Patent Publication Number: US-10307091-B2

Title: Method and apparatus for providing analyte sensor insertion

Description:
RELATED APPLICATION 
     The present application is a continuation of U.S. patent application Ser. No. 15/141,819 filed Apr. 28, 2016, now U.S. Pat. No. 9,795,331, which is a continuation of U.S. patent application Ser. No. 14/500,705 filed Sep. 29, 2014, now U.S. Pat. No. 9,332,933, which is a continuation of U.S. patent application Ser. No. 12/571,349 filed Sep. 30, 2009, now U.S. Pat. No. 8,852,101, which is a continuation of U.S. patent application Ser. No. 11/535,983 filed Sep. 28, 2006, now U.S. Pat. No. 7,697,967, entitled “Method and Apparatus for Providing Analyte Sensor Insertion”, which claims priority to U.S. Provisional Application No. 60/754,870 filed on Dec. 28, 2005 entitled “Medical Device Insertion”, the disclosures of each of which are incorporated in their entirety by reference for all purposes. 
    
    
     BACKGROUND 
     There are many instances in which it is necessary to position at least a portion of a medical device beneath the epidermis of a patient, e.g., in the subcutaneous layer or elsewhere. 
     For example, the monitoring of the level of glucose or other analytes, such as lactate or oxygen or the like, in certain individuals is vitally important to their health. The monitoring of glucose is particularly important to individuals with diabetes, as they must determine when insulin is needed to reduce glucose levels in their bodies or when additional glucose is needed to raise the level of glucose in their bodies. 
     In this regard, devices have been developed for continuous or automatic monitoring of analytes, such as glucose, in the blood stream or interstitial fluid. Many of these analyte measuring devices are configured so that at least a portion of the devices is positioned below the epidermis, e.g., in a blood vessel or in the subcutaneous tissue of a patient. 
     These devices, as well as other medical devices, may be positioned manually, e.g., by a user or a healthcare worker, or automatically or semi-automatically with the aid of a sensor positioning device. Regardless of the manner in which the device is inserted beneath the skin, it is important that the device positioning process does not adversely affect the operation of the device. Furthermore, it is important that pain is minimal. 
     As interest in inserting medical devices, e.g., continuous analyte monitoring devices, beneath the epidermis of a patient continues, there is interest in devices and methods for operably inserting such devices. Of interest are such devices and methods that have minimal impact on device function and which produce minimal pain. Of particular interest are continuous analyte monitoring positioning devices that enable clinically accurate analyte information to be obtained substantially immediately following device positioning in a patient. 
     SUMMARY 
     Generally, the present invention relates to methods and devices for positioning a medical device at least partially beneath the epidermal layer of skin. In certain embodiments, the present invention relates to the continuous and/or automatic in vivo monitoring of the level of an analyte using an analyte sensor and more specifically devices and methods for operably positioning analyte sensors at least partially beneath the epidermal layer of skin. The subject invention is further described with respect to positioning an analyte sensing device (also referred to herein as a “sensor”, “analyte monitoring device/sensor”, and the like) and analyte sensing systems, where such description is in no way intended to limit the scope of the invention. It is understood that the subject invention is applicable to any medical device in which at least a portion of the device is intended to be positioned beneath the epidermis. 
     Embodiments of the subject invention include analyte sensor positioning devices and methods that are adapted to provide clinically accurate analyte data (e.g., analyte-related signal) substantially immediately after a sensor has been operably positioned in a patient (e.g., at least a portion of the sensor in the subcutaneous tissue, or elsewhere). 
     Embodiments of the subject invention include systems in which the period of time after a sensor is positioned in a patient, when a first (or only) sensor calibration is required, is substantially reduced (excluding any factory-set calibration) and/or the number of calibrations (excluding any factory-set calibration) is reduced, e.g., to three or less calibrations, e.g., two or less calibrations, e.g., one calibration or no calibrations. 
     Also provided are sensor positioning devices and methods that at least minimize, and in many instances eliminate, the occurrence of periods of spurious, low analyte readings, e.g., substantially immediately following sensor positioning, during the night, etc. 
     Embodiments include devices and methods that modulate the sensor positioning speed, or stated otherwise the rate at which a sensor is delivered to a site in a patient, e.g., using at least two different velocities. 
     Also provided are positioning devices and methods that operably position a sensor in a site of a patient using an acute angle, relative to the skin. 
     Embodiments also include sensor positioning devices and methods that employ an anesthetic agent. 
     Aspects include minimal pain, including substantially pain-free, sensor positioning methods and devices and sensor positioning methods and devices that do not substantially interfere with sensor function. 
     Also provided are systems and kits. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  shows a block diagram of an exemplary embodiment of an analyte monitor using an implantable analyte sensor, according to the invention; 
         FIG. 2  is a top view of one embodiment of an analyte sensor, according to the invention; 
         FIG. 3A  is a cross-sectional view of the analyte sensor of  FIG. 2 ; 
         FIG. 3B  is a cross-sectional view of another embodiment of an analyte sensor, according to the invention; 
         FIG. 4A  is a cross-sectional view of another embodiment of an analyte sensor, according to the invention; 
         FIG. 4B  is a cross-sectional view of a fourth embodiment of another embodiment of a sensor, according to the invention; 
         FIG. 5  is a cross-sectional view of another embodiment of an analyte sensor, according to the invention; 
         FIG. 6  is an expanded top view of a tip-portion of the analyte sensor of  FIG. 2 ; 
         FIG. 7  is an expanded bottom view of a tip-portion of the analyte sensor of  FIG. 2 ; 
         FIG. 8  is a side view of the analyte sensor of  FIG. 2 ; 
         FIG. 9  is a cross-sectional view of an embodiment of an on-skin sensor control unit, according to the invention; 
         FIG. 10  is a top view of a base of an on-skin sensor control unit; 
         FIG. 11  is a bottom view of a cover of an on-skin sensor control unit; 
         FIG. 12  is a perspective view of an on-skin sensor control unit on the skin of a patient; 
         FIG. 13A  is a block diagram of one embodiment of an on-skin sensor control unit, according to the invention; 
         FIG. 13B  is a block diagram of another embodiment of an on-skin sensor control unit, according to the invention; 
         FIG. 14  is a block diagram of one embodiment of a receiver/display unit, according to the invention; 
         FIG. 15  is an expanded view of an exemplary embodiment of a sensor and a sensor positioning device, according to the invention; 
         FIGS. 16A, 16B, 16C  are cross-sectional views of three embodiments of the insertion device of  FIG. 15 ; 
         FIG. 17  is a perspective view of the internal structure of an exemplary embodiment of an insertion gun, according to the invention; 
         FIGS. 18A-18B  are front component view and perspective view, respectively, of the two stage sensor insertion mechanism including the insertion device armed and ready for insertion, further illustrating the sensor introducer and sensor to make the first stage puncture, and also showing the plunger and the button in accordance with one embodiment of the present invention; 
         FIG. 19A  illustrates a front component view of the two stage sensor insertion mechanism after the firing of the first stage trigger button to achieve the initial puncture, and with the plunger exposed for the second stage insertion activation, and also illustrating the sensor/introducer position after the initial first stage puncture (for example, at 1.55 mm depth) in accordance with one embodiment of the present invention; 
         FIGS. 19B-19D  illustrate a perspective view, a close-up perspective view, and a side view, respectively, of the two stage sensor insertion mechanism after the first stage trigger button firing shown in  FIG. 19A , where the side view shown in  FIG. 19D  further illustrates the special relationship of the carrier and drive spring with the plunger and the trigger button; 
         FIGS. 20A-20B  illustrate the front component view and the perspective view, respectively, of the two stage sensor insertion mechanism after the sensor placement at the predetermined depth with the plunger depressed down to deliver the sensor to the maximum predetermined depth in accordance with one embodiment of the present invention; 
         FIG. 21  illustrates a front perspective component view of the return spring of the two stage sensor insertion mechanism to retain the sensor introducer in a safe position after sensor insertion in accordance with one embodiment of the present invention, where the return spring may be configured to help retract or remove the introducer from the puncture site after sensor deployment to the predetermined depth; 
         FIG. 22A  is a perspective view of a first stage sensor introducer mechanism in accordance with one embodiment of the present invention; 
         FIG. 22B  is a side planar view of the first stage sensor introducer mechanism of  FIG. 22A  in accordance with one embodiment of the present invention; 
         FIG. 22C  is a side planar view of the sensor introducer coupled to the first stage sensor introducer mechanism of  FIG. 22A  in accordance with one embodiment of the present invention; 
         FIG. 23A  is a front planar view of the sensor in accordance with one embodiment of the present invention; 
         FIG. 23B  is a side view of the sensor shown in  FIG. 23A  in accordance with one embodiment of the present invention; 
         FIG. 23C  is a close up view of the tip portion of the sensor shown in  FIG. 23A  in accordance with one embodiment of the present invention; 
         FIG. 23D  is a perspective view of the sensor introducer in the first stage sensor introducer mechanism of  FIG. 22A  in accordance with one embodiment of the present invention; 
         FIG. 23E  is a close up view of the tip portion of the sensor introducer of  FIG. 23D  in accordance with one embodiment of the present invention; 
         FIG. 23F  is a front planar view of the sensor and sensor introducer of the first stage sensor introducer mechanism of  FIG. 22A  in accordance with one embodiment of the present invention; 
         FIG. 23G  is a perspective view of the sensor and sensor introducer shown in  FIG. 23F  in accordance with one embodiment of the present invention; 
         FIG. 23H  is a close up view of the tip portion of the sensor and sensor introducer shown in  FIG. 23F  in accordance with one embodiment of the present invention; 
         FIG. 24  is a front planar view of the first stage sensor insertion of the sensor introducer mechanism of  FIG. 22A  in accordance with one embodiment of the present invention; 
         FIG. 25A  is a side view of a transmitter unit for coupling to the first stage sensor introducer mechanism of FIG. 22 A in accordance with one embodiment of the present invention; 
         FIG. 25B  is a perspective view of the transmitter unit of  FIG. 25A  in accordance with one embodiment of the present invention; 
         FIG. 25C  is a side view of the transmitter unit engaged with the sensor for the second stage sensor insertion in accordance with one embodiment of the present invention; 
         FIG. 25D  is a side view of the transmitter unit mounted to the overall assembly in accordance with one embodiment of the present invention; 
         FIG. 25E  is a perspective view of the transmitter unit mounted to the overall assembly of  FIG. 25D  in accordance with one embodiment of the present invention; 
         FIG. 26A  is a perspective view of the sensor in the final position with respect to the sensor introducer mechanism without the transmitter unit in accordance with one embodiment of the present invention; and 
         FIG. 26B  is a front planar view of the sensor in the final position shown in  FIG. 26A  in accordance with one embodiment of the present invention. 
     
    
    
     DEFINITIONS 
     Throughout the present application, unless a contrary intention appears, the following terms refer to the indicated characteristics. 
     A “biological fluid” or “physiological fluid” or “body fluid”, is any body fluid in which an analyte can be measured, for example, blood, interstitial fluid, dermal fluid, sweat, tears, and urine. “Blood” includes whole blood and its cell-free components, such as, plasma and serum. 
     A “counter electrode” refers to an electrode paired with the working electrode, through which passes a current equal in magnitude and opposite in sign to the current passing through the working electrode. In the context of the invention, the term “counter electrode” is meant to include counter electrodes which also function as reference electrodes (i.e., a counter/reference electrode). 
     An “electrochemical sensor” is a device configured to detect the presence and/or measure the level of an analyte in a sample via electrochemical oxidation and reduction reactions on the sensor. These reactions are transduced to an electrical signal that can be correlated to an amount, concentration, or level of an analyte in the sample. 
     “Electrolysis” is the electrooxidation or electroreduction of a compound either directly at an electrode or via one or more electron transfer agents. 
     A compound is “immobilized” on a surface when it is entrapped on or chemically bound to the surface. 
     A “non-leachable” or “non-releasable” compound or a compound that is “non-leachably disposed” is meant to define a compound that is affixed on the sensor such that it does not substantially diffuse away from the working surface of the working electrode for the period in which the sensor is used (e.g., the period in which the sensor is implanted in a patient or measuring a sample). 
     Components are “immobilized” within a sensor, for example, when the components are covalently, ionically, or coordinatively bound to constituents of the sensor and/or are entrapped in a polymeric or sol-gel matrix or membrane which precludes mobility. For example, in certain embodiments an anesthetic agent or precursor thereof may be immobilized within a sensor. 
     An “electron transfer agent” is a compound that carries electrons between the analyte and the working electrode, either directly, or in cooperation with other electron transfer agents. One example of an electron transfer agent is a redox mediator. 
     A “working electrode” is an electrode at which the analyte (or a second compound whose level depends on the level of the analyte) is electrooxidized or electroreduced with or without the agency of an electron transfer agent. 
     A “working surface” is that portion of the working electrode which is coated with or is accessible to the electron transfer agent and configured for exposure to an analyte-containing fluid. 
     A “sensing layer” is a component of the sensor which includes constituents that facilitate the electrolysis of the analyte. The sensing layer may include constituents such as an electron transfer agent, a catalyst which catalyzes a reaction of the analyte to produce a response at the electrode, or both. In some embodiments of the sensor, the sensing layer is non-leachably disposed in proximity to or on the working electrode. 
     A “non-corroding” conductive material includes non-metallic materials, such as carbon and conductive polymers. 
     When one item is indicated as being “remote” from another, this is referenced that the two items are at least in different buildings, and may be at least one mile, ten miles, or at least one hundred miles apart. When different items are indicated as being “local” to each other they are not remote from one another (for example, they can be in the same building or the same room of a building). “Communicating”, “transmitting” and the like, of information reference conveying data representing information as electrical or optical signals over a suitable communication channel (for example, a private or public network, wired, optical fiber, wireless radio or satellite, or otherwise). Any communication or transmission can be between devices which are local or remote from one another. “Forwarding” an item refers to any means of getting that item from one location to the next, whether by physically transporting that item or using other known methods (where that is possible) and includes, at least in the case of data, physically transporting a medium carrying the data or communicating the data over a communication channel (including electrical, optical, or wireless). “Receiving” something means it is obtained by any possible means, such as delivery of a physical item. When information is received it may be obtained as data as a result of a transmission (such as by electrical or optical signals over any communication channel of a type mentioned herein), or it may be obtained as electrical or optical signals from reading some other medium (such as a magnetic, optical, or solid state storage device) carrying the information. However, when information is received from a communication it is received as a result of a transmission of that information from elsewhere (local or remote). 
     When two items are “associated” with one another they are provided in such a way that it is apparent that one is related to the other such as where one references the other. 
     Items of data are “linked” to one another in a memory when a same data input (for example, filename or directory name or search term) retrieves those items (in a same file or not) or an input of one or more of the linked items retrieves one or more of the others. 
     It will also be appreciated that throughout the present application, that words such as “cover”, “base” “front”, “back”, “top”, “upper”, and “lower” are used in a relative sense only. 
     “May” refers to optionally. 
     When two or more items (for example, elements or processes) are referenced by an alternative “or”, this indicates that either could be present separately or any combination of them could be present together except where the presence of one necessarily excludes the other or others. 
     Any recited method can be carried out in the order of events recited or in any other order which is logically possible. Reference to a singular item, includes the possibility that there are plural of the same item present. 
     DETAILED DESCRIPTION 
     Before the present invention is described, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims. 
     Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges as also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, the preferred methods and materials are now described. 
     It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. 
     As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present invention. 
     The figures shown herein are not necessarily drawn to scale, with some components and features being exaggerated for clarity. 
     As summarized above, the present invention is related to analyte sensor positioning devices and methods (the term “positioning” is used herein interchangeably with “delivery”, “insertion”, and the like). The present invention is applicable to an analyte monitoring system using a sensor—at least a portion of which is positionable beneath the skin of the user for the in vivo determination of a concentration of an analyte, such as glucose, lactate, and the like, in a body fluid. The sensor may be, for example, subcutaneously positionable in a patient for the continuous or periodic monitoring of an analyte in a patient&#39;s interstitial fluid. This may be used to infer the glucose level in the patient&#39;s bloodstream. The sensors of the subject invention also include in vivo analyte sensors insertable into a vein, artery, or other portion of the body containing fluid. A sensor of the subject invention is typically configured for monitoring the level of the analyte over a time period which may range from minutes, hours, days, weeks, or longer. Of interest are analyte sensors, such as glucose sensors, that are capable of providing analyte data for about one hour or more, e.g., about a few hours or more, e.g., about a few days of more, e.g., about three days or more, e.g., about five days or more, e.g., about seven days or more, e.g., about several weeks or months. 
     Embodiments include positioning devices and systems, and methods that provide clinically accurate analyte data (e.g., relative to a reference) substantially immediately, as shown by any suitable technique known to those of skill in the art, e.g., a Clark Error Grid, Parks Error Grid, Continuous Glucose Error Grid, MARD analysis, and the like. For example, in those embodiments in which the sensor is a continuous sensor and at least a portion of the sensor is adapted to be positioned under the skin of a patient, the sensor is adapted to provide clinically accurate analyte data (e.g., relative to a reference) substantially immediately after the sensor is operably positioned in a patient. In other words, the waiting period from the time a sensor is positioned in a user and the time clinically accurate data may be obtained and used by the user, is greatly reduced relative to prior art devices that require a greater waiting period before accurate analyte data may be obtained and used by a user. By “substantially immediately” is meant from about 0 hours to less than about 5 hours, e.g., from about 0 hours to about 3 hours, e.g., from about 0 hours to less than about 1 hour, e.g., from about 30 minutes or less, where in many embodiments the sensors according to the subject invention are capable of providing clinically accurate analyte data once the sensor has been operatively positioned in the patient. 
     As noted above, embodiments also include analyte monitoring devices and methods having substantially reduced (including eliminated) periods of time of spurious, low analyte readings, as compared to a control, i.e., the period of time in which clinically accurate analyte data is obtainable is greater, as compared to a control. The subject invention may be employed to minimize or eliminate spurious low analyte readings obtained at any time during sensor use, including a period of time immediately after sensor activation (e.g., positioning of an analyte sensor in or on a patient) and/or anytime thereafter. Accordingly, embodiments include sensors positioning devices and methods that enable sensors to provide clinically accurate analyte data substantially immediately after the sensor has been operably positioned in a patient (e.g., in the subcutaneous tissue, etc.) and/or without substantial interruption due to spurious analyte readings 
     Embodiments include minimal tissue trauma-producing analyte positioning devices and methods, where embodiments include modulating the rate at which a sensor is delivered to a target site. For example, at least two velocities may be used in the positioning of a sensor, where embodiments include a multiple rate sensor delivery protocol having a first sensor delivery rate, followed by a second sensor delivery rate that is less than the first. Embodiments may include opening the skin with a first velocity, and inserting the sensor through the thus-formed skin opening to a target site (e.g., into the subcutaneous tissue) with a second, minimal tissue trauma-producing velocity, where the second velocity is less than the first velocity. Such may be accomplished automatically or semi-automatically with a sensor positioning device. The positioning device may include a sharp portion and a sensor-carrying portion and may be adapted to provide a skin incision and position a sensor in a patient using variable speeds. It is to be understood that such may be accomplished wholly or at least partially manually. 
     Certain embodiments include two-stage sensor delivery devices and methods and include devices capable of producing at least first and second velocities. Specific embodiments include devices capable of producing a superficial cut in the skin that is no deeper than the epidermis using a first velocity, and inserting the sensor through the thus-formed cut to a target site using a second velocity that is slower than the first velocity. The speed of the first velocity may be selected to minimize the patient&#39;s perception of pain and the speed of the second velocity may be selected to minimize tissue damage at the site of eventual glucose measurements. For example, the high speed of the first velocity (e.g., from about 4 to about 8 m/s in certain embodiments) may minimize the patient&#39;s pain while the slower speed of the second velocity (e.g., from about 0.025 to about 0.5 m/s in certain embodiments) may minimize the damage due to the tissue at the site of the eventual glucose sensor measurements. Accordingly, a user contacts the device to a skin surface and actuates the device to cut the skin and insert the sensor through the cut to the target site, using at least two different velocities for the incision forming and sensor delivery operations. 
     The various velocities employed may differ by any suitable amount. For example, in certain embodiments in which two velocities are employed, the velocities may differ by about 25% to about 95%, e.g., by about 60% to about 90%. Velocity change may be gradual or stepped. The change in velocity may be perceptible to the user or not, where in many embodiments the velocity change is not perceptible by the user. In certain embodiments, the sensor positioning process is automatic in that a user need only activate the device, e.g., actuate a button, lever, contact with a skin surface, or the like, to initiate the sensor positioning process, which process then proceeds to completion without any further user intervention. However, in some embodiments one or more parameters may be controllable by the user, e.g., the timing of velocity change, magnitudes of velocity(ies), etc. 
     Embodiments of the above-described two-speed sensor insertion minimize tissue damage to the superficial layer of the skin, the stratum corneum and epidermis, as a greater force is required to penetrate these outer layers of the skin, and hence a greater likelihood of tissue damage. By limiting the depth of the incision to the upper layers of the skin, i.e., the stratum corneum and epidermis, minimization of tissue damage at the site of the eventual analyte sensor placement in the subcutaneous adipose tissue layer is achieved. 
     Furthermore, since in certain embodiments a separate sharp is not employed to penetrate below the outer layer of skin, not only is the tissue damage in the subcutaneous adipose layer minimized by use of the slower speed in the second velocity portion of the insertion, but the physical size and dimension of the wound is greatly reduced by eliminating the use of a separate sharp device penetrating below the outer layer of the skin. 
     In certain embodiments, the sharp device which disrupts the stratum corneum and epidermis may penetrate from about 0.5 mm to about 1.5 mm below the surface of the skin in certain embodiments. In certain analyte sensing systems, the analyte-sensing chemistry layer on the sensor, by contrast, may be positioned below or deeper than this penetration, e.g., below about 0.5 mm to about 1.5 mm below the surface of the skin. The slow speed of the second velocity portion of the insertion displaces the adipose cells in the subcutaneous adipose tissue layer rather than physically disrupting the cells and effectively coring out a cylinder in which the sensor may be subsequently placed. By contrast, in the present invention, the slow speed of the second velocity portion of the insertion minimizes the volume of tissue which has been removed or even displaced by the sensor insertion. As a result, the sensing portion of the sensor is in immediate proximal contact with the surrounding tissue. In contrast to typical insertion methods in which a cylindrical core of tissue is displaced or removed by a high-speed insertion, in the present invention there is no open volume of tissue in which fluids may accumulate forming edema typical of wound response to trauma of this nature. The absence of or the significant reduction of edema in the present invention associated with the minimization of the perturbed volume of tissue contributes to rapid sensor equilibration with the method of sensor insertion described herein compared with conventional sensor insertion procedures. 
     Embodiments include making a large wide cut through the epidermis, then a much smaller incision in terms of its cross-sectional dimensions through the dermis and into the underlying subcutaneous adipose tissue layer, where in certain embodiments as much as about a fourfold difference in the cross-sectional area (e.g., 0.48 mm 2  for the incision in the epidermis compared to 0.12 mm 2  for the incision in the subcutaneous layer). 
     The subject invention also includes anesthetic agents in sensor positioning. That is, certain embodiments include sensor positioning devices, methods and/or sensors that include an anesthetic agent (“active agent”). The active agent may be any suitable anesthetic agent(s) known or to be discovered. Examples of anesthetic agents include, but are not limited to, lidocaine (with or without epinephrine), prilocaine, bupivacaine, benzocaine, and ropivacaine, marcaine (with or without epinephrine) and the like, and combinations thereof, as well as cold sprays such as ethyl chloride sprays. 
     The active-agent containing devices may be analyte sensors and/or analyte sensor positioning devices in certain embodiments, and/or may be a structure that is positionable near a skin location site at which site an incision is or will be made and sensor is or will be inserted (a body fluid sampling site). In certain embodiments, the structure may be a sensor positioning device, drug delivery device (e.g., insulin delivery device), etc. 
     In certain embodiments, an active agent may not be carried by a device, but rather may be otherwise applied at or substantially near the sensor insertion site. Accordingly, embodiments include systems having an active agent delivery unit and an analyte sensor. 
     Active agent employed in the subject invention may be delivered transdermally, by a topical route, formulated as applicator sticks, solutions, suspensions, emulsions, gels, creams, ointments, pastes, jellies, paints, powders, and aerosols. For example, embodiments may include an active agent in the form of a discrete patch or film or plaster or the like adapted to remain in intimate contact with the epidermis of the recipient for a period of time. For example, such transdermal patches may include a base or matrix layer, e.g., polymeric layer, in which active agent is retained. The base or matrix layer may be operably associated with a support or backing. Active agents suitable for transdermal administration may also be delivered by iontophoresis and may take the form of an optionally buffered aqueous solution that includes the active agent. Suitable formulations may include citrate or bis/tris buffer (pH 6) or ethanol/water and contain a suitable amount of active ingredient. 
     Active agents may be applied via parenteral administration, such as intravenous (“IV”) administration, intramuscular (“IM”), subcutaneous (“SC” or “SQ”), mucosal. The formulations for such administration may include a solution of the active agent dissolved in a pharmaceutically acceptable carrier. Among the acceptable vehicles and solvents that may be employed, include, but are not limited to, water and Ringer&#39;s solution, an isotonic sodium chloride, etc. Active agents may be formulated into preparations for injection by dissolving, suspending or emulsifying them in an aqueous or nonaqueous solvent, such as vegetable or other similar oils, synthetic aliphatic acid glycerides, esters of higher aliphatic acids or propylene glycol; and if desired, with conventional additives such as solubilizers, isotonic agents, suspending agents, emulsifying agents, stabilizers and preservatives. These solutions are sterile and generally free of undesirable matter. 
     In other embodiments, the active agent may be delivered by the use of liposomes which fuse with the cellular membrane or are endocytosed, i.e., by employing ligands attached to the liposome, or attached directly to the oligonucleotide, that bind to surface membrane protein receptors of the cell resulting in endocytosis. By using liposomes, particularly where the liposome surface carries ligands specific for target cells, or are otherwise preferentially directed to a specific organ, one can focus the delivery of the pharmacological agent into the target cells in vivo. (See, e.g., Al-Muhammed, J. Microencapsul. 13:293-306, 1996; Chonn, Curr. Opin. Biotechnol. 6:698-708, 1995; Ostro, Am. J. Hosp. Pharm. 46:1576-1587, 1989). Methods for preparing liposomal suspensions are known in the art and thus will not be described herein in great detail. 
     Embodiments may also include administration of active agent using an active agent administration device other than a sensor positioning device and a sensor such as, but not limited to, pumps (implantable or external devices and combinations of both (e.g., certain components may be implantable and others may be external to the body such as controls for the implantable components)), epidural injectors, syringes or other injection apparatus, catheter and/or reservoir operably associated with a catheter, etc. For example, in certain embodiments a device employed to deliver active agent to a subject may be a pump, syringe, catheter or reservoir operably associated with a connecting device such as a catheter, tubing, or the like. Containers suitable for delivery of active agent to an active agent administration device include instruments of containment that may be used to deliver, place, attach, and/or insert the active agent into the delivery device for administration of the active agent to a subject and include, but are not limited to, vials, ampules, tubes, capsules, bottles, syringes and bags. Embodiments may also include administration of active agent via a biodegradable implant active agent delivery device. Such may be accomplished by employing syringes to deposit such a biodegradable delivery device under the skin of a subject. The implants degrade completely, so that removal is not necessary. 
     Embodiments may include employing an electrode to deliver active agent to a subject. For example, an electrode may be used that has a small port at its tip which is connected to a reservoir or pump containing active agent. The active agent delivery electrode may be implanted using any suitable technique such as surgical cut down, laparoscopy, endoscopy, percutaneous procedure, and the like. In certain embodiments a reservoir or pump may also be implanted in the subject&#39;s body. The active agent delivery electrode, or other analogous device, may be controllable such that the amount of active agent delivered, the rate at which the active agent may be delivered, and the time period over which the active agent may be delivered, etc., may be controllable and may be adjusted, e.g., by a user and/or healthcare worker. 
     Accordingly, embodiments include contacting an analyte determination site with active agent, and determining the concentration of an analyte, where the contacting may be by way of an analyte sensor, analyte sensor positioning device or other structure, transdermal administration, parenteral administration, etc. 
     In those embodiments in which a sensor positioning device and/or sensor or other device includes active agent, the active agent-containing structure may include or incorporate active agent in any suitable manner. For example, at least a portion of a positioning device and/or sensor, e.g., a body fluid-contacting portion, may include active agent, where in certain embodiments substantially the entire positioning device and/or sensor may include active agent. Active agent may be immobilized on a surface of a positioning device and/or sensor or may be configured to diffuse away from a surface of a positioning device and/or sensor. In certain embodiments, at least the portion of the positioning device that is adapted to provide a skin incision, e.g., a sharp of a sensor positioning device, may include active agent. 
     In certain embodiments, active agent is a coating on at least a portion of positioning device and/or sensor. In certain embodiments, active agent is incorporated, e.g., embedded, or otherwise integrated into a positioning device and/or sensor. 
     A positioning device and/or sensor may also have the ability to emit or diffuse active agent at a controllable rate, e.g., may include a controlled release, such as a time release, formulation. For example, a positioning device and/or sensor may include a formulation that is designed to release active agent gradually over time, e.g., over about a period of time commensurate with sensor positioning. A controlled release formulation may employ a polymer or other non-anesthetic agent material to control the release of the active agent. The active agent release rate may be slowed by diffusion through the polymer, or the active agent may be released as the polymer degrades or disintegrates in the body. 
     The active agent may be added to a positioning device and/or sensor during fabrication thereof and/or may be applied after fabrication. For example, a coating containing active agent thereof may be applied to a positioning device and/or sensor after it has been fabricated. 
     Active agent may be applied to a positioning device and/or sensor by any of a variety of methods, e.g., by spraying the active agent onto at least a portion of a positioning device and/or sensor or by dipping a positioning device and/or sensor into the active agent, or otherwise immersing or flooding a positioning device and/or sensor with the active agent. 
     The amount of active agent employed may vary depending on a variety of factors such as the particular active agent used, the particulars of the positioning device and/or sensor, etc. In any event, an effective amount of active agent is used—an amount sufficient to provide the requisite anesthetic result for the desired period of time. 
     Representative analyte sensors, analyte monitoring systems and sensor positioning devices are now described, where such description is for exemplary purposes only and is in no way intended to limit the scope of the invention. 
     Analyte Sensors and Sensor Systems 
     The analyte sensors and analyte monitoring systems of the present invention can be utilized under a variety of conditions. The particular configuration of a sensor and other units used in an analyte monitoring system may depend on the use for which the sensor and system are intended and the conditions under which the sensor and system will operate. As noted above, embodiments include a sensor configured for implantation into a patient or user. The term “implantation” is meant broadly to include wholly implantable sensors and sensors in which only a portion of which is implantable under the skin and a portion of which resides above the skin, e.g., for contact to a transmitter, receiver, transceiver, processor, etc. For example, implantation of the sensor may be made in the arterial or venous systems for direct testing of analyte levels in blood. Alternatively, a sensor may be implanted in the interstitial tissue for determining the analyte level in interstitial fluid. This level may be correlated and/or converted to analyte levels in blood or other fluids. The site and depth of implantation may affect the particular shape, components, and configuration of the sensor. Subcutaneous implantation may be desired, in some cases, to limit the depth of implantation of the sensor. Sensors may also be implanted in other regions of the body to determine analyte levels in other fluids. Examples of suitable sensors for use in the analyte monitoring systems of the invention are described in U.S. Pat. Nos. 6,134,461, 6,175,752, and elsewhere. 
     An exemplary embodiment of an analyte monitoring system  40  for use with an implantable sensor  42 , e.g., for use with a subcutaneously implantable sensor, is illustrated in block diagram form in  FIG. 1 . The analyte monitoring system  40  includes, at minimum, a sensor  42 , at least a portion of the sensor which is configured for implantation (e.g., subcutaneous, venous, or arterial implantation) into a patient, and a sensor control unit  44 . The sensor  42  is coupleable to the sensor control unit  44  which is typically attachable to the skin of a patient. The sensor control unit  44  operates the sensor  42 , including, for example, providing a voltage across the electrodes of the sensor  42  and collecting signals from the sensor  42 . 
     The sensor control unit  44  may evaluate the signals from the sensor  42  and/or transmit the signals to one or more optional receiver/display units  46 ,  48  for evaluation. The sensor control unit  44  and/or the receiver/display units  46 ,  48  may display or otherwise communicate the current level of the analyte. Furthermore, the sensor control unit  44  and/or the receiver/display units  46 ,  48  may indicate to the patient, via, for example, an audible, visual, or other sensory-stimulating alarm, when the level of the analyte is at or near a threshold level. In some embodiments, an electrical shock may be delivered to the patient as a warning through one of the electrodes or the optional temperature probe of the sensor. For example, if glucose is monitored then an alarm may be used to alert the patient to a hypoglycemic or hyperglycemic glucose level and/or to impending hypoglycemia or hyperglycemia. 
     A sensor  42  includes at least one working electrode  58  and a substrate  50 , as shown in  FIG. 2 . The sensor  42  may also include at least one counter electrode  60  (or counter/reference electrode) and/or at least one reference electrode  62  (see for example  FIG. 7 ). The counter electrode  60  and/or reference electrode  62  may be formed on the substrate  50  or may be separate units. For example, the counter electrode and/or reference electrode may be formed on a second substrate which is also implantable in the patient or, for some embodiments of the sensors, the counter electrode and/or reference electrode may be placed on the skin of the patient with the working electrode or electrodes being implanted into the patient. The use of an on-the-skin counter and/or reference electrode with an implantable working electrode is described in, e.g., U.S. Pat. No. 5,593,852. 
     The working electrode or electrodes  58  are formed using conductive materials  52 . The counter electrode  60  and/or reference electrode  62 , as well as other optional portions of the sensor  42 , such as a temperature probe  66  (see for example  FIG. 7 ), may also be formed using conductive material  52 . The conductive material  52  may be formed over a smooth surface of the substrate  50  or within channels  54  formed by, for example, embossing, indenting or otherwise creating a depression in the substrate  50 . 
     A sensing layer  64  (see for example  FIGS. 3, 4, 5 and 6 ) may be provided proximate to or on at least one of the working electrodes  58  to facilitate the electrochemical detection of the analyte and the determination of its level in the sample fluid, particularly if the analyte cannot be electrolyzed at a desired rate and/or with a desired specificity on a bare electrode. 
     In addition to the electrodes  58 ,  60 ,  62  and the sensing layer  64 , the sensor  42  may also include optional components such as one or more of the following: a temperature probe  66  (see for example  FIGS. 5 and 7 ), a mass transport limiting layer  74 , e.g., a matrix such as a membrane or the like, (see for example  FIG. 8 ), a biocompatible layer  75  (see for example  FIG. 8 ), and/or other optional components, as described below. Each of these optional items enhances the functioning of and/or results from the sensor  42 , as discussed below. 
     The substrate  50  may be formed using a variety of non-conducting materials, including, for example, polymeric or plastic materials and ceramic materials. Suitable materials for a particular sensor  42  may be determined, at least in part, based on the desired use of the sensor  42  and properties of the materials. 
     In addition to considerations regarding flexibility, it is often desirable that a sensor  42  should have a substrate  50  which is non-toxic. Preferably, the substrate  50  is approved by one or more appropriate governmental agencies or private groups for in vivo use. Although the substrate  50  in at least some embodiments has uniform dimensions along the entire length of the sensor  42 , in other embodiments, the substrate  50  has a distal end  67  and a proximal end  65  with different widths  53 ,  55 , respectively, as illustrated in  FIG. 2 . 
     At least one conductive trace  52  may be formed on the substrate for use in constructing a working electrode  58 . Typically, the working surface  51  of the working electrode  58  is at least a portion of the conductive trace  52  that is in contact with the analyte-containing fluid (e.g., implanted in the patient). In addition, other conductive traces  52  may be formed on the substrate  50  for use as electrodes (e.g., additional working electrodes, as well as counter, counter/reference, and/or reference electrodes) and other components, such as a temperature probe. The conductive traces  52  may extend most of the distance along a length  57  of the sensor  50 , as illustrated in  FIG. 2 , although this is not necessary. The placement of the conductive traces  52  may depend on the particular configuration of the analyte monitoring system (e.g., the placement of control unit contacts and/or the sample chamber in relation to the sensor  42 ). For implantable sensors, particularly subcutaneously implantable sensors, the conductive traces may extend close to the tip of the sensor  42  to minimize the amount of the sensor that must be implanted. 
     The conductive traces may be formed using a conductive material  56  such as carbon (e.g., graphite), a conductive polymer, a metal or alloy (e.g., gold or gold alloy), or a metallic compound (e.g., ruthenium dioxide or titanium dioxide), and the like. Conductive traces  52  (and channels  54 , if used) may be formed with relatively narrow widths. In embodiments with two or more conductive traces  52  on the same side of the substrate  50 , the conductive traces  52  are separated by distances sufficient to prevent conduction between the conductive traces  52 . The working electrode  58  and the counter electrode  60  (if a separate reference electrode is used) may be made using a conductive material  56 , such as carbon. 
     The reference electrode  62  and/or counter/reference electrode may be formed using conductive material  56  that is a suitable reference material, for example silver/silver chloride or a non-leachable redox couple bound to a conductive material, for example, a carbon-bound redox couple. 
     The electrical contact  49  may be made using the same material as the conductive material  56  of the conductive traces  52 , or alternatively, may be made from a carbon or other non-metallic material, such as a conducting polymer. 
     A number of exemplary electrode configurations are described below, however, it will be understood that other configurations may also be used.  FIG. 3A  is a cross-sectional view of the analyte sensor taken along lines  3 A- 3 A of  FIG. 2 . In certain embodiments, e.g., illustrated in  FIG. 3A , the sensor  42  includes two working electrodes  58   a ,  58   b  and one counter electrode  60 , which also functions as a reference electrode. In another embodiment, the sensor includes one working electrode  58   a , one counter electrode  60 , and one reference electrode  62 , as shown for example in  FIG. 3B . Each of these embodiments is illustrated with all of the electrodes formed on the same side of the substrate  50 . 
     Alternatively, one or more of the electrodes may be formed on an opposing side of the substrate  50 . In another embodiment, two working electrodes  58  and one counter electrode  60  are formed on one side of the substrate  50  and one reference electrode  62  and two temperature probes  66  are formed on an opposing side of the substrate  50 , as illustrated in  FIG. 5 . The opposing sides of the tip of this embodiment of the sensor  42  are illustrated in  FIGS. 6 and 7 . 
     Some analytes, such as oxygen, may be directly electrooxidized or electroreduced on the working electrode  58 . Other analytes, such as glucose and lactate, require the presence of at least one electron transfer agent and/or at least one catalyst to facilitate the electrooxidation or electroreduction of the analyte. Catalysts may also be used for those analytes, such as oxygen, that can be directly electrooxidized or electroreduced on the working electrode  58 . For these analytes, each working electrode  58  has a sensing layer  64  formed proximate to or on a working surface of the working electrode  58 . In many embodiments, the sensing layer  64  is formed near or on only a small portion of the working electrode  58 , e.g., near a tip of the sensor  42 . 
     The sensing layer  64  includes one or more components designed to facilitate the electrolysis of the analyte. The sensing layer  64  may be formed as a solid composition of the desired components (e.g., an electron transfer agent and/or a catalyst). These components may be non-leachable from the sensor  42  and may be immobilized on the sensor  42 . For example, the components may be immobilized on a working electrode  58 . Alternatively, the components of the sensing layer  64  may be immobilized within or between one or more membranes or films disposed over the working electrode  58  or the components may be immobilized in a polymeric or sol-gel matrix. Examples of immobilized sensing layers are described in, e.g., U.S. Pat. Nos. 5,262,035; 5,264,104; 5,264,105; 5,320,725; 5,593,852; and 5,665,222; and PCT Patent Application No. US1998/002403 entitled “Electrochemical Analyte Sensors Using Thermostable Soybean Peroxidase” filed on Feb. 11, 1998, published as WO-1998/035053. 
     Sensors having multiple working electrodes  58   a  may also be used, e.g., and the signals therefrom may be averaged or measurements generated at these working electrodes  58   a  may be averaged. In addition, multiple readings at a single working electrode  58   a  or at multiple working electrodes may be averaged. 
     In many embodiments, the sensing layer  64  contains one or more electron transfer agents in contact with the conductive material  56  of the working electrode  58 , as shown for example in  FIGS. 3A and 3B . Useful electron transfer agents and methods for producing them are described in, e.g., U.S. Pat. Nos. 5,264,104; 5,356,786; 5,262,035; 5,320,725, 6,175,752, 6,329,161, and elsewhere. In another embodiment, the sensing layer  64  is not deposited directly on the working electrode  58   a . Instead, the sensing layer  64  is spaced apart from the working electrode  58   a , as illustrated in  FIG. 4A , and separated from the working electrode  58   a  by a separation layer  61 . 
     The sensing layer  64  may also include a catalyst which is capable of catalyzing a reaction of the analyte. The catalyst may also, in some embodiments, act as an electron transfer agent. In another embodiment, two sensing layers  63 ,  64  are used, as shown in  FIG. 4B . 
     To electrolyze the analyte, a potential (versus a reference potential) is applied across the working and counter electrodes  58 ,  60 . When a potential is applied between the working electrode  58  and the counter electrode  60 , an electrical current will flow. 
     Those skilled in the art will recognize that there are many different reactions that will achieve the same result; namely the electrolysis of an analyte or a compound whose level depends on the level of the analyte. 
     A variety of optional items may be included in the sensor. One optional item is a temperature probe  66  (see for example  FIG. 7 ). One exemplary temperature probe  66  is formed using two probe leads  68 ,  70  connected to each other through a temperature-dependent element  72  that is formed using a material with a temperature-dependent characteristic. An example of a suitable temperature-dependent characteristic is the resistance of the temperature-dependent element  72 . The temperature probe  66  can provide a temperature adjustment for the output from the working electrode  58  to offset the temperature dependence of the working electrode  58 . 
     The sensors of the subject invention are biocompatible. Biocompatibility may be achieved in a number of different manners. For example, an optional biocompatible layer  75  may be formed over at least that portion of the sensor  42  which is inserted into the patient, as shown in  FIG. 8 . 
     An interferant-eliminating layer (not shown) may be included in the sensor  42 . The interferant-eliminating layer may include ionic components, such as Nafion® or the like, incorporated into a polymeric matrix to reduce the permeability of the interferant-eliminating layer to ionic interferants having the same charge as the ionic components. 
     A mass transport limiting layer  74  may be included with the sensor to act as a diffusion-limiting barrier to reduce the rate of mass transport of the analyte, for example, glucose or lactate, into the region around the working electrodes  58 . Exemplary layers that may be used are described for example, in U.S. Pat. No. 6,881,551, and elsewhere. 
     A sensor of the subject invention may be adapted to be a replaceable component in an in vivo analyte monitor, and particularly in an implantable analyte monitor. As described above, in many embodiments the sensor is capable of operation over a period of days or more, e.g., a period of operation may be at least about one day, e.g., at least about three days, e.g., at least about five days, e.g., at least about one week or more, e.g., one month or more. The sensor may then be removed and replaced with a new sensor. 
     As described above, sensor positioning devices are provided. Embodiments of the subject positioning devices include low impact, minimal pain-producing devices, where certain embodiments are configured to obtain clinically accurate analyte information substantially immediately after sensor positioning. Device embodiments include variable insertion speed devices. Embodiments of the two stage sensor inserters described herein include single use, disposable, self-contained Sensor Delivery Units (“SDU”) which may be included in a continuous glucose monitoring system. 
     Referring to  FIG. 15 , sensor positioning device  120  may be used to insert, e.g., subcutaneously insert, at least a portion of the sensor  42  into the patient. The sensor positioning device  120  may be formed using structurally rigid materials, such as metal or rigid plastic. Exemplary materials include, but are not limited to, stainless steel and ABS (acrylonitrile-butadiene-styrene) plastic. In some embodiments, the sensor positioning device  120  is pointed and/or sharp at the tip  121  to facilitate penetration of the skin of the patient. A sharp, thin sensor positioning device may reduce pain felt by the patient upon insertion of the sensor  42 . In other embodiments, the tip  121  of the sensor positioning device  120  has other shapes, including a blunt or flat shape. These embodiments may be particularly useful when the sensor positioning device  120  does not penetrate the skin but rather serves as a structural support for the sensor  42  as the sensor  42  is pushed into the skin. In embodiments in which at least a portion of the positioning device includes an anesthetic agent, such may be included in any suitable location of device  120 , e.g., at least a portion of tip  121 . 
     The sensor positioning device  120  may have a variety of cross-sectional shapes, as shown in  FIGS. 16A, 16B, and 16C . The sensor positioning device  120  illustrated in  FIG. 16A  is a flat, planar, pointed strip of rigid material which may be attached or otherwise coupled to the sensor  42  to ease insertion of the sensor  42  into the skin of the patient, as well as to provide structural support to the sensor  42  during insertion. The sensor positioning devices  120  of  FIGS. 16B and 16C  are U- or V-shaped implements that support the sensor  42  to limit the amount that the sensor  42  may bend or bow during insertion. The cross-sectional width  124  of the sensor positioning devices  120  illustrated in  FIGS. 16B and 16C  may be about 1 mm or less, e.g., about 700 μm or less, e.g., about 500 μm or less, e.g., about 300 μm or less. The cross-sectional height  126  of the sensor positioning device  120  illustrated in  FIGS. 16B and 16C  may be about 1 mm or less, e.g., about 700 μm or less, e.g., about 500 μm or less in certain embodiments. 
     The sensor  42  itself may include optional features to facilitate insertion. For example, the sensor  42  may be pointed at the tip  123  to ease insertion, as illustrated in  FIG. 15 . In addition, the sensor  42  may include a barb  125  which helps retain the sensor  42  in the subcutaneous tissue of the patient. The barb  125  may also assist in anchoring the sensor  42  at the target site, e.g., within the subcutaneous tissue, of the patient during operation of the sensor  42 . However, the barb  125  is typically small enough that little damage is caused to the subcutaneous tissue when the sensor  42  is removed for replacement. The sensor  42  may also include a notch  127  that can be used in cooperation with a corresponding structure (not shown) in the sensor positioning device to apply pressure against the sensor  42  during insertion, but disengage as the sensor positioning device  120  is removed. One example of such a structure in the sensor positioning device is a rod (not shown) between two opposing sides of a sensor positioning device  120  and at an appropriate height of the sensor positioning device  120 . 
     In operation, a sensor is carried by the positioning device to the target site. For example, the sensor  42  is placed within or next to the sensor positioning device  120  (e.g., may be partially or completely held within the sharp of the device, e.g., in a nested configuration or the like) and then a force is provided against the sensor positioning device  120  and/or sensor  42  to carry the sensor  42  into the skin of the patient. As described above, in certain embodiments various speeds may be used in a given insertion, e.g., a first speed followed by a second speed where the first speed is greater relative to the second speed. 
     In one embodiment, the force is applied to the sensor  42  to push the sensor into the skin, while the sensor positioning device  120  remains stationary and provides structural support to the sensor  42 . Alternatively, the force is applied to the sensor positioning device  120  and optionally to the sensor  42  to push a portion of both the sensor  42  and the sensor positioning device  120  through the skin of the patient and into the subcutaneous tissue. In any event, the forces used may be the same or different, as noted herein. The sensor positioning device  120  is optionally pulled out of the skin and subcutaneous tissue with the sensor  42  remaining in the subcutaneous tissue due to frictional forces between the sensor  42  and the patient&#39;s tissue. If the sensor  42  includes the optional barb  125 , then this structure may also facilitate the retention of the sensor  42  within the interstitial tissue as the barb catches in the tissue. The force applied to the sensor positioning device  120  and/or the sensor  42  may be applied manually or mechanically. The sensor  42  is reproducibly inserted through the skin of the patient. 
     In certain embodiments, an insertion gun may be used to insert the sensor. One example of an insertion gun  200  for inserting a sensor  42  is shown in  FIG. 17 . The insertion gun  200  includes a housing  202  and a carrier  204 . The sensor positioning device  120  is typically mounted on the carrier  204  and the sensor  42  is pre-loaded into the sensor positioning device  120 . The carrier  204  drives the sensor  42  and, optionally, the sensor positioning device  120  into the skin of the patient using, for example, a cocked or wound spring, a burst of compressed gas, an electromagnet repelled by a second magnet, or the like, within the insertion gun  200 . In some instances, for example, when using a spring, the carrier  204  and sensor positioning device may be moved, cocked, or otherwise prepared to be directed towards the skin of the patient. 
     After the sensor  42  is inserted, the insertion gun  200  may contain a mechanism which pulls the sensor positioning device  120  out of the skin of the patient. Such a mechanism may use a spring, electromagnet, or the like to remove the sensor positioning device  120 . 
     The insertion gun may be reusable. The sensor positioning device  120  is often disposable to avoid the possibility of contamination. Alternatively, the sensor positioning device  120  may be sterilized and reused. In addition, the sensor positioning device  120  and/or the sensor  42  may be coated with an anticlotting agent to prevent fouling of the sensor  42 . 
     In one embodiment, the sensor  42  is injected between about 2 to about 12 mm into the interstitial tissue of the patient for subcutaneous implantation, e.g., the sensor is injected about 3 to about 9 mm, e.g., about 5 to about 7 mm, into the interstitial tissue. Other embodiments of the invention may include sensors implanted in other portions of the patient, including, for example, in an artery, vein, or organ. The depth of implantation varies depending on the desired implantation target. In any event, in certain embodiments the injection is at a speed that differs from the speed employed to create an opening in the skin through which the sensor is injected. 
     Although the sensor  42  may be inserted anywhere in the body, it is often desirable that the insertion site be positioned so that the on-skin sensor control unit  44  may be concealed. In addition, it is often desirable that the insertion site be at a place on the body with a low density of nerve endings to reduce the pain to the patient. Examples of preferred sites for insertion of the sensor  42  and positioning of the on-skin sensor control unit  44  include the abdomen, thigh, leg, upper arm, and shoulder. 
     Any suitable angle of insertion may be used. An insertion angle is measured from the plane of the skin (i.e., inserting the sensor perpendicular to the skin would be a 90 degree insertion angle). As noted herein, in certain embodiments an angle less than about 90 degrees is used. The orientation of the two stage or two speed sensor inserter device may be either at normal angle to the skin or at an oblique angle to the skin such as but not limited to about 20, about 25, about 30, about 45 or about 60 degrees with respect to the skin surface itself. In contrast with the sensor used in the case of normal or 90 degree insertion, in instances in which other angles are used, the length of the sensor itself may be adjusted by standard trigonometric relations so that the actual depth of placement remains the same (e.g., remains comparable to that achieved using a 90 degree angle), e.g., in certain embodiments about 5.0 mm below the surface of the skin, i.e. in the midst of the subcutaneous adipose tissue layer. 
     The use of an angled insertion (i.e. less than about 90 degrees relative to the skin) in the present achieves physical separation of the superficial incision from the position in the tissue at which the sensor will be measuring the analyte of interest. Furthermore, the use of angled insertion may decrease the physical displacement of the sensor itself relative to the subcutaneous adipose tissue layer when physical pressure is applied to the sensor mount and transmitter in the course of a patient&#39;s normal daily living. This may be especially important for minimizing the occurrences of spurious low readings during periods of sleep. 
     The use of an angled insertion in the present invention takes advantage of the stratum corneum&#39;s reduced susceptibility to shear disruption or penetration compared with rupture due to direct normal insertion. Less force is required to penetrate the stratum corneum and the epidermis using an angled insertion than an insertion conducted at normal incidence. The latter may be accompanied by greater degrees of damage to the underlying tissue as well as the release of various chemical messengers active in the wound response of the epidermis and dermis. 
     Embodiments also include devices and methods for determining the thickness of the subcutaneous adipose tissue layer in a given individual at a given anatomical site such as the lower left or right abdominal quadrant or the posterior or lateral upper arm. Such devices and/or algorithms may be integrated with a positioning device or may be separate. For example, in the event that the subcutaneous adipose tissue layer at the desired location for the placement of the sensor is less than or approximately equal to a predetermined amount, e.g., about 5.0 mm, sensor lengths and/or angles which correctly place the active glucose transduction area of the sensor in the middle of the targeted subcutaneous adipose tissue layer may be determined and used. 
     Sensor positioning devices may involve manual, semi-automatic, or automatic operation, referring to the origin of the force that is used both to insert the sensor and to retract any portion of the sensor positioning device out of the skin of the patient that is not intended to remain inserted during the period of sensor operation. Semi-automatic or automatic operation refers to the incorporation of one or more force-generating methods, e.g., wound springs, compressed gas, electromagnet repulsion of a second magnet, and the like, either in combination with manual force or replacing manual force entirely, for the purpose of inserting the sensor and/or retracting any portion of the sensor positioning device out of the skin of the patient that is not intended to remain inserted during the period of sensor operation. 
     In certain embodiments, a plunger-type button is used as the actuation mechanism of an insertion gun. The button serves the purpose of releasing a compressed spring that drives the sharp tip of the sensor positioning device into the skin of the patient at a fast speed, consistent with minimizing pain, so as to create a superficial skin incision that is no deeper than the epidermis. The sharp tip of the positioning device may then be retracted out of the skin of the patient, manually or using a mechanism such as a spring, electromagnet, or the like. The continued travel of the actuator button would then also serve the purpose of manually driving the sensor into the skin, through the incision created by the sharp tip of the positioning device, at a velocity less than that used to create the incision. 
     In certain other embodiments of the device, the insertion gun includes a housing and a carrier. The sensor positioning device is typically mounted on the carrier and the sensor is pre-loaded into the sensor positioning device. The carrier drives the sensor positioning device into the skin of the patient using, for example, a cocked or wound spring, a burst of compressed gas, an electromagnet repelled by a second magnet, and the like, within the insertion gun. The velocity of the carrier may be decreased, after the creation of the superficial skin incision, through mechanical means e.g., viscous dashpots, air damping, friction, the addition of mass to the carrier, or the like. The continued motion of the carrier, for the purpose of inserting the sensor into the incision created by the sharp tip of the positioning device, would then occur at a velocity less than that used to create the incision. The sharp tip of the positioning device may be retracted out of the skin of the patient, either after the creation of the skin incision or after sensor insertion, manually or using a mechanism such as a spring, electromagnet, or the like. 
     Embodiments include a two stage or two velocity sensor inserter device that includes a base, housing, carrier/introducer/sensor assembly, high speed activation button, drive spring, return spring and manual plunger. These inserters may be provided to users fully assembled and armed with a sensor enclosed inside the introducer. 
     In use, the first stage of the insertion may begin by activating the device, e.g., by pressing the plunger and activation button, to cause the introducer to be propelled into the skin at a higher rate of speed than the speed that will be used at the second stage. The introducer makes a “shallow puncture”, but does not release the sensor. 
     The “shallow puncture” depth may be controlled by the height and location of the latch ledge features on the housing, or the type and force (rate) of the drive spring or in other ways such as hard stop, increase of friction, magnets, safety lock, or dial (similar to a lancet device), and the like. The “shallow puncture” or superficial incision may not provide a channel into which the glucose sensor is placed, but rather may provide an opening in the upper layer of the skin only with mechanical strength. 
     After the “shallow puncture” or superficial incision is made through the stratum corneum and epidermis, the return spring retracts the sharp portion of the introducer out of the skin. The overall (uncompressed) height of the return spring positions the introducer/sensor slightly above the surface of the skin (puncture) for the second stage of the insertion. 
     When the first stage is activated (releasing the latches of the carrier mechanism), the high speed button comes to rest in a lower position on top of the housing, thereby leaving the plunger in the “up” and ready position. The introducer having made the puncture is now in the “next” position (with the sensor still intact). 
     The second stage of the insertion may be accomplished manually (e.g., similar to and approximately as slow or slower than injection via syringe) by the user. Pressing down on the plunger causes the introducer/carrier/sensor to move from the “next” position and continue into the shallow puncture until the prescribed sensor insertion depth is reached. The prescribed insertion depth may be controlled by the compressed (solid) height of the return spring or in some other way such as hard stop, adhesive mount, safety lock or other similar restraining or limiting device. 
     When the prescribed depth is reached, the sensor body may be captured by features on an adhesive mount mounted on the patient&#39;s skin and released from the introducer for contact with the transmitter which is connectable to the mount. The insertion is complete when the first phase has provided an opening through the outer layer of the skin and the second phase has resulted in the placement of the sensor at the desired depth in the subcutaneous adipose tissue layer, 
     The user releases the plunger (e.g., by removing their finger) and the return spring causes the introducer to exit the skin and into the “safe for disposal position”. The SDU may then be detached from the mount and discarded accordingly. 
     A sensor insertion such as described above may be accomplished with one hand and without the benefit of direct line of sight. 
     The two stage insertion process may be achieved in one motion, (e.g., by the user pressing the top of the plunger and pushing down until it comes to rest on the top of the housing). However, the user may make a “2 motion-2 stage” insertion (by pressing the plunger, stopping after the high speed button has been activated then pressing the plunger). 
       FIGS. 18A-18B  are a front component view and perspective view, respectively, of an exemplary embodiment of a two stage sensor insertion mechanism including the insertion device armed and ready for insertion, further illustrating the sensor introducer and sensor to make the first stage puncture, and also showing the plunger and the button in accordance with one embodiment of the present invention. Referring to the Figures, the insertion device in one embodiment includes sensor  1801  operatively coupled to a sensor introducer  1802  substantially provided in the housing  1803  of the sensor insertion mechanism. Also shown in the Figures is a trigger button  1804  operatively coupled to a plunger  1805  in one embodiment, and where the actuation of the trigger button  1804  may be configured to deploy the sensor  1801  to a first insertion depth under the skin layer of the patient, guided by the sensor introducer  1802 . 
       FIG. 19A  illustrates a front component view of the two stage sensor insertion mechanism after the firing of the first stage trigger button to achieve the initial puncture, and with the plunger exposed for the second stage insertion activation, and also illustrating the sensor/introducer position after the initial first stage puncture (for example, at 1.55 mm depth) in accordance with one embodiment of the present invention. 
       FIGS. 19B-19D  illustrate a perspective view, a close-up perspective view, and a side view, respectively, of the two stage sensor insertion mechanism after the first stage trigger button firing shown in  FIG. 19A , where the side view shown in  FIG. 19D  further illustrates the configuration of the carrier or the housing  1803  and drive spring  1806  with the plunger  1805  and the trigger button  1804 . 
       FIGS. 20A-20B  illustrate the front component view and the perspective view, respectively, of the two stage sensor insertion mechanism after the sensor placement at the predetermined depth with the plunger depressed down to deliver the sensor to the maximum predetermined depth in accordance with one embodiment of the present invention. 
       FIG. 21  illustrates a front perspective component view of the return spring of the two stage sensor insertion mechanism to retain the sensor introducer in a safe position after sensor insertion in accordance with one embodiment of the present invention, where the return spring may be configured to help retract or remove the introducer from the puncture site after sensor deployment to the predetermined depth. In one embodiment, the return spring and the drive spring  1806  may be integrally formed and disposed in the housing  1803 . Alternatively, in other embodiments, the return spring and the drive spring  1806  may be separate components disposed substantially within the housing  1803  of the insertion mechanism. 
       FIG. 22A  is a perspective view of a first stage sensor introducer mechanism in accordance with one embodiment of the present invention. Referring to  FIG. 22A  the first stage sensor introducer mechanism in one embodiment includes a mounting unit comprising a base portion  2210  and a sensor guide portion  2220 . As shown, the guide portion  2220  of the mounting unit may be coupled to a sensor introducer assembly housing  2240  configured to operatively couple to a sensor introducer deployment section  2230 . In one embodiment, the sensor introducer deployment section  2230  and the sensor introducer assembly housing  2240  may be configured to be detachably removed from the sensor guide portion  2220  of the mounting unit upon actuation of the sensor introducer deployment section  2230  for transcutaneous positioning of the analyte sensor through the skin layer of the patient, for example, so as to place the sensor at the first deployment position. 
     Referring back to  FIG. 22A , in one embodiment, a sensor introducer  2250  ( FIG. 22C ) is provided substantially within the sensor introducer assembly housing  2240  so as to couple to the sensor introducer deployment section  2230 . As such, in one embodiment, the actuation of the sensor introducer deployment section  2230 , for example, by manual depression thereupon with an application of a predetermined amount of force in a substantially downward direction as shown by directional arrow  2280 . In a further embodiment, the actuation of the sensor introducer deployment section  2230  may include an automated or semi-automated mechanism which the patient or the user may deploy. In such an embodiment, the deployment of the automated or semi-automated mechanism (for example, by triggering a switch) is configured to translate the sensor introducer deployment section  2230  in the downward direction so as to transcutaneously position the sensor introducer  2250  through the skin layer of the patient. 
       FIG. 22B  is a front planar view of the first stage sensor introducer mechanism of  FIG. 22A , and  FIG. 22C  shows the sensor introducer  2250  coupled to the sensor introducer deployment section  2230  of  FIG. 22A  in accordance with one embodiment of the present invention. As shown, in one embodiment, the sensor introducer  2250  is operatively coupled to the sensor introducer deployment section  2230  such that the sensor introducer  2250  is moved in a downward direction upon actuation of the sensor introducer deployment section  2230 . Referring to  FIG. 22C , in one embodiment, the sensor introducer  2250  is provided with a tip portion  2260  which is configured to be coupled to a tip portion  2320  ( FIG. 23A ) of the sensor  2310 , and further to puncture through the skin of the patient upon actuation of the sensor introducer deployment section  2230  so as to position the sensor  2310  at a first predetermined subcutaneous position. Thereafter, as discussed in further detail below, the position of the sensor  2310  is further modified, for example, by the coupling of the transmitter unit  2510  ( FIG. 25A ) substantially on the base portion  2210  of the mounting unit. 
     Referring again to  FIG. 22C , also shown is a return spring  2290  substantially provided in the sensor introducer assembly housing  2240 , and coupled to the sensor introducer deployment section  2230 . In this manner, in one embodiment, the sensor introducer  2250  coupled to the sensor introducer deployment section  2230  may be configured to return to its original pre-deployment position after the initial actuation or deployment of the sensor introducer deployment section  2230  such that the sensor introducer  2250  is substantially removed from the sensor guide portion  2220  of the mounting unit. In this manner, upon completion of the first stage sensor positioning using the sensor introducer deployment section  2230 , the sensor introducer deployment section  2230 , the sensor introducer assembly housing  2240  and the sensor introducer  2250  may be detachably removed from the mounting unit. 
     Optionally, in an alternate embodiment, the sensor introducer assembly housing  2240  may be configured to be retained coupled to the sensor guide portion  2220  after the actuation of the sensor introducer deployment section  2230 , such that the sensor introducer assembly housing  2240  may be configured to substantially entirely house or retain the sensor introducer  2250  to avoid contact with the patient, for example. In such configuration, the sensor introducer deployment section  2230  may be configured to be detachably removed from the sensor introducer assembly housing  2240  and discarded after actuation. 
       FIG. 23A  is a front planar view of the sensor in accordance with one embodiment of the present invention. As shown, the sensor includes a body portion  2310  and a tip portion  2320 , where the tip portion  2320  in one embodiment is configured to couple to the tip portion  2260  of the sensor introducer  2250  for transcutaneous positioning. The body portion  2310  in one embodiment is provided with a plurality of contacts for establishing electrical contact with the transmitter unit. Referring again to  FIG. 23A , the body portion  2310  in one embodiment may be provided with an engagement portion  2330  which is configured to mate with a portion of the transmitter unit housing. 
     As described in further detail below, when the transmitter unit housing  2510  is coupled to the mounting unit, in one embodiment, the transmitter unit housing  2510  may be configured to couple to the engagement portion  2330  of the sensor to displace the sensor from the first predetermined position to the second predetermined position. In this manner, in one embodiment, the sensor introducer  2250  ( FIG. 22C ) may be configured to transcutaneously position the sensor at the first predetermined position under the skin layer of the patient, while the transmitter unit may be configured to further displace the sensor from the first predetermined position to the second predetermined position such that the sensor tip portion  2320  is in fluid contact with the patient&#39;s analyte. 
       FIG. 23B  is a side view of the sensor and  FIG. 23C  is a close up view of the tip portion of the sensor shown in  FIG. 23A  in accordance with one embodiment of the present invention. Referring to  FIG. 23B , in one embodiment, the engagement portion  2330  may be configured to protrude from the sensor body portion  2310  so as to engage with the corresponding portion of the transmitter unit. Referring to  FIG. 23C , the sensor tip portion  2320  in one embodiment includes a sharp tip end  2321  to facilitate the movement of the sensor from the first predetermined position to the second predetermined position substantially in response to the force applied upon the engagement portion  2330  of the sensor by the transmitter unit housing  2510 . Moreover, in one embodiment, the sensor tip portion  2320  may also include a rib portion  2322  configured to provide additional rigidity to the sensor tip portion  2320  to aid the insertion process. 
       FIG. 23D  is a perspective view of the sensor introducer and  FIG. 23E  is a close up view of the tip portion of the sensor introducer in the first stage sensor introducer mechanism of  FIG. 22A  in accordance with one embodiment of the present invention. Referring to  FIG. 23E , in one embodiment, the tip portion  2260  of the sensor introducer  2250  includes a sharp edge section  2261  configured to pierce through the skin layer of the patient when the sensor introducer deployment section  2230  is actuated. 
       FIG. 23F  is a front planar view of the sensor and sensor introducer,  FIG. 23G  is a perspective view of the sensor and sensor introducer, and  FIG. 23H  is a close up view of the tip portion of the sensor and sensor introducer in accordance with one embodiment of the present invention. Moreover,  FIG. 24  is a front planar view of the first stage of sensor insertion using the sensor introducer mechanism of  FIG. 22A  in accordance with one embodiment of the present invention. As can be seen, in one embodiment, the tip portion  2320  of the sensor is substantially provided within the tip portion  2260  of the sensor introducer  2250  such that when the sensor introducer tip portion  2260  pierces through the skin of the patient, the tip portion  2320  of the sensor is configured to transcutaneously move with the movement of the sensor introducer  2250 . 
       FIG. 25A  is a side view of a transmitter unit and  FIG. 25B  is a perspective view of the transmitter unit of  FIG. 25A  in accordance with one embodiment of the present invention. Referring to  FIGS. 25A-25B , in one embodiment, the transmitter unit  2510  includes a plurality of contacts  2520  each configured to establish electrical contact with a corresponding one of a plurality of contacts disposed on the sensor body  2310 . Referring again to  FIG. 25A , the transmitter unit  2510  in one embodiment includes a guide section  2530 . In one embodiment, the guide section  2530  is configured to correspondingly couple to the engagement portion  2330  of the sensor during positioning of the transmitter unit  2510  to couple to the mounting unit. In this manner, in one embodiment, the positioning of the transmitter unit  2510  on the mounting unit provides sufficient force applied on the sensor (and in particular, at the engagement portion  2330  of the sensor) to displace the sensor from the first predetermined position to the second predetermined position. 
     Referring again to the Figures, a temperature detection section  2540  may in one embodiment be provided to the lower surface of the transmitter unit  2510  so as to be in physical contact with the patient&#39;s skin when the transmitter unit  2510  is coupled to the mounting unit. In this manner, the transmitter unit  2510  may be configured to monitor the on skin temperature of the patient, for example, in analyzing and processing signals received from the sensor associated with the detected analyte levels. 
       FIG. 25C  is a side view of the transmitter unit engaged with the sensor for the second stage sensor insertion in accordance with one embodiment of the present invention. Furthermore,  FIG. 25D  is a side view of the transmitter unit and  FIG. 25E  is a perspective view of the transmitter unit mounted to the mounting in accordance with one embodiment of the present invention. Referring to  FIG. 25C , in one embodiment, the positioning of the transmitter unit  2510  to couple to the mounting unit correspondingly engages the guide section  2530  of the transmitter unit  2510  with the engagement portion  2330  of the sensor (where the sensor is already transcutaneously positioned at the first predetermined position by the sensor introducer  2250 ), and with the aid of the sharp tip end  2321 , positions the tip portion  2320  of the sensor at the second predetermined position in fluid contact with the patient&#39;s analyte. 
       FIG. 26A  is a perspective view of the sensor in the final position (second predetermined position) with respect to the sensor introducer mechanism without the transmitter unit, and  FIG. 26B  is a front planar view of the sensor in the final position shown in  FIG. 26A  in accordance with one embodiment of the present invention. 
     In the manner described above, in particular embodiments, the analyte sensor deployment includes a two stage insertion process where the first transcutaneous placement is achieved by the sensor introducer  2250  at a substantially high velocity, and thereafter, a second subsequent positioning of the sensor is obtained using the manual force applied upon the transmitter unit  2510  when the transmitter unit  2510  is coupled to the mounting unit. In this manner, in one embodiment, actual or perceived pain or trauma associated with the initial skin puncture to trancutaneously position the sensor through the skin layer of the patient is substantially minimized using a high speed introduction mechanism, while the subsequent final positioning of the sensor is thereafter achieved at a relatively slower speed (for example, using manual force applied upon the transmitter unit  2510 ). 
     Referring back to  FIG. 1 , the on-skin sensor control unit  44  is configured to be placed on the skin of a patient. One embodiment of the on-skin sensor control unit  44  has a thin, oval shape to enhance concealment, as illustrated in  FIGS. 9-11 . However, other shapes and sizes may be used. The base  74  and cover  76  of the on-skin sensor control unit  44  are formed such that, when the sensor  42  is within the on-skin sensor control unit  44  and the base  74  and cover  76  are fitted together, the sensor  42  is bent. The sensor  42  may be inserted into the subcutaneous tissue of the patient through the sensor port  78 . The on-skin sensor control unit  44  includes a housing  45 , as illustrated in  FIGS. 9-11 .  FIG. 9  is a cross-sectional view of the on-skin sensor control unit taken along lines  14 - 14  of  FIGS. 10-11 . The on-skin sensor control unit  44  is typically attachable to the skin  75  of the patient, as illustrated in  FIG. 12 . Another method of attaching the housing  45  of the on-skin sensor control unit  44  to the skin  75  includes using a mounting unit  77  with opening  79 . 
     The sensor  42  and the electronic components within the on-skin sensor control unit  44  are coupled via conductive contacts  80 . The one or more working electrodes  58 , counter electrode  60  (or counter/reference electrode), optional reference electrode  62 , and optional temperature probe  66  are attached to individual conductive contacts  80 . The on-skin sensor control unit  44  may optionally contain a support structure  82  to hold, support, and/or guide the sensor  42  into the correct position. In the illustrated embodiment of  FIGS. 9-11 , the conductive contacts  80  are provided on the interior of the on-skin sensor control unit  44 . 
     Referring back to the Figures, the on-skin sensor control unit  44  may include at least a portion of the electronic components that operate the sensor  42  and the analyte monitoring device system  40 . One embodiment of the electronics in the on-skin control unit  44  is illustrated as a block diagram in  FIG. 13A . The electronic components of the on-skin sensor control unit  44  may include a power supply  95  for operating the on-skin control unit  44  and the sensor  42 , a sensor circuit  97  for obtaining signals from and operating the sensor  42 ,  42 ′, a measurement circuit  96  that converts sensor signals to a desired format, and a processing circuit  109  that, at minimum, obtains signals from the sensor circuit  97  and/or measurement circuit  96  and provides the signals to an optional transmitter  98 . In some embodiments, the processing circuit  109  may also partially or completely evaluate the signals from the sensor  42  and convey the resulting data to the optional transmitter  98  and/or activate an optional alarm system  94  (see  FIG. 13B ) if the analyte level exceeds a threshold. The processing circuit  109  often includes digital logic circuitry. 
     The on-skin sensor control unit  44  may optionally contain a transmitter or transceiver  98  for transmitting the sensor signals or processed data from the processing circuit  109  to receiver (or transceiver)/display units  46 ,  48 ; a data storage unit  102  for temporarily or permanently storing data from the processing circuit  109 ; a temperature probe circuit  99  for receiving signals from and operating a temperature probe  66 ; a reference voltage generator  101  for providing a reference voltage for comparison with sensor-generated signals; and/or a watch dog circuit  103  that monitors the operation of the electronic components in the on-skin sensor control unit  44 . 
     Moreover, the sensor control unit  44  may include a bias control generator  105  to correctly bias analog and digital semiconductor devices, an oscillator  107  to provide a clock signal, and a digital logic and timing component to provide timing signals and logic operations for the digital components of the circuit. 
       FIG. 13B  illustrates a block diagram of another exemplary on-skin control unit  44  that also includes optional components such as a receiver (or transceiver)  110  to receive, for example, calibration data; a calibration storage unit (not shown) to hold, for example, factory-set calibration data, calibration data obtained via the receiver  110  and/or operational signals received, for example, from a receiver/display unit  46 ,  48  or other external device; an alarm system  94  for warning the patient; and a deactivation switch  111  to turn off the alarm system. 
     The electronics in the on-skin sensor control unit  44  and the sensor  42 ,  42 ′ are operated using a power supply  95 . The sensor control unit  44  may also optionally include a temperature probe circuit  99 . 
     The output from the sensor circuit  97  and optional temperature probe circuit is coupled into a measurement circuit  96  that obtains signals from the sensor circuit  97  and optional temperature probe circuit  99  and, at least in some embodiments, provides output data in a form that, for example can be read by digital circuits. 
     In some embodiments, the data from the processing circuit  109  is analyzed and directed to an alarm system  94  (see  FIG. 13B ) to warn the user. 
     In some embodiments, the data (e.g., a current signal, a converted voltage or frequency signal, or fully or partially analyzed data) from processing circuit  109  is transmitted to one or more receiver/display units  46 ,  48  using a transmitter  98  in the on-skin sensor control unit  44 . The transmitter has an antenna  93 , such as a wire or similar conductor, formed in the housing  45 . 
     In addition to a transmitter  98 , an optional receiver  110  may be included in the on-skin sensor control unit  44 . In some cases, the transmitter  98  is a transceiver, operating as both a transmitter and a receiver. The receiver  110  (and/or receiver display/units  46 ,  48 ) may be used to receive calibration data for the sensor  42 . The calibration data may be used by the processing circuit  109  to correct signals from the sensor  42 . This calibration data may be transmitted by the receiver/display unit  46 ,  48  or from some other source such as a control unit in a doctor&#39;s office. 
     Calibration data may be obtained in a variety of ways. For instance, the calibration data may simply be factory-determined calibration measurements which can be input into the on-skin sensor control unit  44  using the receiver  110  or may alternatively be stored in a calibration data storage unit within the on-skin sensor control unit  44  itself or elsewhere such as, e.g., receiver display/units  46 ,  48 , (in which case a receiver  110  may not be needed). The calibration data storage unit may be, for example, a readable or readable/writeable memory circuit. 
     Alternative or additional calibration data may be provided based on tests performed by a doctor or some other professional or by the patient himself. For example, it is common for diabetic individuals to determine their own blood glucose concentration using commercially available testing kits. The result of this test is input into the on-skin sensor control unit  44  (and/or receiver display/units  46 ,  48 ) either directly, if an appropriate input device (e.g., a keypad, an optical signal receiver, or a port for connection to a keypad or computer) is incorporated in the on-skin sensor control unit  44 , or indirectly by inputting the calibration data into the receiver/display unit  46 ,  48  and transmitting the calibration data to the on-skin sensor control unit  44 . 
     Other methods of independently determining analyte levels may also be used to obtain calibration data. This type of calibration data may supplant or supplement factory-determined calibration values. 
     In some embodiments of the invention, calibration data may be required at periodic intervals, for example, about every ten hours, eight hours, about once a day, or about once a week, to confirm that accurate analyte levels are being reported. Calibration may also be required each time a new sensor  42  is implanted or if the sensor exceeds a threshold minimum or maximum value or if the rate of change in the sensor signal exceeds a threshold value. In some cases, it may be necessary to wait a period of time after the implantation of the sensor  42  before calibrating to allow the sensor  42  to achieve equilibrium. In some embodiments, the sensor  42  is calibrated only after it has been inserted. In other embodiments, no calibration of the sensor  42  is needed (e.g., a factory calibration may be sufficient). 
     Regardless of the type of analyte monitoring system employed, it has been observed that transient, low readings may occur for a period of time. These anomalous low readings may occur during the first hours of use, or anytime thereafter. In certain embodiments, spurious low readings may occur during the night and may be referred to as “night time dropouts”. For example, in the context of an operably positioned continuous monitoring analyte sensor under the skin of a user, such spurious low readings may occur for a period of time following sensor positioning and/or during the first night post-positioning. In many instances, the low readings resolve after a period of time. However, these transient, low readings put constraints on analyte monitoring during the low reading period. Attempts to address this problem vary and include delaying calibration and/or reporting readings to the user until after this period of low readings passes after positioning of the sensor or frequent calibration of the sensor—both of which are inconvenient and neither of which are desirable. 
     However, as noted above embodiments of the subject invention have at least a minimal period, if at all, of spurious low readings, i.e., a substantially reduced sensor equilibration period, including substantially no equilibration period. In this regard, in those embodiments in which an initial post-positioning calibration is required, such may be performed substantially immediately after sensor positioning. For example, in certain embodiments a calibration protocol may include a first post-positioning calibration at less than about 10 hours after a sensor has been operably positioned, e.g., less than about 5 hours, e.g., less than about 3 hours, e.g., less than about 1 hour, e.g., less than about 0.5 hours. One or more additional calibrations may not be required, or may be performed at suitable times thereafter. 
     The on-skin sensor control unit  44  may include an optional data storage unit  102  which may be used to hold data (e.g., measurements from the sensor or processed data). 
     In some embodiments of the invention, the analyte monitoring device  40  includes only an on-skin control unit  44  and a sensor  42 . 
     One or more receiver/display units  46 ,  48  may be provided with the analyte monitoring device  40  for easy access to the data generated by the sensor  42  and may, in some embodiments, process the signals from the on-skin sensor control unit  44  to determine the concentration or level of analyte in the subcutaneous tissue. The receiver may be a transceiver. Receivers may be palm-sized and/or may be adapted to fit on a belt or within a bag or purse that the patient carries. 
     The receiver/display units  46 ,  48 , as illustrated in block form at  FIG. 14 , typically include a receiver  150  to receive data from the on-skin sensor control unit  44 , an analyzer  152  to evaluate the data, a display  154  to provide information to the patient, and an alarm system  156  to warn the patient when a condition arises. The receiver/display units  46 ,  48  may also optionally include a data storage device  158 , a transmitter  160 , and/or an input device  162 . 
     Data received by the receiver  150  is then sent to an analyzer  152 . 
     The output from the analyzer  152  is typically provided to a display  154 . The receiver/display units  46 ,  48  may also include a number of optional items such as a data storage unit  158  to store data, a transmitter  160  which can be used to transmit data, and an input device  162 , such as a keypad or keyboard. 
     In certain embodiments, the receiver/display unit  46 ,  48  is integrated with a calibration unit (not shown). For example, the receiver/display unit  46 ,  48  may, for example, include a conventional blood glucose monitor. Devices may be used including those that operate using, for example, electrochemical and colorimetric blood glucose assays, assays of interstitial or dermal fluid, and/or non-invasive optical assays. When a calibration of the implanted sensor is needed, the patient uses the integrated in vitro monitor to generate a reading. The reading may then, for example, automatically be sent by the transmitter  160  of the receiver/display unit  46 ,  48  to calibrate the sensor  42 . 
     In certain embodiments, analyte data (processed or not) may be forwarded (such as by communication) to a remote location such as a doctor&#39;s office if desired, and received there for further use (such as further processing). 
     Integration with a Drug Administration System 
     The subject invention also includes sensors used in sensor-based drug delivery systems. The system may provide a drug to counteract the high or low level of the analyte in response to the signals from one or more sensors. Alternatively, the system may monitor the drug concentration to ensure that the drug remains within a desired therapeutic range. The drug delivery system may include one or more (e.g., two or more) sensors, a sensor positioning device, an on-skin sensor control unit, a receiver/display unit, a data storage and controller module, and a drug administration system. In some cases, the receiver/display unit, data storage and controller module, and drug administration system may be integrated in a single unit. The sensor-based drug delivery system may use data from the one or more sensors to provide necessary input for a control algorithm/mechanism in the data storage and controller module to adjust the administration of drugs. As an example, a glucose sensor could be used to control and adjust the administration of insulin. According to certain embodiments of the subject invention, accurate data from the one or more sensors may be obtained substantially immediately after sensor positioning to provide necessary input for a control algorithm/mechanism in the data storage and controller module to adjust the administration of drugs substantially immediately. 
     Kits 
     Finally, kits for use in practicing the subject invention are also provided. The subject kits may include one or more sensors as described herein. Embodiments may also include a sensor and/or a sensor positioning device and/or transmitter and/or receiver and/or anesthetic agent, which may or may not be independent of the sensor and/or sensor positioning device. 
     In addition to one or more of the above-described components, the subject kits may also include written instructions for using a sensor, e.g., positioning a sensor using a sensor positioning device and/or using a sensor to obtain analyte information. The instructions may be printed on a substrate, such as paper or plastic, etc. As such, the instructions may be present in the kits as a package insert, in the labeling of the container of the kit or components thereof (i.e., associated with the packaging or sub-packaging) etc. In other embodiments, the instructions are present as an electronic storage data file present on a suitable computer readable storage medium, e.g., CD-ROM, diskette, etc. In yet other embodiments, the actual instructions are not present in the kit, but means for obtaining the instructions from a remote source, e.g., via the Internet, are provided. An example of this embodiment is a kit that includes a web address where the instructions can be viewed and/or from which the instructions can be downloaded. As with the instructions, this means for obtaining the instructions is recorded on a suitable substrate. 
     In many embodiments of the subject kits, the components of the kit are packaged in a kit containment element to make a single, easily handled unit, where the kit containment element, e.g., box or analogous structure, may or may not be an airtight container, e.g., to further preserve the one or more sensors and additional reagents (e.g., control solutions), if present, until use. 
     Various other modifications and alterations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.