrates a cross-section of insertor  170  with a cutting plane taking parallel to the page. The x-y-z axes of  FIGS. 6A, 6B, and 7  are included for reference, with the z axis proceeding out of the page. Insertor  170  includes serter member  174  and is holding second unit  108  in the disengaged position relative to first unit  106 . Serter member  174  is engaged with first unit  106  such that axis A 1  ( FIG. 5 ) passes through first insertion needle  140  and first unit first channel  148 , axis A 2  passes through second insertion needle  142  and first unit second channel  154 , axis A 3  passes through cannula second end  126  and fluid access  130 , and axis A 4  to passes through second connector  136  and first connector  132 . 
     Insertor  170  includes a charging member  184  configured to compress an insertion spring  186 . Charging member  184  may be configured to compress insertion spring  186  when plunger  172  imparts a force (e.g., in the distal direction D) on charging member  184 . In examples, charging member  184  is configured to translate relative to serter member  174  when plunger  172  translates relative to serter member  174  to cause the compression of insertion spring  186 . Charging member  184  may be configured to exert a force (e.g., in the distal direction D) on insertion spring  186  to cause the compression of insertion spring  186 . 
     In examples, insertor  170  includes a discharging member  188  configured to exert a force on insertion spring  186  opposite the force exerted by charging member  184  when charging member  184  compresses insertion spring  186 . In examples, insertion spring  186  is configured to compress substantially between charging member  184  and discharging member  188 . Discharging member  188  may be configured to remain substantially stationary with respect to serter member  174  when charging member  184  translates relative to serter member  174  (e.g., under the influence of plunger  172 ), such that the relative motion between charging member  184  and discharging member  188  causes the compression of insertion spring  186 . In some examples, inserter  170  includes a release device  190  configured to maintain discharging member  188  substantially stationary relative to serter member  174  as charging member  184  translates relative to serter member  174 . 
     As an example,  FIG. 8B  illustrates insertor  170  with plunger  172  having translated distally relative to serter member  174  to cause the compression of insertion spring  186 . Having compressed insertion spring. The translation of plunger  172  has caused charging member  184  to translate relative to serter member  174 . Insertion spring  186  has been compressed as charging member  184  translated relative to serter member  174  as discharging member  184  remained substantially stationary relative to serter member  174  (e.g., due to release device  190 ). In examples, and as depicted at  FIGS. 8A and 8B , first unit  106  may be configured to remain substantially stationary with respect to discharging member  188  when charging member  184  translates relative to discharging member  188 . 
     Discharging member  188  may be configured to exert a force on second unit  108  to cause movement of second unit  108  relative to first unit  106 , and relative to serter member  174 . In examples, insertion spring  186  is configured to exert a spring force on discharging member  188  to cause the movement. Insertor  170  may be configured such that, when charging member  184  translates relative to serter member  174  by a certain amount (causing charging of insertion spring  186 ), insertor  170  causes release device  190  to disengage from discharging member  188 , such that insertion spring  186  may cause movement of discharging member  188  relative to serter member  174 . The movement of discharging member  188  relative to serter member  174  may cause discharging member  188  to exert a force on second unit  108  (e.g., in the distal direction D), causing second unit  108  to move towards first unit  106 . 
       FIG. 8C  illustrates insertor  170  having caused release device  190  to disengage from discharging member  188 . Insertion spring  186  has exerted a force (e.g., in the distal direction D) on discharging member  188 , causing movement of discharging member  188  (e.g., in the distal direction D). The movement of discharging member  188  has caused discharging member  188  to exert a force on second unit  108 , causing movement of second unit  108  (e.g., in the distal direction D) toward first unit  106 . 
     In  FIG. 8C , the movement of second unit  108  toward first unit  106  has caused second housing  116  to engage first housing  110 , such that first housing  110  substantially limits movement of second housing  116  relative to first housing  110 . For example, movement of second unit  108  toward first unit  106  may cause fixation device  120  ( FIGS. 1, 3 ) to substantially secure second housing  116  in a position relative to first housing  110 . Further, the movement of second unit  108  toward first unit  106  has extended first insertion needle  140  and cannula first end  124  through first unit first channel  148  ( FIGS. 4A-4C ) and second insertion needle  142  and sensor  104  through first unit second channel  154  ( FIGS. 4A-4C ). The extension of first insertion needle  140  and second insertion needle  142  causes implantation of cannula first end  124  and sensor  104  into the user. 
     Further, in  FIG. 8C , the movement of second unit  108  toward first unit  106  has caused cannula second end  126  ( FIG. 6B ) to insert within fluid access  130  ( FIG. 7 ) to establish fluid communication between fluid reservoir  112  ( FIGS. 3, 4A-4C ) and cannula first end  124  ( FIG. 6B ). The movement of second unit  108  toward first unit  106  has caused second connector  136  ( FIG. 6B ) to electrically connect with first connector  132  ( FIG. 7 ) to establish electrical connection between processing circuitry  114  ( FIGS. 3, 4A-4C ) and sensor  104 . 
     In examples, and as depicted at  FIG. 8C , needle carrier  156  may be configured to remain substantially stationary with to second unit  108  when discharging member  188  causes movement of second unit  108 . In examples, inserter  170  includes a release device  192  configured to maintain needle carrier  156  substantially stationary relative to second unit  108  when discharging member  188  causes second unit  108  to move toward first unit  106 . 
     In examples, insertor  170  includes a withdrawal spring  194  configured to cause first insertion needle  140  and second insertion needle  142  to move relative to second unit  108  (and first unit  106  when second housing  116  is engaged with first housing  110 ). In examples, withdrawal spring is configured to cause a force (e.g., in the proximal direction P) to impart on first insertion needle  140  and second insertion needle  142  to cause the movement. Insertor  170  may be configured to cause first insertion needle  140  to move relative to second unit  108  through second unit first channel  160  ( FIGS. 4A-4C ). Insertor  170  may be configured to cause second insertion needle  142  to move relative to second unit  108  through second unit second channel  162  ( FIGS. 4A-4C ). In some examples, withdrawal spring  194  is configured to exert a force on needle carrier  156  to cause movement of needle carrier  156  relative to second unit  108 , and needle carrier  156  is configured to transmit at least some portion of the force to first insertion needle  140  and second insertion needle  142  to cause the movement of first insertion needle  140  and second insertion needle  142  relative to second unit  108 . 
       FIG. 8D  illustrates insertor  170  having caused release device  192  to disengage from needle carrier  156 , such that first insertion needle  140  and second insertion needle  142  may move proximally relative to second unit  108 . Withdrawal spring  194  has discharged to cause a force (e.g., in the proximal direction P) to impart on first insertion needle  140  and second insertion needle  142  to cause the movement. In examples, withdrawal spring  194  exerts a force in the proximal direction P on needle carrier  156  causing movement of needle carrier  156  in the proximal direction, and the movement of needle carrier  156  cause movement of first insertion needle  140  and second insertion needle  142  relative to second unit  108 . 
     Release device  190  may have any configuration sufficient engage to maintain discharging member  188  substantially stationary relative to serter member  174  as charging member  184  translates relative to serter member  174 , and sufficient to disengage to allow insertion spring  186  to cause movement of discharging member  188  relative to serter member  174 . Release device  192  may have any configuration sufficient to engage to maintain needle carrier  156  substantially stationary relative to second unit  108  when discharging member  188  causes second unit  108  to move toward first unit  106 , and sufficient to disengage to allow first insertion needle  140  and second insertion needle  142  to translate proximally relative to second unit  108 . Release device  190 ,  192  may include, for example, a mechanical switch configured to be actuated by some portion of or structure of inserter  170 , a magnetic switch configured to actuate based on a position of some portion of or structure of inserter  170 , an actuated device configured to be actuated by a position sensor included in inserter  170 , a specific structure of inserter  170 , and/or a device configured to actuate using other arrangements. 
     As discussed, first insertion needle  140  is configured to implant cannula first end  124  within a user, and configured to release cannula first end  124  when first insertion needle  140  is withdrawn, such that first insertion needle  140  may displace from second unit  108  as cannula first end  124  remains implanted in the user. Second insertion needle  142  is configured to implant sensor  104  within a user, and configured to release sensor  104  when second insertion needle  142  is withdrawn, such that second insertion needle  142  may displace from second unit  108  as sensor  104  remains implanted in the user. 
     In examples, first insertion needle  140  is configured to engage cannula  102  to cause cannula first end  124  to translate in the first direction toward the user (e.g., the distal direction D) when first insertion needle  140  translates in the first direction. First insertion needle  140  may be configured to exert a force on cannula  102  in the first direction to cause the translation of cannula first end  124 . In examples, first insertion needle  140  and/or cannula  102  includes a first structural feature configured to cause first insertion needle  140  to exert the force in the first direction on cannula  102 . First insertion needle  140  may be configured to disengage from (e.g., release) cannula  102  when first insertion needle  140  is subsequently withdrawn in the second direction opposite the first direction (e.g., in the proximal direction P). For example, first insertion needle  140  and/or cannula  102  may include a structural feature (the same as the first structural feature or a different structural feature) configured to allow first insertion needle  140  to move substantially independently of cannula  102  when first insertion needle  140  retracts in the second direction. 
     First insertion needle  140  may be configured to engage cannula  102  in any manner sufficient to cause first insertion needle  140  to translate cannula first end  124  in the first direction toward the user, and/or to allow first insertion needle  140  to move substantially independently of cannula  102  as first insertion needle  140  retracts in the second direction. In some examples, first insertion needle  140  is configured to at least partially extend within lumen  128  to cause the translation of cannula first end  124  in the first direction (e.g., the distal direction D). First insertion needle  140  may be configured to extend within lumen  128  when first insertion needle  140  moves substantially independently of cannula  102  as first insertion needle  140  retracts in the second direction. In other examples, first insertion needle  140  may be configured to substantially surround some portion of cannula  102  to cause the translation of cannula first end  124  in the first direction and/or allow first insertion needle  140  to move substantially independently of cannula  102  as first insertion needle  140  retracts in the second direction. 
     In some examples, cannula  102  defines a T-junction wherein cannula first end  124  defines an opening at one end of the cross bar of the T-junction and cannula  102  defines a needle access  196  ( FIG. 4A ) at the other end of the cross bar. In examples, first insertion needle  140  is configured to extend through lumen  128  of cannula  102  by extending through needle access  196  and cannula first end  124 . First insertion needle  196  may be configured to withdraw through needle access  196  when infusion device  100  causes first insertion needle  140  to retract as cannula first end  124  remains implanted in the user. In examples, second septum  165  is configured to fluidly isolate needle access  196  and portions of infusion device  100  such as first connector  132 , second connector  136 , first facing surface  150 , second facing surface  152 , and/or other components of first unit  106  and/or second unit  108 . Second septum  165  may be configured to self-seal when first insertion needle  140  withdraws (e.g., in the distal direction D) through second septum  165  to substantially maintain the fluid isolation. 
     In examples, first insertion needle  140  is configured to substantially mate with cannula  102  when first insertion needle  140  exerts the force in the first direction (e.g., the distal direction D) on cannula  102 . First insertion needle  140  may be configured such that a subsequent force in the second direction (e.g., the proximal direction P) causes first insertion needle  140  to unmate (e.g., disengage) and move independently of cannula  102 . In examples, first insertion needle  140  includes a bearing surface configured such that, when the force in the first direction is exerted on first insertion needle  140 , the bearing surface engages a portion of cannula  102  and transmits some portion of the force to cannula  102 , and when a force in the second direction is exerted on first insertion needle  140 , the bearing surface displaces from cannula  102 , such that first insertion needle  140  moves independently of cannula  102 . Hence, infusion device  100  may be configured to withdraw first insertion needle  140  independently from cannula  102 , such that cannula first end  124  remains implanted as first insertion needle  140  retracts. 
     As discussed, second insertion needle  142  is configured to releasably engage sensor  104  to cause the implantation of sensor  104  within the user. Second insertion needle  142  and sensor  104  may be cooperatively configured and arranged such that the second insertion needle  142  releasably carries at least a portion (e.g., a distal portion) of sensor  104  as second insertion needle  142  extends in the first direction toward the user (e.g., in the distal direction D). Second insertion needle  142  may be configured to engage sensor  104  in any manner sufficient to cause second insertion needle  142  to translate sensor  104  in the first direction toward the user, and/or to allow second insertion needle  142  to move substantially independently of sensor  104  as second insertion needle  142  retracts in the second direction. 
     In some examples, second insertion needle  142  configured to at least partially surround sensor  104  to carry sensor  104  as second insertion needle  142  extends through first unit  110  in the first direction. Second insertion needle  142  may be configured as a substantially hollow needle defining a void that accommodates sensor  104  within the void. Second insertion needle  142  and/or the sensor  104  may be configured such that second insertion needle  142  mechanically engages the sensor  104  when second insertion needle  142  extends in the first direction (e.g., the distal direction D) and disengages from sensor  104  when second insertion needle  142  retracts in the second direction (e.g., in the proximal direction P). In other examples, second insertion needle  142  may be configured to substantially insert into some portion of sensor  104  (e.g., such that some portion of sensor  104  substantially surrounds some portion of second insertion needle  142 ) to cause the translation of sensor  104  in the first direction, and/or to allow second insertion needle  142  to move substantially independently of sensor  104  as second insertion needle  142  retracts in the second direction. 
     In examples, second insertion needle  142  and/or sensor  104  include a second structural feature (e.g., the void defined by second insertion needle) configured to cause second insertion needle  142  to exert a force on sensor  104  when second insertion needle  142  extends in the first direction. Second insertion needle  142  may be configured to engage sensor  104  to cause implantation of sensor  104  in the user as second insertion needle  142  extends in the first direction. Sensor  104  may be configured to extend from first housing  110  when second insertion needle  142  causes the implantation of sensor  104  within the user. 
     Second insertion needle  142  may be configured to disengage from (e.g., release) sensor  104  when second insertion needle  142  is subsequently retracted in the second direction (e.g., in the proximal direction P). For example, second insertion needle  142  and/or sensor  104  may include a structural feature (the same as the second structural feature or a different structural feature) configured to allow second insertion needle  142  to move substantially independently of sensor  104  when second insertion needle  142  retracts in the second direction. In some examples, second insertion needle  142  is configured such that body tissue within the user engages with sensor  104  when second insertion needle  142  retracts, such that sensor  104  remains implanted in the user when second insertion needle  142  is withdrawn from the user. For example, second insertion needle  142  may include a portion (e.g., a distal portion) defining a longitudinal opening, such that a portion of sensor  104  is exposed to body tissue when second insertion needle  142  is inserted in the user. The body tissue may act to grip (e.g., frictionally engage) the exposed portion of sensor  104  as second insertion needle  142  is retracted, such that second insertion needle  142  may be withdrawn as sensor  104  remains implanted in the user. In examples, sensor  104  may include one or more structural features configured to assist the frictional engagement with the body tissue. In some examples, infusion device  100  may be configured to mechanically engage sensor  104  to hold sensor  104  in place (e.g., within the user) when second insertion needle  142  is retracted in the second direction. 
     Sensor  104  may be fabricated using a flexible or pliable substrate or carrier. In examples, sensor  104  is initially provided in a folded, serpentine, coiled, or accordion shape to, for example, provide a desired amount of slack to accommodate extension sensor  104  while sensor  104  is electrically coupled to second connector  136 . Sensor  104  may be configured such that, as second insertion needle  142  carries sensor  104  in the second direction (e.g., in the distal direction D), sensor  104  extends without losing electrical contact with second connector  136 . 
     First connector  132  and/or second connector  136  may have any configuration sufficient to allows first connector  132  and second connector  136  to establish electrical communication when second housing  116  engages first housing  110 . In examples, first connector  132  and/or second connector  136  includes a flexible or pliable block substantially encasing and/or supporting one or more conductors. In examples, the block comprises silicone and/or some other substantially insulative material. In examples, the conductors comprise carbon pillars and/or some other substantially conductive material. In some examples, first connector  132  and/or second connector  136  is configured to compress (e.g., decrease volume) when second housing  116  engages first housing  110  and first connector  132  and second connector  136  establish electrical communication. 
     A technique for implanting a cannula and a sensor within a user is illustrated in  FIG. 9 . Although the technique is described mainly with reference to infusion device  100  of  FIG. 1  through  FIG. 8D , the technique may be applied to other infusion devices in other examples. 
     The technique includes engaging a second housing  116  of a second unit  108  with a first housing  110  of a first unit  106  ( 202 ). The first housing  110  may be proximate the skin  118  of the user. The first unit  106  may include a fluid reservoir  112  and processing circuitry  114 . The second unit  108  may include a first insertion needle  140  and a second insertion needle  142 . The technique may include releasably engaging cannula  102  using first insertion needle  140  and releasably engaging sensor  104  using second insertion needle  142 . 
     The technique includes inserting cannula  102  through a first unit first channel  148  ( 192 ) and inserting sensor  104  through first unit second channel  154  through a first unit second channel  154  ( 204 ). First unit first channel  148  and first unit second channel  154  may be defined by first housing  110 . In examples, the technique includes inserting cannula  102  through first unit first channel  148  using first insertion needle  140 . The technique may include inserting sensor  104  through first unit second channel  154  using second insertion needle  142 . In examples, the technique includes causing first insertion needle  140  to extend through first unit first channel  148  by translating second unit  108  towards (e.g., in a distal direction D) first unit  110 . The technique may include causing second insertion needle  142  to extend through first unit second channel  154  by translating second unit  108  towards (e.g., in the distal direction D) first unit  110 . 
     The technique may include extending first insertion needle  140  through first unit first channel  148  and through a first hole  115  defined by first housing  110 . The technique may include translating cannula first end  124  through first hole  115 . First hole  115  may be defined in a base surface  111  of first housing  110  configured to position proximate the skin  118  of the user. The technique may include piercing the skin  118  of the user with a first needle distal end  144  and causing cannula first end  124  to implant within the user. In examples, the technique includes extending second insertion needle  142  through first unit second channel  154  and through a second hole  117  defined by first housing  110 . The technique may include translating at least a portion of sensor  104  through second hole  117  ( 206 ). Second hole  117  may be defined in base surface  111  of first housing  110 . The technique may include piercing the skin  118  of the user with a second needle distal end  146  and causing at least the portion of sensor  104  to implant within the user. 
     The technique may include causing second housing  116  to engage first housing  110  such that first housing  110  substantially limits motions of second housing  116  relative to first housing  110 . In examples, the technique includes causing second housing  116  to limit relative motion of second housing  116  using a fixation device  120 . 
     In examples, the technique includes inserting cannula second end  126  into a fluid access  130  when second housing  116  engages first housing  110 . Fluid access  130  may be defined by first housing  110 . In examples, fluid access  130  is in fluid communication with fluid reservoir  112  of first unit  110 . In examples, the technique includes establishing fluid communication with cannula  102  and fluid reservoir  112  when cannula second end  126  inserts into fluid access  130 . In examples, the technique includes establishing fluid communication between fluid reservoir  112  and cannula first end  124  via a lumen  128  of cannula  102 . The technique may include causing cannula second end  126  to puncture a access septum  163  configured to fluidly isolate fluid access  130  and portions of infusion device  100 . In some examples, the technique includes causing a fluid flow from fluid reservoir  112  to cannula first end  124  using a fluid pump  127 . The technique may include controlling an operation of fluid pump  127  using processing circuitry  114 . 
     In examples, the technique includes electrically connecting a first connector  132  of first unit  106  and a second connector  136  of second unit  108  when second housing  116  engages first housing  110 . First connector  132  may be in electrical communication with processing circuitry  114 . Second connector  136  may be in electrical communication with processing circuitry  114 . In examples, the technique includes establishing electrical connectivity between sensor  104  and processing circuitry  114  when first connector  132  electrically connects with second connector  136 . The technique may include transmitting a signal indicative of a physiological characteristic of a user to processing circuitry  114  using the electrical connection between sensor  104  and the processing circuitry. In examples, the technique includes controlling an operation of fluid pump  127  using processing circuitry  114  based on the indicative signal. 
     The technique may include retracting first insertion needle  140  and second insertion needle  142  when second unit  108  is engaged with first unit  106 . The technique may include retracting first insertion needle  140  and second insertion needle  142  in a direction away from first housing  110  (e.g., in the proximal direction P). In examples, the technique includes causing first insertion needle  140  to release cannula  102  when first insertion needle  140  retracts. The technique may include mechanically disengaging first insertion needle  140  from cannula  102  such that cannula first end  124  remains implanted in the user when first insertion needle  140  retracts. In examples, the technique includes causing second insertion needle  142  to release sensor  104  when second insertion needle  142  retracts. The technique may include mechanically disengaging second insertion needle  142  from sensor  104  such that at least some portion of sensor  104  remains implanted in the user when second insertion needle  142  retracts. 
     In examples, the technique includes positioning second unit  108  in an inserter  170 . The technique may include moving second unit  108  toward first unit  106  using the inserter  170 . In some examples, the technique includes causing inserter  170  to move second unit  108  toward first unit  106  by actuating a plunger  172 . In examples, plunger  172  is configured to compress an insertion spring  186 . The technique may include causing second unit  108  to move toward first unit  106  using an expansion of the insertion spring. 
     In examples, inserter  170  is configured to displace first insertion needle  140  over an axis A 1  extending through first unit first channel  148  when second unit  108  moves toward first unit  106 . In examples, inserter  170  is configured to displace second insertion needle  142  over an axis A 2  extending through first unit second channel  154  when second unit  108  moves toward first unit  106 . In examples, inserter  170  is configured to displace cannula second end  126  over an axis A 3  extending through fluid access  130  when second unit  108  moves toward first unit  106 . In examples, inserter  170  is configured to displace second connector  136  over an axis A 4  extending through first connector  132  when second unit  108  moves toward first unit  106 . In examples, the technique includes mating a serter member  174  of insertor  170  with first unit  106 . The technique may include displacing first insertion needle  140  over axis A 1 , displacing second insertion needle  142  over axis A 2 , displacing cannula second end  126  over axis A 3 , and/or displacing second connector  136  over axis A 4  when serter member  174  mates with first unit  106  using insertor  170 . 
     In examples, the technique includes withdrawing first insertion needle  140  and second insertion needle  142  (e.g., in the proximal direction P) from first unit  106  and second unit  108  using insertor  170 . The technique may include causing first insertion needle  140  and second insertion needle  142  to withdraw using an expansion of a withdrawal spring  194 . In examples, the technique includes causing first insertion needle  140  to release cannula  102  and causing second insertion needle  142  to release sensor  104  by withdrawing first insertion needle  140  and second insertion needle  142 . The technique may include withdrawing first insertion needle  140  and second insertion needle  142  by causing a needle carrier  156  to move in a direction away (e.g., in the proximal direction P) from first housing  110  and second housing  116 . 
     The techniques and functionalities described in this disclosure, including those attributed to processor  143 , processing circuitry, sensors, and/or various constituent components, may be implemented, at least in part, in hardware, software, firmware or any combination thereof. For example, various aspects of the techniques may be implemented within one or more processors, including one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components, embodied in any suitable device. Processing circuitry, control circuitry, and sensing circuitry, as well as other processors, controllers, and sensors described herein, may be implemented at least in part as, or include, one or more executable applications, application modules, libraries, classes, methods, objects, routines, subroutines, firmware, and/or embedded code, for example. In addition, analog circuits, components and circuit elements may be employed to construct one, some or all of the control circuitry and sensors, instead of or in addition to the partially or wholly digital hardware and/or software described herein. Accordingly, analog or digital hardware may be employed, or a combination of the two. 
     In one or more examples, the techniques and functionalities described in this disclosure may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on, as one or more instructions or code, a computer-readable medium and executed by a hardware-based processing unit. The computer-readable medium may be an article of manufacture including a non-transitory computer-readable storage medium encoded with instructions. Instructions embedded or encoded in an article of manufacture including a non-transitory computer-readable storage medium encoded, may cause one or more programmable processors, or other processors, to implement one or more of the techniques described herein, such as when instructions included or encoded in the non-transitory computer-readable storage medium are executed by the one or more processors. Example non-transitory computer-readable storage media may include RAM, ROM, programmable ROM (PROM), erasable programmable ROM (EPROM), electronically erasable programmable ROM (EEPROM), flash memory, a hard disk, a compact disc ROM (CD-ROM), a floppy disk, a cassette, magnetic media, optical media, or any other computer readable storage devices or tangible computer readable media. 
     In some examples, a computer-readable storage medium comprises non-transitory medium. The term “non-transitory” may indicate that the storage medium is not embodied in a carrier wave or a propagated signal. In certain examples, a non-transitory storage medium may store data that can, over time, change (e.g., in RAM or cache). 
     The functionality described herein may be provided within dedicated hardware and/or software modules. Depiction of different features as modules or units is intended to highlight different functional aspects and does not necessarily imply that such modules or units must be realized by separate hardware or software components. Rather, functionality associated with one or more modules or units may be performed by separate hardware or software components, or integrated within common or separate hardware or software components. Also, the techniques could be fully implemented in one or more circuits or logic elements 
     The present disclosure includes the following examples. 
     Example 1: An infusion device comprising: a first unit defining a first housing, wherein the first housing defines a first channel extending through the first housing, a second channel extending through the first housing, and a fluid access, wherein the first unit includes processing circuitry and a fluid reservoir in fluid communication with the fluid access; and a second unit defining a second housing configured to engage the first housing, the second unit comprising: a cannula having a first end and a second end; a sensor; a first insertion needle releasably carrying the cannula; and a second insertion needle releasably carrying the sensor, wherein: the first insertion needle is configured to insert the first end of the cannula through the first channel when the second housing engages the first housing, the second insertion needle is configured to insert the sensor through the second channel when the second housing engages the first housing, the second end of the cannula is configured to insert through the fluid access when the second housing engages the first housing, and the infusion device is configured to electrically connect the sensor and the processing circuitry when the second housing engages the first housing. 
     Example 2: The infusion device of example 1, wherein: the first insertion needle defines a first needle distal end configured to pierce the skin of a user, the second insertion needle defines a second needle distal end configured to pierce the skin of a user, the first insertion needle is configured to extend through the first channel and displace the first needle distal end away from the first housing to pierce the skin of the user when the second housing engages the first housing, and the second insertion needle is configured to extend through the second channel and translate the second needle distal end in a direction away from the first housing to pierce the skin of the user when the second housing engages with the first housing. 
     Example 3: The infusion device of example 1 or 2, wherein the second unit defines a second unit first channel extending through the second housing and a second unit second channel extending through the second housing, wherein the first insertion needle extends through the second unit first channel and the second insertion needle extends through the second unit first channel. 
     Example 4: The infusion device of any of examples 1-3, wherein the first insertion needle is configured to withdraw from the second unit first channel and the first channel of the first unit when the second housing is engaged with the first housing, and wherein the second insertion needle is configured to withdraw from the second unit second channel and the second channel of the first unit when the second housing is engaged with the first housing. 
     Example 5: The infusion device of any of examples 1-4, wherein the sensor is a glucose sensor and the cannula is configured to deliver insulin from the fluid reservoir when the second housing engages the first housing. 
     Example 6: The infusion device of any of examples 1-5, wherein the second unit is configured to withdraw the first end of the cannula from the first channel, withdraw the second end of the cannula from the fluid access, and withdraw the sensor from the second channel when the second housing is disengaged from the first housing. 
     Example 7: The infusion device of any of examples 1-6, wherein the second unit is configured to translate in a first direction to cause the second housing to engage the first housing, and wherein: the first insertion needle is configured to translate in the first direction to implant the first end of the cannula in a user when the second unit translates in the first direction, the second insertion needle is configured to translate in the first direction to implant the sensor in the user when the second unit translates in the first direction, the first insertion needle is configured to release the first end of the cannula when the first insertion needle translates in a second direction opposite the first direction, such that the first end of the cannula remains implanted in the user when the first insertion needle withdraws from the user, and the second insertion needle is configured to release the sensor when the first insertion needle translates in a second direction opposite the first direction, such that the first end of the cannula remains implanted in the user when the first insertion needle withdraws from the user. 
     Example 8: The infusion device of any of examples 1-7, wherein the cannula is configured to establish fluid communication from the first end of the cannula to the fluid reservoir when the second end of the cannula inserts through the fluid access. 
     Example 9: The infusion device of any of examples 1-8, wherein the first unit comprises an access septum configured to fluidly isolate the fluid reservoir and the second housing, wherein the second end of the cannula is configured to pierce the access septum when the second end of the cannula inserts through the fluid access. 
     Example 10: The infusion device of any of examples 1-9, wherein: the first insertion needle defines a first needle distal end configured to pierce the skin of a user, the second insertion needle defines a second needle distal end configured to pierce the skin of a user, and the infusion device is configured to cause the first needle distal end and the second needle distal end to pierce the skin of the user substantially concurrently. 
     Example 11: The infusion device of any of examples 1-10, wherein the first unit includes a first electrical contact in electrical communication with the processing circuitry, and wherein the second unit includes a second electrical contact in electrical communication with the sensor, and wherein the first electrical contact and the second electrical contact are configured to establish the electrical connection between the sensor and the processing circuitry when the second housing engages the first housing. 
     Example 12: The infusion device of any of examples 1-11, wherein the first unit further comprises a fluid pump configured to deliver a fluid from the fluid reservoir to the fluid access. 
     Example 13: The infusion device of any of examples 1-12, wherein the processing circuitry is configured to: receive a signal indicative of a physiological characteristic of the user from the sensor; and control an operation of the fluid pump based on the indicative signal. 
     Example 14: The infusion device of any of examples 1-13, further comprising an inserter configured to translate the second unit toward the first unit to cause the second housing to engage the first housing, wherein: the inserter is configured to mate with the first housing, the inserter is configured to align a first needle distal end of the first insertion needle with the first channel when the inserter is mated with the first housing, the inserter is configured to align a second needle distal end of the second insertion needle with the second channel when the inserter is mated with the first housing, and the inserter is configured to align the second end of the cannula with the fluid access when the inserter is mated with the first housing. 
     Example 15: The infusion device of any of examples 1-14, further comprising an inserter and a carrier attached to the first insertion needle and the second insertion needle, wherein: the inserter is configured to translate the second unit and the carrier in a first direction toward the first unit to cause the second housing to engage the first housing; and the inserter is configured to translate the carrier, the first insertion needle, and the second insertion needle away from the second housing in a second direction opposite the first direction when the second housing engages the first housing. 
     Example 16: An infusion device comprising: a first unit defining a first housing, wherein the first housing defines a first channel extending through the first housing, a second channel extending through the first housing, and a fluid access, and wherein the first unit includes processing circuitry, a fluid reservoir, and a fluid pump in fluid communication with the fluid access; and a second unit defining a second housing configured to engage the first housing, the second unit comprising: a cannula having a first end and a second end; and a sensor; a first insertion needle releasably carrying the cannula and configured to extend through the first channel; and a second insertion needle releasably carrying the sensor and configured to extend through the second channel, wherein: the first housing is configured to substantially secure the second housing from movement relative to the first housing when the second housing engages the first housing, the first insertion needle is configured to extend a portion of the cannula including the first end through the first channel when the first insertion needle extends through the first channel, the second insertion needle is configured to extend a portion of the sensor through the second channel when the second insertion needle extends through the second channel, the second end of the cannula is configured to insert through the fluid access when the second housing engages the first housing, wherein the cannula is configured to establish fluid communication from the first end of the cannula to the fluid reservoir when the second end of the cannula inserts through the fluid access, and the infusion device is configured to electrically connect the sensor and the processing circuitry when the second housing engages the first housing. 
     Example 17: The infusion device of example 16, further comprising an inserter, and wherein the second unit includes a carrier attached to the first insertion needle and the second insertion needle, wherein the inserter is configured to: releasably mate with the first housing, align the first needle distal end with the first channel when the inserter is releasably mated with the first housing, align the second needle distal end with the second channel when the inserter is releasably mated with the first housing, and align the second end of the cannula with the fluid access when the inserter is releasably mated with the first housing, and wherein the inserter is configured to translate the second unit in a first direction toward the first unit to cause the second housing to engage the first housing, and wherein the inserter is configured to translate the carrier, the first insertion needle, and the second insertion needle away from the second housing in a second direction opposite the first direction when the second housing is engaged with the first housing. 
     Example 18: The infusion device of example 16 or 17, wherein the processing circuitry is configured to: receive a signal indicative of a physiological characteristic of the user from the sensor; and control an operation of the fluid pump based on the indicative signal. 
     Example 19: A method comprising: engaging a first housing defined by a first unit and a second housing defined by a second unit, wherein the first unit includes processing circuitry and a fluid reservoir, and wherein the second unit includes a first insertion needle, a second insertion needle, a cannula, and a sensor; inserting, when the second housing engages the first housing, a first end of the cannula through a first channel defined by the first housing using the first insertion needle; inserting, when the second housing engages the first housing, the sensor through a second channel defined by the first housing using the second insertion needle; inserting, when the second housing engages the first housing, a second end of the cannula into a fluid access defined by the first housing; and electrically connecting, when the second housing engages the first housing, the sensor and the processing circuitry. 
     Example 20: The method of example 19, further comprising: translating the second unit in a first direction to engage the first housing and the second housing; and translating the first insertion needle and the second insertion needle in a second direction opposite the first direction when the first housing is engaged with the second direction. 
     Various examples have been described. These are other examples are within the scope of the disclosure.