as shown in the drawings for purposes of illustration , the invention is embodied in a reusable analyte sensor site that is used with a replaceable analyte sensor that determines body characteristics on a continuous , intermittent or near continuous basis . in preferred embodiments of the present invention , the replaceable analyte sensor is for determining glucose levels in the blood and / or bodily fluids of the user . however , it will be recognized that further embodiments of the invention may be used to determine the levels of other agents , characteristics or compositions , such as hormones , cholesterol , medications concentrations , viral loads ( e . g ., hiv ), or the like . the reusable analyte sensor site and replaceable analyte sensor are primarily adapted for use in sub - dermal or inter - peritoneal human tissue . however , still further embodiments may be placed in other types tissue , such as muscle , lymph , organ tissue , veins , arteries or the like , and used in animal tissue . in other embodiments , the reusable analyte sensor site may be used with a medication or fluid infusion catheter that requires regular replacement . as shown in fig1 - 3 , the reusable analyte sensor site 10 in accordance with a preferred embodiments of the present invention includes a site housing 12 that is implanted in sub - dermal or inter - peritoneal tissue 2 . initially , the implanted site housing 12 is sutured in position by sutures ( not shown ). preferably , the sutures are absorbable by the body . however , in alternative embodiments , the sutures are permanent . in preferred embodiments , the material of the site housing 12 is formed to create an interior cavity 14 to support an ingrown and vascularized tissue ( see fig2 and 3 ) structure around a replaceable analyte sensor 100 . in addition to the interior cavity 14 , the site housing 12 will form a conduit 16 that provides an entrance from the body tissue 2 to the interior cavity 14 of the site housing 12 . the conduit 16 should be sized long enough so that the trauma associated with inserting and replacing the replaceable analyte sensor 100 ( or catheter ) will not effect the ingrown and vascularized tissue structure formed around the interior cavity 14 and an active sensing portion 102 of the replaceable analyte sensor 100 ( or the outlet port of the replaceable catheter ). preferably , the length of the conduit is 5 millimeters . however , larger or smaller lengths ( ranging from 2 millimeters 20 millimeters or more ) may be used with the selection being dependent on the size of the site housing 12 , the type of tissue 2 the site housing 12 is implanted in , the frequency within which the replaceable analyte sensor 100 ( or replaceable catheter ) must be replaced , or the like . preferred embodiments of the site housing 12 can remain implanted in the tissue between 1 to 2 years . however , the actual long term life of the reusable analyte sensor site 10 will be a function of the material , the surface texture , the chemistry and the type of tissue 2 the reusable analyte sensor site 10 is implanted in . the goal is to achieve a reusable analyte sensor site 10 that is stable for multiple use to minimize the effects of frequent sensor replacement . preferably , the site housing 12 is formed from textured teflon ® or dacron ® that has a porosity selected to allow tissue ingrowth ( i . e ., cellular attachment ) and vascularization of the material of the site housing 12 . many suitable materials are available from gore & amp ; associates inc . and / or baxter international . in alternative embodiments , other materials may be used , such as glass , sintered or woven metal meshes ( e . g ., titanium or other bio - compatible metal ), composites , laminates , or the like , as long as they provide good vascularization and ingrowth of tissue . in preferred embodiments , the porosity ( or mesh ) size of the selected material is selected to be between 2 - 25 microns . however , in alternative embodiments , the porosity may be selected to be other sizes such that cells can attach and vascularization will occur . however , the porosity ( or mesh ) size must be selected to inhibit tissue ingress and keep tissue cells away from the active sensing portion 102 of the replaceable analyte sensor 100 . also , the sizing must be selected to avoid , or substantially minimize formation of a fibrin sheath that attaches to the material of the site housing 12 or which causes encapsulation of the site housing 12 . for example , as shown in fig3 the porosity of the material of the site housing 12 is selected to promote cellular attachment of cells c to the mesh pores so as to encourage the vascularization of the site housing 12 with capillary vessels v to assure a good supply of bodily fluid f in and around the active sensing portion 102 of the replaceable analyte sensor 100 ( or catheter ), while at the same time inhibiting the ingress of tissues and cells c . in addition to promoting tissue ingrowth and vascularization , the material used to form the site housing 12 must be sufficiently permeable to the analyte which is to be measured by the active sensing portion 102 of the replaceable analyte sensor 100 . for instance , if glucose levels are being measured , the ingrown and vascularized site housing 12 must pass glucose ( and transporting fluids ) from the tissue 2 to the active sensing portion 102 of the replaceable analyte sensor 100 so that the glucose levels can be measured . alternatively , if a replaceable catheter ( not shown ) is being used , such as for example to deliver insulin , the ingrown and vascularized site housing 12 must pass insulin out of the interior cavity 14 into the tissue 2 . in particular embodiments , the material of the site housing 12 may also be selected to keep out certain materials or bodily fluid constituents that could interfere with or alter the readings of the active sensing portion 102 of the replaceable analyte sensor 100 . it is further preferred that the material of the site housing 12 be selected so that any by products given off by the analyte sensing reaction will pass through the vascularized site housing and removed by the body of the user . in alternative embodiments , the material of the site housing 12 will be selected to pass the analyte but to contain any materials , or by products , produced by the replaceable analyte sensor 100 . as shown in fig2 the interior cavity 14 of the site housing 12 is selected to be sufficiently large to accommodate the active sensing portion 102 of the replaceable analyte sensor 100 ( or replaceable catheter ). however , the interior cavity 14 should not be so large that the fluid within the interior cavity 14 ( i . e ., passed through the vascularized site housing 12 ) becomes stagnant . it is important that the fluid within the interior cavity 14 change frequently to provide an accurate reading on the active sensing portion 102 of the replaceable analyte sensor 100 . in preferred embodiments of the present invention , the replaceable analyte sensor is a electrochemical sensor that uses either enzymatic or fluorescent techniques to determine the analyte level . in the illustrated embodiments , the active sensing portion 102 of the replaceable analyte sensor 100 is inserted through the conduit 16 of the site housing 12 and slid into the interior cavity 14 of the site housing 12 to take analyte level readings . a connection end 104 of the replaceable analyte sensor 100 is generally attached to an analyte monitor 200 that is implanted in the body tissues 2 of the user . preferably , the analyte monitor 200 transmits the signal values , raw data , operational information and parameters , or the like , to an external information retrieving device ( not shown ) or another implanted medical device ( not shown ). in alternative embodiments , the analyte monitor 200 may be incorporated into an implantable medication ( or fluid ) delivery pump , such as discussed above . although it is possible to have the connection end 104 of the replaceable analyte sensor 100 extend outside of the body , this approach is not preferred due to the risk of infection , irritation , the possible inconvenience of having external connections , or the like . once in place , the traumatized tissue 4 around the conduit entrance 22 will tend to attach and encapsulate the entrance 22 to the conduit that opens out to the body tissue 2 and the mid - portion 106 of the replaceable analyte sensor 100 that is not within the interior cavity 14 of the site housing 12 . this also tends to bind the entrance 22 of the conduit 16 to the mid - portion 106 of the replaceable analyte sensor 100 . however , due to the length of the conduit 16 and the distance from of the traumatized tissue 4 from the interior cavity 14 , there is little , or no , trauma and encapsulation that will occur around the already established interior cavity 14 of the site housing 12 . thus , the active sensing portion 102 can begin to take readings relatively soon after implantation , stabilization and calibration of the replaceable analyte sensor 100 . the entrance 22 of the conduit and the mid - portion 106 of the replaceable analyte sensor 100 that are encapsulated do not present an issue , since they do not effect operation of the active sensing portion 102 of the replaceable analyte sensor 100 . when the replaceable analyte sensor 100 is replaced , the encapsulation and tissue attachment 4 around the entrance 22 of the conduit 16 is cut to free the replaceable analyte sensor 100 , after which the replaceable analyte sensor 100 is withdrawn from the conduit 16 and interior cavity 14 of the site housing 12 . the removal process causes trauma around the entrance 22 of the conduit 16 ; however , it does not traumatize the interior cavity 14 , since it is located a sufficient distance away from the entrance 22 of the conduit 16 to avoid the effects of the new trauma . after removal of the old replaceable analyte sensor 100 , a new replaceable analyte sensor 100 is inserted through the conduit 16 and into the interior cavity 14 of the site housing 12 . as discussed preferred embodiments of the replaceable analyte sensor 100 are electrochemical in nature . for instance , the sensor may use electrodes and glucose oxidase to measure the glucose levels in the bodily fluids . examples of electrochemical sensors is shown and described in u . s . pat . nos . 5 , 391 , 250 ; 5 , 390 , 671 , to lord et al ., entitled “ transcutaneous sensor insertion set ”; u . s . pat . no . 5 , 165 , 407 , to wilson et al ., entitled “ implantable glucose sensor ”; and u . s . pat . no . 4 , 890 , 620 , to gough , entitled “ two - dimensional diffusion glucose substrate sensing electrode ”, which are incorporated herein in their entireties by reference . alternative embodiments may utilize optical properties such as shown and described in u . s . pat . no . 5 , 605 , 152 , to lord et al ., entitled “ optical glucose sensor ” or fiber optic structures and / or optical / fluorescent compounds such as shown and described in u . s . patent application ser . no . 08 / 752 , 945 ( corresponding to pct / us96 / 18720 ), to van antwerp et al ., entitled “ detection of biological molecules using chemical amplification and optical sensor ”, all of which are herein incorporated by reference . other sensor technologies suitable for implantation and working with bodily fluids are acceptable for use with the reusable analyte sensor site 10 . in alternative embodiments , the replaceable analyte sensor does not need to extend outside and beyond the entrance 22 of the conduit 16 . for instance , the replaceable analyte sensor may be in the form of a patch that is placed inside the interior cavity 14 and integrated by light transmitted through the skin . in this situation , the entrance 22 of the conduit 16 would be capped and uncapped to provide access to the replaceable analyte sensor and the interior cavity 14 . in another alternative embodiment , the entrance 22 of the conduit 16 is covered by a pierceable septum , or the like , and analyte sensor material , such as contained in microspheres , gels , or the like , is injected through the conduit 16 into the interior cavity 14 , and withdrawn ( if necessary ) by the same route when new sensor material is to be placed into the interior cavity 14 . while the description above refers to particular embodiments of the present invention , it will be understood that many modifications may be made without departing from the spirit thereof . the accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention . the presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive , the scope of the invention being indicated by the appended claims , rather than the foregoing description , and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein .
a reusable analyte sensor site for use with a replaceable analyte sensor for determining a level of an analyte includes a site housing . preferably , the site housing material is formed to have an interior cavity with an opening and a conduit that is connected to the opening of the interior cavity to provide access to the interior cavity . the site housing material is selected to promote tissue ingrowth and vasuclarization , and yet be free of tissue ingress . also , the site housing material permits the analyte to pass through the site housing material to the interior cavity to permit measurement by the replaceable analyte sensor . in addition , the conduit has a predetermined length to inhibit trauma and encapsulation of tissue occurring at the conduit , which is associated with placing the replaceable analyte sensor in the interior cavity of the site housing , from interfering with the tissue ingrowth and vascularization surrounding the interior cavity of the site housing material .