Patent Publication Number: US-2020289038-A1

Title: Analyte sensing biointerface

Description:
INCORPORATION BY REFERENCE TO RELATED APPLICATIONS 
     Any and all priority claims identified in the Application Data Sheet, or any correction thereto, are hereby incorporated by reference under 37 CFR 1.57. This application is a continuation of U.S. application Ser. No. 16/742,848, filed Jan. 14, 2020, which is a continuation of U.S. application Ser. No. 16/654,621, filed Oct. 16, 2019, which is a continuation of U.S. application Ser. No. 16/455,468, filed Jun. 27, 2019, now U.S. Pat. No. 10,667,729, which is a continuation of U.S. application Ser. No. 15/711,225, filed Sep. 21, 2017, now U.S. Pat. No. 10,376,188, which is a continuation of U.S. application Ser. No. 14/281,697, filed May 19, 2014, now U.S. Pat. No. 9,788,766, which is a continuation of U.S. application Ser. No. 13/285,880, filed Oct. 31, 2011, now abandoned, which is a continuation of U.S. application Ser. No. 11/404,929, filed Apr. 14, 2006, now U.S. Pat. No. 8,060,174, which claims the benefit of U.S. Provisional Application No. 60/671,622, filed Apr. 15, 2005, and U.S. Provisional Application No. 60/683,923, filed May 23, 2005. Each of the aforementioned applications is incorporated by reference herein in its entirety, and each is hereby expressly made a part of this specification. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of biosensing. More particularly, the present invention relates to analyte sensors for implantation into a host. 
     BACKGROUND OF THE INVENTION 
     Biosensors are devices that can be used to detect the presence or amount of analytes, such as biomolecules, in a biological sample. Some biosensors are designed to detect analytes in a living host. Such detection can advantageously be done through the use of implantable biosensors, which are implanted intravascularly or within tissue to detect the presence or amount of analyte at the implantation location. One practical application of implantable biosensors is implantable glucose sensors that continuously monitor a patient&#39;s blood glucose level. 
     One type of implantable glucose sensor utilizes glucose oxidase that catalyzes the reaction between glucose and oxygen to produce gluconic acid and hydrogen peroxide. The hydrogen peroxide can be detected by measuring the electrochemical oxidation of the hydrogen peroxide at an appropriate electrode, such as a platinum electrode. The current generated by this oxidation can be related to the amount of hydrogen peroxide in the vicinity of the electrode, and hence, the amount of glucose in the vicinity of the sensor. In some glucose sensors, the electrode is coated with an analyte membrane system. The analyte membrane system may contain the glucose oxidase enzyme as well as one or more polymeric membranes that control the diffusion of glucose or block or limit certain undesired species from reaching the electrode, such as is described further in U.S. application Ser. No. 10/153,356, filed on May 22, 2002, which is incorporated herein by reference in its entirety. 
     One difficulty encountered with implantable biosensors, such as implantable glucose sensors, is that many of these devices tend to lose their function with time after implantation. While not being bound by any particular theory, this decrease in function can at least partially be attributed to the host&#39;s foreign body response (FBR) to the implant. Typical FBR response to an implantable biosensor is illustrated in  FIG. 1 . FBR is a local inflammatory response that results in the formation of a barrier cell layer  40  around the surface of the implant  47 . This layer generally consists of macrophages and foreign body giant cells  41 . An intermediate layer  42 , consisting of fibroblasts  43  and a fibrous matrix  44 , typically form over the barrier cell layer  40 . Finally, an outer layer  46  consisting of loose connective granular tissue and new blood vessels  45  forms over the intermediate layer. The barrier cell layer  40  can have the adverse effect of blocking transport of the analyte to the analyte sensor  47 . Furthermore, lack of vascularization in the intermediate  42  and barrier cell  40  layers decreases analyte availability to the sensor. Thus, once the FBR acts to induce the above-described tissue growth around the implanted biosensor, sensing ability decreases. 
     SUMMARY OF THE INVENTION 
     Devices and methods are needed that address the FBR&#39;s negative effects on biosensing. The devices and methods of the preferred embodiments address these negative effects, and offer other benefits and advantages, as described herein. 
     Accordingly, in a first aspect, an implantable analyte sensor is provided, comprising an electrically non-conductive biocompatible matrix comprising a plurality of passageways extending from openings in an exterior surface of the matrix into an interior portion of the matrix; a working electrode comprising an electrically conductive material, wherein an electroactive surface of the working electrode is within the matrix; and electronics electrically coupled to the working electrode, the electronics configured to detect a current flowing through the working electrode or a voltage of the working electrode, wherein the current or voltage is indicative of a quantity of an analyte within the passageways. 
     In an embodiment of the first aspect, the implantable sensor comprises a membrane coating surfaces of at least some of the passageways in the interior portion of the matrix, the membrane comprising a component that affects a rate of diffusion of the analyte through the membrane. 
     In an embodiment of the first aspect, the implantable sensor comprises a membrane coating surfaces of at least some of the passageways in the interior portion of the matrix, the membrane comprising a component that is capable of reacting with the analyte to produce a species that is capable of electrochemically reacting at a surface of the working electrode. 
     In an embodiment of the first aspect, the implantable sensor comprises a reference electrode disposed within the matrix, wherein an electroactive surface of the reference electrode is within the matrix, and wherein at least a portion of the matrix electrically insulates the reference electrode from the working electrode. 
     In an embodiment of the first aspect, the implantable sensor comprises a reference electrode disposed on an exterior surface of the matrix, wherein at least a portion of the matrix electrically insulates the reference electrode from the working electrode. 
     In an embodiment of the first aspect, the implantable sensor comprises a counter electrode disposed within the matrix, wherein an electroactive surface of the counter electrode is within the matrix, and wherein at least a portion of the matrix electrically insulates the counter electrode from the working electrode. 
     In an embodiment of the first aspect, the implantable sensor comprises a counter electrode disposed on an exterior surface of the matrix, wherein at least a portion of the matrix electrically insulates the counter electrode from the working electrode. 
     In an embodiment of the first aspect, the matrix is a substantially solid material and the passageways comprise pores within the substantially solid material. The matrix can be a mesh of fibers. The fibers can comprise an electrically non-conductive material or an electrically conductive material. The fibers can further comprise a membrane coating the fibers. 
     In an embodiment of the first aspect, the working electrode has a membrane coating disposed thereon. 
     In a second aspect, a sensor is provided for measuring an analyte in a host, the sensor comprising a biointerface comprising a porous biocompatible matrix, wherein electroactive surfaces are distributed within at least some pores in the biointerface. 
     In an embodiment of the second aspect, the electroactive surfaces have a membrane coating disposed thereon. 
     In an embodiment of the second aspect, the porous biocompatible matrix is configured to support tissue ingrowth and comprises a plurality of interconnected cavities and a solid portion, and wherein a substantial number of the interconnected cavities are greater than or equal to about 20 microns in at least one dimension. 
     In an embodiment of the second aspect, the porous biocompatible matrix comprises a length of greater than one cavity in each of three dimensions substantially throughout the matrix. 
     In an embodiment of the second aspect, the cavities and a plurality of cavity interconnections are formed in a plurality of layers, wherein each layer has different cavity dimensions. 
     In an embodiment of the second aspect, the porous biocompatible matrix is configured to promote vascularization and interfere with barrier cell layer formation within the matrix, whereby the biocompatible matrix is suitable for long-term analyte transport in vivo. 
     In an embodiment of the second aspect, the porous biocompatible matrix comprises a plurality of fibers. 
     In an embodiment of the second aspect, the plurality of fibers are selected from the group consisting of woven fibers and non-woven fibers. 
     In an embodiment of the second aspect, at least a portion of the plurality of fibers comprise an electrode core. 
     In an embodiment of the second aspect, the fibers comprising an electrode core comprise a membrane surrounding the electrode core. 
     In an embodiment of the second aspect, the electroactive surfaces comprise an electroactive surface of at least one working electrode. 
     In an embodiment of the second aspect, the electroactive surfaces further comprise an electroactive surface of at least one reference electrode. 
     In an embodiment of the second aspect, the sensor further comprises sensor electronics operably connected to the electroactive surfaces. 
     In an embodiment of the second aspect, the sensor is configured to measure glucose. 
     In a third aspect, a sensor for measuring an analyte in a host is provided, the sensor comprising an analyte sensing mechanism disposed within a porous biocompatible matrix. 
     In an embodiment of the third aspect, dimensions of the porous biocompatible matrix comprising the sensing mechanism are less than about 1000 microns. 
     In an embodiment of the third aspect, the porous biocompatible matrix and the sensing mechanism are configured to resist barrier cell layer formation. 
     In an embodiment of the third aspect, the sensing mechanism comprises a working electrode. 
     In an embodiment of the third aspect, the working electrode is a wire electrode. 
     In an embodiment of the third aspect, the working electrode is a substantially planar electrode. 
     In an embodiment of the third aspect, the sensing mechanism further comprises an enzyme domain disposed over the working electrode. 
     In an embodiment of the third aspect, the sensing mechanism further comprises an electrode domain disposed between the enzyme domain in the working electrode. 
     In an embodiment of the third aspect, the sensing mechanism further comprises a resistance domain disposed over the enzyme domain. 
     In an embodiment of the third aspect, the sensing mechanism further comprises a bioprotective domain disposed over the enzyme domain. 
     In an embodiment of the third aspect, the sensing mechanism further comprises a resistance domain disposed between the enzyme domain and the bioprotective domain. 
     In a fourth aspect, a method of detecting an analyte within tissue of a host is provided, comprising implanting an analyte sensor according to the first aspect within the tissue; allowing tissue to grow within at least a portion of some of the passageways; and detecting current flowing through the working electrode or a voltage of the working electrode, thereby detecting the quantity of an analyte within the passageways. 
     In an embodiment of the fourth aspect, allowing tissue to grow within at least a portion of some of the passageways comprises formation of one or more host tissue materials within the passageways, the host tissue materials selected from the group consisting of a fibrous matrix, blood vessels, fibroblasts, connective granular tissue, macrophages, and foreign body giant cells. 
     In a fifth aspect, a method of manufacturing an analyte sensor according to the first aspect is provided, comprising forming a first layer of an electrically non-conductive material; depositing a layer of an electrically conductive material on the first layer of the electrically non-conductive material; depositing a second layer of electrically non-conductive material onto the layer of electrically conductive material; and etching the deposited layers to form interconnected pores in the layers, at least some of the pores extending from an exterior surface of the electrically non-conductive material to the electrically conductive material. 
     In an embodiment of the fifth aspect, at least one of the forming and depositing steps comprises dip coating. 
     In an embodiment of the fifth aspect, at least one of the forming and depositing steps comprises vapor deposition. 
     In an embodiment of the fifth aspect, at least one of the forming and depositing steps comprises lamination. 
     In an embodiment of the fifth aspect, at least one of the forming and depositing steps comprises spin coating. 
     In an embodiment of the fifth aspect, the method further comprises coating the pores&#39; interior surfaces with an analyte membrane system, the analyte membrane system comprising at least one of a component that affects the analyte&#39;s rate of diffusion through the analyte membrane system and a component that reacts with the analyte to produce a species that electrochemically reacts at the surface of the electrically conductive material. 
     In an embodiment of the fifth aspect, coating comprises dip coating. 
     In an embodiment of the fifth aspect, coating comprises vapor deposition. 
     In an embodiment of the fifth aspect, coating comprises spin coating. 
     In a sixth aspect, a method of manufacturing an analyte sensor according to the first aspect is provided, comprising depositing a first layer of an electrically non-conductive material onto a substrate; depositing a first layer of an electrically conductive material onto the first layer of the electrically non-conductive material; depositing a second layer of electrically non-conductive material onto the first layer of electrically conductive material; depositing a second layer of electrically conductive material onto the second layer of electrically non-conductive material; depositing a third layer of electrically non-conductive material onto the second layer of electrically conductive material; depositing a third layer of electrically conductive material onto the third layer of electrically non-conductive material; depositing a fourth layer of electrically non-conductive material onto the third layer of electrically conductive material; and etching the deposited layers to form pores in the layers, at least some of the pores extending from an exterior surface of the electrically non-conductive material and contacting all three layers of electrically conductive material. 
     In an embodiment of the sixth aspect, at least one of the depositing steps comprises dip coating. 
     In an embodiment of the sixth aspect, at least one of the depositing steps comprises vapor deposition. 
     In an embodiment of the sixth aspect, at least one of the depositing steps comprises lamination. 
     In an embodiment of the sixth aspect, the method further comprises coating the pores&#39; interior surfaces with an analyte membrane system, the analyte membrane system comprising at least one of a component that affects the analyte&#39;s rate of diffusion through the analyte membrane system and a component that reacts with the analyte to produce a species that electrochemically reacts at the surface of the electrically conductive material. 
     In an embodiment of the sixth aspect, coating comprises dip coating. 
     In an embodiment of the sixth aspect, coating comprises vapor deposition. 
     In a seventh aspect a method of manufacturing an analyte sensor according to the first aspect is provided, comprising depositing a first layer of an electrically conductive material onto a substrate; depositing a first layer of an electrically non-conductive material onto the first layer of electrically conductive material; depositing a second layer of electrically conductive material onto the first layer of electrically non-conductive material; depositing a second layer of electrically non-conductive material onto the second layer of electrically conductive material; depositing a third layer of electrically conductive material onto the second layer of electrically non-conductive material; and etching the deposited layers to form pores in the layers, at least some of the pores extending from an exterior surface of either the first or third layers of electrically conductive material and contacting the second layer of electrically conductive material and the other first or third layers of electrically conductive material. 
     In an embodiment of the seventh aspect, at least one of the depositing steps comprises dip coating. 
     In an embodiment of the seventh aspect, at least one of the depositing steps comprises vapor deposition. 
     In an embodiment of the seventh aspect, at least one of the depositing steps comprises lamination. 
     In an embodiment of the seventh aspect, at least one of the depositing steps comprises spin coating. 
     In an embodiment of the seventh aspect, the method further comprises coating the pores&#39; interior surfaces with an analyte membrane system, the analyte membrane system comprising at least one of a component that affects the analyte&#39;s rate of diffusion through the analyte membrane system and a component that reacts with the analyte to produce a species that electrochemically reacts at the surface of the electrically conductive material. 
     In an embodiment of the seventh aspect, coating comprises dip coating. 
     In an embodiment of the seventh aspect, coating comprises vapor deposition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts foreign body response near the surface of an implanted biosensor. 
         FIG. 2  depicts foreign body response near the surface of an implanted biosensor in the presence of a cell-disruptive biointerface. 
         FIG. 3  depicts cell disruptive material comprising a solid matrix with cavities therein. 
         FIG. 4  depicts an implantable analyte sensor comprising a membrane system and a cell-disruptive biointerface. 
         FIG. 5A  depicts a side schematic view of a sensing biointerface that incorporates electrodes within fibers. 
         FIG. 5B  depicts a cross-sectional view through line  5 B- 5 B of  FIG. 5A  showing some fibers that form the biointerface in the embodiment of  FIG. 5A . 
         FIG. 5C  depicts a cross-sectional expanded view of one fiber shown in  FIG. 5B , showing an electrode surrounded by a membrane system. 
         FIG. 6A  depicts a schematic surface view of a sensing cell-disruptive biointerface that comprises pores and incorporates electrodes within the biointerface. 
         FIG. 6B  depicts a cross-sectional view through line  6 B- 6 B of  FIG. 6A , showing the through porosity of the biointerface. 
         FIG. 6C  depicts a cross-sectional expanded view of a portion of the cross-sectional view of  FIG. 6B  showing the membrane system within the pores of the biointerface. 
         FIG. 7  depicts a flowchart of a process for manufacturing a sensing biointerface. 
         FIGS. 8A and 8B  depict cross-sectional views of a sensing cell-disruptive biointerface comprising a mesh structure that incorporates electrodes as part of the biointerface. 
         FIG. 9A  depicts a schematic surface view of a sensing biointerface that includes a biointerface matrix surrounding a sensing mechanism. 
         FIG. 9B  depicts a wire-like analyte sensing mechanism in one exemplary embodiment, which can be incorporated into the biointerface matrix to form the sensing biointerface. 
         FIG. 9C  depicts a schematic cross-sectional view of a sensing biointerface in one embodiment, including a single wire sensor incorporated into a cylindrical shaped biointerface matrix. 
         FIG. 9D  is a schematic cross-sectional view of a sensing biointerface in another embodiment wherein the sensing biointerface includes a plurality of wire electrodes. 
         FIG. 9E  is a schematic cross-sectional view of a sensing biointerface in yet another embodiment wherein the sensing biointerface comprises a spiral wire electrode. 
         FIG. 9F  is a schematic cross-sectional view of a sensing biointerface in yet another embodiment wherein the sensing biointerface comprises a wire electrode in a tortuous path within the biointerface matrix. 
         FIG. 9G  is a schematic cross-sectional view of a sensing biointerface in yet another embodiment, wherein the sensing biointerface comprises a wire electrode, a biointerface matrix, and a sensor body. 
         FIG. 10  is a flow chart that illustrates a process of dispensing a fibrous biointerface matrix onto a sensing mechanism. 
         FIG. 11  is a flow chart that illustrates a process of molding a biointerface matrix onto a sensing mechanism. 
         FIG. 12  is a flow chart that illustrates an alternative process of molding a biointerface matrix onto a sensing mechanism. 
         FIG. 13  is a flow chart that illustrates a process of wrapping a sensing mechanism with a biointerface membrane. 
         FIG. 14  is a flow chart that illustrates a process of inserting a sensing mechanism into a biointerface matrix. 
         FIG. 15  depicts a block diagram of the sensor electronics in one embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Described herein are structures for use in implantable analyte sensors. The term “analyte” as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to a substance or chemical constituent in a biological fluid (e.g., blood, urine, extracellular fluid) that is intended to be analyzed. In one embodiment, the analyte is blood glucose. The term “detection” of an analyte as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to detecting the presence of an analyte and/or detecting the amount of an analyte present. In one embodiment, detection of an analyte provides a measure of the concentration of the analyte in a biological fluid. 
     One method of addressing foreign body response to implanted biosensors is illustrated in  FIG. 2 , which depicts FBR in the presence of a biointerface. The biointerface is constructed of a barrier cell disruptive material  49  that is incorporated on the surface of the biosensor  50 . The material  49  disrupts the continuity of cells  41  so that an impermeable barrier cell layer does not form. All of the FBR cells and materials may be present, such as foreign body giant cells  41 , fibroblasts  43 , fibrous matrix  44 , blood vessels  45 , and connective granular tissue; however, orderly formation of the barrier cell, intermediate, and outer layers is inhibited. Thus, transport of the analyte to the sensor is not blocked and blood vessels can grow closer to the biosensor  50 . 
     The biointerface comprising barrier cell disruptive material  49  may be constructed from non-woven fibers, such as illustrated in  FIG. 2 . Alternatively, the biointerface may be woven fiber mesh.  FIG. 3  depicts another barrier cell disruptive layer  66 . The barrier cell disruptive layer  66  consists of a solid material  62  in which cavities  64  are formed as depicted in  FIG. 3 . In order to ensure that the analyte can pass from the tissue side of the barrier cell disruptive layer  66  to the sensor side, it is advantageous that at least some cavities  64  extend uninterrupted from the tissue side to the sensor side. 
       FIG. 4  depicts an implantable analyte sensor configuration that makes use of a barrier cell disruptive biointerface  33 . The sensor typically comprises a working electrode  21 , a reference electrode  20 , and an optional counter electrode  22 . These electrodes may be insulated from each other by insulative layers  28  and further be housed in housing  10 . The reference electrode  20  provides a stable-voltage electrode relative to which the voltage of the working electrode  21  can be controlled or detected. The optional counter electrode  22  draws the current flowing through the working electrode  20  so that the current through the reference electrode  20  is kept at a minimum, thus maintaining its voltage stability. The end surfaces of all three electrodes are advantageously coated with membrane system  32 , which may comprise a layer of glucose oxidase enzyme. The membrane system  32  may then be coated with the barrier cell disruptive biointerface  33 . 
     Biointerfaces for promoting tissue in-growth adjacent to a biosensor are described in more detail in U.S. Pat. No. 6,702,857, which is incorporated herein by reference in its entirety. 
     The Sensing Biointerface 
     Some embodiments of the present invention comprise combining the sensor electrodes and membrane system into the cell-disruptive biointerface (for example, a porous biointerface material). Thus, the cell-disruptive biointerface becomes a sensing biointerface. This combination may be embodied by providing an electrically non-conductive biocompatible matrix within which at least the working electrode can be incorporated, which together function as the biointerface. However, in some alternative embodiments, the biocompatible matrix comprises electrically conductive materials, for example, that function as a working electrode, without a requirement for a non-conductive material within the matrix. Passageways (e.g., interconnected pores or cavities) can be formed within the matrix or designed into the matrix to provide openings for FBR tissue in-growth near to the working electrode and provide analyte access to the working electrode. 
     The term “electrically non-conductive” material as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to material that allows no current or little current to flow under the voltages typically applied to the electrodes in biosensors. Non-limiting examples of electrically non-conductive materials include insulators such as plastics and other non-conductive polymers, non-conductive inorganics such as oxides, and semi-conductors exhibiting low conductivity such as un-doped silicon. 
     The term “electrically conductive” material as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to material through which readily detectable current flows under the voltages typically applied to the electrodes in biosensors. Non-limiting examples of electrically conductive material include metallic conductors, conductive polymers, and semi-conductors with appreciable conductivity. 
     Thus, some embodiments provide a porous biointerface, also referred to as a biointerface matrix, comprising a biocompatible matrix material wherein electroactive surfaces, with or without a membrane system coating, are distributed within at least some pores in the biointerface. In one embodiment, the sensing mechanism, being within the biointerface, includes a morphology substantially similar, complementary, or integral with the morphology of the cell disruptive biointerface, thereby enabling manipulation of the foreign body response to the sensing mechanism in a manner substantially similar to that of a porous biointerface material (e.g., cell-disruptive biointerface). While not wishing to be bound by theory, it is believed that by providing a sensing mechanism integrated throughout the biointerface, the cellular mechanisms of the foreign body response will not substantially attack or attempt to block the sensing mechanism as a foreign body as happens with conventional analyte sensors. A few exemplary embodiments are provided herein, however one skilled in the art will appreciate the variety of systems and methods that can be implemented in order to form the sensing biointerface described herein. 
     The biocompatible matrix, also referred to as biointerface matrix, can comprise a variety of porous structures. In one embodiment, the biocompatible matrix comprises a plurality of woven or non-woven fibers, one example of which is described with reference to  FIGS. 5A to 5C . In another embodiment, the biocompatible matrix comprises a solid structure with interconnected pores or cavities formed therein, one example of which is described with reference to  FIGS. 6A to 6C . In another example, the biocompatible matrix comprises a scaffolding defined by uniform or non-uniform mesh, wire, or fiber layers, formed with a through-porosity, one example of which is described with reference to  FIGS. 8A and 8B . 
     In some embodiments, the interconnected cavities (also referred to as pores or passageways) are dimensioned such that fibrous matrix, blood vessels, fibroblasts, connective granular tissue, macrophages, and/or foreign body giant cells can migrate through or grow therein. In some embodiments, the interconnected cavities can be defined by one dimension (for example, shortest, average, or longest) of the cavity. In some embodiments, the dimensions can be defined by measuring techniques known in the art of porous materials (for example, bubble point test). It should be noted, that the term “nominal pore size” in the context of the biocompatible matrix in certain embodiments is derived from methods of analysis common to the membrane trade, such as the ability of the membrane to filter particles of a particular size, or the resistance of the membrane to the flow of fluids. It is noted, however that the term “nominal pore size” may not actually indicate the size or shape of the cavities, which in reality may have some degree of variability. Accordingly, in some embodiments, the term “nominal pore size” is a manufacturer&#39;s convention used to identify a particular membrane of a particular commercial source which can have a certain bubble point. One example of a bubble point measurement is described in Pharmaceutical Technology May 1983 pp. 36 to 42, which is incorporated herein by reference in its entirety. 
     In some embodiments, a substantial number of the cavities defined using any of the methods described above, are greater than or equal to about 20 microns in one dimension. In some other embodiments, a substantial number of the cavities are greater than or equal to about 30, 40, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180, 200, 240, 280, 320, 360, 400, 500, 600, 700 microns in one dimension. 
     In some embodiments, a substantial number of the cavities, defined using any of the methods described above, are less than or equal to about 1000 microns in one dimension. In other embodiments, a substantial number of the cavities are less than or equal to about 900, 800, 700, 600, 500, 400, 360, 320, 280, 240, 200, 180, 160, 140, 120, 100 microns in one dimension. 
     In one alternative embodiment, wherein a substantial number of cavities are greater than or equal to about 20 microns in one dimension, there can be additional cavities that are less than or equal to about 20 microns in their shortest dimension interspersed therein. In another alternative embodiment, wherein a substantial number of cavities are greater than or equal to about 20 microns in one dimension, cavity dimensions can be gradually increased or decreased progressively through the layer, including some cavities that are less than or equal to about 20 microns in one dimension. 
     In some embodiments, the biocompatible matrix includes non-woven materials, woven materials, or other such materials, such that a porous structure is formed from the cavities between the fibers. In these embodiments, for example, the fibers are formed by structural elements that provide the three-dimensional conformation. Therefore in these embodiments, the biocompatible matrix may be defined by a fiber size of between about 1 and 100 microns in all but the longest dimension and a sufficient number of cavities of a size and structure to allow inflammatory cells (for example, macrophages) to completely enter therein through the apertures that define the cavities. 
     In some alternative embodiments, a bioactive agent is incorporated into the above described biocompatible matrix, which then diffuses out into the environment adjacent to the sensing region in order to modify the tissue response of the host to the device, for example, such as is described in U.S. Publication No. US-2005-0031689-A1. Additionally or alternately, a bioactive agent can be administered locally at the exit-site or implantation-site. Suitable bioactive agents are those that modify the host&#39;s tissue response to the sensor, for example anti-inflammatory agents, anti-infective agents, anesthetics, inflammatory agents, growth factors, immunosuppressive agents, antiplatelet agents, anti-coagulants, anti-proliferates, ACE inhibitors, cytotoxic agents, anti-barrier cell compounds, vascularization-inducing compounds, anti-sense molecules, or mixtures thereof, such as are described in more detail in co-pending U.S. Patent Publication No. US-2005-0031689-A1. 
     In embodiments wherein the biocompatible matrix is designed to enhance short-term (e.g., from about 1 to about 30 days) lifetime or performance of the sensor, a suitable bioactive agent can be chosen to ensure that tissue ingrowth does not substantially occur within the cavities of the biocompatible matrix. For example, by providing a tissue modifying bioactive agent, such as an anti-inflammatory agent (for example, Dexamethasone), substantial tissue ingrowth can be inhibited, at least in the short term, in order to maintain sufficient glucose transport through the pores of the biocompatible matrix to maintain a stable sensitivity. 
     In embodiments wherein the biocompatible matrix is designed to enhance long-term (e.g., from about a day to about a year or more) lifetime or performance of the sensor, a suitable bioactive agent, such as a vascularization-inducing compound or anti-barrier cell compound, can be chosen to encourage tissue ingrowth without barrier cell formation. 
     The architectures of the first and second domains may support vascularized tissue growth in or around the sensing biointerface, interfere with and resist barrier cell layer formation, and allow the transport of analytes through the porous biocompatible matrix to the sensing mechanism. However, certain outside influences, for example, faulty surgical techniques, acute or chronic movement of the implant, or other surgery-, patient-, and/or implantation site-related conditions, can create acute and/or chronic inflammation at the implant site. When this occurs, the sensing biointerface architecture alone may not be sufficient to overcome the acute and/or chronic inflammation. Accordingly, in some embodiments, the membrane architecture can benefit from additional mechanisms that aid in reducing this acute and/or chronic inflammation that can produce a barrier cell layer and/or a fibrotic capsule surrounding the implant, resulting in compromised solute transport through the membrane. 
     In general, the inflammatory response to biomaterial implants can be divided into two phases. The first phase consists of mobilization of mast cells and then infiltration of predominantly polymorphonuclear (PMN) cells. This phase is termed the acute inflammatory phase. Over the course of days to weeks, chronic cell types that comprise the second phase of inflammation replace the PMNs. Macrophage and lymphocyte cells predominate during this phase. While not wishing to be bound by any particular theory, it is believed that short-term stimulation of vascularization, or short-term inhibition of scar formation or barrier cell layer formation, provides protection from scar tissue formation, thereby providing a stable platform for sustained maintenance of the altered foreign body response. 
     Accordingly, bioactive intervention can modify the foreign body response in the early weeks of foreign body capsule formation, thereby fundamentally altering the long-term behavior of the foreign body capsule. Additionally, it is believed that the sensing biointerfaces of the preferred embodiments can advantageously benefit from bioactive intervention to overcome sensitivity of the membrane to implant procedure, motion of the implant, or other factors, which are known to otherwise cause inflammation, scar formation, and hinder device function in vivo. 
     In general, bioactive agents that are believed to modify tissue response include anti-inflammatory agents, anti-infective agents, anesthetics, inflammatory agents, growth factors, angiogenic (growth) factors, adjuvants, wound factors, resorbable device components, immunosuppressive agents, antiplatelet agents, anticoagulants, ACE inhibitors, cytotoxic agents, anti-barrier cell compounds, vascularization compounds, anti-sense molecules, and the like. In some embodiments, preferred bioactive agents include S1P (Sphingosine-1-phosphate), Monobutyrin, Cyclosporin A, Anti-thrombospondin-2, Rapamycin (and its derivatives), and Dexamethasone. However, other bioactive agents, biological materials (for example, proteins), or even non-bioactive substances can be preferred for incorporation into the membranes of preferred embodiments. 
     Bioactive agents suitable for use in the preferred embodiments are loosely organized into two groups: anti-barrier cell agents and vascularization agents. These designations reflect functions that are believed to provide short-term solute transport through the sensing biointerface, and additionally extend the life of a healthy vascular bed and hence solute transport through the sensing biointerface long term in vivo. However, not all bioactive agents can be clearly categorized into one or other of the above groups; rather, bioactive agents generally comprise one or more varying mechanisms for modifying tissue response and can be generally categorized into one or both of the above-cited categories. 
     Anti-Barrier Cell Agents 
     Generally, anti-barrier cell agents include compounds exhibiting affects on macrophages and foreign body giant cells (FBGCs). It is believed that anti-barrier cell agents prevent closure of the barrier to solute transport presented by macrophages and FBGCs at the device-tissue interface during FBC maturation. 
     Anti-barrier cell agents generally include mechanisms that inhibit foreign body giant cells and/or occlusive cell layers. For example, Super Oxide Dismutase (SOD) Mimetic, which utilizes a manganese catalytic center within a porphyrin like molecule to mimic native SOD and effectively remove superoxide for long periods, thereby inhibiting FBGC formation at the surfaces of biomaterials in vivo, may be incorporated into a sensing biointerface of a preferred embodiment. 
     Anti-barrier cell agents can include anti-inflammatory and/or immunosuppressive mechanisms that affect the wound healing process, for example, healing of the wound created by the incision into which an implantable device is inserted. Cyclosporine, which stimulates very high levels of neovascularization around biomaterials, can be incorporated into a sensing biointerface of a preferred embodiment [see U.S. Pat. No. 5,569,462 to Martinson et al., which is incorporated herein by reference in its entirety.] Alternatively, Dexamethasone, which abates the intensity of the FBC response at the tissue-device interface, can be incorporated into a sensing biointerface of a preferred embodiment. Alternatively, Rapamycin, which is a potent specific inhibitor of some macrophage inflammatory functions, can be incorporated into a sensing biointerface of a preferred embodiment. 
     Other suitable medicaments, pharmaceutical compositions, therapeutic agents, or other desirable substances can be incorporated into the membranes of preferred embodiments, including, but not limited to, anti-inflammatory agents, anti-infective agents, and anesthetics. 
     Generally, anti-inflammatory agents reduce acute and/or chronic inflammation adjacent to the implant, in order to decrease the formation of a FBC capsule to reduce or prevent barrier cell layer formation. Suitable anti-inflammatory agents include but are not limited to, for example, nonsteroidal anti-inflammatory drugs (NSAIDs) such as acetaminophen, aminosalicylic acid, aspirin, celecoxib, choline magnesium trisalicylate, diclofenac potassium, diclofenac sodium, diflunisal, etodolac, fenoprofen, flurbiprofen, ibuprofen, indomethacin, interleukin (IL)-10, IL-6 mutein, anti-IL-6 iNOS inhibitors (for example, L-NAME or L-NMDA), Interferon, ketoprofen, ketorolac, leflunomide, melenamic acid, mycophenolic acid, mizoribine, nabumetone, naproxen, naproxen sodium, oxaprozin, piroxicam, rofecoxib, salsalate, sulindac, and tolmetin; and corticosteroids such as cortisone, hydrocortisone, methylprednisolone, prednisone, prednisolone, betamethesone, beclomethasone dipropionate, budesonide, dexamethasone sodium phosphate, flunisolide, fluticasone propionate, paclitaxel, tacrolimus, tranilast, triamcinolone acetonide, betamethasone, fluocinolone, fluocinonide, betamethasone dipropionate, betamethasone valerate, desonide, desoximetasone, fluocinolone, triamcinolone, triamcinolone acetonide, clobetasol propionate, and dexamethasone. 
     Generally, immunosuppressive and/or immunomodulatory agents interfere directly with several key mechanisms necessary for involvement of different cellular elements in the inflammatory response. Suitable immunosuppressive and/or immunomodulatory agents include anti-proliferative, cell-cycle inhibitors, (for example, paclitaxel, cytochalasin D, infiximab), taxol, actinomycin, mitomycin, thospromote VEGF, estradiols, NO donors, QP-2, tacrolimus, tranilast, actinomycin, everolimus, methothrexate, mycophenolic acid, angiopeptin, vincristing, mitomycine, statins, C MYC antisense, sirolimus (and analogs), RestenASE, 2-chloro-deoxyadenosine, PCNA Ribozyme, batimstat, prolyl hydroxylase inhibitors, PPARγ ligands (for example troglitazone, rosiglitazone, pioglitazone), halofuginone, C-proteinase inhibitors, probucol, BCP671, EPC antibodies, catchins, glycating agents, endothelin inhibitors (for example, Ambrisentan, Tesosentan, Bosentan), Statins (for example, Cerivasttin),  E. coli  heat-labile enterotoxin, and advanced coatings. 
     Generally, anti-infective agents are substances capable of acting against infection by inhibiting the spread of an infectious agent or by killing the infectious agent outright, which can serve to reduce immuno-response without inflammatory response at the implant site. Anti-infective agents include, but are not limited to, anthelmintics (mebendazole), antibiotics including aminoclycosides (gentamicin, neomycin, tobramycin), antifungal antibiotics (amphotericin b, fluconazole, griseofulvin, itraconazole, ketoconazole, nystatin, micatin, tolnaftate), cephalosporins (cefaclor, cefazolin, cefotaxime, ceftazidime, ceftriaxone, cefuroxime, cephalexin), beta-lactam antibiotics (cefotetan, meropenem), chloramphenicol, macrolides (azithromycin, clarithromycin, erythromycin), penicillins (penicillin G sodium salt, amoxicillin, ampicillin, dicloxacillin, nafcillin, piperacillin, ticarcillin), tetracyclines (doxycycline, minocycline, tetracycline), bacitracin; clindamycin; colistimethate sodium; polymyxin b sulfate; vancomycin; antivirals including acyclovir, amantadine, didanosine, efavirenz, foscarnet, ganciclovir, indinavir, lamivudine, nelfinavir, ritonavir, saquinavir, silver, stavudine, valacyclovir, valganciclovir, zidovudine; quinolones (ciprofloxacin, levofloxacin); sulfonamides (sulfadiazine, sulfisoxazole); sulfones (dapsone); furazolidone; metronidazole; pentamidine; sulfanilamidum crystallinum; gatifloxacin; and sulfamethoxazole/trimethoprim. 
     Vascularization Agents 
     Generally, vascularization agents include substances with direct or indirect angiogenic properties. In some cases, vascularization agents may additionally affect formation of barrier cells in vivo. By indirect angiogenesis, it is meant that the angiogenesis can be mediated through inflammatory or immune stimulatory pathways. It is not fully known how agents that induce local vascularization indirectly inhibit barrier-cell formation; however it is believed that some barrier-cell effects can result indirectly from the effects of vascularization agents. 
     Vascularization agents include mechanisms that promote neovascularization and accelerate wound healing around the membrane and/or minimize periods of ischemia by increasing vascularization close to the tissue-device interface. Sphingosine-1-Phosphate (S1P), which is a phospholipid possessing potent angiogenic activity, is incorporated into a sensing biointerface of a preferred embodiment. Monobutyrin, which is a potent vasodilator and angiogenic lipid product of adipocytes, is incorporated into a sensing biointerface of a preferred embodiment. In another embodiment, an anti-sense molecule (for example, thrombospondin-2 anti-sense), which increases vascularization, is incorporated into a sensing biointerface. 
     Vascularization agents can include mechanisms that promote inflammation, which is believed to cause accelerated neovascularization and wound healing in vivo. In one embodiment, a xenogenic carrier, for example, bovine collagen, which by its foreign nature invokes an immune response, stimulates neovascularization, and is incorporated into a sensing biointerface of the preferred embodiments. In another embodiment, Lipopolysaccharide, which is a potent immunostimulant, is incorporated into a sensing biointerface. In another embodiment, a protein, for example, a bone morphogenetic protein (BMP), which is known to modulate bone healing in tissue, is incorporated into a sensing biointerface of a preferred embodiment. 
     Generally, angiogenic agents are substances capable of stimulating neovascularization, which can accelerate and sustain the development of a vascularized tissue bed at the tissue-device interface. Angiogenic agents include, but are not limited to, Basic Fibroblast Growth Factor (bFGF), (also known as Heparin Binding Growth Factor-II and Fibroblast Growth Factor II), Acidic Fibroblast Growth Factor (aFGF), (also known as Heparin Binding Growth Factor-I and Fibroblast Growth Factor-I), Vascular Endothelial Growth Factor (VEGF), Platelet Derived Endothelial Cell Growth Factor BB (PDEGF-BB), Angiopoietin-1, Transforming Growth Factor Beta (TGF-Beta), Transforming Growth Factor Alpha (TGF-Alpha), Hepatocyte Growth Factor, Tumor Necrosis Factor-Alpha (TNF-Alpha), Placental Growth Factor (PLGF), Angiogenin, Interleukin-8 (IL-8), Hypoxia Inducible Factor-I (HIF-1), Angiotensin-Converting Enzyme (ACE) Inhibitor Quinaprilat, Angiotropin, Thrombospondin, Peptide KGHK, Low Oxygen Tension, Lactic Acid, Insulin, Copper Sulphate, Estradiol, prostaglandins, cox inhibitors, endothelial cell binding agents (for example, decorin or vimentin), glenipin, hydrogen peroxide, nicotine, and Growth Hormone. 
     Generally, pro-inflammatory agents are substances capable of stimulating an immune response in host tissue, which can accelerate or sustain formation of a mature vascularized tissue bed. For example, pro-inflammatory agents are generally irritants or other substances that induce chronic inflammation and chronic granular response at the wound-site. While not wishing to be bound by theory, it is believed that formation of high tissue granulation induces formation of blood vessels, which supply an adequate or rich supply of analytes to the device-tissue interface. Pro-inflammatory agents include, but are not limited to, xenogenic carriers, Lipopolysaccharides,  S. aureus  peptidoglycan, and proteins. 
     Other substances that can be incorporated into membranes of preferred embodiments include various pharmacological agents, excipients, and other substances well known in the art of pharmaceutical formulations. 
     Bioactive Agent Delivery Systems and Methods 
     There are a variety of systems and methods by which the bioactive agent may be incorporated into the sensing biointerface of the preferred embodiments. In some embodiments, the bioactive agent is incorporated at the time of manufacture of the sensing biointerface. For example, the bioactive agent can be blended into any of the membrane and/or biointerface materials prior to curing, or subsequent to their manufacture, for example, by coating, imbibing, solvent-casting, or sorption of the bioactive agent into the sensing biointerface. Although the bioactive agent is preferably incorporated into the sensing biointerface, in some embodiments the bioactive agent can be administered concurrently with, prior to, or after implantation of the device systemically, for example, by oral administration, or locally, for example, by subcutaneous injection near the implantation site. In some embodiments, a combination of bioactive agent(s) incorporated in the sensing biointerface and bioactive agent(s) administration locally and/or systemically is used. 
     In embodiments wherein the sensing biointerface of the preferred embodiments include a bioactive agent, the bioactive agent can be incorporated into numerous aspects of the device, including electrically conductive or non-conductive portions, insulator or membrane materials, and/or other materials that form the biointerface. In some embodiments, the bioactive agent is sprinkled or sprayed on the surface of the device, for example, using known deposition techniques. 
     The bioactive agent can include a carrier matrix, wherein the matrix includes one or more of collagen, a particulate matrix, a resorbable or non-resorbable matrix, a controlled-release matrix, and/or a gel. In some embodiments, the carrier matrix includes a reservoir, wherein a bioactive agent is encapsulated within a microcapsule. The carrier matrix can include a system in which a bioactive agent is physically entrapped within a polymer network. In some embodiments, the bioactive agent is cross-linked with certain materials of the sensing biointerface, while in others the bioactive agent is sorbed into the materials, for example, by adsorption, absorption, or imbibing. The bioactive agent can be deposited in or on the sensing biointerface, for example, by coating, filling, or solvent casting. In certain embodiments, ionic and nonionic surfactants, detergents, micelles, emulsifiers, demulsifiers, stabilizers, aqueous and oleaginous carriers, solvents, preservatives, antioxidants, or buffering agents are used to incorporate the bioactive agent into the sensing biointerface. The bioactive agent can be incorporated into a polymer using techniques such as described above, and the polymer can be used to form the sensing biointerface, coatings on the sensing biointerface, portions of the sensing biointerface, and/or a portion of an implantable device. 
     The sensing biointerface can be manufactured using techniques such as described here and/or as are known in the art. The bioactive agent can be sorbed into the sensing biointerface, for example, by soaking the sensing biointerface for a length of time (for example, from about an hour or less to about a week or more, preferably from about 4, 8, 12, 16, or 20 hours to about 1, 2, 3, 4, 5, or 7 days). 
     The bioactive agent can be blended into uncured polymer prior to forming the sensing biointerface (or a portion thereof). The sensing biointerface (or a portion thereof) is then cured and the bioactive agent thereby cross-linked and/or encapsulated within the polymer that forms the sensing biointerface (or a portion thereof). For example, Monobutyrin was covalently bonded to a silicone matrix in such a manner that is slowly cleavable under in vivo conditions. The alcohol groups of Monobutyrin react with a silanol group, resulting in a C—O—Si bond. This bond is known to be susceptible to hydrolysis, and is therefore cleaved to yield the original alcohol and silanol. Thus, the Monobutyrin is released from the silicone matrix according to the rate of hydrolysis. Other bioactive agents, such as Dexamethasone, comprise alcohol groups and can be bound to a silicone matrix in a similar manner. 
     In yet another embodiment, microspheres are used to encapsulate the bioactive agent. The microspheres can be formed of biodegradable polymers, most preferably synthetic polymers or natural polymers such as proteins and polysaccharides. In this context, the term polymer is used to refer to both to synthetic polymers and proteins. U.S. Pat. No. 6,281,015, which is incorporated herein by reference in its entirety, discloses some systems and methods that can be used in conjunction with the preferred embodiments. In general, bioactive agents can be incorporated in (1) the polymer matrix forming the microspheres, (2) microparticle(s) surrounded by the polymer which forms the microspheres, (3) a polymer core within a protein microsphere, (4) a polymer coating around a polymer microsphere, (5) mixed in with microspheres aggregated into a larger form, or (6) a combination thereof. Bioactive agents can be incorporated as particulates or by co-dissolving the factors with the polymer. Stabilizers can be incorporated by addition of the stabilizers to the factor solution prior to formation of the microspheres. 
     The bioactive agent can be incorporated into a hydrogel and coated or otherwise deposited in or on the sensing biointerface. Some hydrogels suitable for use in the preferred embodiments include cross-linked, hydrophilic, three-dimensional polymer networks that are highly permeable to the bioactive agent and are triggered to release the bioactive agent based upon exposure to a stimulus. 
     The bioactive agent can be incorporated into the sensing biointerface by solvent casting, wherein a solution including dissolved bioactive agent is disposed on the surface of the sensing biointerface, after which the solvent is removed to form a coating on the membrane surface. 
     In yet another embodiment, the interconnected cavities of the sensing biointerface are filled with the bioactive agent. Preferably, a bioactive agent, with or without a carrier matrix, fills the cavities of the membrane, depending on the loading and release properties desired, which are discussed in more detail below. 
     Short-term release of the bioactive agent in the preferred embodiments generally refers to release over a period of from about 1 day or less to about 2, 3, 4, 5, 6, or 7 days, 2 or 3 weeks, 1 month, or more. More preferably, the short-term release of the bioactive agent occurs over from about 14, 15, 16, 17, or 18 days up to about 19, 20, or 21 days. 
     Some devices, such as implantable analyte measuring-devices, drug delivery devices, and cell transplantation devices, that require transport of solutes across the device-tissue interface for proper function, tend to lose their function after the first few days following implantation. At least one reason for this loss of function is the lack of direct contact with circulating fluid for appropriate analyte transport to the device. Therefore, in some embodiments, short-term release of certain bioactive agents, for example vascularization agents, can increase the circulating fluid to the device for an extended period of time. 
     Additionally, it is believed that short-term release of the bioactive agent can have a positive effect of the functionality of porous sensing biointerface during the initial tissue ingrowth period prior to formation of a capillary bed. For example, when a device requiring analyte transport across its device-tissue interface is implanted, a “sleep period” can occur which begins as early as the first day after implantation and extends as far as one month after implantation. However shorter sleep periods are more common. During this sleep period, extensive ingrowth of tissue into the porous structure causes the inflammatory cells responsible for facilitating wound healing to proliferate within the local environment of the wound region. Because these cells are respiring, they consume some or all of the analyte (e.g., glucose) and oxygen that is within the wound environment, which has been shown to block adequate flow of analytes to the implantable device. Accordingly in some embodiments, it is believed that short-term release of certain bioactive agents, for example vascularization agents, can aid in providing adequate vascularization to substantially overcome the effects of the sleep period, and thereby allow sufficient analytes to pass through to the implantable device. 
     Additionally, it is believed that short-term release of the bioactive agent can have an enhanced effect on neovascularization at the tissue-device interface. Although neovascularization alone is generally not sufficient to provide sufficient analyte transport at the device-tissue interface, in combination with other mechanisms, enhanced neovascularization can result in enhanced transport of analytes from the host to the implanted device. Therefore in some embodiments, short-term release of certain bioactive agents, for example angiogenic agents, can have a positive effect on neovascularization and thereby enhance transport of analytes at the device-tissue interface. 
     Additionally, it is believed that short-term release of the bioactive agent may be sufficient to reduce or prevent barrier cell layer formation. Formation of a cohesive monolayer of closely opposed cells, e.g., macrophages and foreign body giant cells, interfere with the transport of analytes across the tissue-device interface, also known as a barrier cell layer, and are large contributors to poor device performance. See U.S. Pat. No. 6,702,857, which is incorporated herein by reference in its entirety. Therefore in some embodiments, it is believed that short-term release of certain bioactive agents, for example, anti-barrier cell agents, can aid in preventing barrier cell layer formation. 
     Additionally, it is believed that short-term release of the bioactive agent may be sufficient to prevent negative effects of acute inflammation caused, for example, by surgical trauma, micro-motion, or macro-motion of the device in the soft tissue. Short-term release of anti-inflammatory agents may be sufficient to rescue a sensing biointerface from the negative effects associated with such acute inflammation, rendering adequate analyte transport. 
     Long-term release of the bioactive agent in the preferred embodiments generally occurs over a period of from about 1 month to about 2 years or more, preferably from at least about 2 months to at least about 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, or 23 months, and more preferably from at least about 3 months to at least about 4, 5, 6, 7, 8, 9, 10, 11, or 12 months. 
     Long-term glucose-measuring device experiments demonstrate that many biointerface materials experience a distinct and continual decline in sensitivity, for example, reduced analyte transport, beginning at three months after implantation in some cases. It is believed that this decline in analyte transport can be a result of barrier cell layer formation, cellular growth at the membrane, and/or thickening of the fibrous elements of the foreign body capsule. Other contributing factors can include chronic inflammation, which is believed to be due to micro-motion or macro-motion of the device; delamination of the sensing biointerface, which is believed to be due to cellular ingrowth within and under the sensing biointerface; compression of the sensing biointerface due to increasing compression of the foreign body capsule around the device; and distortion of the sensing biointerface, which is believed to be a result of a combination of compression and cellular ingrowth, for example. 
     Accordingly, long-term release of certain bioactive agents can modulate the foreign body response sufficiently to prevent long-term thickening of the foreign body capsule, reduce or prevent barrier cell layer formation, reduce or prevent chronic inflammation, reduce or prevent extensive cellular ingrowth, and/or reduce or prevent compression of the foreign body capsule on the sensing biointerface. 
     Loading of Bioactive Agents 
     The amount of loading of the bioactive agent into the sensing biointerface can depend upon several factors. For example, the bioactive agent dosage and duration can vary with the intended use of the sensing biointerface, for example, cell transplantation, analyte measuring-device, and the like; differences among patients in the effective dose of bioactive agent; location and methods of loading the bioactive agent; and release rates associated with bioactive agents and optionally their carrier matrix. Therefore, one skilled in the art will appreciate the variability in the levels of loading the bioactive agent, for the reasons described above. 
     In some embodiments, wherein the bioactive agent is incorporated into the sensing biointerface without a carrier matrix, the preferred level of loading of the bioactive agent into the sensing biointerface can vary depending upon the nature of the bioactive agent. The level of loading of the bioactive agent is preferably sufficiently high such that a biological effect is observed. Above this threshold, bioactive agent can be loaded into the sensing biointerface so as to imbibe up to 100% of the solid portions, cover all accessible surfaces of the membrane, and/or fill up to 100% of the accessible cavity space. Typically, the level of loading (based on the weight of bioactive agent(s), sensing biointerface, and other substances present) is from about 1 ppm or less to about 1000 ppm or more, preferably from about 2, 3, 4, or 5 ppm up to about 10, 25, 50, 75, 100, 200, 300, 400, 500, 600, 700, 800, or 900 ppm. In certain embodiments, the level of loading can be 1 wt. % or less up to about 50 wt. % or more, preferably from about 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, or 20 wt. % up to about 25, 30, 35, 40, or 45 wt. %. 
     When the bioactive agent is incorporated into the sensing biointerface with a carrier matrix, such as a gel, the gel concentration can be optimized, for example, loaded with one or more test loadings of the bioactive agent. It is generally preferred that the gel contain from about 0.1 or less to about 50 wt. % or more of the bioactive agent(s), preferably from about 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9 wt. % to about 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, or 45 wt. % or more bioactive agent(s), more preferably from about 1, 2, or 3 wt. % to about 4 or 5 wt. % of the bioactive agent(s). Substances that are not bioactive can also be incorporated into the matrix. 
     Referring now to microencapsulated bioactive agents, the release of the agents from these polymeric systems generally occurs by two different mechanisms. The bioactive agent can be released by diffusion through aqueous filled channels generated in the dosage form by the dissolution of the agent or by voids created by the removal of the polymer solvent or a pore forming agent during the original micro-encapsulation. Alternatively, release can be enhanced due to the degradation of the polymer. With time, the polymer erodes and generates increased porosity and microstructure within the device. This creates additional pathways for release of the bioactive agent. 
     Sensing Biointerface Based Sensors 
     Generally, the sensor electrodes comprise at least one working electrode incorporated into the biocompatible matrix. In some embodiments, at least one reference electrode is incorporated into the biocompatible matrix; however it is possible that the reference electrode exists at a location other than within the biocompatible matrix. In some embodiments, a counter electrode is incorporated into the biocompatible matrix, however it is possible that the counter electrode exists at a location other than within the biocompatible matrix or does not exist at all. 
       FIG. 5A  is a schematic surface view of a sensing biointerface in one embodiment, wherein a biocompatible matrix is formed from a plurality of fibers formed into a porous biointerface structure  50 . In this embodiment, the plurality of fibers that form the biocompatible matrix  50  include at least some fibers formed from an electrode material surrounded by a membrane system (also referred to as an electrode-membrane fiber), which fibers are configured to measure an analyte. In some embodiments, all of the fibers of the biocompatible matrix are electrode-membrane fibers, however in some alternative embodiments some portion of the fibers that form the biocompatible matrix may be formed from insulating materials or other non-conductive materials. Furthermore, in some embodiments, fibers may be present that comprise an electrode material that is exposed directly to the cavities between the fibers without a membrane coating. 
     In the illustrated embodiment, the fibers are formed into a non-woven fiber matrix, which is generally manufactured by interlocking or bonding a plurality of fibers together. In some alternative embodiments, the sensing biointerface can be formed into a woven fiber matrix, which is generally manufactured by weaving the fibers together. In some other alternative embodiments, the sensing biointerface can be formed into a uniform or non-uniform scaffolding, which is generally manufactured by bonding an array of fibers together. 
       FIG. 5B  is a cross-sectional view taken along line  5 B- 5 B of  FIG. 5A , showing a plurality of electrode-membrane fibers  52  in cross-section. In this embodiment, at least one working electrode  54  may be provided, wherein the working electrode  54  may be surrounded by a multi-layer membrane system. In some embodiments, the biocompatible matrix comprises a plurality of working, reference, and/or counter electrodes, some or all of which may be electrode-membrane fibers. However, not all embodiments include a membrane system and/or electrode cores in some or all of the fibers. Generally, the biocompatible matrix comprises between about 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10% to about 20, 30, 40, 50, 60, 70, 80, 90, or 100% of its fibers with an electrode core. Although  FIG. 5B  depicts an electrode-membrane fiber  52  near the periphery of the fiber bundle, those of skill in the art will appreciate that such electrode-membrane fibers may be located at any location within the fiber bundle. For example, in some embodiments, electrode-membrane fibers are located at a central position within the fiber bundle. 
       FIG. 5C  is an expanded view of one of the electrode-membrane fibers  52  of  FIG. 5B  in cross-section, showing the membrane system surrounding the fiber. The electrode-membrane fiber comprises a core formed from an electrode material (e.g. working electrode  54 ). In some embodiments, the electrode core comprises a wire or other bulk metal formed into an appropriate size and shape. Alternatively, the electrode core comprises a substrate onto which the electrode material is deposited utilizing techniques known in the art, for example, thin or thick film techniques. Preferably, the electrode core comprises any suitable conductive material, for example gold, silver, platinum, palladium, iridium, lead, conducting polymers, or other non-metal electrodes such as graphite or other carbon materials. 
     In general, the membrane system functions to control the flux of a biological fluid therethrough and/or to protect sensitive regions of the sensor from contamination by the biological fluid, for example. Some conventional electrochemical enzyme-based analyte sensors generally include a membrane system that controls the flux of the analyte being measured, protects the electrodes from contamination of the biological fluid, and/or provides an enzyme that catalyzes the reaction of the analyte with a co-factor, for example. See, e.g., co-pending U.S. patent application Ser. No. 10/838,912, filed May 3, 2004 entitled “IMPLANTABLE ANALYTE SENSOR,” U.S. patent application Ser. No. 11/077,715, filed Mar. 10, 2005 and entitled “TRANSCUTANEOUS ANALYTE SENSOR,” and U.S. patent application Ser. No. 11/360,819, filed Feb. 22, 2006 entitled “ANALYTE SENSOR,” all of which are incorporated herein by reference in their entirety. 
     The membrane systems of the preferred embodiments can include any membrane configuration suitable for use with any analyte sensor (such as described in more detail above). In general, the membrane systems of the preferred embodiments include a plurality of domains, all or some of which can be adhered to or deposited on the analyte sensor as is appreciated by one skilled in the art. In one embodiment, the membrane system generally provides one or more of the following functions: 1) protection of the exposed electrode surface from the biological environment, 2) diffusion resistance (limitation) of the analyte, 3) a catalyst for enabling an enzymatic reaction, 4) limitation or blocking of interfering species, and 5) hydrophilicity at the electrochemically reactive surfaces of the sensor interface, such as described in more detail in co-pending U.S. patent application Ser. No. 10/838,912, filed May 3, 2004 entitled “IMPLANTABLE ANALYTE SENSOR,” U.S. patent application Ser. No. 11/077,715, filed Mar. 10, 2005 and entitled “TRANSCUTANEOUS ANALYTE SENSOR,” and U.S. patent application Ser. No. 11/360,250, filed Feb. 22, 2006, and entitled “ANALYTE SENSOR”, all of which are incorporated herein by reference in their entirety. Accordingly, in one embodiment, the membrane system include a plurality of domains or layers, for example, an electrode (or electrolyte) domain  56 , an interference domain  57 , an enzyme domain  58 , and a resistance domain  59 , and may include additional domains, such as a bioprotective domain, a cell impermeable domain, and/or an oxygen domain (not shown), such as described in more detail in the above-cited co-pending U.S. Patent Applications and below. However, it is understood that a membrane system modified for other sensors, for example, by including fewer or additional domains is within the scope of the preferred embodiments. 
     In one example of a glucose sensor, one or more working electrode-membrane fibers, one or more reference electrode fibers, and one or more counter electrode fibers are formed into a non-woven biocompatible matrix. The matrix is connected to appropriate electronics (e.g., as shown in  FIG. 10 ) and implanted in a host. The connection between the electrodes and electronics may be accomplished by electrically coupling each electrode within each electrode-containing fiber with the electronics. For example, in one embodiment, each working electrode  54  is electrically coupled, such as by spot welding or other suitable technique, to a single electrical contact, which is then coupled to the sensing electronics. Over time, as the host&#39;s tissue grows in and through the passageways of the biocompatible matrix, a mature bed of vascularized tissue will form, enabling long term measurement of the analyte in vivo. While not wishing to be bound by theory, it is believed that the host response to a sensing biointerface of the preferred embodiments will enable long term measurement of an analyte due to tissue ingrowth into and through the passageways of the porous biocompatible matrix. It is noted, however, that measurement of the analyte prior to tissue ingrowth is also possible and/or with the incorporation of bioactive agents can be manipulated for a variety of lengths and/or time periods. 
       FIG. 6A  depicts a schematic surface view of a sensing cell-disruptive biointerface  90  that incorporates electrodes as part of the biointerface in an alternative embodiment. In this embodiment, openings  115  are shown on the surface of the biointerface that provide access to pores within the through-porous biointerface. Openings  115  may be formed on all sides of the biointerface. In some embodiments, the openings and pores are formed into the biointerface by drilling or etching (e.g., laser drilling or photolithography), which is described in more detail below. 
       FIG. 6B  depicts a cross-sectional view through line  6 B- 6 B of  FIG. 6A  showing the through-porosity of the biointerface. In this embodiment, the biointerface  90  includes a sheet having multiple layers, for example, alternating non-conductive and conductive layers. Conductive layer  100  is the working electrode, conductive layer  102  is the counter electrode, and conductive layer  104  is the reference electrode. The working electrode  100  is separated from the counter electrode  102  and reference electrode  104  by non-conductive layers  106  and  108 . The counter electrode  102  and reference electrode  104  are separated from tissue surrounding the implant by non-conductive layers  110  and  112 . Because the electrodes  100 ,  102 , and  104  are incorporated within the biointerface  90  rather than being adjacent to it as depicted in  FIG. 4 , the biointerface  90  may contact body tissue on both sides. In other words, both non-conductive layers  110  and  112  will be in contact with surrounding body tissue. 
     Non-limiting examples of material that may be used for non-conductive layers  106 ,  108 ,  110 , and  112  include polytetrafluoroethylene, polyethylene-co-tetrafluoroethylene, polyurethanes, block copolymers, and silicone. 
     In some embodiments, the working electrode  100  and the counter electrode  102  are constructed from platinum. The counter electrode  102  advantageously has low impedance. Thus, in some embodiments, the counter electrode  102  has an exposed surface area (e.g., surface area exposed to the inside surface of pores  114 ) at least several fold greater than the exposed surface area of the working electrode  100 . In some embodiments, the reference electrode  104  is a silver/silver chloride reference electrode (e.g., it consists of a layer of silver and an adjacent layer of silver chloride). 
     The sensing biointerface  90  advantageously has the form of a thin membrane. In one embodiment, the biointerface  90  is approximately 300 microns thick. 
     A plurality of pores  114  extend throughout biointerface  90 . The size of pores  114  are such that tissue may grow into the pores during the body&#39;s FBR response. However, the size and distribution of the pores  114  are sufficient to disrupt the continuity of cells growing within the pores  114 , thus preventing formation of a barrier cell layer on the interior surfaces of the pores  114 . The porous structure throughout sensing biointerface  90  creates a structure similar to the biointerface depicted in  FIG. 3 , with the exception that electrodes  100 ,  102 , and  104  are part of the porous structure. It will be appreciated that tissue growth can occur in pores that have an opening  115  on the exterior surface of the biointerface  90 . Tissue in-growth may also occur in pores that do not have an opening on the exterior surface of biointerface  90  but that intersect other pores that do have openings on the exterior surface of biointerface  90 . 
     It will be appreciated that the pores  114  within biointerface  90  may make any angle relative to the surfaces of the biointerface  90  and relative to each other and that they may follow a non-straight path within the biointerface  90 . It will also be appreciated that the pores  114  may have non-uniform size. In one embodiment, a substantial number of the pores  114  are not less than 20 microns in the shortest dimension and not more than 1000 microns in the longest dimension. In one embodiment, a substantial number of the pores are not less than 25 microns in the shortest dimension and not more than 500 microns in the longest dimension. 
     As depicted in  FIGS. 6B  and C, where a pore  114  extends through one of the conductive layers  100 ,  102 , or  104 , an interface point  116  is formed between the inside surface of the pore  114  and the electrode  100 ,  102 , or  104 . Thus, electrochemical reactions between agents in the pores  114  are possible at interface points  116 . Interface points  130  between a pore  114  and working electrode  100  provide a point where analyte sensing can occur. In some embodiments, a plurality of interface points  130  are provided that enable analyte sensing in locations substantially distributed throughout the biointerface. Alternatively, only one or a few interface points could be provided that enable analyte sensing in locations substantially distributed throughout the biointerface. In general, it is advantageous to have a substantial number of pores  114  that extend from one of the surfaces of the biointerface  90  to at least the working electrode  100 . 
       FIG. 6C  depicts a cross-sectional expanded view of a portion of the cross-sectional view of  FIG. 6B  showing the membrane system within the pores of the biointerface in one embodiment. In some implantable analyte sensors, analyte detection using a structure such as sensing biointerface  90  may be used without a membrane system. In other sensors, such as a glucose sensor, it is advantageous to include a membrane system. Membrane system  137  may be coated on the interior surfaces of pores  114 . The coating  137  will then provide an interface between the interior of pores  114  and interface points  130 ,  133 , and  135 . Thus, for example, in a glucose sensor, glucose supplied within pores  114  by in-grown vasculature will diffuse through membrane system  137 , contact glucose oxidase enzyme situated at the interface between the membrane system  137  and the working electrode, where it will be oxidized producing hydrogen peroxide as a by-product. The hydrogen peroxide can then be electrochemically oxidized, and thus detected, at interface point  130  on working electrode  100 . 
     The membrane systems for use in the sensing biointerfaces described herein may be any single- or multiple-component membrane that enhances detection of the desired analyte. Some embodiments of membranes useful for glucose detection are disclosed in U.S. application Ser. No. 10/153,356, filed on May 22, 2002, which is incorporated herein by reference in its entirety. 
     In the embodiment of  FIGS. 6A to 6C , the working  100 , counter  102 , and reference electrodes  104  are incorporated within non-conductive layers  106 ,  108 ,  110 , and  112 . Thus, all of the electrodes are internal to the sensing biointerface  90 . Alternatively, only the working electrode  100  is internal to the biointerface  150 . As in the embodiment of  FIGS. 6A to 6C , the optional counter electrode  102  and reference electrode  104  are separated from the working electrode  100  by non-conductive layers  106  and  108 . Alternatively, reference and/or counter electrode(s) can be external to the device (e.g., not implanted) and may exist in any known configuration as is appreciate by one skilled in the art. However, additional non-conductive layers are not provided on the tissue sides of the counter  102  and reference  104  electrodes. This configuration is advantageous because the total number of layers are decreased, thus simplifying manufacture. Furthermore, because the counter electrode  102  will have one surface exposed to surrounding tissue without being blocked by one of the non-conductive layers, it will have a much greater surface area exposed to the extracellular electrolyte solution, thus decreasing its impedance and lowering solution resistance. In such an alternative embodiment wherein reference and counter electrodes are on external (tissue-facing) surfaces of the biointerface, the membrane system can be coated onto the exterior surfaces and of the counter electrode  102  and  104  respectively, if desired, in addition to the interior surfaces of pores the  114 . 
     It will be appreciated that the working, reference, and optional counter electrodes may be placed in any location within the sensing biointerfaces described herein. However, it is advantageous to place the working electrode within the interior of the sensing biointerface so that it will be exposed to the optimal cell-disrupted FBR environment. Such a placement inhibits the formation of a barrier cell layer on the working electrode. 
     Some embodiments of the present invention relate to methods for manufacturing a sensing biointerface such as depicted in  FIGS. 6A to 6C . Because the general structure of these biointerfaces is a plurality of planar layers deposited on top of each other, manufacturing can be done using any of a number of known techniques for manufacturing multi-layer thin or thick film structures. For example, techniques typically used in semiconductor manufacturing may be applied.  FIG. 7  depicts a flowchart of one process sequence that can be used. At block  200 , the first layer of the biointerface is deposited onto a substrate. The substrate may be any suitable surface such as glass or plastic. In one advantageous embodiment, the substrate allows for easy removal of the biointerface after manufacturing is complete. In one embodiment, the substrate comprises polytetrafluoroethylene. The first layer deposited onto the substrate may be either a non-conductive layer or a conductive layer, depending on the desired final configuration of layers. For example, in the embodiment of  FIGS. 6A to 6C , the first layer may be non-conductive layer  110 . Alternatively, the first layer may be counter electrode  102 . At block  202 , an additional layer is deposited onto the first layer. The composition of the additional layer depends on the final desired configuration. At decision block  204 , it is determined whether additional layers are required. If so, the process returns to block  202  for the deposition of a new layer. The process continues until all desired layers have been deposited. Then, at block  206 , an interconnected pore structure is induced into the structure. The interconnected pore structure may be induced by any method known in the art for generating pore structures. In one embodiment, the pores are generated by chemical etching of the layered structure. In another embodiment, porosity is generated by plasma etching of the layered structure. At block  208 , a membrane system is deposited on all exposed surfaces if desired. Any suitable method for depositing the membrane system may be used, including but not limited to dip coating and vapor deposition. Where the membrane system comprises multiple layers, such as an enzyme layer and a polymeric diffusion layer, the layers can be deposited sequentially. Finally, at block  210 , the completed biointerface is removed from the substrate and incorporated with the other components required for the biosensor, such as sensing electronics and a power source. 
     It will be appreciated that the method illustrated in  FIG. 7  may be modified as desired. For example, it may be advantageous to remove the biointerface from the substrate at an earlier step, such as prior to deposition of a membrane system. Furthermore, it may be advantageous to induce porosity in some of the deposited layers prior to deposition of all of the layers. 
     Deposition of each layer may be by any suitable deposition technique. Non-limiting examples of deposition techniques include dip coating, vapor deposition, sputtering, lamination, brush application, thick film, spin coating, and ink jet application. In some embodiments, different deposition techniques are used for different layers. 
     In some embodiments, photolithography techniques may be used to define the location and size of pore openings in the surface of the biointerface. For example, masks may be applied on one or more of the surfaces of the biointerface and appropriate pore openings photolithographically defined. Etching may then induce pore formation into the biointerface through the photolithographically defined openings. Finally, the masks can be removed by an appropriate etch process. In one embodiment, pore openings are defined in a regular array on the surfaces of the biointerface. In one embodiment, when the counter electrode and reference electrode comprise the outer surface of the biointerface, such as the embodiment of  FIG. 6B , they can be used as masks in the porosity inducing etch step. 
       FIGS. 8A and 8B  depict cross-sectional views of other embodiments of a sensing biointerface. In  FIG. 8A , a mesh structure or scaffolding  250  is provided. Incorporated within the mesh structure  250  are working electrode  100 , counter electrode  102 , and reference electrode  104 . In some embodiments, the mesh structure  250  consists of non-woven fibers. In other embodiments, the mesh structure  250  consists of woven fibers. The fibers of mesh structure  250  may be electrically non-conductive in order to insulate the electrodes  100 ,  102 , and  104  from each other. However, in one embodiment, the mesh structure  250  is constructed of an electrically conductive material, such a metal threading, that is then coated with a non-conductive material. The mesh structure  250  provides spaces in between the fibers for FBR tissue in-growth, however, barrier cell layers are disrupted as is depicted in  FIG. 2 . Analyte sensing can occur at any location  252  where the surface of working electrode  102  interfaces with an opening between the mesh fibers. If desired the electrodes  100 ,  102 , and  104  may be coated with a membrane system as described herein. In the alternative embodiment of  FIG. 8B , working electrode  100  is still embedded within mesh  250 ; however, counter electrode  102  and reference electrode  104  are deposited onto the surfaces of the mesh structure  250 . 
     Non-limiting examples of fibers that may be used for mesh structure  250  include polypropylene, polytetrafluoroethylene, polyvinylchloride, polyvinylidene fluoride, polybutylene terephthalate, polymethylmethacrylate, polyether ether ketone, polyurethanes, polyolefins, polyesters, polycarbonates, cellosic polymers, polysulfones, and block copolymers thereof including, for example, di-block, tri-block, alternating, random, and graft copolymers. In some embodiments, non-woven fibers are greater than about 5 microns in the shortest dimension. In some embodiments, non-woven fibers are greater than about 10 microns in the shortest dimension. 
     The electrodes  100 ,  102 , and  104  are formed from electrically conductive mesh, wires, or other porous structure such that tissue grows therebetween, which together with the insulating mesh structure  250 , forms the sensing biointerface configured for substantial tissue ingrowth including a mature bed of vascularization. 
     It will be appreciated that other methods besides inducing porosity and using mesh structures can be used to form matrixes that comprise passageways from the exterior of the matrix to a working electrode disposed within the matrix. Thus, many other sensing biointerface structures are within the scope of this disclosure. 
     For example, in one alternative embodiment, a biointerface matrix, also referred to as a biocompatible matrix, surrounds a sensing mechanism.  FIG. 9A  depicts a schematic surface view of a porous biointerface  300  that surrounds a sensing mechanism  302 . The sensing mechanism may include a working electrode, with or without a membrane system as described herein. In some embodiments, the sensing mechanism may also include a reference and/or counter electrode. Reference and/or Counter electrode(s) may also be incorporated into the biointerface matrix and/or provided external to the biointerface matrix as described elsewhere herein. 
     In some embodiments, the sensing mechanism does not itself include passageways or pores, but rather is sized small enough (e.g., in at least one dimension) and/or incorporated into the biointerface matrix in such a way so as to manage the foreign body response with the biointerface matrix to resist barrier cell formation and allow the transport of analytes to the sensing mechanism long-term (e.g., 1 day to 1 year or more). For example, in some embodiments, at least one dimension of the sensing mechanism may be sized similarly to the size of passageways or structures within the non-sensing portion of the biointerface matrix (e.g., pore diameter of a porous structure or fiber diameter of a fibrous structure). In one embodiment, the sensing mechanism has a size in at least one dimension of less than about 1000 microns, however other configurations are possible. In some embodiments, both the biointerface matrix and the sensing mechanism are substantially indistinguishable to cells in a host&#39;s biological response, such that cells grow in and around the sensing biointerface substantially without barrier cells, thereby enabling sensing of the analyte (e.g., glucose) throughout the sensing biointerface for a long time (e.g., from days to years). 
     In the embodiment illustrated in  FIG. 9A  of a porous biointerface matrix, also referred to as a biocompatible matrix, a substantial number of the cavities may be greater than or equal to about 20 microns in one dimension (e.g., a substantial number of the cavities are greater than or equal to about 30, 40, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180, 200, 240, 280, 320, 360, 400, 500, 600, 700 microns in one dimension). In some embodiments, a substantial number of the cavities are less than or equal to about 1000 microns in one dimension (e.g., a substantial number of the cavities are less than or equal to about 900, 800, 700, 600, 500, 400, 360, 320, 280, 240, 200, 180, 160, 140, 120, 100 microns in one dimension.) Although  FIG. 9A  illustrates a porous biointerface matrix, in some embodiments, the biocompatible matrix includes non-woven materials, woven materials, or other such materials, such that a porous structure is formed from the cavities between the fibers. In these embodiments, for example, the fibers are formed by structural elements that provide the three-dimensional conformation. Therefore in these embodiments, the biocompatible matrix may be defined by a fiber size of between about 1 and 100 microns in all but the longest dimension and a sufficient number of cavities of a size and structure to allow inflammatory cells (for example, macrophages) to completely enter therein through the apertures that define the cavities. 
     A variety of sensing mechanisms having the characteristics described above can be incorporated into a biointerface matrix. For example, in some embodiments, the sensing mechanism is a wire structure, with or without membrane systems as described herein.  FIG. 9B  depicts a longitudinal cross-section of one exemplary wire-type sensor. The wire-type sensor in  FIG. 9B  includes a working electrode wire  306  with exposed electroactive portions  308 . The electroactive portions  308  are optionally coated with a membrane system as described herein. Although a wire-type sensor is described herein, the sensing biointerface of the preferred embodiments can utilize other appropriately sized sensors, for example, a planar substrate-based sensor such as described in U.S. Pat. No. 6,565,509, which is incorporated herein by reference in its entirety. 
     The electrode  306  is formed from a fine wire with a diameter of from about 0.001 inches or less to about 0.010 inches or more. In preferred embodiments, the working electrode  306  comprises a wire formed from a conductive material, such as platinum, platinum-iridium, palladium, graphite, gold, carbon, conductive polymer, alloys, or the like. Although the electrodes can by formed by a variety of manufacturing techniques (bulk metal processing, deposition of metal onto a substrate, or the like), it can be advantageous to form the electrodes from plated wire (e.g., platinum on steel wire) or bulk metal (e.g., platinum wire). It is believed that an electrode formed from bulk metal wire provide superior performance (e.g., in contrast to deposited electrodes), including increased stability of assay, simplified manufacturability, resistance to contamination (e.g., which can be introduced in deposition processes), and improved surface reaction (e.g., due to purity of material) without peeling or delamination. 
     In some embodiments, the working electrode  306  is covered with an insulating material  312 , for example, a non-conductive polymer. Dip-coating, spray-coating, vapor-deposition, or other coating or deposition techniques can be used to deposit the insulating material on the working electrode. In one embodiment, the insulating material comprises parylene, which can be an advantageous polymer coating for its strength, lubricity, and electrical insulation properties. Generally, parylene is produced by vapor deposition and polymerization of para-xylylene (or its substituted derivatives). While not wishing to be bound by theory, it is believed that the lubricious (e.g., smooth) coating (e.g., parylene) on the sensors of the preferred embodiments contributes to minimal trauma and extended sensor life. However, an insulator material is not required. 
     In embodiments wherein an outer insulator  312  is used, portion(s) of the coated assembly structure can be stripped or otherwise removed, for example, by hand, excimer lasing, chemical etching, laser ablation, grit-blasting (e.g., with sodium bicarbonate or other suitable grit), or the like, to expose the electroactive surfaces  308 . Alternatively, a portion of the electrode can be masked prior to depositing the insulator in order to maintain an exposed electroactive surface area  308 . In one exemplary embodiment, grit blasting is implemented to expose the electroactive surfaces  308 , preferably utilizing a grit material that is sufficiently hard to ablate the polymer material, while being sufficiently soft so as to minimize or avoid damage to the underlying metal electrode (e.g., a platinum electrode). Although a variety of “grit” materials can be used (e.g., sand, talc, walnut shell, ground plastic, sea salt, and the like), in some preferred embodiments, sodium bicarbonate is an advantageous grit-material because it is sufficiently hard to ablate, e.g., a parylene coating without damaging, e.g., an underlying platinum conductor. One additional advantage of sodium bicarbonate blasting includes its polishing action on the metal as it strips the polymer layer, thereby eliminating a cleaning step that might otherwise be necessary. 
     In some alternative embodiments, multiple electrodes can be included within the sensing biointerface  300 . For example, a three-electrode system (working, reference, and counter electrodes) and/or additional working electrode(s) (e.g., an electrode which can be used to generate oxygen, which is configured as a baseline subtracting electrode, or which is configured for measuring additional analytes). U.S. Publication No. US-2005-0161346-A1 and U.S. Publication No. US-2005-0143635-A1 describe some systems and methods for implementing and using additional working, counter, and/or reference electrodes. The resulting electrode system can be configured with an appropriate membrane system, such as the membranes described herein, wherein the first working electrode is configured to measure a first signal comprising glucose and baseline and the additional working electrode is configured to measure a baseline signal consisting of baseline only (e.g., configured to be substantially similar to the first working electrode without an enzyme disposed thereon). In this way, the baseline signal can be subtracted from the first signal to produce a glucose-only signal that is substantially not subject to fluctuations in the baseline and/or interfering species on the signal. 
     A membrane system can be deposited on the exposed electroactive surfaces (and/or portions or entirety of sensing mechanism) using known thin film techniques (for example, vapor deposition, spraying, electro-depositing, dipping, or the like) as described in more detail elsewhere herein. 
     In some embodiments, the biointerface matrix  304  may be formed around wire electrode(s) or other electrodes using a pre-formed shape and structure. In some embodiments, the sensing biointerface can be manufactured by forming particles (e.g., sugar, salt, or other natural or synthetic uniform or non-uniform particles) in a mold including the sensing mechanism (e.g., a wire electrode), wherein the particles have shapes and sizes substantially corresponding to the desired cavity dimensions. Most often, the particles are made to coalesce within the mold (e.g. around the sensing mechanism) to provide the desired interconnectivity between the cavities. 
     The desired material for the solid portion (e.g., silicone, elastomeric conductive carbon, and the like) can be introduced into the mold using methods common in the art of polymer processing, for example injecting, pressing, vacuuming, or pouring, while taking care to work with the sensing mechanism within the mold. After the solid portion material is cured or solidified, the coalesced particles are then dissolved, melted, etched, or otherwise removed leaving interconnecting cavities within the solid portion. 
     Accordingly, the nominal cavity size of the cavities of the biointerface matrix can be substantially defined by the particle size used in creating the cavities. It is noted that in some embodiments, the particles used to form the cavities can be substantially spherical, thus the dimensions below describe a diameter of the particle and/or a diameter of the cavity. In some alternative embodiments, the particles used to form the cavities can be non-spherical (e.g., rectangular, square, diamond, or other geometric or non-geometric shapes), thus the dimensions below describe one dimension (e.g., shortest, average, or longest, for example) of the particle and/or cavity. 
     In some embodiments, a variety of different particle sizes can be used in the manufacture of the biointerface matrix. In some embodiments, the dimensions of the particles can be somewhat smaller or larger than the dimensions of the resulting cavities due to dissolution or other precipitation that can occurring during the manufacturing process, for example. 
     One preferred material that can be used to form the solid portion of the biointerface matrix is a material that allows the passage of the analyte (e.g., glucose) therethrough. For example, the biointerface matrix may be formed from a silicone polymer/hydrophobic-hydrophilic polymer blend. In one embodiment, The hydrophobic-hydrophilic polymer for use in the blend may be any suitable hydrophobic-hydrophilic polymer, including but not limited to components such as polyvinylpyrrolidone (PVP), polyhydroxyethyl methacrylate, polyvinylalcohol, polyacrylic acid, polyethers such as polyethylene glycol or polypropylene oxide, and copolymers thereof, including, for example, di-block, tri-block, alternating, random, comb, star, dendritic, and graft copolymers (block copolymers are discussed in U.S. Pat. Nos. 4,803,243 and 4,686,044, which are incorporated herein by reference). In one embodiment, the hydrophobic-hydrophilic polymer is a copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO). Suitable such polymers include, but are not limited to, PEO-PPO diblock copolymers, PPO-PEO-PPO triblock copolymers, PEO-PPO-PEO triblock copolymers, alternating block copolymers of PEO-PPO, random copolymers of ethylene oxide and propylene oxide, and blends thereof. In some embodiments, the copolymers may be optionally substituted with hydroxy substituents. Commercially available examples of PEO and PPO copolymers include the PLURONIC® brand of polymers available from BASF®. In one embodiment, PLURONIC® F-127 is used. Other PLURONIC® polymers include PPO-PEO-PPO triblock copolymers (e.g., PLURONIC® R products). Other suitable commercial polymers include, but are not limited to, SYNPERONICS® products available from UNIQEMA®. 
     The silicone polymer for use in the silicone/hydrophobic-hydrophilic polymer blend may be any suitable silicone polymer. In some embodiments, the silicone polymer is a liquid silicone rubber that may be vulcanized using a metal- (e.g., platinum), peroxide-, heat-, ultraviolet-, or other radiation-catalyzed process. In some embodiments, the silicone polymer is a dimethyl- and methylhydrogen-siloxane copolymer. In some embodiments, the copolymer has vinyl substituents. In some embodiments, commercially available silicone polymers may be used. For example, commercially available silicone polymer precursor compositions may be used to prepare the blends, such as described below. In one embodiment, MED-4840 available from NUSIL® Technology LLC is used as a precursor to the silicone polymer used in the blend. MED-4840 consists of a 2-part silicone elastomer precursor including vinyl-functionalized dimethyl- and methylhydrogen-siloxane copolymers, amorphous silica, a platinum catalyst, a crosslinker, and an inhibitor. The two components may be mixed together and heated to initiate vulcanization, thereby forming an elastomeric solid material. Other suitable silicone polymer precursor systems include, but are not limited to, MED-2174 peroxide-cured liquid silicone rubber available from NUSIL® Technology LLC, SILASTIC® MDX4-4210 platinum-cured biomedical grade elastomer available from DOW CORNING®, and Implant Grade Liquid Silicone Polymer (durometers 10-50) available from Applied Silicone Corporation. 
     Silicone polymer/hydrophobic-hydrophilic polymer blends are described in more detail in U.S. patent application Ser. No. 11/404,417, entitled “SILICONE BASED MEMBRANES FOR USE IN IMPLANTABLE GLUCOSE SENSORS,” filed on Apr. 14, 2006, which is incorporated herein by reference in its entirety. 
     Although one method of manufacturing porous domains is described above, a variety of methods known to one of ordinary skill in the art can be employed to create the structures of preferred embodiments. For example, molds can be used in the place of the particles described above, such as coral, self-assembly beads, etched or broken silicon pieces, glass frit pieces, and the like. The dimensions of the mold can define the cavity sizes, which can be determined by measuring the cavities of a model final product, and/or by other measuring techniques known in the art, for example, by a bubble point test. In U.S. Pat. No. 3,929,971, Roy discloses a method of making a synthetic membrane having a porous microstructure by converting calcium carbonate coral materials to hydroxyapatite while at the same time retaining the unique microstructure of the coral material. 
     Other methods of forming a three-dimensional biointerface matrix can be used, for example holographic lithography, stereolithography, and the like, wherein cavity sizes are defined and precisely formed by the lithographic or other such process to form a lattice of unit cells, as described in Published U.S. Patent Application 2005-0251083, entitled “Macro-Micro Architecture for Biointerface Membrane,” which is incorporated herein by reference in its entirety and as described by Pekkarinen et al. in U.S. Pat. No. 6,520,997, which discloses a photolithographic process for creating a porous membrane. In another embodiment, the biointerface matrix can “written” using computer-aided manufacturing techniques. In one such example, the biointerface matrix is fabricated by dispensing of very small volumes of liquid silicone rubber (LSR), by a robot, onto a heated platform, in an array called out by a CAD-like code programmed into the computer controlling the robot, whereby layers of LSR would be added onto the structure as the layers beneath them are cured by the heat. One such method has been disclosed in U.S. Published Patent Application No. 2004-0253365-A1 entitled “Architecture Tool and Methods of Use,” which is incorporated herein by reference in its entirety. Alternatively, the biointerface matrix includes non-woven materials, woven materials, or other such materials, such that a porous structure is formed from the cavities between the fibers. In any of the above methods of forming the biointerface matrix, the matrix can be formed around a sensing mechanism as illustrated with reference to  FIG. 9A .  FIGS. 10 to 14  are flowcharts illustrating various exemplary processes for obtaining a structure having a biointerface matrix surrounding a sensing mechanism, such as depicted in  FIG. 9A . 
       FIG. 10  is a flow chart that illustrates the process  151  of forming a biointerface-coated small structured sensing mechanism in one embodiment. In this embodiment, the biointerface matrix includes woven or non-woven fibers formed directly onto the sensing mechanism. Generally, fibers can be deposited onto the sensing mechanism using methods suitable for formation of woven- or non-woven fibrous materials. In some embodiments, the biointerface matrix is electrospun directly onto the sensing mechanism; electrospinning advantageously allows the biointerface matrix to be made with small consistent fiber diameters that are fused at the nodes and are without aggregation. In some embodiments, the biointerface matrix is directly written onto the sensing mechanism; direct-writing can advantageously allow uniform deposition of stored patterns for providing consistent and reproducible architectures. 
     At block  153 , one or more dispensers dispense a polymeric material used to form the fibers, such as any of the polymeric materials described herein. The coating process can be performed in a vacuum or in a gaseous medium, which environment may affect the architecture of the biointerface matrix as is appreciated by one skilled in the art. 
     In embodiments wherein the biointerface is electro spun onto the sensing mechanism, the dispenser dispenses a charged liquefied polymer within an electric field, to thereby form a jet of polymer fibers, for example, such as described in International Published Patent Application No. WO 2005/032400, which is incorporated herein by reference in its entirety. In embodiments wherein the biointerface is directly-written onto the sensing mechanism, the dispenser dispenses a polymer solution using a nozzle with a valve, or the like, for example as described in U.S. Published Patent Application No. 2004/0253365, which is incorporated herein by reference in its entirety. In general, a variety of nozzles and/or dispensers can be used to dispense a polymeric material to form the woven or non-woven fibers of the biointerface matrix. 
     At block  154 , the dispenser(s) is moved relative to the sensing mechanism and/or the sensing mechanism is moved relative to the dispenser(s) so as to coat the sensing mechanism with the fibers. In embodiments wherein the biointerface matrix is electrospun onto the sensing mechanism, the dispenser(s) can change the direction and/or magnitude of the electric field during motion in order to effect the orientation of the polymer fibers on the sensing mechanism. Additionally, the path of the dispenser is preferably selected so as to coat the portions of or the entire object. In one exemplary embodiment, wherein it is desirable for the biointerface matrix to substantially circumscribe the sensing mechanism (e.g., a substantially cylindrical shape), such as illustrated in  FIG. 9C  described below, the dispenser can be moved along a helix path, a circular path, a zigzag path, or the like. Additionally, the dispenser can move rotationally and/or translationally relative to the sensing mechanism. The number of sweeps is preferably selected according to the desired architecture of the biointerface matrix. Additionally, the density of the fibers and/or the type of liquefied polymer can be changed from one sweep to the other to thereby control the architecture of the membrane. 
     In embodiments where the biointerface matrix is directly written onto the sensing mechanism, the dispenser may be programmed to write a pattern that creates the desired membrane architecture, including the interconnected cavities and solid portion(s). Namely, the dispenser is programmed to move in the x, y, and optionally z direction in order to create the desired membrane architecture. See, for example, U.S. Published Patent Application No. 2004/0253365 cited above. 
     Although the preferred embodiments described moving the dispenser(s) relative to the sensing mechanism, alternatively, the dispenser can remain stationary and the sensing mechanism moved, as is appreciated by one skilled in the art. 
     In some embodiments, the sensing mechanism is moved in a rotational or translational motion, which can be performed in combination with, or instead of, movement of the dispenser. In this step, the sensing mechanism is moved so as to ensure coating throughout the entirety of the biointerface region (or a portion thereof). In one exemplary embodiment, wherein a substantially circumscribing biointerface matrix is desired (e.g., for a substantially cylindrically shaped sensing mechanism) such as illustrated in  FIG. 9C , the sensing mechanism can be rotated so to aid in coating the entire circumference of the sensing mechanism. In another exemplary embodiment, wherein a substantially planar biointerface matrix is desired (e.g., for a substantially planar sensing mechanism), the sensing mechanism can be translated so as to aid in coating the desired planar surface area. 
       FIG. 11  is a flow chart that illustrates the process  160  of forming a biointerface-coated sensing mechanism in an alternative embodiment. In this embodiment, the biointerface matrix is porous in configuration, such as illustrated in  FIG. 3 , for example. 
     At block  162 , a selectively removable porous mold is formed by spraying, coating, rolling, or otherwise forming selectively removable particles, for example, sugar crystals, onto the surface of the sensing mechanism. Additional examples of materials suitable as selectively removable mold material include thermoplastic polymers such as waxes, paraffin, polyethylene, nylon, polycarbonate, or polystyrene in naturally available particles or processed into specific sizes, shapes, molded forms, spheres or fibers, salt or other particles which cannot be made to inherently stick together coated with sugar, and certain drug crystals such as gentamycin, tetracycline, or cephalosporins. In general, any dissolvable, burnable, meltable, or otherwise removable particle which can be made to stick together could be used. Preferably, the particles have shapes and sizes substantially corresponding to the desired cavity dimensions, such as described in more detail above. In some embodiments, the particles are made to adhere to the sensing mechanism by environmental conditions, for example, humidity can be used to cause sugar to adhere to the sensing mechanism. 
     In some embodiments, the particles are made to coalesce to provide the desired interconnectivity between the cavities. In an exemplary porous silicone embodiment, sugar crystals are exposed to a humid environment sufficient to cause coalescence of the sugar crystals. In some alternative embodiments, other molds may be used in the place of the particles described above, for example, coral, self-assembly beads, etched and broken silicon pieces, glass frit pieces, and the like. 
     At block  164 , a material (e.g., a moldable or conformable material) is filled or coated into the interconnected cavities of the mold using methods common in the art of polymer processing, for example, injecting, pressing, vacuuming, vapor depositing, pouring, and the like. Examples of materials suitable for the resulting porous device include polymers, metals, metal alloys, ceramics, biological derivatives, and combinations thereof, in solid or fiber form. In an exemplary porous silicone embodiment, silicone is pressed into the interconnected cavities of the mold. 
     At block  166 , the material is substantially cured or solidified to form the solid portion(s) of the biointerface matrix. Solidification of the material can be accelerated by supplying dry air (which may be heated) to the material, for example. Additionally, freezing, freeze drying or vacuum desiccation, with or without added heat, may also be utilized to cause the material to solidify. In some circumstances, a skin or any excess material can be removed (e.g., shaved, etched, or the like) after curing. In the exemplary porous silicone embodiment, an outer skin of silicone is removed to expose the interconnected cavities at an outer surface. 
     At block  168 , the selectively removable porous mold is dissolved, melted, etched, or otherwise removed, leaving interconnecting cavities within the solid portion. Preferably, the selectively removable porous mold is readily removable without significantly altering the final product (or product material). This removal may be by dissolution by some solvent that does not significantly dissolve the final product material. Alternatively, the mold material may be melted (or burned) out of the final product material if the melting point (or burning point) of the mold material is below that of the final product material. In the exemplary porous silicone embodiment, water is used to dissolve the sugar crystals. 
       FIG. 12  is a flow chart that illustrates the process  170  of forming a biointerface-coated small structured sensing mechanism in another alternative embodiment. In this embodiment, the biointerface matrix is porous in configuration, such as illustrated in  FIG. 3 , for example. 
     At block  172 , a selectively removable porous mold is formed by filling a shaped cavity with selectively removable particles, for example, sugar crystals, wherein the sensing mechanism is located within the shaped cavity, and wherein the selectively removable particles substantially surround the sensing mechanism. Additional examples of materials suitable as selectively removable mold material are described with reference to block  162 , above. In some embodiments, the shaped cavity mold is formed from a selectively removable material (e.g., sacrificial cavity mold) similar the selectively removable particles described above. One such example includes a tube formed from a dissolvable polymer. Alternatively, the shaped cavity can be a non-selectively removable material, and instead, a sacrificial layer of selectively removable material is formed directly onto the cavity walls, enabling the removal of the biointerface matrix after dissolution of the sacrificial layer. 
     Preferably the shape of the cavity mold substantially corresponds to the desired final shape of the biointerface matrix. In one exemplary embodiment, the cavity mold is substantially cylindrical, for example using a syringe or cannula as the cavity mold. 
     In some embodiments, the particles are made to coalesce to provide the desired interconnectivity between the cavities. In an exemplary porous silicone embodiment, sugar crystals are exposed to humidity or spray of water sufficient to cause coalescence of the sugar crystals. In some alternative embodiments, other molds may be used in the place of the particles described above, for example, coral, self-assembly beads, etched and broken silicon pieces, glass frit pieces, and the like. 
     At block  174 , a material (e.g., a moldable or conformable material) is filled into the interconnected cavities of the mold using methods common in the art of polymer processing, for example, injecting, pressing, vacuuming, vapor depositing, pouring, and the like. Examples of materials suitable for the resulting porous device are described in more detail with reference to block  164 , above. In an exemplary porous silicone embodiment, silicone is pressed into the interconnected cavities of the mold. 
     At block  176 , the material is substantially cured or solidified to form the solid portion(s) of the biointerface matrix. Solidification of the material can be accelerated as described in more detail with reference to block  166 , above. 
     At block  178 , the selectively removable porous mold is dissolved, melted, etched, or otherwise removed, leaving interconnecting cavities within the solid portion surrounding the sensing mechanism. In some embodiments, wherein a sacrificial layer is formed as described above, the sacrificial layer can be remove before, during, or after the removal of the selectively removable porous mold. In some embodiments, the final product is removed from the cavity mold before, during, or after the removal of the selectively removable porous mold. 
     Preferably, the selectively removable porous mold is readily removable without significantly altering the final product (or product material). This removal may be by dissolution by some solvent that does not significantly dissolve the final product material. Alternatively, the mold material may be melted (or burned) out of the final product material if the melting point (or burning point) of the mold material is below that of the final product material. In one exemplary embodiment, a sacrificial tube forms the mold cavity; wherein the sacrificial tube is removed prior to, during, or after dissolution of the selectively removable porous mold. One skilled in the art can appreciate a variety of modifications or combinations of the above described removal step without departing from the spirit of the invention. 
       FIG. 13  is a flow chart that illustrates the process  180  of forming a biointerface-wrapped sensing mechanism in one embodiment. In this embodiment, the interconnected cavities and solid portion(s) of the biointerface matrix can be fibrous or porous in configuration. In fact, substantially any biointerface matrix with an architecture as described in more detail above, which is formed in substantially any manner, can be used with this embodiment. 
     At block  182 , a sensing mechanism is manufactured and provided, wherein the sensing mechanism is formed with a small structure as discussed above. 
     At block  184 , a biointerface membrane with an architecture as described herein is manufactured in substantially any desired manner, wherein the biointerface membrane is formed substantially as a sheet or tube of membrane. 
     At block  186 , the biointerface membrane is wrapped around the sensing mechanism manually, or using an automated device, as can be appreciated by one skilled in the art. Namely, the biointerface membrane is wrapped such that it substantially surrounds the sensing mechanism. The number of wraps can be from less than 1 to about 100, preferably 1, 1½, 2, 2½, 3, 3½, 4, 5, 6, 7, 8, 9, 10, or more. The number of wraps depends on the architecture of the sheet of biointerface membrane, and the desired architecture of the biointerface surrounding the sensing mechanism. 
     In some embodiments, the circumference (or a portion thereof (e.g., an edge)) of the biointerface membrane with an architecture as described herein can be adhered or otherwise attached or sealed to form a substantially consistent outer surface (of the biointerface membrane). In an aspect of this embodiment, the biointerface membrane is wrapped around the sensing mechanism one time, wherein the “wrap” includes a tubular biointerface membrane configured to slide over the sensing mechanism, for example, be stretching the tubular biointerface membrane and inserting the sensing mechanism therein. 
       FIG. 14  is a flow chart that illustrates the process  190  of forming a sensing biointerface in one embodiment. In this embodiment, the sensing mechanism is inserted into the biointerface matrix so that it is encompassed therein. 
     At block  192 , a biointerface matrix is manufactured in substantially any desired manner, such as the methods described above. In some embodiments, the biointerface matrix is molded into the desired final shape to surround the sensing mechanism and implant into a host. Alternatively, the biointerface matrix can be provided as a sheet of bulk material. 
     At block  194 , a particularly shaped or sized biointerface matrix can be optionally cut. Namely, in embodiments wherein the biointerface matrix is provided in bulk, e.g., as a sheet of material, the desire shape or size can be cut therefrom. In these embodiments, bulk biointerface matrix sheet is preferably of the appropriate thickness for the desired final product. In one exemplary embodiment, the biointerface matrix (bulk sheet) is compressed, for example between two substantially rigid structures, and the final size/shape biointerface matrix cut there from, after which the biointerface matrix is released. While not wishing to be bound by theory, it is believed that by compressing the biointerface matrix during cutting, a more precise shape can be achieved. It is noted that biointerface matrix can have sufficient elasticity, such that the thickness is returned after release from compression, as is appreciated by one skilled in the art. 
     At block  196 , a sensing mechanism is inserted into the biointerface matrix. Preferably, the sensing mechanism is inserted into the membrane such that the sensing mechanism contacts at least one or more of the interconnected cavities so that the host analyte can be measured. Alternatively, the biointerface can be formed from a material that allows the flux of the analyte there through. In some embodiments, the sensing mechanism is inserted with the aid of a needle. Alternatively, the sensing mechanism is formed with appropriate sharpness and rigidity to enable insertion through the biointerface matrix. 
     In some embodiments, an anchoring mechanism, such as a barb, is provided on the sensing mechanism, in order to anchor the sensing mechanism within the biointerface matrix (and/or host tissue). A variety of additional or alternative aspects can be provided to implement the biointerface matrix surrounded sensing mechanisms of the preferred embodiments. 
     All of the above manufacturing techniques can be used to form the sensing biointerface around a sensing mechanism, as is appreciated by one skilled in the art. Additionally, the sensing biointerface of the preferred embodiments can be implemented in a variety of shapes, sizes, and configurations and with any number of electrodes and electrode configurations. The figures below ( FIGS. 9C to 9G ) describe a few exemplary embodiments. 
       FIG. 9C  is a schematic cross-sectional view of a sensing biointerface in one embodiment, including a single wire sensor  306  incorporated into a cylindrical shaped biointerface matrix  300  (shown in a side-view cross-section). In one exemplary embodiment, the biointerface is formed as described above, within a cylindrical mold. Although a method of molding the sensing mechanism into the biointerface matrix is described above, other methods may include manufacturing the sensing mechanism and biointerface separately and then assembling them in a final step (e.g., pushing the wire through the biointerface matrix (e.g., with a protective introducer in some embodiments)). The use of a wire electrode provides multiple sensing points along the length of the wire (either a continuous series of sensing points due to an uninsulated length of wire or discrete sensing points due to periodic removal of an insulative coating as described above with reference to  FIG. 9B ). Accordingly, blockage or dysfunction of certain sensing points along the wire (e.g., due to cellular attack or other malfunction) but not others will not affect the function of the overall sensor. 
       FIG. 9D  is a schematic cross-sectional view of a sensing biointerface in another embodiment wherein the sensing biointerface includes a plurality of electrodes  306  (e.g., in fork configuration). For example, working, reference, and/or counter wire electrodes may be separately disposed in the biointerface matrix  300 . In addition, multiple working electrodes may be used. A variety of configurations are possible for implementing the plurality of electrodes within the biointerface matrix. One advantage of deploying multiple working electrodes within the porous sensing biointerface is that some blockage or dysfunction on one working electrode (e.g., due to cellular attack or other malfunction) but not others will not affect the function of the overall sensor. In addition, using multiple working wire electrodes increases the overall number of sensing points along the combined lengths of the wires. By using a fork configuration, the sensing points are more widely distributed throughout the biointerface matrix. 
       FIG. 9E  is a schematic cross-sectional view of a sensing biointerface in yet another embodiment wherein the sensing biointerface comprises a spiral wire sensor  306 . Similar to the above-described plurality of electrodes, the spiral electrode provides an increase distribution of sensing points throughout the sensing biointerface, increasing the probability of good sensor performance. 
       FIG. 9F  is a schematic cross-sectional view of a sensing biointerface in yet another embodiment wherein the electrode  306  forms a tortuous path within the biointerface matrix  300 , again increasing the distribution of sensing points. 
       FIG. 9G  is a schematic cross-sectional view of a wholly implantable analyte sensor  300  in one embodiment. The sensor  300  includes a sensor body  302  (e.g., for containing the electronics described below) suitable for subcutaneous implantation. Published U.S. Patent Application No. 2004/0199059 to Brauker et al. describes systems and methods suitable for the sensor body  302 , and is incorporated herein by reference in its entirety. The sensor  300  also includes a small-structured sensing mechanism consisting of a wire electrode  304  with a biointerface matrix coating  306 . The biointerface matrix  306  depicted in  FIG. 9G  is not necessarily drawn to scale and may be any suitable size and shape when surrounding wire electrode  304 . In some embodiments, the wire electrode  304  is coated with a membrane system such as described herein. 
     One material that may be particularly suitable for either the biointerface matrix and/or the electrode(s) of the sensing mechanism of the preferred embodiments is elastomeric carbon. Elastomeric carbon is a material that is both elastic, which is a preferable property for the biointerface matrix, and is conductive, which is a preferable property for the working electrode(s). Some embodiments contemplate only a portion of the biointerface formed from elastomeric carbon. Because the elastomeric carbon can be used for the sensing mechanism and the biointerface matrix portions of the sensing biointerface, manufacture of the sensing biointerface is made simple. Namely, the biointerface matrix can be formed at least in part from elastomeric carbon using any of the methods described above, after which a membrane system can be directly applied to at least portions thereof, enabling sensing of the analyte. As one example of the elastomeric carbon embodiments described above, the biointerface matrix is formed from woven or non-woven material (e.g., ePTFE) over which elastomeric carbon or other electrode material is deposited (e.g., sprayed), and over which the membrane is deposited (e.g., vapor deposited). Although one example has been suggested, a variety of implementations of the sensing biointerface comprising elastomeric carbon (e.g., as a part of the biointerface matrix and sensing mechanism) are contemplated and included in the preferred embodiments. 
     In general, sterilization of the transcutaneous sensor can be completed after final assembly, utilizing methods such as electron beam radiation, gamma radiation, glutaraldehyde treatment, or the like. The sensor can be sterilized prior to or after packaging. In an alternative embodiment, one or more sensors can be sterilized using variable frequency microwave chamber(s), which can increase the speed and reduce the cost of the sterilization process. In another alternative embodiment, one or more sensors can be sterilized using ethylene oxide (EtO) gas sterilization, for example, by treating with 100% ethylene oxide, which can be used when the sensor electronics are not detachably connected to the sensor and/or when the sensor electronics must undergo a sterilization process. 
     Membrane Systems 
     As discussed above, in some embodiments, the sensing biointerfaces described herein include membranes systems deposited on one or more electroactive surfaces within the biointerface. In some embodiments, one or more domains of the membrane systems are formed from materials such as silicone, polytetrafluoroethylene, polyethylene-co-tetrafluoroethylene, polyolefin, polyester, polycarbonate, biostable polytetrafluoroethylene, homopolymers, copolymers, terpolymers of polyurethanes, polypropylene (PP), polyvinylchloride (PVC), polyvinylidene fluoride (PVDF), polybutylene terephthalate (PBT), polymethylmethacrylate (PMMA), polyether ether ketone (PEEK), polyurethanes, cellulosic polymers, polysulfones and block copolymers thereof including, for example, di-block, tri-block, alternating, random and graft copolymers. Co-pending U.S. patent application Ser. No. 10/838,912, which is incorporated herein by reference in its entirety, describes some biointerface and membrane system configurations and materials that may be applied to the preferred embodiments. Additional examples are described below. 
     The membrane system can be deposited on the exposed electroactive surfaces using known thin film techniques (for example, vapor deposition, spraying, electro-depositing, dipping, or the like). In alternative embodiments, however, other vapor deposition processes (e.g., physical and/or chemical vapor deposition processes) can be useful for providing one or more of the insulating and/or membrane layers, including ultrasonic vapor deposition, electrostatic deposition, evaporative deposition, deposition by sputtering, pulsed laser deposition, high velocity oxygen fuel deposition, thermal evaporator deposition, electron beam evaporator deposition, deposition by reactive sputtering molecular beam epitaxy, atmospheric pressure chemical vapor deposition (CVD), atomic layer CVD, hot wire CVD, low-pressure CVD, microwave plasma-assisted CVD, plasma-enhanced CVD, rapid thermal CVD, remote plasma-enhanced CVD, and ultra-high vacuum CVD, for example. However, the membrane system can be disposed over (or deposited on) the electroactive surfaces using any known method, as will be appreciated by one skilled in the art. It is noted that the membrane system that surrounds the working electrode does not have to be the same structure as the membrane system that surrounds a reference electrode, etc. For example, the enzyme domain deposited over the working electrode does not necessarily need to be deposited over the reference and/or counter electrodes. 
     Electrode/Electrolyte Domain 
     In selected embodiments, the membrane system comprises an electrode domain  56 . The electrode domain  56  is preferably situated more proximal to the electroactive surfaces than the interference and/or enzyme domain. Preferably, the electrode domain includes a coating that maintains a layer of water at the electrochemically reactive surfaces of the sensor. For example, a humectant in a binder material can be employed as an electrode domain; this allows for the full transport of ions in the aqueous environment. The electrode domain can also assist in stabilizing the operation of the sensor by accelerating electrode start-up and drifting problems caused by inadequate electrolyte. The material that forms the electrode domain can also provide an environment that protects against pH-mediated damage that can result from the formation of a large pH gradient due to the electrochemical activity of the electrodes. 
     In one embodiment, the electrode domain  56  includes a flexible, water-swellable, hydrogel film having a “dry film” thickness of from about 0.05 micron or less to about 20 microns or more, more preferably from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns, and more preferably still from about 2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns. “Dry film” thickness refers to the thickness of a cured film cast from a coating formulation by standard coating techniques. 
     In certain embodiments, the electrode domain  56  is formed of a curable mixture of a urethane polymer and a hydrophilic polymer. Particularly preferred coatings are formed of a polyurethane polymer having carboxylate or hydroxyl functional groups and non-ionic hydrophilic polyether segments, wherein the polyurethane polymer is crosslinked with a water soluble carbodiimide (e.g., 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)) in the presence of polyvinylpyrrolidone and cured at a moderate temperature of about 50° C. 
     In some preferred embodiments, the electrode domain  56  is formed from a hydrophilic polymer such as polyvinylpyrrolidone (PVP). An electrode domain formed from PVP has been shown to reduce break-in time of analyte sensors; for example, a glucose sensor utilizing a cellulosic-based interference domain such as described in more detail below. 
     In one embodiment, the electrode domain  56  is formed from a silicone polymer/hydrophobic-hydrophilic polymer blend. In one embodiment, The hydrophobic-hydrophilic polymer for use in the blend may be any suitable hydrophobic-hydrophilic polymer, including but not limited to components such as polyvinylpyrrolidone (PVP), polyhydroxyethyl methacrylate, polyvinylalcohol, polyacrylic acid, polyethers such as polyethylene glycol or polypropylene oxide, and copolymers thereof, including, for example, di-block, tri-block, alternating, random, comb, star, dendritic, and graft copolymers (block copolymers are discussed in U.S. Pat. Nos. 4,803,243 and 4,686,044, which are incorporated herein by reference). In one embodiment, the hydrophobic-hydrophilic polymer is a copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO). Suitable such polymers include, but are not limited to, PEO-PPO diblock copolymers, PPO-PEO-PPO triblock copolymers, PEO-PPO-PEO triblock copolymers, alternating block copolymers of PEO-PPO, random copolymers of ethylene oxide and propylene oxide, and blends thereof. In some embodiments, the copolymers may be optionally substituted with hydroxy substituents. Commercially available examples of PEO and PPO copolymers include the PLURONIC® brand of polymers available from BASF®. In one embodiment, PLURONIC® F-127 is used. Other PLURONIC® polymers include PPO-PEO-PPO triblock copolymers (e.g., PLURONIC® R products). Other suitable commercial polymers include, but are not limited to, SYNPERONICS® products available from UNIQEMA®. 
     The silicone polymer for use in the silicone/hydrophobic-hydrophilic polymer blend may be any suitable silicone polymer. In some embodiments, the silicone polymer is a liquid silicone rubber that may be vulcanized using a metal- (e.g., platinum), peroxide-, heat-, ultraviolet-, or other radiation-catalyzed process. In some embodiments, the silicone polymer is a dimethyl- and methylhydrogen-siloxane copolymer. In some embodiments, the copolymer has vinyl substituents. In some embodiments, commercially available silicone polymers may be used. For example, commercially available silicone polymer precursor compositions may be used to prepare the blends, such as described below. In one embodiment, MED-4840 available from NUSIL® Technology LLC is used as a precursor to the silicone polymer used in the blend. MED-4840 consists of a 2-part silicone elastomer precursor including vinyl-functionalized dimethyl- and methylhydrogen-siloxane copolymers, amorphous silica, a platinum catalyst, a crosslinker, and an inhibitor. The two components may be mixed together and heated to initiate vulcanization, thereby forming an elastomeric solid material. Other suitable silicone polymer precursor systems include, but are not limited to, MED-2174 peroxide-cured liquid silicone rubber available from NUSIL® Technology LLC, SILASTIC® MDX4-4210 platinum-cured biomedical grade elastomer available from DOW CORNING®, and Implant Grade Liquid Silicone Polymer (durometers 10-50) available from Applied Silicone Corporation. 
     Silicone polymer/hydrophobic-hydrophilic polymer blends are described in more detail in U.S. patent application Ser. No. 11/404,417, entitled “SILICONE BASED MEMBRANES FOR USE IN IMPLANTABLE GLUCOSE SENSORS,” filed on Apr. 14, 2006, which is incorporated herein by reference in its entirety. 
     Preferably, the electrode domain is deposited by vapor deposition, spray coating, dip coating, casting, or other thin film techniques on the electroactive surfaces of the sensor. In one preferred embodiment, the electrode domain is formed by dip-coating the electroactive surfaces in an electrode domain solution and curing the domain for a time of from about 15 minutes to about 30 minutes at a temperature of from about 40° C. to about 55° C. (and can be accomplished under vacuum (e.g., 20 to 30 mmHg)). In embodiments wherein dip-coating is used to deposit the electrode domain, a preferred insertion rate of from about 1 to about 3 inches per minute into the electrolyte layer solution, with a preferred dwell time of from about 0.5 to about 2 minutes in the electrolyte layer solution, and a preferred withdrawal rate of from about 0.25 to about 2 inches per minute from the electrolyte layer solution provide a functional coating. However, values outside of those set forth above can be acceptable or even desirable in certain embodiments, for example, depending upon solution viscosity and solution surface tension, as is appreciated by one skilled in the art. In one embodiment, the electroactive surfaces of the electrode system are dip-coated one time (one layer) and cured at 50° C. under vacuum for 20 minutes. 
     Although an independent electrode domain  56  is described herein, in some embodiments sufficient hydrophilicity can be provided in the interference domain and/or enzyme domain (the domain adjacent to the electroactive surfaces) so as to provide for the full transport of ions in the aqueous environment (e.g. without a distinct electrode domain). In these embodiments, an electrode domain is not necessary. 
     Interference Domain 
     Interferents are molecules or other species that are reduced or oxidized at the electrochemically reactive surfaces of the sensor, either directly or via an electron transfer agent, to produce a false positive analyte signal. In preferred embodiments, an interference domain  57  is provided that substantially restricts, resists, or blocks the flow of one or more interfering species. Some known interfering species for a glucose sensor, as described in more detail above, include acetaminophen, ascorbic acid, bilirubin, cholesterol, creatinine, dopamine, ephedrine, ibuprofen, L-dopa, methyl dopa, salicylate, tetracycline, tolazamide, tolbutamide, triglycerides, and uric acid. In general, the interference domain of the preferred embodiments is less permeable to one or more of the interfering species than to the analyte, e.g., glucose. 
     In some embodiments, the interference domain  57  is formed from one or more cellulosic derivatives. In general, cellulosic derivatives include polymers such as cellulose acetate, cellulose acetate butyrate, 2-hydroxyethyl cellulose, cellulose acetate phthalate, cellulose acetate propionate, cellulose acetate trimellitate, and the like. 
     In one embodiment, the interference domain  57  is formed from cellulose acetate butyrate. Cellulose acetate butyrate with a molecular weight of about 10,000 daltons to about 75,000 daltons, preferably from about 15,000, 20,000, or 25,000 daltons to about 50,000, 55,000, 60,000, 65,000, or 70,000 daltons, and more preferably about 20,000 daltons is employed. In certain embodiments, however, higher or lower molecular weights can be preferred. Additionally, a casting solution or dispersion of cellulose acetate butyrate at a weight percent of about 15% to about 25%, preferably from about 15%, 16%, 17%, 18%, 19% to about 20%, 21%, 22%, 23%, 24% or 25%, and more preferably about 18% is preferred. Preferably, the casting solution includes a solvent or solvent system, for example an acetone:ethanol solvent system. Higher or lower concentrations can be preferred in certain embodiments. A plurality of layers of cellulose acetate butyrate can be advantageously combined to form the interference domain in some embodiments, for example, three layers can be employed. It can be desirable to employ a mixture of cellulose acetate butyrate components with different molecular weights in a single solution, or to deposit multiple layers of cellulose acetate butyrate from different solutions comprising cellulose acetate butyrate of different molecular weights, different concentrations, and/or different chemistries (e.g., functional groups). It can also be desirable to include additional substances in the casting solutions or dispersions, e.g., functionalizing agents, crosslinking agents, other polymeric substances, substances capable of modifying the hydrophilicity/hydrophobicity of the resulting layer, and the like. 
     In one alternative embodiment, the interference domain  57  is formed from cellulose acetate. Cellulose acetate with a molecular weight of about 30,000 daltons or less to about 100,000 daltons or more, preferably from about 35,000, 40,000, or 45,000 daltons to about 55,000, 60,000, 65,000, 70,000, 75,000, 80,000, 85,000, 90,000, or 95,000 daltons, and more preferably about 50,000 daltons is preferred. Additionally, a casting solution or dispersion of cellulose acetate at a weight percent of about 3% to about 10%, preferably from about 3.5%, 4.0%, 4.5%, 5.0%, 5.5%, 6.0%, or 6.5% to about 7.5%, 8.0%, 8.5%, 9.0%, or 9.5%, and more preferably about 8% is preferred. In certain embodiments, however, higher or lower molecular weights and/or cellulose acetate weight percentages can be preferred. It can be desirable to employ a mixture of cellulose acetates with molecular weights in a single solution, or to deposit multiple layers of cellulose acetate from different solutions comprising cellulose acetates of different molecular weights, different concentrations, or different chemistries (e.g., functional groups). It can also be desirable to include additional substances in the casting solutions or dispersions such as described in more detail above. 
     Layer(s) prepared from combinations of cellulose acetate and cellulose acetate butyrate, or combinations of layer(s) of cellulose acetate and layer(s) of cellulose acetate butyrate can also be employed to form the interference domain  57 . 
     In some alternative embodiments, additional polymers, such as Nafion®, can be used in combination with cellulosic derivatives to provide equivalent and/or enhanced function of the interference domain  57 . As one example, a 5 wt % Nafion® casting solution or dispersion can be used in combination with a 8 wt % cellulose acetate casting solution or dispersion, e.g., by dip coating at least one layer of cellulose acetate and subsequently dip coating at least one layer Nafion® onto a sensor such as described with reference to the preferred embodiments. Any number of coatings or layers formed in any order may be suitable for forming the interference domain of the preferred embodiments. 
     In some alternative embodiments, more than one cellulosic derivative can be used to form the interference domain  57  of the preferred embodiments. In general, the formation of the interference domain on a surface utilizes a solvent or solvent system in order to solvate the cellulosic derivative (or other polymer) prior to film formation thereon. In preferred embodiments, acetone and ethanol are used as solvents for cellulose acetate; however one skilled in the art appreciates the numerous solvents that are suitable for use with cellulosic derivatives (and other polymers). Additionally, one skilled in the art appreciates that the preferred relative amounts of solvent can be dependent upon the cellulosic derivative (or other polymer) used, its molecular weight, its method of deposition, its desired thickness, and the like. However, a percent solute of from about 1% to about 25% is preferably used to form the interference domain solution so as to yield an interference domain having the desired properties. The cellulosic derivative (or other polymer) used, its molecular weight, method of deposition, and desired thickness can be adjusted, depending upon one or more other of the parameters, and can be varied accordingly as is appreciated by one skilled in the art. 
     In some alternative embodiments, other polymer types that can be utilized as a base material for the interference domain  57  including polyurethanes, polymers having pendant ionic groups, and polymers having controlled pore size, for example. In one such alternative embodiment, the interference domain includes a thin, hydrophobic membrane that is non-swellable and restricts diffusion of low molecular weight species. The interference domain  57  is permeable to relatively low molecular weight substances, such as hydrogen peroxide, but restricts the passage of higher molecular weight substances, including glucose and ascorbic acid. Other systems and methods for reducing or eliminating interference species that can be applied to the membrane system of the preferred embodiments are described in U.S. Publication No. US-2005-0115832-A1, U.S. Publication No. US-2005-0176136-A1, U.S. Publication No. US-2005-0161346-A1, and U.S. Publication No. US-2005-0143635-A1. In some alternative embodiments, a distinct interference domain is not included (e.g., wherein interferants are resisted by other domains and/or wherein the sensing mechanism does not have interferants). 
     In one embodiment, the interference domain  57  is formed from a silicone polymer/hydrophobic-hydrophilic polymer blend. In one embodiment, The hydrophobic-hydrophilic polymer for use in the blend may be any suitable hydrophobic-hydrophilic polymer, including but not limited to components such as polyvinylpyrrolidone (PVP), polyhydroxyethyl methacrylate, polyvinylalcohol, polyacrylic acid, polyethers such as polyethylene glycol or polypropylene oxide, and copolymers thereof, including, for example, di-block, tri-block, alternating, random, comb, star, dendritic, and graft copolymers (block copolymers are discussed in U.S. Pat. Nos. 4,803,243 and 4,686,044, which are incorporated herein by reference). In one embodiment, the hydrophobic-hydrophilic polymer is a copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO). Suitable such polymers include, but are not limited to, PEO-PPO diblock copolymers, PPO-PEO-PPO triblock copolymers, PEO-PPO-PEO triblock copolymers, alternating block copolymers of PEO-PPO, random copolymers of ethylene oxide and propylene oxide, and blends thereof. In some embodiments, the copolymers may be optionally substituted with hydroxy substituents. Commercially available examples of PEO and PPO copolymers include the PLURONIC® brand of polymers available from BASF®. In one embodiment, PLURONIC® F-127 is used. Other PLURONIC® polymers include PPO-PEO-PPO triblock copolymers (e.g., PLURONIC® R products). Other suitable commercial polymers include, but are not limited to, SYNPERONICS® products available from UNIQEMA®. 
     The silicone polymer for use in the silicone/hydrophobic-hydrophilic polymer blend may be any suitable silicone polymer. In some embodiments, the silicone polymer is a liquid silicone rubber that may be vulcanized using a metal- (e.g., platinum), peroxide-, heat-, ultraviolet-, or other radiation-catalyzed process. In some embodiments, the silicone polymer is a dimethyl- and methylhydrogen-siloxane copolymer. In some embodiments, the copolymer has vinyl substituents. In some embodiments, commercially available silicone polymers may be used. For example, commercially available silicone polymer precursor compositions may be used to prepare the blends, such as described below. In one embodiment, MED-4840 available from NUSIL® Technology LLC is used as a precursor to the silicone polymer used in the blend. MED-4840 consists of a 2-part silicone elastomer precursor including vinyl-functionalized dimethyl- and methylhydrogen-siloxane copolymers, amorphous silica, a platinum catalyst, a crosslinker, and an inhibitor. The two components may be mixed together and heated to initiate vulcanization, thereby forming an elastomeric solid material. Other suitable silicone polymer precursor systems include, but are not limited to, MED-2174 peroxide-cured liquid silicone rubber available from NUSIL® Technology LLC, SILASTIC® MDX4-4210 platinum-cured biomedical grade elastomer available from DOW CORNING®, and Implant Grade Liquid Silicone Polymer (durometers 10-50) available from Applied Silicone Corporation. 
     Silicone polymer/hydrophobic-hydrophilic polymer blends are described in more detail in U.S. patent application Ser. No. 11/404,417, entitled “SILICONE BASED MEMBRANES FOR USE IN IMPLANTABLE GLUCOSE SENSORS,” filed on Apr. 14, 2006, which is incorporated herein by reference in its entirety. 
     In preferred embodiments, the interference domain  57  is deposited directly onto the electroactive surfaces of the sensor for a domain thickness of from about 0.05 micron or less to about 20 microns or more, more preferably from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns, and more preferably still from about 1, 1.5 or 2 microns to about 2.5 or 3 microns. Thicker membranes can also be desirable in certain embodiments, but thinner membranes are generally preferred because they have a lower impact on the rate of diffusion of hydrogen peroxide from the enzyme membrane to the electrodes. 
     In general, the membrane systems of the preferred embodiments can be formed and/or deposited on the exposed electroactive surfaces (e.g., one or more of the working and reference electrodes) using known thin film techniques (for example, casting, spray coating, drawing down, electro-depositing, dip coating, and the like), however casting or other known application techniques can also be utilized. Preferably, the interference domain is deposited by vapor deposition, spray coating, or dip coating. In one exemplary embodiment, the interference domain is formed by dip coating the sensor into an interference domain solution using an insertion rate of from about 20 inches/min to about 60 inches/min, preferably 40 inches/min, a dwell time of from about 0 minute to about 5 seconds, preferably 0 seconds, and a withdrawal rate of from about 20 inches/minute to about 60 inches/minute, preferably about 40 inches/minute, and curing (drying) the domain from about 1 minute to about 30 minutes, preferably from about 3 minutes to about 15 minutes (and can be accomplished at room temperature or under vacuum (e.g., 20 to 30 mmHg)). In one exemplary embodiment including cellulose acetate butyrate interference domain, a 3-minute cure (i.e., dry) time is preferred between each layer applied. In another exemplary embodiment employing a cellulose acetate interference domain, a 15 minute cure (i.e., dry) time is preferred between each layer applied. 
     The dip process can be repeated at least one time and up to 10 times or more. The preferred number of repeated dip processes depends upon the cellulosic derivative(s) used, their concentration, conditions during deposition (e.g., dipping) and the desired thickness (e.g., sufficient thickness to provide functional blocking of (or resistance to) certain interferents), and the like. In some embodiments, 1 to 3 microns may be preferred for the interference domain thickness; however, values outside of these can be acceptable or even desirable in certain embodiments, for example, depending upon viscosity and surface tension, as is appreciated by one skilled in the art. In one exemplary embodiment, an interference domain is formed from three layers of cellulose acetate butyrate. In another exemplary embodiment, an interference domain is formed from 10 layers of cellulose acetate. In alternative embodiments, the interference domain can be formed using any known method and combination of cellulose acetate and cellulose acetate butyrate, as will be appreciated by one skilled in the art. 
     In some embodiments, the electroactive surface can be cleaned prior to application of the interference domain  57 . In some embodiments, the interference domain  57  of the preferred embodiments can be useful as a bioprotective or biocompatible domain, namely, a domain that interfaces with host tissue when implanted in an animal (e.g., a human) due to its stability and biocompatibility. 
     Enzyme Domain 
     In preferred embodiments, the membrane system further includes an enzyme domain  58  disposed more distally from the electroactive surfaces than the interference domain  57 ; however other configurations can be desirable. In the preferred embodiments, the enzyme domain provides an enzyme to catalyze the reaction of the analyte and its co-reactant. In the preferred embodiments of a glucose sensor, the enzyme domain includes glucose oxidase; however other oxidases, for example, galactose oxidase or uricase oxidase, can also be used. 
     For an enzyme-based electrochemical glucose sensor to perform well, the sensor&#39;s response is preferably limited by neither enzyme activity nor co-reactant concentration. Because enzymes, including glucose oxidase, are subject to deactivation as a function of time even in ambient conditions, this behavior is compensated for in forming the enzyme domain. Preferably, the enzyme domain is constructed of aqueous dispersions of colloidal polyurethane polymers including the enzyme. However, in alternative embodiments the enzyme domain is constructed from an oxygen enhancing material, for example, silicone, or fluorocarbon. Preferably, the enzyme is immobilized within the domain. See, e.g., U.S. patent application Ser. No. 10/896,639 filed on Jul. 21, 2004 and entitled “Oxygen Enhancing Membrane Systems for Implantable Device.” 
     In one embodiment, the enzyme domain  58  is formed from a silicone polymer/hydrophobic-hydrophilic polymer blend. In one embodiment, The hydrophobic-hydrophilic polymer for use in the blend may be any suitable hydrophobic-hydrophilic polymer, including but not limited to components such as polyvinylpyrrolidone (PVP), polyhydroxyethyl methacrylate, polyvinylalcohol, polyacrylic acid, polyethers such as polyethylene glycol or polypropylene oxide, and copolymers thereof, including, for example, di-block, tri-block, alternating, random, comb, star, dendritic, and graft copolymers (block copolymers are discussed in U.S. Pat. Nos. 4,803,243 and 4,686,044, which are incorporated herein by reference). In one embodiment, the hydrophobic-hydrophilic polymer is a copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO). Suitable such polymers include, but are not limited to, PEO-PPO diblock copolymers, PPO-PEO-PPO triblock copolymers, PEO-PPO-PEO triblock copolymers, alternating block copolymers of PEO-PPO, random copolymers of ethylene oxide and propylene oxide, and blends thereof. In some embodiments, the copolymers may be optionally substituted with hydroxy substituents. Commercially available examples of PEO and PPO copolymers include the PLURONIC® brand of polymers available from BASF®. In one embodiment, PLURONIC® F-127 is used. Other PLURONIC® polymers include PPO-PEO-PPO triblock copolymers (e.g., PLURONIC® R products). Other suitable commercial polymers include, but are not limited to, SYNPERONICS® products available from UNIQEMA®. 
     The silicone polymer for use in the silicone/hydrophobic-hydrophilic polymer blend may be any suitable silicone polymer. In some embodiments, the silicone polymer is a liquid silicone rubber that may be vulcanized using a metal- (e.g., platinum), peroxide-, heat-, ultraviolet-, or other radiation-catalyzed process. In some embodiments, the silicone polymer is a dimethyl- and methylhydrogen-siloxane copolymer. In some embodiments, the copolymer has vinyl substituents. In some embodiments, commercially available silicone polymers may be used. For example, commercially available silicone polymer precursor compositions may be used to prepare the blends, such as described below. In one embodiment, MED-4840 available from NUSIL® Technology LLC is used as a precursor to the silicone polymer used in the blend. MED-4840 consists of a 2-part silicone elastomer precursor including vinyl-functionalized dimethyl- and methylhydrogen-siloxane copolymers, amorphous silica, a platinum catalyst, a crosslinker, and an inhibitor. The two components may be mixed together and heated to initiate vulcanization, thereby forming an elastomeric solid material. Other suitable silicone polymer precursor systems include, but are not limited to, MED-2174 peroxide-cured liquid silicone rubber available from NUSIL® Technology LLC, SILASTIC® MDX4-4210 platinum-cured biomedical grade elastomer available from DOW CORNING®, and Implant Grade Liquid Silicone Polymer (durometers 10-50) available from Applied Silicone Corporation. 
     Silicone polymer/hydrophobic-hydrophilic polymer blends are described in more detail in U.S. patent application Ser. No. 11/404,417, entitled “SILICONE BASED MEMBRANES FOR USE IN IMPLANTABLE GLUCOSE SENSORS,” filed on Apr. 14, 2006, which is incorporated herein by reference in its entirety. 
     In preferred embodiments, the enzyme domain is deposited onto the interference domain for a domain thickness of from about 0.05 micron or less to about 20 microns or more, more preferably from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns, and more preferably still from about 2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns. However in some embodiments, the enzyme domain can be deposited directly onto the electroactive surfaces. Preferably, the enzyme domain is deposited by spray or dip coating. In one embodiment, the enzyme domain is formed by dip coating the interference domain coated sensor into an enzyme domain solution and curing the domain for from about 15 to about 30 minutes at a temperature of from about 40° C. to about 55° C. (and can be accomplished under vacuum (e.g., 20 to 30 mmHg)). In embodiments wherein dip coating is used to deposit the enzyme domain at room temperature, a preferred insertion rate of from about 0.25 inch per minute to about 3 inches per minute, with a preferred dwell time of from about 0.5 minutes to about 2 minutes, and a preferred withdrawal rate of from about 0.25 inch per minute to about 2 inches per minute provides a functional coating. However, values outside of those set forth above can be acceptable or even desirable in certain embodiments, for example, depending upon viscosity and surface tension, as is appreciated by one skilled in the art. In one embodiment, the enzyme domain is formed by dip coating two times (namely, forming two layers) in an enzyme domain solution and curing at 50° C. under vacuum for 20 minutes. However, in some embodiments, the enzyme domain can be formed by dip coating and/or spray coating one or more layers at a predetermined concentration of the coating solution, insertion rate, dwell time, withdrawal rate, and/or desired thickness. 
     Resistance Domain 
     In preferred embodiments, the membrane system includes a resistance domain  59  disposed more distal from the electroactive surfaces than the enzyme domain. Although the following description is directed to a resistance domain for a glucose sensor, the resistance domain can be modified for other analytes and co-reactants as well. 
     There exists a molar excess of glucose relative to the amount of oxygen in blood; that is, for every free oxygen molecule in extracellular fluid, there are typically more than 100 glucose molecules present (see Updike et al., Diabetes Care 5:207-21(1982)). However, an immobilized enzyme-based glucose sensor employing oxygen as co-reactant is preferably supplied with oxygen in non-rate-limiting excess in order for the sensor to respond linearly to changes in glucose concentration, while not responding to changes in oxygen concentration. Specifically, when a glucose-monitoring reaction is oxygen limited, linearity is not achieved above minimal concentrations of glucose. Without a semipermeable membrane situated over the enzyme domain to control the flux of glucose and oxygen, a linear response to glucose levels can be obtained only for glucose concentrations of up to about 40 mg/dL. However, in a clinical setting, a linear response to glucose levels is desirable up to at least about 400 mg/dL. 
     The resistance domain includes a semipermeable membrane that controls the flux of oxygen and glucose to the underlying enzyme domain, preferably rendering oxygen in a non-rate-limiting excess. As a result, the upper limit of linearity of glucose measurement is extended to a much higher value than that which is achieved without the resistance domain. In one embodiment, the resistance domain exhibits an oxygen to glucose permeability ratio of from about 50:1 or less to about 400:1 or more, preferably about 200:1. As a result, one-dimensional reactant diffusion is adequate to provide excess oxygen at all reasonable glucose and oxygen concentrations found in the subcutaneous matrix (See Rhodes et al., Anal. Chem., 66:1520-1529 (1994)). 
     In alternative embodiments, a lower ratio of oxygen-to-glucose can be sufficient to provide excess oxygen by using a high oxygen solubility domain (for example, a silicone or fluorocarbon-based material or domain) to enhance the supply/transport of oxygen to the enzyme domain. If more oxygen is supplied to the enzyme, then more glucose can also be supplied to the enzyme without creating an oxygen rate-limiting excess. In alternative embodiments, the resistance domain is formed from a silicone composition, such as is described in U.S. Publication No. US-2005-0090607-A1. 
     In a preferred embodiment, the resistance domain includes a polyurethane membrane with both hydrophilic and hydrophobic regions to control the diffusion of glucose and oxygen to an analyte sensor, the membrane being fabricated easily and reproducibly from commercially available materials. A suitable hydrophobic polymer component is a polyurethane, or polyetherurethaneurea. Polyurethane is a polymer produced by the condensation reaction of a diisocyanate and a difunctional hydroxyl-containing material. A polyurethaneurea is a polymer produced by the condensation reaction of a diisocyanate and a difunctional amine-containing material. Preferred diisocyanates include aliphatic diisocyanates containing from about 4 to about 8 methylene units. Diisocyanates containing cycloaliphatic moieties can also be useful in the preparation of the polymer and copolymer components of the membranes of preferred embodiments. The material that forms the basis of the hydrophobic matrix of the resistance domain can be any of those known in the art as appropriate for use as membranes in sensor devices and as having sufficient permeability to allow relevant compounds to pass through it, for example, to allow an oxygen molecule to pass through the membrane from the sample under examination in order to reach the active enzyme or electrochemical electrodes. Examples of materials which can be used to make non-polyurethane type membranes include vinyl polymers, polyethers, polyesters, polyamides, inorganic polymers such as polysiloxanes and polycarbosiloxanes, natural polymers such as cellulosic and protein based materials, and mixtures or combinations thereof. 
     In a preferred embodiment, the hydrophilic polymer component is polyethylene oxide. For example, one useful hydrophobic-hydrophilic copolymer component is a polyurethane polymer that includes about 20% hydrophilic polyethylene oxide. The polyethylene oxide portions of the copolymer are thermodynamically driven to separate from the hydrophobic portions of the copolymer and the hydrophobic polymer component. The 20% polyethylene oxide-based soft segment portion of the copolymer used to form the final blend affects the water pick-up and subsequent glucose permeability of the membrane. 
     In one preferred embodiment, the resistance domain  59  is formed from a silicone polymer/hydrophobic-hydrophilic polymer blend. In one embodiment, The hydrophobic-hydrophilic polymer for use in the blend may be any suitable hydrophobic-hydrophilic polymer, including but not limited to components such as polyvinylpyrrolidone (PVP), polyhydroxyethyl methacrylate, polyvinylalcohol, polyacrylic acid, polyethers such as polyethylene glycol or polypropylene oxide, and copolymers thereof, including, for example, di-block, tri-block, alternating, random, comb, star, dendritic, and graft copolymers (block copolymers are discussed in U.S. Pat. Nos. 4,803,243 and 4,686,044, which are incorporated herein by reference). In one embodiment, the hydrophobic-hydrophilic polymer is a copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO). Suitable such polymers include, but are not limited to, PEO-PPO diblock copolymers, PPO-PEO-PPO triblock copolymers, PEO-PPO-PEO triblock copolymers, alternating block copolymers of PEO-PPO, random copolymers of ethylene oxide and propylene oxide, and blends thereof. In some embodiments, the copolymers may be optionally substituted with hydroxy substituents. Commercially available examples of PEO and PPO copolymers include the PLURONIC® brand of polymers available from BASF®. In one embodiment, PLURONIC® F-127 is used. Other PLURONIC® polymers include PPO-PEO-PPO triblock copolymers (e.g., PLURONIC® R products). Other suitable commercial polymers include, but are not limited to, SYNPERONICS® products available from UNIQEMA®. 
     The silicone polymer for use in the silicone/hydrophobic-hydrophilic polymer blend may be any suitable silicone polymer. In some embodiments, the silicone polymer is a liquid silicone rubber that may be vulcanized using a metal- (e.g., platinum), peroxide-, heat-, ultraviolet-, or other radiation-catalyzed process. In some embodiments, the silicone polymer is a dimethyl- and methylhydrogen-siloxane copolymer. In some embodiments, the copolymer has vinyl substituents. In some embodiments, commercially available silicone polymers may be used. For example, commercially available silicone polymer precursor compositions may be used to prepare the blends, such as described below. In one embodiment, MED-4840 available from NUSIL® Technology LLC is used as a precursor to the silicone polymer used in the blend. MED-4840 consists of a 2-part silicone elastomer precursor including vinyl-functionalized dimethyl- and methylhydrogen-siloxane copolymers, amorphous silica, a platinum catalyst, a crosslinker, and an inhibitor. The two components may be mixed together and heated to initiate vulcanization, thereby forming an elastomeric solid material. Other suitable silicone polymer precursor systems include, but are not limited to, MED-2174 peroxide-cured liquid silicone rubber available from NUSIL® Technology LLC, SILASTIC® MDX4-4210 platinum-cured biomedical grade elastomer available from DOW CORNING®, and Implant Grade Liquid Silicone Polymer (durometers 10-50) available from Applied Silicone Corporation. 
     Silicone polymer/hydrophobic-hydrophilic polymer blends are described in more detail in U.S. patent application Ser. No. 11/404,417, entitled “SILICONE BASED MEMBRANES FOR USE IN IMPLANTABLE GLUCOSE SENSORS,” filed on Apr. 14, 2006, which is incorporated herein by reference in its entirety. 
     In preferred embodiments, the resistance domain is deposited onto the enzyme domain to yield a domain thickness of from about 0.05 microns or less to about 20 microns or more, more preferably from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns, and more preferably still from about 2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns. Preferably, the resistance domain is deposited onto the enzyme domain by vapor deposition, spray coating, or dip coating. In one preferred embodiment, spray coating is the preferred deposition technique. The spraying process atomizes and mists the solution, and therefore most or all of the solvent is evaporated prior to the coating material settling on the underlying domain, thereby minimizing contact of the solvent with the enzyme. 
     In another preferred embodiment, physical vapor deposition (e.g., ultrasonic vapor deposition) is used for coating one or more of the membrane domain(s) onto the electrodes, wherein the vapor deposition apparatus and process include an ultrasonic nozzle that produces a mist of micro-droplets in a vacuum chamber. In these embodiments, the micro-droplets move turbulently within the vacuum chamber, isotropically impacting and adhering to the surface of the substrate. Advantageously, vapor deposition as described above can be implemented to provide high production throughput of membrane deposition processes (e.g., at least about 20 to about 200 or more electrodes per chamber), greater consistency of the membrane on each sensor, and increased uniformity of sensor performance, for example, as described below. 
     In some embodiments, depositing the resistance domain (for example, as described in the preferred embodiments above) includes formation of a membrane system that substantially blocks or resists ascorbate (a known electrochemical interferant in hydrogen peroxide-measuring glucose sensors). While not wishing to be bound by theory, it is believed that during the process of depositing the resistance domain as described in the preferred embodiments, a structural morphology is formed that is characterized in that ascorbate does not substantially permeate therethrough. 
     In a preferred embodiment, the resistance domain is deposited on the enzyme domain by spray coating a solution of from about 1 wt. % to about 5 wt. % polymer and from about 95 wt. % to about 99 wt. % solvent. In spraying a solution of resistance domain material, including a solvent, onto the enzyme domain, it is desirable to mitigate or substantially reduce any contact with enzyme of any solvent in the spray solution that can deactivate the underlying enzyme of the enzyme domain. Tetrahydrofuran (THF) is one solvent that minimally or negligibly affects the enzyme of the enzyme domain upon spraying. Other solvents can also be suitable for use, as is appreciated by one skilled in the art. 
     Although a variety of spraying or deposition techniques can be used, spraying the resistance domain material and rotating the sensor at least one time by 180° can typically provide adequate coverage by the resistance domain. Spraying the resistance domain material and rotating the sensor at least two times by 120° provides even greater coverage (one layer of 360° coverage), thereby ensuring resistivity to glucose, such as is described in more detail above. 
     In preferred embodiments, the resistance domain is spray coated and subsequently cured for a time of from about 15 minutes to about 90 minutes at a temperature of from about 40° C. to about 60° C. (and can be accomplished under vacuum (e.g., from 20 to 30 mmHg)). A cure time of up to about 90 minutes or more can be advantageous to ensure complete drying of the resistance domain. 
     In one embodiment, the resistance domain is formed by spray coating at least six layers (namely, rotating the sensor seventeen times by 120° for at least six layers of 360° coverage) and curing at 50° C. under vacuum for 60 minutes. However, the resistance domain can be formed by dip coating or spray coating any layer or plurality of layers, depending upon the concentration of the solution, insertion rate, dwell time, withdrawal rate, and/or the desired thickness of the resulting film. Additionally, curing in a convention oven can also be employed. 
     In certain embodiments, a variable frequency microwave oven can be used to cure the membrane domains/layers. In general, microwave ovens directly excite the rotational mode of solvents. Consequently, microwave ovens cure coatings from the inside out rather than from the outside in as with conventional convection ovens. This direct rotational mode excitation is responsible for the typically observed “fast” curing within a microwave oven. In contrast to conventional microwave ovens, which rely upon a fixed frequency of emission that can cause arcing of dielectric (metallic) substrates if placed within a conventional microwave oven, Variable Frequency Microwave (VFM) ovens emit thousands of frequencies within 100 milliseconds, which substantially eliminates arcing of dielectric substrates. Consequently, the membrane domains/layers can be cured even after deposition on metallic electrodes as described herein. While not wishing to be bound by theory, it is believe that VFM curing can increase the rate and completeness of solvent evaporation from a liquid membrane solution applied to a sensor, as compared to the rate and completeness of solvent evaporation observed for curing in conventional convection ovens. 
     In certain embodiments, VFM is can be used together with convection oven curing to further accelerate cure time. In some sensor applications wherein the membrane is cured prior to application on the electrode (see, for example, U.S. Publication No. US-2005-0245799-A1, which is incorporated herein by reference in its entirety), conventional microwave ovens (e.g., fixed frequency microwave ovens) can be used to cure the membrane layer. 
     Bioprotective Domain 
     The bioprotective domain, if present, is positioned less distal to the implantable device than the cell disruptive layer, and can be resistant to cellular attachment, impermeable to cells, and/or is composed of a biostable material. When the bioprotective domain is resistant to cellular attachment (for example, attachment by inflammatory cells, such as macrophages, which are therefore kept a sufficient distance from other domains, for example, the enzyme domain), hypochlorite and other oxidizing species are short-lived chemical species in vivo, and biodegradation does not occur. Additionally, the materials preferred for forming the bioprotective domain may be resistant to the effects of these oxidative species and have thus been termed biodurable. See, for example, U.S. Pat. No. 6,702,857, filed Jul. 27, 2001, and entitled “MEMBRANE FOR USE WITH IMPLANTABLE DEVICES” and U.S. patent application Ser. No. 10/647,065, filed Aug. 22, 2003, published in Publication No. 20050112169 and entitled, “POROUS MEMBRANES FOR USE WITH IMPLANTABLE DEVICES,” both of which are incorporated herein by reference in their entirety. 
     In one embodiment, bioprotective domain is formed from high oxygen soluble materials such as polymers formed from silicone, fluorocarbons, perfluorocarbons, or the like. In one embodiment, the cell impermeable domain is formed from a silicone composition with a hydrophile such as such as polyethylene glycol, propylene glycol, pyrrolidone, esters, amides, carbonates, or polypropylene glycol covalently incorporated or grafted therein. In still other embodiments, the bioprotective domain is formed from a monomer, polymer, copolymer, or blend including one or more of: lactic acid, glycolic acid, anhydrides, phospazenes, vinyl alcohol, ethylene vinyl alcohol, acetates, ε-caprolactone, β-hydroxybutyrate, γ-ethyl glutamate, DTH iminocarbonate, Bisphenol A iminocarbonate, sebacic acid, hexadecanoic acid, saccharides, chitosan, hydyoxyethyl methacrylate (HEMA), ceramics, hyaluronic acid (HA), collagen, gelatin, starches, hydroxy apatite, calcium phosphates, bioglasses, amino acid sequences, proteins, glycoproteins, protein fragments, agarose, fibrin, n-butylene, isobutylene, dioxanone, nylons, vinyl chlorides, amides, ethylenes, n-butyl methacrylate (BMA), metal matrix composites (MMCs), metal oxides (e.g. aluminum), DETOSU-1,6 HD-t-CDM ortho ester, styrene, and plasma treated surfaces of any of the above. 
     In one preferred embodiment, the bioprotective domain is formed from a silicone polymer/hydrophobic-hydrophilic polymer blend. In one embodiment, The hydrophobic-hydrophilic polymer for use in the blend may be any suitable hydrophobic-hydrophilic polymer, including but not limited to components such as polyvinylpyrrolidone (PVP), polyhydroxyethyl methacrylate, polyvinylalcohol, polyacrylic acid, polyethers such as polyethylene glycol or polypropylene oxide, and copolymers thereof, including, for example, di-block, tri-block, alternating, random, comb, star, dendritic, and graft copolymers (block copolymers are discussed in U.S. Pat. Nos. 4,803,243 and 4,686,044, which are incorporated herein by reference). In one embodiment, the hydrophobic-hydrophilic polymer is a copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO). Suitable such polymers include, but are not limited to, PEO-PPO diblock copolymers, PPO-PEO-PPO triblock copolymers, PEO-PPO-PEO triblock copolymers, alternating block copolymers of PEO-PPO, random copolymers of ethylene oxide and propylene oxide, and blends thereof. In some embodiments, the copolymers may be optionally substituted with hydroxy substituents. Commercially available examples of PEO and PPO copolymers include the PLURONIC® brand of polymers available from BASF®. In one embodiment, PLURONIC® F-127 is used. Other PLURONIC® polymers include PPO-PEO-PPO triblock copolymers (e.g., PLURONIC® R products). Other suitable commercial polymers include, but are not limited to, SYNPERONICS® products available from UNIQEMA®. 
     The silicone polymer for use in the silicone/hydrophobic-hydrophilic polymer blend may be any suitable silicone polymer. In some embodiments, the silicone polymer is a liquid silicone rubber that may be vulcanized using a metal- (e.g., platinum), peroxide-, heat-, ultraviolet-, or other radiation-catalyzed process. In some embodiments, the silicone polymer is a dimethyl- and methylhydrogen-siloxane copolymer. In some embodiments, the copolymer has vinyl substituents. In some embodiments, commercially available silicone polymers may be used. For example, commercially available silicone polymer precursor compositions may be used to prepare the blends, such as described below. In one embodiment, MED-4840 available from NUSIL® Technology LLC is used as a precursor to the silicone polymer used in the blend. MED-4840 consists of a 2-part silicone elastomer precursor including vinyl-functionalized dimethyl- and methylhydrogen-siloxane copolymers, amorphous silica, a platinum catalyst, a crosslinker, and an inhibitor. The two components may be mixed together and heated to initiate vulcanization, thereby forming an elastomeric solid material. Other suitable silicone polymer precursor systems include, but are not limited to, MED-2174 peroxide-cured liquid silicone rubber available from NUSIL® Technology LLC, SILASTIC® MDX4-4210 platinum-cured biomedical grade elastomer available from DOW CORNING®, and Implant Grade Liquid Silicone Polymer (durometers 10-50) available from Applied Silicone Corporation. 
     Silicone polymer/hydrophobic-hydrophilic polymer blends are described in more detail in U.S. patent application Ser. No. 11/404,417, entitled “SILICONE BASED MEMBRANES FOR USE IN IMPLANTABLE GLUCOSE SENSORS,” filed on Apr. 14, 2006, which is incorporated herein by reference in its entirety. 
     It is advantageous that the bioprotective domain have both high oxygen and aqueous analyte solubility so that sufficient reactants reach the enzyme layer. Accordingly, in one embodiment, the concentration of hydrophobic-hydrophilic polymer (e.g., PLURONIC® F-127) relative to silicone polymer (e.g., MED-4840) is relatively high, e.g., from about 10% to about 30% in the bioprotective layer  42 . In one embodiment, the concentration of hydrophobic-hydrophilic polymer is from about 15% to about 25% (e.g., about 20%). 
     In preferred embodiments, the thickness of the bioprotective domain is from about 10 or 15 microns or less to about 125, 150, 175, 200 or 250 microns or more. In more preferred embodiments, the thickness of the bioprotective domain is from about 20, 25, 30, or 35 microns to about 60, 65, 70, 75, 80, 85, 90, 95, or 100 microns. In even more preferred embodiments, the bioprotective domain is from about 20 or 25 microns to about 50, 55, or 60 microns thick. 
     Electronics 
     The electrodes of the sensing biointerfaces described above may be electrically coupled at their ends to contacts (or the like) on the sensor electronics, which are connected to a power source. In some embodiments, the sensing biointerface and accompanying electronics are packaged in a complete sensor device that can be implanted (e.g., transcutaneously or wholly) into a host. In one embodiment, the sensing biointerface is stretched linearly between two supports on the complete sensor device. In one embodiment, the sensing biointerface is coiled or looped to reduce the footprint of the device. However, whatever configuration of sensing biointerface is used, it is advantageous to allow both (or numerous) sides of the biointerface to contact tissue. 
     In some embodiments, the biocompatible matrix is electrically coupled to the sensor electronics and includes a power source. Namely, the sensor electronics comprise contacts (or the like) configured to contact the electrodes of the biocompatible matrix. In one embodiment, a portion of the electrodes of the biocompatible matrix are exposed at their ends and configured to engage contacts on the sensor electronics device body. Alternately, the electrodes are hard-wired into the sensor electronics to provide electrical coupling. The power source is connected to the sensor electronics and includes a battery, inductor, or the like, as is appreciated by one skilled in the art. 
       FIG. 15  is a block diagram that illustrates the sensor electronics in one embodiment. In this embodiment, a potentiostat  134  is shown, which is operably connected to an electrode system (such as described above) and provides a voltage to the electrodes, which biases the sensor to enable measurement of an current signal indicative of the analyte concentration in the host (also referred to as the analog portion). In some embodiments, the potentiostat includes a resistor (not shown) that translates the current into voltage. In some alternative embodiments, a current to frequency converter is provided that is configured to continuously integrate the measured current, for example, using a charge counting device. 
     An A/D converter  136  digitizes the analog signal into a digital signal, also referred to as “counts” for processing. Accordingly, the resulting raw data stream in counts, also referred to as raw sensor data, is directly related to the current measured by the potentiostat  134 . 
     A processor module  138  includes the central control unit that controls the processing of the sensor electronics  132 . In some embodiments, the processor module includes a microprocessor, however a computer system other than a microprocessor can be used to process data as described herein, for example an ASIC can be used for some or all of the sensor&#39;s central processing. The processor typically provides semi-permanent storage of data, for example, storing data such as sensor identifier (ID) and programming to process data streams (for example, programming for data smoothing and/or replacement of signal artifacts such as is described in U.S. Publication No. US-2005-0043598-A1). The processor additionally can be used for the system&#39;s cache memory, for example for temporarily storing recent sensor data. In some embodiments, the processor module comprises memory storage components such as ROM, RAM, dynamic-RAM, static-RAM, non-static RAM, EEPROM, rewritable ROMs, flash memory, or the like. 
     In some embodiments, the processor module comprises a digital filter, for example, an infinite impulse response (IIR) or finite impulse response (FIR) filter, configured to smooth the raw data stream from the A/D converter. Generally, digital filters are programmed to filter data sampled at a predetermined time interval (also referred to as a sample rate). In some embodiments, wherein the potentiostat is configured to measure the analyte at discrete time intervals, these time intervals determine the sample rate of the digital filter. In some alternative embodiments, wherein the potentiostat is configured to continuously measure the analyte, for example, using a current-to-frequency converter as described above, the processor module can be programmed to request a digital value from the A/D converter at a predetermined time interval, also referred to as the acquisition time. In these alternative embodiments, the values obtained by the processor are advantageously averaged over the acquisition time due the continuity of the current measurement. Accordingly, the acquisition time determines the sample rate of the digital filter. In preferred embodiments, the processor module is configured with a programmable acquisition time, namely, the predetermined time interval for requesting the digital value from the A/D converter is programmable by a user within the digital circuitry of the processor module. An acquisition time of from about 2 seconds to about 512 seconds is preferred; however any acquisition time can be programmed into the processor module. A programmable acquisition time is advantageous in optimizing noise filtration, time lag, and processing/battery power. 
     Preferably, the processor module is configured to build the data packet for transmission to an outside source, for example, an RF transmission to a receiver. Generally, the data packet comprises a plurality of bits that can include a preamble, a unique identifier identifying the electronics unit, the receiver, or both, (e.g., sensor ID code), data (e.g., raw data, filtered data, and/or an integrated value) and/or error detection or correction. Preferably, the data (transmission) packet has a length of from about 8 bits to about 128 bits, preferably about 48 bits; however, larger or smaller packets can be desirable in certain embodiments. The processor module can be configured to transmit any combination of raw and/or filtered data. In one exemplary embodiment, the transmission packet contains a fixed preamble, a unique ID of the electronics unit, a single five-minute average (e.g., integrated) sensor data value, and a cyclic redundancy code (CRC). 
     In some embodiments, the processor module further comprises a transmitter portion that determines the transmission interval of the sensor data to a receiver, or the like. In some embodiments, the transmitter portion, which determines the interval of transmission, is configured to be programmable. In one such embodiment, a coefficient can be chosen (e.g., a number of from about 1 to about 100, or more), wherein the coefficient is multiplied by the acquisition time (or sampling rate), such as described above, to define the transmission interval of the data packet. Thus, in some embodiments, the transmission interval is programmable from about 2 seconds to about 850 minutes, more preferably from about 30 second to about 5 minutes; however, any transmission interval can be programmable or programmed into the processor module. However, a variety of alternative systems and methods for providing a programmable transmission interval can also be employed. By providing a programmable transmission interval, data transmission can be customized to meet a variety of design criteria (e.g., reduced battery consumption, timeliness of reporting sensor values, etc.) 
     Conventional glucose sensors measure current in the nanoAmp range. In some embodiments, the preferred embodiments are configured to measure the current flow in the picoAmp range, and in some embodiments, femtoAmps. Namely, for every unit (mg/dL) of glucose measured, at least one picoAmp of current is measured. Preferably, the analog portion of the A/D converter  136  is configured to continuously measure the current flowing at the working electrode and to convert the current measurement to digital values representative of the current. In one embodiment, the current flow is measured by a charge counting device (e.g., a capacitor). Preferably, a charge counting device provides a value (e.g., digital value) representative of the current flow integrated over time (e.g., integrated value). In some embodiments, the value is integrated over a few seconds, a few minutes, or longer. In one exemplary embodiment, the value is integrated over 5 minutes; however, other integration periods can be chosen. Thus, a signal is provided, whereby a high sensitivity maximizes the signal received by a minimal amount of measured hydrogen peroxide (e.g., minimal glucose requirements without sacrificing accuracy even in low glucose ranges), reducing the sensitivity to oxygen limitations in vivo (e.g., in oxygen-dependent glucose sensors). 
     In some embodiments, the electronics unit is programmed with a specific ID, which is programmed (automatically or by the user) into a receiver to establish a secure wireless communication link between the electronics unit and the receiver. Preferably, the transmission packet is Manchester encoded; however, a variety of known encoding techniques can also be employed. 
     A battery  144  is operably connected to the sensor electronics  132  and provides the power for the sensor. In one embodiment, the battery is a lithium manganese dioxide battery; however, any appropriately sized and powered battery can be used (for example, AAA, nickel-cadmium, zinc-carbon, alkaline, lithium, nickel-metal hydride, lithium-ion, zinc-air, zinc-mercury oxide, silver-zinc, and/or hermetically-sealed). In some embodiments, the battery is rechargeable, and/or a plurality of batteries can be used to power the system. The sensor can be transcutaneously powered via an inductive coupling, for example. In some embodiments, a quartz crystal  96  is operably connected to the processor  138  and maintains system time for the computer system as a whole, for example for the programmable acquisition time within the processor module. 
     Optional temperature probe  140  is shown, wherein the temperature probe is located on the electronics assembly or the glucose sensor itself. The temperature probe can be used to measure ambient temperature in the vicinity of the glucose sensor. This temperature measurement can be used to add temperature compensation to the calculated glucose value. 
     An RF module  148  is operably connected to the processor  138  and transmits the sensor data from the sensor to a receiver within a wireless transmission  150  via antenna  152 . In some embodiments, a second quartz crystal  154  provides the time base for the RF carrier frequency used for data transmissions from the RF transceiver. In some alternative embodiments, however, other mechanisms, such as optical, infrared radiation (IR), ultrasonic, or the like, can be used to transmit and/or receive data. 
     In the RF telemetry module of the preferred embodiments, the hardware and software are designed for low power requirements to increase the longevity of the device (for example, to enable a life of from about 3 to about 24 months, or more) with maximum RF transmittance from the in vivo environment to the ex vivo environment for wholly implantable sensors (for example, a distance of from about one to ten meters or more). Preferably, a high frequency carrier signal of from about 402 MHz to about 433 MHz is employed in order to maintain lower power requirements. In some embodiments, the RF module employs a one-way RF communication link to provide a simplified ultra low power data transmission and receiving scheme. The RF transmission can be OOK or FSK modulated, preferably with a radiated transmission power (EIRP) fixed at a single power level of typically less than about 100 microwatts, preferably less than about 75 microwatts, more preferably less than about 50 microwatts, and most preferably less than about 25 microwatts. 
     Additionally, in wholly implantable devices, the carrier frequency may be adapted for physiological attenuation levels, which is accomplished by tuning the RF module in a simulated in vivo environment to ensure RF functionality after implantation; accordingly, the preferred glucose sensor can sustain sensor function for 3 months, 6 months, 12 months, or 24 months or more. 
     The above description of sensor electronics associated with the electronics unit is applicable to a variety of continuous analyte sensors, such as non-invasive, minimally invasive, and/or invasive (e.g., transcutaneous and wholly implantable) sensors. For example, the sensor electronics and data processing as well as the receiver electronics and data processing described below can be incorporated into the wholly implantable glucose sensor disclosed in U.S. Publication No. US-2005-0245799-A1 and U.S. patent application Ser. No. 10/885,476 filed Jul. 6, 2004 and entitled, “SYSTEMS AND METHODS FOR MANUFACTURE OF AN ANALYTE-MEASURING DEVICE INCLUDING A MEMBRANE SYSTEM.” 
     Methods and devices that are suitable for use in conjunction with aspects of the preferred embodiments are disclosed in U.S. Pat. Nos. 4,994,167; 4,757,022; 6,001,067; 6,741,877; 6,702,857; 6,558,321; 6,931,327; and 6,862,465. 
     Methods and devices that are suitable for use in conjunction with aspects of the preferred embodiments are disclosed in U.S. Publication No. US-2005-0176136-A1; U.S. Publication No. US-2005-0251083-A1; U.S. Publication No. US-2005-0143635-A1; U.S. Publication No. US-2005-0181012-A1; U.S. Publication No. US-2005-0177036-A1; U.S. Publication No. US-2005-0124873-A1; U.S. Publication No. US-2005-0051440-A1; U.S. Publication No. US-2005-0115832-A1; U.S. Publication No. US-2005-0245799-A1; U.S. Publication No. US-2005-0245795-A1; U.S. Publication No. US-2005-0242479-A1; U.S. Publication No. US-2005-0182451-A1; U.S. Publication No. US-2005-0056552-A1; U.S. Publication No. US-2005-0192557-A1; U.S. Publication No. US-2005-0154271-A1; U.S. Publication No. US-2004-0199059-A1; U.S. Publication No. US-2005-0054909-A1; U.S. Publication No. US-2005-0112169-A1; U.S. Publication No. US-2005-0051427-A1; U.S. Publication No. US-2003-0032874; U.S. Publication No. US-2005-0103625-A1; U.S. Publication No. US-2005-0203360-A1; U.S. Publication No. US-2005-0090607-A1; U.S. Publication No. US-2005-0187720-A1; U.S. Publication No. US-2005-0161346-A1; U.S. Publication No. US-2006-0015020-A1; U.S. Publication No. US-2005-0043598-A1; U.S. Publication No. US-2003-0217966-A1; U.S. Publication No. US-2005-0033132-A1; U.S. Publication No. US-2005-0031689-A1; U.S. Publication No. US-2004-0045879-A1; U.S. Publication No. US-2004-0186362-A1; U.S. Publication No. US-2005-0027463-A1; U.S. Publication No. US-2005-0027181-A1; U.S. Publication No. US-2005-0027180-A1; U.S. Publication No. US-2006-0020187-A1; U.S. Publication No. US-2006-0036142-A1; U.S. Publication No. US-2006-0020192-A1; U.S. Publication No. US-2006-0036143-A1; U.S. Publication No. US-2006-0036140-A1; U.S. Publication No. US-2006-0019327-A1; U.S. Publication No. US-2006-0020186-A1; U.S. Publication No. US-2006-0020189-A1; U.S. Publication No. US-2006-0036139-A1; U.S. Publication No. US-2006-0020191-A1; U.S. Publication No. US-2006-0020188-A1; U.S. Publication No. US-2006-0036141-A1; U.S. Publication No. US-2006-0020190-A1; U.S. Publication No. US-2006-0036145-A1; U.S. Publication No. US-2006-0036144-A1; and U.S. Publication No. US-2006-0016700A1. 
     Methods and devices that are suitable for use in conjunction with aspects of the preferred embodiments are disclosed in U.S. application Ser. No. 09/447,227 filed Nov. 22, 1999 and entitled “DEVICE AND METHOD FOR DETERMINING ANALYTE LEVELS”; U.S. application Ser. No. 11/280,672 filed Nov. 16, 2005, and entitled “TECHNIQUES TO IMPROVE POLYURETHANE MEMBRANES FOR IMPLANTABLE GLUCOSE SENSORS”; U.S. application Ser. No. 11/280,102 filed Nov. 16, 2005, and entitled “TECHNIQUES TO IMPROVE POLYURETHANE MEMBRANES FOR IMPLANTABLE GLUCOSE SENSORS”; U.S. application Ser. No. 11/201,445 filed Aug. 10, 2005 and entitled “SYSTEM AND METHODS FOR PROCESSING ANALYTE SENSOR DATA”; U.S. application Ser. No. 11/335,879 filed Jan. 18, 2006 and entitled “CELLULOSIC-BASED INTERFERENCE DOMAIN FOR AN ANALYTE SENSOR”; U.S. application Ser. No. 11/334,876 filed Jan. 18, 2006 and entitled “TRANSCUTANEOUS ANALYTE SENSOR”; U.S. application Ser. No. 11/333,837 filed Jan. 17, 2006 and entitled “LOW OXYGEN IN VIVO ANALYTE SENSOR”. 
     In one embodiment of the present invention, a method is provided for detecting an analyte, such as glucose, using an implantable sensing biointerface such as described herein. The implantable sensing biointerface can be surgically implanted at a tissue site in a host. Tissue can then be allowed to grow in the passageways in the matrix of the sensing biointerface. Finally, the analyte can be detected at a working electrode disposed within the sensing biointerface matrix, with or without the aid of a membrane system disposed on the working electrode. 
     The term “host” as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to a mammal. In one embodiment, the host is a human. 
     The use of the sensing biointerfaces described herein has several advantages. First, because the biointerface can be constructed in the form of a thin membrane, it can be implanted with much less trauma to surrounding tissue. Furthermore, the thinness of the biointerface can promote rapid in-growth of tissue and thus, allow the sensor to reach optimal detecting condition faster. The fact that tissue in-growth can occur from both sides of the biointerface also promotes fast start-up and an improvement in the quality of detection. The small scale of the passageways within the biointerface allows tissue, including vasculature, to grow closer to the electrodes than would otherwise be possible. Thus the time lag between a change in analyte levels in the blood stream and detection of those levels is decreased because the transport distance of the analyte from blood vessels to the electrodes is shorter. 
     The term “substantially” as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to being largely but not necessarily wholly that which is specified. 
     All references cited herein, including but not limited to published and unpublished applications, patents, and literature references, are incorporated herein by reference in their entirety and are hereby made a part of this specification. To the extent publications and patents or patent applications incorporated by reference contradict the disclosure contained in the specification, the specification is intended to supersede and/or take precedence over any such contradictory material. 
     The term “comprising” as used herein is synonymous with “including,” “containing,” or “characterized by,” and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. 
     All numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that may vary depending upon the desired properties sought to be obtained. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of any claims in any application claiming priority to the present application, each numerical parameter should be construed in light of the number of significant digits and ordinary rounding approaches. 
     The above description discloses several methods and materials of the present invention. This invention is susceptible to modifications in the methods and materials, as well as alterations in the fabrication methods and equipment. Such modifications will become apparent to those skilled in the art from a consideration of this disclosure or practice of the invention disclosed herein. Consequently, it is not intended that this invention be limited to the specific embodiments disclosed herein, but that it cover all modifications and alternatives coming within the true scope and spirit of the invention.