Abstract:
An analyte monitor includes a sensor, a sensor control unit, and a display unit. The sensor control unit typically has a housing adapted for placement on skin and is adapted to receive a portion of an electrochemical sensor. The sensor control unit also includes two or more conductive contacts disposed on the housing and configured for coupling to two or more contact pads on the sensor. A transmitter is disposed in the housing and coupled to the plurality of conductive contacts for transmitting data obtained using the sensor. The display unit has a receiver for receiving data transmitted by the transmitter of the sensor control unit and a display coupled to the receiver for displaying an indication of a level of an analyte, such as blood glucose. An inserter having a retractable introducer is provided for subcutaneously implanting the sensor in a predictable and reliable fashion.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This non-provisional application is related to and claims priority based on U.S. Provisional Application No. 60/424,099, entitled “Sensor Inserter Device and Methods of Use,” filed on Nov. 5, 2002, which is incorporated herein in its entirety by this reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is, in general, directed to devices and methods for the in vivo monitoring of an analyte, such as glucose or lactate, using a sensor to provide information to a patient about the level of the analyte. More particularly, the present invention relates to devices and methods for inserting a subcutaneously implantable electrochemical sensor in a patient for such monitoring. 
       BACKGROUND OF THE INVENTION 
       [0003]    The monitoring of the level of glucose or other analytes, such as lactate or oxygen, in certain individuals is vitally important to their health. High or low levels of glucose or other analytes may have detrimental effects. 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. 
         [0004]    A conventional technique used by many diabetics for personally monitoring their blood glucose level includes the periodic drawing of blood, the application of that blood to a test strip, and the determination of the blood glucose level using colorimetric, electrochemical, or photometric detection. This technique does not permit continuous or automatic monitoring of glucose levels in the body, but typically must be performed manually on a periodic basis. Unfortunately, the consistency with which the level of glucose is checked varies widely among individuals. Many diabetics find the periodic testing inconvenient and they sometimes forget to test their glucose level or do not have time for a proper test. In addition, some individuals wish to avoid the pain associated with the test. These situations may result in hyperglycemic or hypoglycemic episodes. An in vivo glucose sensor that continuously or automatically monitors the individual&#39;s glucose level would enable individuals to more easily monitor their glucose, or other analyte, levels. 
         [0005]    A variety of devices have been developed for continuous or automatic monitoring of analytes, such as glucose, in the blood stream or interstitial fluid. A number of these devices use electrochemical sensors which are directly implanted into a blood vessel or in the subcutaneous tissue of a patient. However, these devices are often difficult to reproducibly and inexpensively manufacture in large numbers. In addition, these devices are typically large, bulky, and/or inflexible, and many can not be used effectively outside of a controlled medical facility, such as a hospital or a doctor&#39;s office, unless the patient is restricted in his activities. 
         [0006]    Some devices include a sensor guide which rests on or near the skin of the patient and may be attached to the patient to hold the sensor in place. These sensor guides are typically bulky and do not allow for freedom of movement. In addition, the sensor guides or the sensors include cables or wires for connecting the sensor to other equipment to direct the signals from the sensors to an analyzer. The size of the sensor guides and presence of cables and wires hinders the convenient use of these devices for everyday applications. The patient&#39;s comfort and the range of activities that can be performed while the sensor is implanted are important considerations in designing extended-use sensors for continuous or automatic in vivo monitoring of the level of an analyte, such as glucose. There is a need for a small, comfortable device which can continuously monitor the level of an analyte, such as glucose, while still permitting the patient to engage in normal activities. Continuous and/or automatic monitoring of the analyte can provide a warning to the patient when the level of the analyte is at or near a threshold level. For example, if glucose is the analyte, then the monitoring device might be configured to warn the patient of current or impending hyperglycemia or hypoglycemia. The patient can then take appropriate actions. 
       SUMMARY OF THE INVENTION 
       [0007]    Generally, the present invention relates to methods and devices for the continuous and/or automatic in vivo monitoring of the level of an analyte using a subcutaneously implantable sensor. Many of these devices are small and comfortable when used, thereby allowing a wide range of activities. One embodiment includes a sensor control unit having a housing adapted for placement on skin. The housing is also adapted to receive a portion of an electrochemical sensor. The sensor control unit includes two or more conductive contacts disposed on the housing and configured for coupling to two or more contact pads on the sensor. A transmitter is disposed in the housing and coupled to the plurality of conductive contacts for transmitting data obtained using the sensor. The sensor control unit may also include a variety of optional components, such as, for example, adhesive for adhering to the skin, a mounting unit, a receiver, a processing circuit, a power supply (e.g., a battery), an alarm system, a data storage unit, a watchdog circuit, and a measurement circuit. The sensor itself has at least one working electrode and at least one contact pad coupled to the working electrode or electrodes. The sensor may also include optional components, such as, for example, a counter electrode, a counter/reference electrode, a reference electrode, and a temperature probe. The analyte monitoring system also includes a display unit that has a receiver for receiving data from the sensor control unit and a display coupled to the receiver for displaying an indication of the level of an analyte. The display unit may optionally include a variety of components, such as, for example, a transmitter, an analyzer, a data storage unit, a watchdog circuit, an input device, a power supply, a clock, a lamp, a pager, a telephone interface, a computer interface, an alarm or alarm system, a radio, and a calibration unit. In addition, the analyte monitoring system or a component of the analyte monitoring system may optionally include a processor capable of determining a drug or treatment protocol and/or a drug delivery system. 
         [0008]    According to one aspect of the invention, an insertion kit is disclosed for inserting an electrochemical sensor into a patient. The insertion kit includes an introducer. A portion of the introducer has a sharp, rigid, planer structure adapted to support the sensor during insertion of the electrochemical sensor. The insertion kit also includes an insertion gun having a port configured to accept the electrochemical sensor and the introducer. The insertion gun has a driving mechanism for driving the introducer and electrochemical sensor into the patient, and a retraction mechanism for removing the introducer while leaving the sensor within the patient. 
         [0009]    According to another aspect of the invention, a method of using an electrochemical sensor is disclosed. A mounting unit is adhered to skin of a patient. An insertion gun is aligned with a port on the mounting unit. The electrochemical sensor is disposed within the insertion gun and then the electrochemical sensor is inserted into the skin of the patient using the insertion gun. The insertion gun is removed and a housing of the sensor control unit is mounted on the mounting base. A plurality of conductive contacts disposed on the housing is coupled to a plurality of contact pads disposed on the electrochemical sensor to prepare the sensor for use. 
         [0010]    The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The Figures and the detailed description which follow more particularly exemplify these embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    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: 
           [0012]      FIG. 1  is a block diagram of one embodiment of a subcutaneous analyte monitor using a subcutaneously implantable analyte sensor, according to the invention. 
           [0013]      FIG. 2  is a top view of one embodiment of an analyte sensor, according to the invention. 
           [0014]      FIG. 3  is an expanded side view of one embodiment of a sensor and an introducer, according to the invention. 
           [0015]      FIGS. 4A ,  4 B,  4 C are cross-sectional views of three embodiments of the introducer of  FIG. 3 . 
           [0016]      FIG. 5  is a cross-sectional view of one embodiment of a on-skin sensor control unit, according to the invention. 
           [0017]      FIG. 6  is a top view of a base of the on-skin sensor control unit of  FIG. 5 . 
           [0018]      FIG. 7  is a bottom view of a cover of the on-skin sensor control unit of  FIG. 5 . 
           [0019]      FIG. 8  is a perspective view of the on-skin sensor control unit of  FIG. 5  on the skin of a patient. 
           [0020]      FIG. 9  is a perspective view of the internal structure of an insertion gun, according to the invention. 
           [0021]      FIG. 10A  is a top view of one embodiment of an on-skin sensor control unit, according to the invention. 
           [0022]      FIG. 10B  is a top view of one embodiment of a mounting unit of the on-skin sensor control unit of  FIG. 10A . 
           [0023]      FIG. 11A  is a top view of another embodiment of an on-skin sensor control unit after insertion of an introducer and a sensor, according to the invention. 
           [0024]      FIG. 11B  is a top view of one embodiment of a mounting unit of the on-skin sensor control unit of  FIG. 11A . 
           [0025]      FIG. 11C  is a top view of one embodiment of a housing for at least a portion of the electronics of the on-skin sensor control unit of  FIG. 11A . 
           [0026]      FIG. 11D  is a bottom view of the housing of  FIG. 11C . 
           [0027]      FIG. 11E  is a top view of the on-skin sensor control unit of  FIG. 11A  with a cover of the housing removed. 
           [0028]      FIG. 12  depicts an introducer, sensor, insertion gun and mounting unit, which can be assembled and sold together in an insertion kit. 
           [0029]      FIG. 13  is a perspective view showing a preferred commercial embodiment of a sensor inserter and adhesive mount constructed according to the invention. 
           [0030]      FIG. 14  is a perspective view of the adhesive mount and sensor attached to the patient&#39;s skin. 
           [0031]      FIG. 15  is a perspective view of the transmitter attached to the adhesive mount. 
           [0032]      FIG. 16  is an exploded perspective view of the preferred commercial embodiment of  FIG. 13 . 
           [0033]      FIG. 17  is a side elevation view of the preferred commercial embodiment of  FIG. 13 . 
           [0034]      FIG. 18  is an end elevation view of the preferred commercial embodiment of  FIG. 13 . 
           [0035]      FIG. 19  is a cross-sectional view taken along line  19 - 19  in  FIG. 18 . 
           [0036]      FIG. 20  is a cross-sectional view taken along line  20 - 20  in  FIG. 17 . 
           [0037]      FIG. 21  is a broken away view similar to  FIG. 20 , showing the shuttle in the neutral position. 
           [0038]      FIG. 22  is a broken away view similar to  FIG. 20 , showing the shuttle in the cocked position. 
           [0039]      FIG. 23  is a broken away view similar to  FIG. 20 , showing the shuttle in the insertion position. 
           [0040]      FIG. 24  is a cross-sectional view taken along line  24 - 24  in  FIG. 17 . 
           [0041]      FIG. 25  is a perspective view of a transcutaneously implantable sensor. 
           [0042]      FIG. 26A  is a perspective view of a sensor introducer. 
           [0043]      FIG. 26B  is a bottom view of the introducer shown in  FIG. 26A . 
           [0044]      FIG. 27  is a perspective view of a shuttle member. 
           [0045]      FIG. 28  is a top plan view of an oversized adhesive tape. 
           [0046]      FIG. 29  is a perspective view of the transmitter attached to the adhesive mount and showing the sensor sandwiched therebetween. 
           [0047]      FIG. 30  is a perspective view of the interconnect on one end of the transmitter. 
           [0048]      FIG. 31  is an enlarged perspective view of the interconnect of  FIG. 30  with the seal and one spring removed for clarity. 
           [0049]      FIG. 32  is an enlarged perspective view of the interconnect seal. 
           [0050]      FIG. 33A  is a perspective view of an alternative embodiment of a sensor inserter kit. 
           [0051]      FIG. 33B  is an exploded view of some of the components shown assembled in  FIG. 33A . 
       
    
    
       [0052]    While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0053]    The present invention is applicable to an analyte monitoring system using an implantable sensor for the in vivo determination of a concentration of an analyte, such as glucose or lactate, in a fluid. The sensor can be, for example, subcutaneously implanted in a patient for the continuous or periodic monitoring an analyte in a patient&#39;s interstitial fluid. This can then be used to infer the glucose level in the patient&#39;s bloodstream. Other in vivo analyte sensors can be made, according to the invention, for insertion into a vein, artery, or other portion of the body containing fluid. The analyte monitoring system is typically configured for monitoring the level of the analyte over a time period which may range from days to weeks or longer. 
         [0054]    The 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 the analyte monitoring system may depend on the use for which the analyte monitoring system is intended and the conditions under which the analyte monitoring system will operate. One embodiment of the analyte monitoring system includes a sensor configured for implantation into a patient or user. 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 preferred, 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 sensor for use in the analyte monitoring systems of the invention are described in U.S. patent application Ser. No. 09/034,372 and Ser. No. 09/753,746 (the complete parent application to this CIP), both incorporated herein by reference. 
         [0055]    One embodiment of the analyte monitoring system  40  for use with an implantable sensor  42 , and particularly 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 , a portion of 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 coupled to the sensor control unit  44  which is typically attached 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, a electrical shock can 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. 
         [0056]    A sensor  42  includes at least one working electrode  58  formed on 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 . The substrate  50  of the sensor 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. 
         [0057]    In some embodiments, the substrate is flexible. For example, if the sensor  42  is configured for implantation into a patient, then the sensor  42  may be made flexible (although rigid sensors may also be used for implantable sensors) to reduce pain to the patient and damage to the tissue caused by the implantation of and/or the wearing of the sensor  42 . A flexible substrate  50  often increases the patient&#39;s comfort and allows a wider range of activities. Suitable materials for a flexible substrate  50  include, for example, non-conducting plastic or polymeric materials and other non-conducting, flexible, deformable materials. Examples of useful plastic or polymeric materials include thermoplastics such as polycarbonates, polyesters (e.g., Mylar™ and polyethylene terephthalate (PET)), polyvinyl chloride (PVC), polyurethanes, polyethers, polyamides, polyimides, or copolymers of these thermoplastics, such as PETG (glycol-modified polyethylene terephthalate). 
         [0058]    In other embodiments, the sensors  42  are made using a relatively rigid substrate  50  to, for example, provide structural support against bending or breaking. Examples of rigid materials that may be used as the substrate  50  include poorly conducting ceramics, such as aluminum oxide and silicon dioxide. One advantage of an implantable sensor  42  having a rigid substrate is that the sensor  42  may have a sharp point and/or a sharp edge to aid in implantation of a sensor  42  without an additional introducer. 
         [0059]    It will be appreciated that for many sensors  42  and sensor applications, both rigid and flexible sensors will operate adequately. The flexibility of the sensor  42  may also be controlled and varied along a continuum by changing, for example, the composition and/or thickness of the substrate  50 . 
         [0060]    In addition to considerations regarding flexibility, it is often desirable that implantable sensors  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. 
         [0061]    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 . In these embodiments, the distal end  67  of the substrate  50  may have a relatively narrow width  53 . For sensors  42  which are implantable into the subcutaneous tissue or another portion of a patient&#39;s body, the narrow width  53  of the distal end  67  of the substrate  50  may facilitate the implantation of the sensor  42 . Often, the narrower the width of the sensor  42 , the less pain the patient will feel during implantation of the sensor and afterwards. The sensor  42  is designed to be a replaceable component in an implantable analyte monitor. Typically, the sensor  42  is capable of operation over a period of days. Preferably, the period of operation is at least three days. The sensor  42  can then be removed and replaced with a new sensor. 
         [0062]    An introducer  120  can be used to subcutaneously insert the sensor  42  into the patient, as illustrated in  FIG. 3 . The introducer  120  is typically formed using structurally rigid materials, such as metal or rigid plastic. Preferred materials include stainless steel and ABS (acrylonitrile-butadiene-styrene) plastic. In some embodiments, the introducer  120  is pointed and/or sharp at the tip  121  to facilitate penetration of the skin of the patient. A sharp, thin introducer may reduce pain felt by the patient upon insertion of the sensor  42 . In other embodiments, the tip  121  of the introducer  120  has other shapes, including a blunt or flat shape. These embodiments may be particularly useful when the introducer  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. 
         [0063]    The introducer  120  may have a variety of cross-sectional shapes, as shown in  FIGS. 4A ,  4 B, and  4 C. The introducer  120  illustrated in  FIG. 4A  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 introducers  120  of  FIGS. 4B and 4C  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 introducers  120  illustrated in  FIGS. 4B and 4C  is typically 1 mm or less, preferably 700 μm or less, more preferably 500 μm or less, and most preferably 300 μm or less. The cross-sectional height  126  of the introducer  120  illustrated in  FIGS. 4B and 4C  is typically about 1 mm or less, preferably about 700 μM or less, and more preferably about 500 μm or less. 
         [0064]    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. 3 . 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  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 introducer to apply pressure against the sensor  42  during insertion, but disengage as the introducer  120  is removed. One example of such a structure in the insertion device is a rod (not shown) between two opposing sides of an introducer  120  and at an appropriate height of the introducer  120 . 
         [0065]    In operation, the sensor  42  is placed within or next to the introducer  120  and then a force is provided against the introducer  120  and/or sensor  42  to carry the sensor  42  into the skin of the patient. In one embodiment, the force is applied to the sensor  42  to push the sensor into the skin, while the introducer  120  remains stationary and provides structural support to the sensor  42 . Alternatively, the force is applied to the introducer  120  and optionally to the sensor  42  to push a portion of both the sensor  42  and the introducer  120  through the ski of the patient and into the subcutaneous tissue. The introducer  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. 
         [0066]    The force applied to the introducer  120  and/or the sensor  42  may be applied manually or mechanically. Preferably, the sensor  42  is reproducibly inserted through the skin of the patient. In one embodiment, an insertion gun is used to insert the sensor. One example of an insertion gun  200  for inserting a sensor  42  is shown in  FIG. 9 . The insertion gun  200  includes a housing  202  and a carrier  204 . The introducer  120  is typically mounted on the carrier  204  and the sensor  42  is pre-loaded into the introducer  120 . The carrier  204  drives the sensor  42  and, optionally, the introducer  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 introducer  120  may be moved, cocked, or otherwise prepared to be directed towards the skin of the patient. 
         [0067]    After the sensor  42  is inserted, the insertion gun  200  may contain a mechanism which pulls the introducer  120  out of the skin of the patient. Such a mechanism may use a spring, electromagnet, or the like to remove the introducer  120 . 
         [0068]    The insertion gun may be reusable. The introducer  120  is often disposable to avoid the possibility of contamination. Alternatively, the introducer  120  may be sterilized and reused. In addition, the introducer  120  and/or the sensor  42  may be coated with an anticlotting agent to prevent fouling of the sensor  42 . 
         [0069]    In one embodiment, the sensor  42  is injected between 2 to 12 mm into the interstitial tissue of the patient for subcutaneous implantation. Preferably, the sensor is injected 3 to 9 mm, and more preferably 5 to 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. 
         [0070]    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  can 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. 
         [0071]    An insertion angle is measured from the plane of the skin (i.e., inserting the sensor perpendicular to the skin would be a 90° insertion angle). Insertion angles usually range from 10 to 90°, typically from 15 to 60°, and often from 30 to 45°. 
       On-Skin Sensor Control Unit 
       [0072]    The on-skin sensor control unit  44  is configured to be placed on the skin of a patient. The on-skin sensor control unit  44  is optionally formed in a shape that is comfortable to the patient and which may permit concealment, for example, under a patient&#39;s clothing. The thigh, leg, upper arm, shoulder, or abdomen are convenient parts of the patient&#39;s body for placement of the on-skin sensor control unit  44  to maintain concealment. However, the on-skin sensor control unit  44  may be positioned on other portions of the patient&#39;s body. One embodiment of the on-skin sensor control unit  44  has a thin, oval shape to enhance concealment, as illustrated in  FIGS. 5-7 . However, other shapes and sizes may be used. 
         [0073]    The particular profile, as well as the height, width, length, weight, and volume of the on-skin sensor control unit  44  may vary and depends, at least in part, on the components and associated functions included in the on-skin sensor control unit  44 , as discussed below. For example, in some embodiments, the on-skin sensor control unit  44  has a height of 1.3 cm or less, and preferably 0.7 cm or less. In some embodiments, the on-skin sensor control unit  44  has a weight of 90 grams or less, preferably 45 grams or less, and more preferably 25 grams or less. In some embodiments, the on-skin sensor control unit  44  has a volume of about 15 cm 3  or less, preferably about 10 cm 3  or less, more preferably about 5 cm 3  or less, and most preferably about 2.5 cm 3  or less. 
         [0074]    The on-skin sensor control unit  44  includes a housing  45 , as illustrated in  FIGS. 5-7 . The housing  45  is typically formed as a single integral unit that rests on the skin of the patient. The housing  45  typically contains most or all of the electronic components, described below, of the on-skin sensor control unit  44 . The on-skin sensor control unit  44  usually includes no additional cables or wires to other electronic components or other devices. If the housing includes two or more parts, then those parts typically fit together to form a single integral unit. 
         [0075]    In some embodiments, conductive contacts  80  are provided on the exterior of the housing  45 . In other embodiments, the conductive contacts  80  are provided on the interior of the housing  45 , for example, within a hollow or recessed region. 
         [0076]    In some embodiments, the housing  45  of the on-skin sensor control unit  44  is a single piece. The conductive contacts  80  may be formed on the exterior of the housing  45  or on the interior of the housing  45  provided there is a port  78  in the housing  45  through which the sensor  42  can be directed to access the conductive contacts  80 . 
         [0077]    In other embodiments, the housing  45  of the on-skin sensor control unit  44  is formed in at least two separate portions that fit together to form the housing  45 , for example, a base  74  and a cover  76 , as illustrated in  FIGS. 5-7 . The two or more portions of the housing  45  may be entirely separate from each other. Alternatively, at least some of the two or more portions of the housing  45  may be connected together, for example, by a hinge, to facilitate the coupling of the portions to form the housing  45  of the on-skin sensor control unit  44 . 
         [0078]    These two or more separate portions of the housing  45  of the on-skin sensor control unit  44  may have complementary, interlocking structures, such as, for example, interlocking ridges or a ridge on one component and a complementary groove on another component, so that the two or more separate components may be easily and/or firmly coupled together. This may be useful, particularly if the components are taken apart and fit together occasionally, for example, when a battery or sensor  42  is replaced. However, other fasteners may also be used to couple the two or more components together, including, for example, screws, nuts and bolts, nails, staples, rivets, or the like. In addition, adhesives, both permanent or temporary, may be used including, for example, contact adhesives, pressure sensitive adhesives, glues, epoxies, adhesive resins, and the like. 
         [0079]    Typically, the housing  45  is at least water resistant to prevent the flow of fluids into contact with the components in the housing, including, for example, the conductive contacts  80 . Preferably, the housing is waterproof. In one embodiment, two or more components of the housing  45 , for example, the base  74  and the cover  76 , fit together tightly to form a hermetic, waterproof, or water resistant seal so that fluids can not flow into the interior of the on-skin sensor control unit  44 . This may be useful to avoid corrosion currents and/or degradation of items within the on-skin sensor control unit  44 , such as the conductive contacts, the battery, or the electronic components, particularly when the patient engages in such activities as showering, bathing, or swimming. 
         [0080]    Water resistant, as used herein, means that there is no penetration of water through a water resistant seal or housing when immersed in water at a depth of one meter at sea level. Waterproof, as used herein, means that there is no penetration of water through the waterproof seal or housing when immersed in water at a depth of ten meters, and preferably fifty meters, at sea level. It is often desirable that the electronic circuitry, power supply (e.g., battery), and conductive contacts of the on-skin sensor control unit, as well as the contact pads of the sensor, are contained in a water resistant, and preferably, a waterproof, environment. 
         [0081]    The on-skin sensor control unit  44  is typically attached to the skin  75  of the patient, as illustrated in  FIG. 8 . The on-skin sensor control unit  44  may be attached by a variety of techniques including, for example, by adhering the on-skin sensor control unit  44  directly to the skin  75  of the patient with an adhesive provided on at least a portion of the housing  45  of the on-skin sensor control unit  44  which contacts the skin  75 , by suturing the on-skin sensor control unit  44  to the skin  75  through suture openings (not shown) in the sensor control unit  44 , or by strapping the on-skin sensor control unit  44  to the skin  75 . 
         [0082]    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 . The mounting unit  77  is often a part of the on-skin sensor control unit  44 . One example of a suitable mounting unit  77  is a double-sided adhesive strip, one side of which is adhered to a surface of the skin of the patient and the other side is adhered to the on-skin sensor control unit  44 . In this embodiment, the mounting unit  77  may have an optional opening  79  which is large enough to allow insertion of the sensor  42  through the opening  79 . Alternatively, the sensor may be inserted through a thin adhesive and into the skin. 
         [0083]    A variety of adhesives may be used to adhere the on-skin sensor control unit  44  to the skin  75  of the patient, either directly or using the mounting unit  77 , including, for example, pressure sensitive adhesives (PSA) or contact adhesives. Preferably, an adhesive is chosen which is not irritating to all or a majority of patients for at least the period of time that a particular sensor  42  is implanted in the patient. Alternatively, a second adhesive or other skin-protecting compound may be included with the mounting unit so that a patient, whose skin is irritated by the adhesive on the mounting unit  77 , can cover his skin with the second adhesive or other skin-protecting compound and then place the mounting unit  77  over the second adhesive or other skin-protecting compound. This should substantially prevent the irritation of the skin of the patient because the adhesive on the mounting unit  77  is no longer in contact with the skin, but is instead in contact with the second adhesive or other skin-protecting compound. 
         [0084]    Returning to  FIG. 8 , when the sensor  42  is changed, the on-skin sensor control unit  44  may be moved to a different position on the skin  75  of the patient, for example, to avoid excessive irritation. Alternatively, the on-skin sensor control unit  44  may remain at the same place on the skin of the patient until it is determined that the unit  44  should be moved. 
         [0085]    Another embodiment of a mounting unit  77  used in an on-skin sensor control unit  44  is illustrated in  FIGS. 10A and 10B . The mounting unit  77  and a housing  45  of an on-skin sensor control unit  44  are mounted together in, for example, an interlocking manner, as shown in  FIG. 10A . The mounting unit  77  is formed, for example, using plastic or polymer materials, including, for example, polyvinyl chloride, polyethylene, polypropylene, polystyrene, ABS polymers, and copolymers thereof. The mounting unit  77  may be formed using a variety of techniques including, for example, injection molding, compression molding, casting, and other molding methods. 
         [0086]    The mounting unit  77  typically includes an adhesive on a bottom surface of the mounting unit  77  to adhere to the skin of the patient or the mounting unit  77  is used in conjunction with, for example, double-sided adhesive tape or the like. The mounting unit  77  typically includes an opening  79  through which the sensor  42  is inserted, as shown in  FIG. 10B . The mounting unit  77  may also include a support structure  220  for holding the sensor  42  in place and against the conductive contacts  80  on the on-skin sensor control unit  42 . The mounting unit  77 , also, optionally, includes a positioning structure  222 , such as an extension of material from the mounting unit  77 , that corresponds to a structure (not shown), such as an opening, on the sensor  42  to facilitate proper positioning of the sensor  42 , for example, by aligning the two complementary structures. 
         [0087]    In another embodiment, a coupled mounting unit  77  and housing  45  of an on-skin sensor control unit  44  is provided on an adhesive patch  204  with an optional cover  206  to protect and/or confine the housing  45  of the on-skin sensor control unit  44 , as illustrated in  FIG. 11A . The optional cover may contain an adhesive or other mechanism for attachment to the housing  45  and/or mounting unit  77 . The mounting unit  77  typically includes an opening  49  through which a sensor  42  is disposed, as shown in  FIG. 11B . The opening  49  may optionally be configured to allow insertion of the sensor  42  through the opening  49  using an introducer  120  or insertion gun  200  (see  FIG. 9 ). The housing  45  of the on-skin sensor control unit  44  has a base  74  and a cover  76 , as illustrated in  FIG. 11C . A bottom view of the housing  45 , as shown in  FIG. 11D , illustrates ports  230  through which conductive contacts (not shown) extend to connect with contact pads on the sensor  42 . A board  232  for attachment of circuit components may optionally be provided within the on-skin sensor control unit  44 , as illustrated in  FIG. 11E . 
         [0088]    In some embodiments, the adhesive on the on-skin sensor control unit  44  and/or on any of the embodiments of the mounting unit  77  is water resistant or waterproof to permit activities such as showering and/or bathing while maintaining adherence of the on-skin sensor control unit  44  to the skin  75  of the patient and, at least in some embodiments, preventing water from penetrating into the sensor control unit  44 . The use of a water resistant or waterproof adhesive combined with a water resistant or waterproof housing  45  protects the components in the sensor control unit  44  and the contact between the conductive contacts  80  and the sensor  42  from damage or corrosion. An example of a non-irritating adhesive that repels water is Tegaderm (3M, St. Paul, Minn.). 
         [0089]    In one embodiment, the on-skin sensor control unit  44  includes a sensor port  78  through which the sensor  42  enters the subcutaneous tissue of the patient, as shown in  FIGS. 5 to 7 . 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  may then be placed on the skin of the patient with the sensor  42  being threaded through the sensor port  78 . If the housing  45  of the sensor  42  has, for example, a base  74  and a cover  76 , then the cover  76  may be removed to allow the patient to guide the sensor  42  into the proper position for contact with the conductive contacts  80 . 
         [0090]    Alternatively, if the conductive contacts  80  are within the housing  45  the patient may slide the sensor  42  into the housing  45  until contact is made between the contact pads  49  and the conductive contacts  80 . The sensor control unit  44  may have a structure which obstructs the sliding of the sensor  42  further into the housing once the sensor  42  is properly positioned with the contact pads  49  in contact with the conductive contacts  80 . 
         [0091]    In other embodiments, the conductive contacts  80  are on the exterior of the housing  45  (see e.g.,  FIGS. 10A-10B  and  11 A- 11 E). In these embodiments, the patient guides the contacts pads  49  of the sensor  42  into contact with the conductive contacts  80 . In some cases, a guiding structure may be provided on the housing  45  which guides the sensor  42  into the proper position. An example of such a structure includes a set of guiding rails extending from the housing  45  and having the shape of the sensor  42 . 
         [0092]    In some embodiments, when the sensor  42  is inserted using an introducer  120  (see  FIG. 3 ), the tip of the introducer  120  or optional insertion gun  200  (see  FIG. 9 ) is positioned against the skin or the mounting unit  77  at the desired insertion point. In some embodiments, the introducer  120  is positioned on the skin without any guide. In other embodiments, the introducer  120  or insertion gun  200  is positioned using guides (not shown) in the mounting unit  77  or other portion of the on-skin sensor control unit  44 . In some embodiments, the guides, opening  79  in the mounting unit  77  and/or sensor port  78  in the housing  45  of the on-skin sensor control unit  44  have a shape which is complementary to the shape of the tip of the introducer  120  and/or insertion gun  200  to limit the orientation of the introducer  120  and/or insertion gun  200  relative to the opening  79  and/or sensor port  78 . The sensor can then be subcutaneously inserted into the patient by matching the complementary shape of the opening  79  or sensor port  78  with the introducer  120  and/or insertion gun  200 . 
         [0093]    In some embodiments, the shapes of a) the guides, opening  79 , or sensor port  78 , and (b) the introducer  120  or insertion gun  200  are configured such that the two shapes can only be matched in a single orientation. This aids in inserting the sensor  42  in the same orientation each time a new sensor is inserted into the patient. This uniformity in insertion orientation may be required in some embodiments to ensure that the contact pads  49  on the sensor  42  are correctly aligned with appropriate conductive contacts  80  on the on-skin sensor control unit  44 . In addition, the use of the insertion gun, as described above, may ensure that the sensor  42  is inserted at a uniform, reproducible depth. 
         [0094]    An exemplary on-skin sensor control unit  44  can be prepared and used in the following manner. A mounting unit  77  having adhesive on the bottom is applied to the skin. An insertion gun  200  (see  FIG. 9 ) carrying the sensor  42  and the introducer  120  is positioned against the mounting unit  77 . The insertion gun  200  and mounting unit  77  are optionally designed such that there is only one position in which the two properly mate. The insertion gun  200  is activated and a portion of the sensor  42  and optionally a portion of the introducer  120  are driven through the skin into, for example, the subcutaneous tissue. The insertion gun  200  withdraws the introducer  120 , leaving the portion of the sensor  42  inserted through the skin. The housing  45  of the on-skin control unit  44  is then coupled to the mounting unit  77 . Optionally, the housing  45  and the mounting unit  77  are formed such that there is only one position in which the two properly mate. The mating of the housing  45  and the mounting unit  77  establishes contact between the contact pads  49  (see e.g.,  FIG. 2 ) on the sensor  42  and the conductive contacts  80  on the on-skin sensor control unit  44 . Optionally, this action activates the on-skin sensor control unit  44  to begin operation. 
         [0095]    The introducer, sensor, insertion gun and mounting unit can be manufactured, marketed, or sold as a unit. For example,  FIG. 12  depicts an introducer  270 , sensor  272 , insertion gun  274  and mounting unit  276 , which can be assembled (as indicated by the arrows) and sold together in an insertion kit. In such an embodiment of an insertion kit, the insertion gun  274  can be packaged in a pre-loaded fashion, with an introducer  270  and sensor  272  mated or otherwise coupled, the mated sensor  272  and introducer  270  loaded upon the carrier  278  of the insertion gun, and with a mounting unit  276  already mated with the end of the insertion gun  274 . 
         [0096]    In one embodiment, the insertion gun  274  is packaged in a state where it is ready to thrust the sensor  272  (and perhaps introducer  270 ) into subcutaneous tissue. For example, the insertion gun  274  can be packaged in a “cocked” state, such that the thrusting force used to introduce the sensor  272  into the subcutaneous tissue is stored in the device as potential energy (in the case of the embodiment depicted in  FIG. 12 , the insertion gun  274  would be “cocked” by compressing its spring  280 , thus storing potential energy within the coils of the spring). Preferably, an insertion gun  274  packaged in such a manner employs a “safety”, a barrier to prevent the release of the stored potential energy. The barrier is removed in order to permit the potential energy to be released. Within the context of the embodiment presented in  FIG. 12 , an example of a safety is a pin (not pictured) that impedes the spring from expanding, once compressed. Thus, an insertion kit so embodied can be obtained at a place of purchase, removed from its package, and used after removal of the safety, without necessitating additional steps. Alternatively, the insertion gun  274  can be packaged in the above-described pre-loaded configuration, but without being “cocked”. Thus, an insertion kit with an “uncocked” insertion gun  274  can be obtained at a place of purchase, removed from its package, cocked, and used. To facilitate the insertion kit being ready to use with minimal user-exercised steps, the insertion kit can be sterilized prior to packaging. Examples of acceptable sterilizing techniques include exposing the elements of the insertion kit to gamma radiation or an e-beam. 
         [0097]    Referring to  FIGS. 13-33 , preferred commercial embodiments of a sensor inserter constructed according to the invention will now be described.  FIG. 13  shows an overall perspective view of a sensor inserter kit  300  comprising a single-use sensor inserter  310  and a single-use adhesive mount  312  removably attached to the bottom thereof. 
         [0098]    As an overview of the operation of inserter kit  300 , the kit comes packaged generally as shown in  FIG. 13  with a sensor  314  (best seen in  FIGS. 16 and 25 ) preloaded within inserter  310  and with inserter  310  in a “cocked” state. After preparing an insertion site on the skin, typically in the abdominal region, the patient removes upper liner  316  and lower liner  318  from adhesive mount  312  to expose the bottom surface and a portion of the top surface of an adhesive tape  320  (best seen in  FIG. 16 ) located beneath mount  312 . Mount  312 , with inserter  310  attached, is then applied to the patient&#39;s skin at the insertion site. Safety lock tabs  322  are squeezed together to allow actuator button  324  to be pressed causing inserter  310  to fire, thereby inserting sensor  314  into the patient&#39;s skin with a predetermined velocity and force. Once sensor  314  has been inserted into the skin, the patient removes inserter  310  from mount  312  by pressing release tabs  326  on opposite sides of inserter  310  and lifting inserter  310  away from mount  312 . 
         [0099]    Referring to  FIG. 14 , mount  312  is shown adhered to a patient&#39;s skin  328  with sensor  314  already inserted. Once inserter  310  is removed from mount  312 , transmitter  330  can be slid into place. The circuitry of transmitter  330  makes electrical contact with the contact pads on sensor  314  after transmitter  330  is fully seated on mount  312 . Once initialization and synchronization procedures are completed, electrochemical measurements from sensor  314  can be sent wirelessly from transmitter  330  to a portable receiver  332 , as shown in  FIG. 15 . Sensor  314 , mount  312  and transmitter  330  remain in place on the patient for a predetermined period, currently envisioned to be three days. These components are then removed so that sensor  314  and mount  312  can be properly discarded. The entire procedure above can then be repeated with a new inserter  310 , sensor  314  and mount  312 , reusing transmitter  330  and receiver  332 . 
         [0100]    Referring to  FIG. 16 , the preferred inserter kit  300  is assembled as shown from the following components: housing  334 , actuator button  324 , drive spring  336 , shuttle  338 , introducer sharp  340 , sensor  314 , retraction spring  342 , inserter base  344 , upper liner  316 , adhesive mount  312 , adhesive tape  320 , and lower liner  318 . 
         [0101]    Sensor  314  has a main surface  346  slidably mounted between U-shaped rails  348  of introducer sharp  340  and releasably retained there by sensor dimple  350  which engages introducer dimple  352 . Introducer sharp  340  is mounted to face  354  of shuttle  338 , such as with adhesive, heat stake or ultrasonic weld. Sensor  314  also has a surface  356  that extends orthogonally from main surface  346  and just beneath a driving surface  358  of shuttle  338  when mounted thereon (details of these features are better shown in FIGS.  19  and  25 - 27 .) 
         [0102]    Shuttle  338  is slidably and non-rotabably constrained on base  344  by arcuate guides  360 . As best seen in  FIGS. 19 ,  24  and  27 , shuttle  338  is generally formed by an outer ring  362  and an inner cup-shaped post  364  connected by two bridges  366 . Bridges  366  slide between the two slots  368  formed between guides  360  and allow shuttle  338  to travel along guides  360  without rotating. Retraction spring  342  is captivated at its outer circumference by guides  360 , at its bottom by the floor  370  of base  344 , at its top by bridges  366 , and at its inner circumference by the outer surface of shuttle post  364 . Drive spring  336  is captivated at its bottom and outer circumference by the inside surface of shuttle post  364 , at its top by the ceiling  372  inside actuator button  324 , and at its inner circumference by stem  374  depending from ceiling  372 . When drive spring  336  is compressed between actuator button  324  and shuttle  338  it urges shuttle  338  towards base  344 . When retraction spring  342  is compressed between shuttle  338  and base  344 , it urges shuttle  338  towards actuator button  324 . 
         [0103]    Actuator button  324  is slidably received within housing  334  from below and resides in opening  376  at the top of housing  334  with limited longitudinal movement. Arms  378  on each side of actuator button  324  travel in channels  380  along the inside walls of housing  334 , as best seen in  FIG. 20 . Longitudinal movement of actuator button  324  is limited in one direction by the base  378  of arms  378  contacting the edge of opening  376  at the top of housing  334 , and in the other direction by the distal ends  384  of arms  378  contacting stops  386  in channels  380 . Slots  388  are preferably provided in the top of housing  334  for ease of housing manufacture and so tools can be inserted to inwardly compress arms  378  beyond stops  386  to allow actuator button  324  to be removed from housing  334  if needed. 
         [0104]    When sensor  314 , introducer  340 , shuttle  338 , retraction spring  342 , drive spring  336  and actuator button  324  are assembled between base  344  and housing  334  as shown in  FIG. 16  and described above, housing  334  is snapped into place on base  344 . Base  344  is held onto housing  334  by upper base barbs  390  that engage upper openings  392  in housing  334 , and lower base barbs  394  (best seen in  FIG. 17 ) that engage lower openings  396  in housing  334 . Slots  398  and  400  are provided for ease of manufacture of housing  334 , and base  344  is preferably removable from housing  334  with tools if needed. 
         [0105]    Referring to  FIG. 19 , actuator button  324  is preferably provided with safety lock tabs  322  hingedly formed on opposite ends. Tabs  322  can be urged from a relaxed outward position to a flexed inward position. When in the normal outward position, shoulders  402  on the outer surfaces of tabs  322  engage the rim  404  of opening  376  to prevent the actuator button  324  from being depressed, thereby avoiding accidental firing of inserter  310 . Tabs  322  can be squeezed inward just enough to clear the rim  404  of opening  376  while pressing the actuator button  324  down to fire the inserter. Alternatively, tabs  322  can be squeezed further inward so that barbs  406  on the inside edges can engage catches  408  located on a center portion of actuator button  324 , thereby defeating the safety lock to allow later firing by simply pressing down on the actuator button  324 . Preferably, upwardly extending grips  410  are provided on tabs  322  for better visual indication of safety lock status and actuation control. 
         [0106]    Referring to  FIG. 20 , shuttle  338  is provided with laterally extending barbed fingers  412  which travel in channels  380  along the inside walls of housing  334 . When shuttle  338  is inserted up into housing  334  far enough, barbed fingers  412  momentarily deflect inward and then snap outward again to catch on stops  386 . In this “cocked” position as shown, drive spring  336  is compressed and urging shuttle  338  towards base  344 , but barbed fingers  412  catching on stops  386  prevent such travel. 
         [0107]    Referring to  FIGS. 21-23 , the sequence of loading, cocking, arming, firing, and automatic retraction of inserter  310  will be described. It is envisioned that in production, inserters  310  will be fabricated and fully assembled by one vender except for sensor  314 , which will be supplied and installed by a second vendor in a sterile environment. Accordingly, inserter  310  will be manufactured and shipped to the sensor vendor in a neutral state, as shown in  FIG. 21 . A hole  414  provided through the center of actuator button  324  allows the sensor vendor to insert a pin (manually or by automated machinery, not shown) through hole  414  to drive shuttle  338  towards base  344  in a controlled fashion and hold it there against the force of retraction spring  342 . This will cause introducer sharp  340  to be extended through base  344  (as shown in  FIG. 23 ) so that sensor  314  can be loaded into introducer  340 . When the pin is removed, shuttle  338 , introducer  340  and sensor  314  will retract to the neutral position. The sensor vendor can then cock the loaded inserter  310  before shipment by pushing another pin (not shown) from the opposite direction through a central hole  416  in base  344  (with mount  312  removed) until the pin contacts dimple  418  formed in the bottom of shuttle  338 . By pushing shuttle  338  towards actuator button  324  until barbed fingers  412  clear stops  386 , the inserter  310  is cocked (as shown in  FIG. 22 .) 
         [0108]    Referring to  FIG. 22 , inserter  310  is preferably received by the patient in the cocked position as shown. To use inserter  310 , the patient applies mount  312  to the mounting site and disables the safety mechanism as previously described, and then pushes actuator button  324  against the force of drive spring  336 . As actuator button  324  travels toward base  344 , drive cam surfaces  420  on arms  378  contact ramped surfaces  422  of barbed fingers  412  and urge them inward. When fingers  412  are driven inward enough to clear stops  386 , shuttle  338  is driven by drive spring  336  with a predetermined speed and force to an insertion position, as shown in  FIG. 23 . 
         [0109]    Referring to  FIG. 23 , inserter  310  is shown in the insertion position with the tail  424  of introducer sharp  340  extending through base  344  and mount  312  into the skin of the patient.  FIG. 23  shows shuttle  338  in a fully extended position with its lower surface  426  bottomed out on base  344 . However, the lower orthogonal surface  356  of sensor  314  will contact an exposed sensor contact portion  428  (best seen in  FIGS. 14 and 16 ) on top of adhesive tape  320  supported from below by the patient&#39;s skin, and therefore will typically stop traveling before reaching the fully bottomed out position shown. Tail  424  of introducer sharp  340  provides rigidity and a skin piercing edge  430  for allowing the flexible tail  431  of sensor  314  to be implanted in the patient&#39;s skin. After providing this function, introducer sharp  340  is immediately removed from the patient and retracted into a safe position inside housing  334  as retraction spring  342  (which has been compressed by the travel of the shuttle) pushes shuttle  338  back towards actuator cap. Sensor  314  is pulled from introducer  340  and held in place by the sensor contact portion  428  on top of adhesive tape  320  adhering to orthogonal surface  356  of sensor  314 . The geometries of sensor dimple  350  and mating introducer dimple  352  are chosen to create a separation force between them that is less than the adhesion force of tape  320  on orthogonal surface  356 , but great enough to retain sensor  314  in introducer  340  during typical shipping and product handling shock loads. Driving surface  358  beneath shuttle  338  presses down on top of orthogonal surface  356  to ensure good contact with adhesive tape  320  before shuttle  338  retracts with introducer  340 . As discussed above with previous embodiments, barb(s) on sensor tail  431  can be employed to further anchor the sensor in its operating position. 
         [0110]    Referring again to  FIG. 21 , retraction spring  342  will return shuttle  338  to the neutral position as shown after firing, but without sensor  314  which remains inserted in patient&#39;s skin (not still in introducer  340  as shown here.) Drive spring  336  is preferably designed to be stiffer than retraction spring  342  so that shuttle  338  oscillations are quickly dampened out, and so introducer sharp  340  does not return to sensor  314  or the patient to cause injury. With sensor  314  now inserted in the patient&#39;s skin, inserter  310  can be removed from mount  312  by inwardly flexing release tabs  326  on opposite sides of inserter  310  to remove latch hooks  432  from mount channels  434  and then lifting inserter  310  away from mount  312 . Introducer sharp  340  remains protected inside housing  334  during disposal of inserter  310 . Transmitter  330  can now be slid into place on mount  312  as previously described. 
         [0111]    Referring to  FIG. 28 , an alternative embodiment of adhesive tape  320 ′ is shown. This oversized tape  320 ′ has the advantage of holding transmitter  330  in place even when fairly large forces are placed on it. In this embodiment adhesive tape  320 ′ has a double-sided portion  436  (adhesive on both top and bottom sides) residing between mount  312  and the patient&#39;s skin, and a single-sided portion  438  outwardly extending from the double-sided portion  436 , preferably in all directions, for adhering just to the patient&#39;s skin. In the previous embodiment, it is difficult to separate mount  312  from the skin merely with tension forces, but applying a force to just one side of mount  312  results in a high peeling force being applied to that edge of the adhesive tape  320  which causes tape  320  to peel off of the skin. In contrast, any force applied to transmitter  330  in this alternative embodiment results in a tension force rather than a peeling force being applied to tape  320 ′, inhibiting inadvertent removal until an edge of tape  320 ′ is intentionally peeled up. Preferably, single-sided portion  438  has a width roughly double the width of double-sided portion  436 . In the preferred embodiment, theses widths are 2.14 and 1.14 inches, respectively. Preferably, the length that single-sided portion  438  extends beyond double-sided portion  436  is roughly equivalent to the combined height of transmitter  330  attached to mount  312 , in this case about 0.5 inches. 
         [0112]    In the preferred embodiment, sensor  314  is made from a 0.005 inch thick Mylar substrate, such as Dupont Melinex ST-505, print treated both sides, heat stabilized and bi-axially oriented. Main surface  346  is 0.315 tall by 0.512 wide, and orthogonal surface  356  is 0.374 wide by 0.202 deep. Sensor tail  431  is 0.230 long by 0.023 wide. Semispherical sensor dimple  350  is 0.050 inches wide and 0.026 inches deep. Introducer  340  is made from SUS 301 medical grade stainless steel, 0.004 inches thick, having a surface roughness less than or equal to 0.5 micrometers. The height of the main portion of introducer  340  is 0.614 inches, and the inside width is 0.513 inches. The overall thickness of rolled rails  348  is 0.026 inches. The length and width of introducer tail  424  are 0.354 and 0.036 inches, respectively. The preferred angle of the sharp  340  is 21 degrees. Preferably, semispherical introducer dimple  352  has a radius of 0.024 inches. In the preferred embodiment, shuttle  338  has an average speed of at least 1 meter/second, and has a momentum at its end of travel of about 2.65 lb-m/sec. 
         [0113]    Preferably, housing  334 , button  324 , shuttle  338 , base  344  and mount  312  are all injection molded from G.E. Lexan PC. Inside and outside working surfaces of arms  378  on button  324  are preferably lubricated with Dow Corning 360 Medical Fluid. Drive spring  336  has a free length of 1.25 inches, a working length of 1.00 inch, and a rate between 20 and 30 pounds per inch. Retraction spring  342  has a free length of 1.5 inches, a working length of 0.35 inches, and a rate between 0.15 and 0.35 pounds per inch. Adhesive tape  320  preferably is medical grade acrylic adhesive on polyester film (such as Acutek 0396013) with a semi-bleached kraft liner having silicon release. 
         [0114]    Referring to  FIG. 29 , an interconnect  440  is shown for providing waterproof electrical connections between sensor  314  and transmitter  330 . Interconnect  440  includes a seal  442  mounted on an end of transmitter  330  that contacts one side of sensor  314  when transmitter  330  is slid onto mount  312 . When transmitter  330  is locked into place on mount  312 , seal  442  is compressed between transmitter  330  and sensor  314  and urges sensor  314  against raised end stop  444  of mount  312 . 
         [0115]    Referring to  FIG. 30 , further details of interconnect  440  are shown. Seal  442  has an exterior wall  446  for surrounding electrical contacts  448  (in this case four), and interior walls  450  for isolating electrical contacts  448  from each other. Rim  452  formed on the transmitter housing  330  surrounds the base  454  of seal  442  to prevent it from collapsing outward when compressed. 
         [0116]    Referring to  FIG. 31 , an enlarged partial view of  FIG. 30  is shown with seal  442  and one spring removed for clarity. Electrical contacts  448  are preferably constructed from compression springs  456  mounted on connector lugs  458 . Lugs  458  are stamped rearward on their edges to form protrusions  460  that retain springs  456 . Alternately or in conjunction with this stamping, plastic rings (not shown) can be melted over the base of each spring  456  for attaching it to its respective lug  458 . Connector lugs  458  can protrude through slots in transmitter housing  330 , or be insert molded integral with the plastic housing  330  when it is molded. 
         [0117]    Referring to  FIG. 32 , and enlarged perspective view of the seal  442  is shown. It has been discovered through experimentation that two lips  462  of equal height along the distal edge of exterior wall  446  provide the best seal from exterior elements. Good isolation between electrical contacts  448  is best achieved by having interior walls  450  with a height equal to that of lips  462 . Recesses  464  should be sized large enough so that seal  442  does not interfere with the movement of springs  456  when seal  442  and springs  456  are compressed. In the preferred embodiment, the distal face of seal  442  defined by lips  462  is formed at a 1 degree angle to match the draft angle of mount end stop  444 . 
         [0118]    Seal  442  is preferably made of shore A 30 durometer compression molded silicone. It is envisioned that seal  442  can be shortened in the axial direction (parallel to springs  456 ) to reduce the force required to compress it when attaching transmitter  330  to mount  312 . Best results for fastening seal  442  to transmitter housing  330  have been achieved with double sided adhesive tape  320 , silicone adhesive on one side and acrylic adhesive on the other for sticking to the PC-ABS blend of the transmitter housing  330 , such as product number 9731 manufactured by 3M Company of St. Paul, Minn. Springs  456  are preferably made from gold-plated beryllium copper so as to deter galvanic current effects. Preferably, main surface  346  of sensor  314  that contacts seal  442  has a uniform thickness dielectric coating with a window in it (i.e. no dielectric) where springs  456  contact sensor  314 . An interconnect  440  constructed as described above remains water proof when submerged to a depth of at least 1 meter for 45 minutes. 
         [0119]    To increase the reliability of sensor insertion, the following enhancements can be added to the inserter kit  300  described above. First, a sensor flap  466 , as shown in  FIG. 25 , can be formed along the top edge of sensor  314 . When sensor  314  reaches the extended, delivered position as shown in  FIG. 23 , flap  466  catches on bottom edge  468  of base  344 , shown in  FIG. 19 , to ensure that sensor  314  separates from introducer  340  as shuttle  338  returns upward to the retracted position. Adhesive can also be located on the bottom of orthogonal sensor surface  356  to ensure that sensor  314  adheres to the sensor contact portion  428  on the top of adhesive mount tape  320 , as shown in  FIG. 16 . 
         [0120]    Referring to  FIGS. 33A and 33B , actuator button  324 ′ can be made easier for elderly patients to push by anchoring the upper end of drive spring  336  on a housing bridge  470  instead of button  324 . This change also makes the insertion force of inserter  310  more consistent, and allows stronger spring forces to be used if desired. Bridge  470  spans across opening  376 ′ and divides it into two openings  472  in the top of housing  334 ′. The top portion of button  324 ′ is bifurcated into two protrusions  474  that each extend through an opening  472 . A clearance hole (not shown) is provided through the center of button  324 ′ to allow drive spring  336  to pass through and secure around a post (not shown) depending from the bottom center of bridge  470 . 
         [0121]    Safety lock key  476  can be provided to prevent actuator button  324 ′ from being pressed until key  476  is removed. Aperture  478  is provided in the top center of bridge  470  for receiving boss  480  located at the bottom of key  476 , thereby allowing key  476  to rotate. When key handle  482  is rotated perpendicular to button protrusions  474  as shown, two opposing perpendicular fins  484  on key  476  swing into inwardly facing slots (not shown) on the inside of protrusions  474  and prevent button  324 ′ from being actuated. When key handle  482  and fins  484  are rotated parallel to button protrusions  474  such that fins  484  disengage therefrom, key  476  can be removed and button  324 ′ can then be actuated. Other than these modifications, this inserter kit  300 ′ functions the same as the embodiment previous described. 
         [0122]    To provide an easier and more consistent release of shuttle  338  by actuator button  324  or  324 ′, it is envisioned that less aggressive finger engagement with stops  386  can be employed, or the above designs can be modified to have a single, more centrally located shuttle release finger (not shown) instead of the two outboard fingers  412  shown. 
         [0123]    The present invention should not be considered limited to the particular examples described above. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable and which fall within the general scope of the invention will be readily apparent to those of skill in the art to which the present invention is directed upon review of the instant specification.