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 . 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 applications , ser . no . 09 / 034 , 372 , issued as u . s . pat . no . 6 , 134 , 461 on oct . 17 , 2000 , and ser . no . 09 / 753 , 746 , issued as u . s . pat . no . 6 , 560 , 471 on may 6 , 2003 , both incorporated herein by reference . 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 fig1 . 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 . a sensor 42 includes at least one working electrode 58 formed on a substrate 50 , as shown in fig2 . 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 . 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 ). 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 . 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 . 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 . 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 fig2 . 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 . an introducer 120 can be used to subcutaneously insert the sensor 42 into the patient , as illustrated in fig3 . 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 . the introducer 120 may have a variety of cross - sectional shapes , as shown in fig4 a , 4 b , and 4 c . the introducer 120 illustrated in fig4 a 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 fig4 b 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 fig4 b 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 fig4 b and 4c is typically about 1 mm or less , preferably about 700 μm or less , and more preferably about 500 μm or less . 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 fig3 . 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 . 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 . 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 fig9 . 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 . 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 . 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 . 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 . 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 . 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 °. 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 fig5 - 7 . however , other shapes and sizes may be used . 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 . the on - skin sensor control unit 44 includes a housing 45 , as illustrated in fig5 - 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 . 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 . 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 . 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 fig5 - 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 . 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 . 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 . 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 . the on - skin sensor control unit 44 is typically attached to the skin 75 of the patient , as illustrated in fig8 . 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 . 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 . 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 . returning to fig8 , 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 . another embodiment of a mounting unit 77 used in an on - skin sensor control unit 44 is illustrated in fig1 a 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 fig1 a . 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 . 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 fig1 b . 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 . 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 fig1 a . 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 fig1 b . 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 fig9 ). the housing 45 of the on - skin sensor control unit 44 has a base 74 and a cover 76 , as illustrated in fig1 c . a bottom view of the housing 45 , as shown in . fig1 d , 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 fig1 e . 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 .). 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 fig5 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 . 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 . in other embodiments , the conductive contacts 80 are on the exterior of the housing 45 ( see e . g ., fig1 a - 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 . in some embodiments , when the sensor 42 is inserted using an introducer 120 ( see fig3 ), the tip of the introducer 120 or optional insertion gun 200 ( see fig9 ) 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 . 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 . 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 fig9 ) 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 ., fig2 ) 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 . the introducer , sensor , insertion gun and mounting unit can be manufactured , marketed , or sold as a unit . for example , fig1 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 . 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 fig1 , 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 fig1 , 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 . referring to fig1 - 33 , preferred commercial embodiments of a sensor inserter constructed according to the invention will now be described . fig1 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 . as an overview of the operation of inserter kit 300 , the kit comes packaged generally as shown in fig1 with a sensor 314 ( best seen in fig1 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 fig1 ) 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 . referring to fig1 , 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 fig1 . 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 . referring to fig1 , 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 . 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 fig1 and 25 - 27 .) shuttle 338 is slidably and non - rotabably constrained on base 344 by arcuate guides 360 . as best seen in fig1 , 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 . 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 fig2 . 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 . 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 fig1 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 fig1 ) 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 . referring to fig1 , 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 . referring to fig2 , 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 . referring to fig2 - 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 fig2 . 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 fig2 ) 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 fig2 .) referring to fig2 , 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 fig2 . referring to fig2 , 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 . fig2 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 fig1 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 . referring again to fig2 , 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 . referring to fig2 , 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 . 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 . 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 . referring to fig2 , 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 . referring to fig3 , 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 . referring to fig3 , an enlarged partial view of fig3 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 . referring to fig3 , 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 . 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 . 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 fig2 , can be formed along the top edge of sensor 314 . when sensor 314 reaches the extended , delivered position as shown in fig2 , flap 466 catches on bottom edge 468 of base 344 , shown in fig1 , 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 fig1 . referring to fig3 a 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 . 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 . 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 . 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 .
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 .