Abstract:
An implantable biosensor assembly and system includes an enzymatic sensor probe from which subcutaneous and interstitial glucose levels may be inferred. The assembly may be associated by direct percutaneous connection with electronics, such as for signal amplification, sensor polarization, and data download, manipulation, display, and storage. The biosensor comprises a miniature probe characterized by lateral flexibility and tensile strength and has a large electrode surface area for increased sensitivity. Irritation of tissues surrounding the probe is minimized due to ease of flexibility and small cross section of the sensor. Foreign body reaction is diminished due to a microscopically rough porous probe surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation of U.S. application Ser. No. 10/401,224, filed Mar. 26, 2003, the contents of the entirety of which are herein incorporated by this reference. 
     
    
     TECHNICAL FIELD  
       [0002]     This invention relates generally to medical devices and associated methods such as measuring glucose for ongoing diabetes management. The invention described herein can also be used for enzymatic determination of other analytes. This invention provides a particularly useful implantable biosensor.  
       BACKGROUND OF THE INVENTION  
       [0003]     Heretofore, treatment and management of diabetes has been undertaken through many and varied techniques. Formerly, glucose in urine was measured, though recognized as less than adequate due to the time delay inherent in the metabolism and voiding process. Currently, the approach predominantly used for self-monitoring of blood glucose requires periodic pricks of the skin with a needle, whereby a blood sample is obtained and tested directly to provide information about blood glucose levels. This information is then utilized as a basis from which to schedule the administration of insulin to maintain glucose equilibrium within the patient. Direct measurement of glucose levels in periodic blood samples from diabetes patients provides reasonably useful information about insulin levels at certain selected points in time. However, the dynamic nature of blood glucose chemistry and the complexity of factors influencing blood sugar levels render such periodic information less than optimal.  
         [0004]     The glucose level in the subcutaneous interstitial fluid very closely approximates the glucose level in the blood, with a negligible time lag. The variables of patient food selection, physical activity and insulin dosage, regime and protocol for a person with diabetes each have a dynamic impact on physiologic balance within the patient&#39;s body that can change dramatically over a short period of time. If the net result of changes in these variables and dynamics results in disequilibrium expressed as too much glucose (“hyperglycemia”), then more insulin is required, whereas too little glucose (“hypoglycemia”) requires immediate intervention to raise the glucose levels. A deleterious impact on physiology follows either such disequilibrium.  
         [0005]     Hyperglycemia is the source of most of the long-term consequences of diabetes, such as blindness, nerve degeneration, and kidney failure. Hypoglycemia, on the other hand, poses the more serious short-term danger. Hypoglycemia can occur at any time of the day or night and can cause the patient to lose consciousness. Guarding against hypoglycemia may require frequent monitoring of blood glucose levels and render the skin-prick approach tedious, painful and, in some cases, impractical. Even diligent patients who perform finger-sticking procedures many times each day achieve only a poor approximation of continuous monitoring. Accordingly, extensive attention has been given to development of improved means of monitoring patient glucose levels for treatment of diabetes.  
         [0006]     Many efforts to continuously monitor glucose levels have involved implantable electrochemical biosensors. These amperometric sensors utilize an immobilized form of the enzyme glucose oxidase to catalyze the conversion of oxygen and glucose to gluconic acid and hydrogen peroxide. Such sensors may be used to measure hydrogen peroxide resulting from the enzymatic reaction. Alternatively, these glucose oxidase-based biosensors measure oxygen consumption to infer glucose concentrations.  
         [0007]     Typical implantable, subcutaneous needle-type biosensors are disclosed in various publications, such as the following examples: “An Amperometric Needle-type Glucose Sensor Tested in Rats and Man,” by D. R. Matthews, E. Bown, T. W. Beck, E. Plotkin, L. Lock, E. Gosden, and M. Wickham, which discloses an amperometric glucose-measuring 25-gauge (0.5 mm diameter) needle-type sensor using a glucose oxidase and dimethyl ferrocene paste behind a semipermeable membrane situated over a window in the needle, “Performance of Subcutaneously Implanted Needle-Type Glucose Sensors Employing a Novel Trilayer Coating,” by Francis Moussy, D. Jed Harrison, Darryl W. O&#39;Brien, and Ray V. Rajotte, which teaches a miniature, needle-type glucose sensor utilizing a perfluorinated ionomer, Nafion, as a protective, biocompatible, outer coating, and poly(o-phenylenediamine) as an inner coating to reduce interference by small, electroactive compounds. Glucose oxidase immobilized in a bovine serum albumin matrix was sandwiched between these coatings. The entire assembly of a platinum working electrode and an Ag/AgCl reference electrode was 0.5 mm in diameter and could be inserted subcutaneously through an 18-gauge needle. Other examples include, “Needle Enzyme Electrodes for Biological Studies,” by S. J. Churchouse, C. M. Battersby, W. H. Mullen and P. M. Vadgama, which presents yet another needle enzyme electrode characterized as the most promising approach to miniaturization for invasive use, “A Miniaturized Nafion-based Glucose Sensor,” by F. Moussy, D. J. Harrison, and R. V. Rajotte, which, while teaching a high sensitivity (due in part to greater surface area of the electrode) needle-type sensor with a spear-shaped point, acknowledges the need for more protection against abrasion, “Design and In Vitro Studies of a Needle-Type Glucose Sensor for Subcutaneous Monitoring,” by Dilbir S. Bindra, Yanan Zhang, George S. Wilson, Robert Sternberg, Daniel R. Thevenot, Dinah Moatti and Gerard Reach, which sets forth yet another needle-type glucose microsensor having a 26-gauge (0.45-mm) outside diameter.  
         [0008]     Additional needle-type implantable biosensors are disclosed in certain United States patent documents. Relevant documents include: “Subcutaneous Glucose Electrode” to Heller et al., U.S. Pat. No. 6,329,161 B1; “Subcutaneous Implantable Sensor Set Having the Capability to Remove Deliver Fluids to an Insertion Site” to Mastrototaro et al., U.S. Pat. No. 5,951,521; “Transcutaneous Sensor Insertion Set” to Halili et al., U.S. Pat. No. 5,586,553; “Transcutaneous Sensor Insertion Set” to Cheney, II et al., U.S. Pat. No. 5,568,806; “Transcutaneous Sensor Insertion Set” to Lord et. al., U.S. Pat. No. 5,390,671; and “Implantable Glucose Sensor” to Wilson et al., U.S. Pat. No. 5,165,407.  
         [0009]     To provide continuous measurement, biosensors can be placed for extended periods of time in various locations within the body. One method of placement is percutaneously with an indwelling sensor having an attached external wire associated with a readout device. A risk of infection is associated with percutaneous biosensors, and they must typically be replaced at regular intervals because of the risk of infection at the insertion site.  
         [0010]     Another problem with implanted sensors is irritation of the tissues surrounding the implanted biosensors. Such irritation is typically due, in part, to the lateral rigidity of prior art biosensors. Related to this problem is the scarring of surrounding tissue due not only to rigidity but also to abrupt edges associated with the implants. Scar tissue surrounding reference electrodes of the prior art is not desirable, but may be tolerated in some cases. However, scar tissue can be materially detrimental to the sensor function in the vicinity of the working electrode because it impedes the diffusion of oxygen and glucose.  
         [0011]     Further, to protect itself against a perceived invader, the body commonly experiences a foreign body reaction by encapsulating the implanted biosensors with protein, which may shorten the life of the implant and adversely affect the accuracy of information provided. The size of the sensor may also be regarded as a problem; smaller is better for comfort. Further yet, interfering compounds, such as, for example, ascorbic acid, and acetaminophen, can reduce the accuracy of prior art amperometric glucose sensors given the membranes selected historically to envelop such sensors. Additionally, the quantity of dissolved oxygen is limited at high glucose concentrations, thus leading to nonlinear output of sensor signals at high glucose concentrations.  
         [0012]     A need remains for a sensor including a miniaturized probe of suitable materials and characteristics that may facilely be placed percutaneously. A need exists for a miniaturized, albeit durable, implantable biosensor percutaneously deployable wherein irritation to tissues surrounding the biosensor is minimized. A need also exists to achieve a rough exterior of the portion of an implantable biosensor exposed to surrounding tissue so that foreign body reaction may be reduced. Similarly, there is a need for a selected membrane or membrane combination suitable to correction of nonlinear diffusion of glucose. Further needed is a method of manufacturing such a miniaturized yet strong and durable implantable biosensor with resilient flexibility and minimal surface relief while achieving a microscopically porous surface.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     The invention includes an implantable needle-type biosensor wherein an electric signal is produced between first and second electrical contacts responsive to an electrochemical reaction in a body. A needle-like probe element is typically inserted through an introducer cannula into tissues of a subject&#39;s body. An implantable needle element of an exemplary biosensor includes an elongate core having a distal end spaced apart axially from a proximal end. A workable core may be formed as a single element, or may include a plurality of axially oriented fibers arranged in a bundle. Certain cores are nonconductive to electric current. Workable cores may be made from natural and synthetic fibers, metal, polymers, and plastics. Currently preferred cores are made from polymer material.  
         [0014]     In general, a working electrode is associated with a distal end of the core. Desirably, the working electrode is arranged to protrude beyond a distal end of an introducer cannula into intimate contact with tissue of a subject&#39;s body. A reference electrode is included in abiosensor to produce an electrical signal, in combination with the working electrode, responsive to the electrochemical reaction. Structure included in a biosensor is adapted to resist direct physical contact between the working electrode and the reference electrode to prevent forming a direct electrical short between those electrodes.  
         [0015]     A first electrically conductive path exists between the working electrode and a first electrical contact. Similarly, a second electrically conductive path exists between the reference electrode and a second electrical contact. The first and second electrical contacts typically are associated with a hub operable to secure a probe in relation to a cannula. A signal may be received from the first and second contacts for data reduction and correlation to a physiological state in a body, such as glucose concentration. In general, the signal is transmitted through a sensor cable affixed to structure of the hub. A workable sensor cable includes first and second wires, each wire having a first end arranged to make respective electrical connections with one of the first and second electrical contacts, and a second end of each wire typically being affixed to a sensor module operable to impose a conditioning signal on the biosensor probe.  
         [0016]     A working electrode can include a metal element (usually including platinum) formed as a wrap about a portion of the core. An exemplary working electrode includes a length of a first wire arranged to circumscribe a plurality of revolutions about the core. In such an exemplary working electrode, a diameter of the first wire is between about 0.001 and about 0.005 inch. Desirably, the first wire is arranged to form a spiral path. Usually, at least a portion of the core is disposed substantially coaxial with an axis of the spiral path. A currently preferred spiral path has an axial spacing, between the centerlines of a pair of adjacent wire wraps, sized between about one and about two diameters of the first wire. A larger spacing, up to about five diameters (or even more in some cases), is also workable, although it is recognized that the electrode&#39;s active surface area decreases with larger pitch spacing. Typically, the working electrode is arranged to reinforce the core so as to enable a reinforced core to carry an axial compression load permitting insertion of a distal tip of the biosensor through an introducer catheter for placement of the working electrode into intimate contact with tissue of the subject&#39;s body.  
         [0017]     The reference electrode typically includes a metal element (usually including silver, and preferably including chlorided silver) and can also be associated with the distal end of a core. A reference electrode may alternatively be associated with an introducer cannula, or some other structure. In the latter case, a reference electrode may sometimes be recessed into an exterior surface of the introducer cannula. In any event, it is currently preferred for a reference electrode to be placed into intimate contact with tissue of a subject&#39;s body. One embodiment of a reference electrode includes a length of a second wire formed as a wrap about a portion of the core. Another embodiment of a reference electrode may be fashioned as a length of wire, wire coil, foil, film, or coating associated with a cannula.  
         [0018]     A preferred electrode (either working or reference) maybe characterized as having: an axially interrupted load path between first and second ends, a maximum equivalent outside diameter, a minimum equivalent inside diameter, and a surface texture disposed between the first and second ends that has a radially oriented component. Such an electrode has a larger reactive surface area and a lower bending stiffness compared to a hollow cylinder structured from an equivalent material and having equivalent maximum outside and minimum inside diameters.  
         [0019]     The core of a biosensor probe according to the instant invention can function to assist in retraction of the various components of the biosensor probe. One structure operable to assist in such retraction includes a plug carried on a distal end of the core. The plug can be structured as a stopper that is too large to pass through an electrode. Such a stopper operates to resist extraction of the core from within a portion of the working electrode as the biosensor is removed from the subject&#39;s body, so as not to leave a detached portion of the working electrode in the body. One functional plug is preferably formed, at least in part, with a polymer coating. Another functional plug can include a droplet of dielectric adhesive. A functional plug typically forms an enlargement in a cross-section of the core, with a portion of the enlargement being disposed distal to the working electrode.  
         [0020]     In probes carrying both working and reference electrodes, a dielectric spacer is usually interposed between the electrodes to resist direct physical contact between them. A functional dielectric spacer can be made from a droplet of dielectric adhesive bonded to a portion of the core. Such a droplet desirably also is arranged as a stopper to resist extraction of the core from within a portion of the reference electrode as a biosensor is removed from a subject&#39;s body, so as not to leave a detached portion of the reference electrode in the body.  
         [0021]     A probe portion of a biosensor includes a sensor shaft disposed between the working electrode and the hub. The sensor shaft generally includes a cylinder disposed circumferentially about an axial length of the core proximal to the working electrode. A currently preferred cylinder includes a plurality of circumferential wrappings of a component wire having a smaller diameter than a diameter of the formed cylinder. Wrappings forming the cylinder desirably are closely spaced, or even touching, in an axial direction along an axis of the cylinder whereby to enable the shaft to carry an axial compression load effective to install the biosensor probe portion through an introducer cannula and into a body. Usually, a dielectric spacer is disposed at a distal end of the cylinder to resist direct physical contact between the shaft and an electrode. One such dielectric spacer can be formed from a droplet, or small quantity, of dielectric adhesive bonded to a portion of the core.  
         [0022]     Desirably, an exterior coating of a negatively charged polymer is applied to the working electrode. One operable negatively charged polymer includes sulfonated polyethersulfone. It is also sometimes desirable to provide a microscopically roughed-up outer surface on the coating to enhance biocompatibility of the biosensor with tissue of the subject&#39;s body. Desirable surface texture is formed by elements having a size of between about 5 and 50 microns. Multifiber cores typically include a plurality of spaces between the fibers operable to carry glucose oxidase whereby to enhance a volume of glucose oxidase associated with a working electrode.  
         [0023]     The instant invention may be embodied broadly as an implantable biosensor including an introducer cannula and a probe element. The introducer cannula includes a lumen extending axially between its proximal and distal ends. The cannula&#39;s proximal end carries affixing structure adapted to resist motion of the proximal end relative to a skin surface of a subject and further carries holding structure configured to receive a probe. A distal end of the cannula carries a first electrode. A probe includes an elongate core having a distal end spaced apart axially from a proximal end and is structured for sliding installation, through the cannula lumen, into a subject. A proximal end of the probe is associated with a hub adapted to be held by the cannula-holding structure. The distal end of the probe carries a second electrode. The probe and cannula are cooperatively structured on assembly to resist direct physical contact between the first electrode and the second electrode. Desirably, the first and second electrodes are installed to be in intimate contact with the tissue of a subject.  
         [0024]     A method for manufacturing an implantable, needle-type biosensor probe with a transversely flexible first electrode effective to resist irritation at a site of implantation in a subject includes the steps of: a) providing a core comprising a first nonconductive material; b) disposing a first electrode in a reinforcing path about the core; c) disposing a first electrical conductor between the first electrode and a hub associated with a proximal end of the probe; and d) disposing a second electrical conductor between a second electrode and the hub. The method can also include the step of: e) forming a stopper carried by the core, a portion of the stopper being disposed distal to the first electrode and operable to resist extraction of the core from within the interior of the electrode, whereby to retain an association between the core and the electrode to resist leaving a portion of the electrode in a subject subsequent to removal of the probe.  
         [0025]     Sometimes, step b) includes: forming the first electrode as an axially interrupted first cylinder having a first length between a first end and a second end, a maximum equivalent outside diameter, and a minimum equivalent inside diameter. The first cylinder desirably includes a surface texture disposed between its first and second ends that has a radially oriented component so as to provide a larger reactive surface area and a lower bending stiffness than a second cylinder having an equivalent maximum outside diameter and first length. An exemplary electrode having such conformation can be formed from a wire of between about 0.001 and about 0.005 inch in diameter, with the wire being disposed to occupy a spiral path about the core.  
         [0026]     In some cases, the method may further include disposing a second wire circumferentially about the core in a spiral reinforcing path operable to enhance an axial load-carrying capability of the core, whereby to form the second electrode. Typically, the step of applying an insulation to a conductive path extending proximally from one or both electrodes is further included. When two electrodes are carried on a core, the method additionally can include affixing a dielectric element between the working and the reference electrodes. Such dielectric element desirably is also adapted to resist extraction of the core from retention in an electrode, whereby to resist leaving a portion of that electrode inside a subject subsequent to extraction of the probe. Generally, the method includes the step of wrapping, or otherwise disposing, a third wire circumferentially about the core in a spiral reinforcing path to form a shaft of the probe. Furthermore, the method includes affixing the hub to a proximal portion of the shaft.  
         [0027]     Coatings are typically applied in additional steps subsequent to assembly of basic probe structure. An inner exclusion membrane is formed in a first coating step by applying a solution, such as 5% polyethersulfone, to the working electrode. A second coating step includes applying a solution, such as 1% glucose oxidase, 0.6% albumin and 0.5% glutaraldehyde, to the working electrode to form a middle enzymatic membrane. In a third coating step, a solution, such as 5% polyurethane is applied to both the working and the reference electrodes to form an outer polymer membrane. The final polyurethane coating desirably is microscopically roughed-up by performing a phase inversion polymerization procedure. In general, a workable phase inversion polymerization procedure includes immediately dipping the final polyurethane layer into a water bath to largely rinse away the miscible solvent soon after the first of the polymer molecules comprising the 5% solution have begun to bond with the second-to-last layer. Desirably, the resulting surface includes protruding particles sized between about 5 and 50 microns. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0028]     In the drawings, which illustrate what are currently regarded as the best modes for carrying out the invention:  
         [0029]      FIG. 1  is a schematic of one configuration of a preferred embodiment;  
         [0030]      FIG. 2A  is a perspective side view in elevation of a first implantable biosensor according to the present invention;  
         [0031]      FIG. 2B  is an enlarged perspective view in elevation of a probe portion of the implantable biosensor illustrated in  FIG. 2A ;  
         [0032]      FIG. 2C  is a perspective side view in elevation of the implantable biosensor of  FIG. 2A , in a partially assembled configuration;  
         [0033]      FIG. 2D  is a perspective side view in elevation of the implantable biosensor of  FIG. 2A , in an assembled configuration;  
         [0034]      FIG. 3  is a side view illustrating a stage of construction of a probe portion of the implantable biosensor of the invention;  
         [0035]      FIG. 4  is a side view of an assembled but uncoated miniature probe portion of the biosensor of the invention;  
         [0036]      FIG. 5  is an enlarged cross-sectional side view of a miniature, flexible probe portion of the implantable biosensor;  
         [0037]      FIG. 6A  is a perspective side view in elevation of a second implantable biosensor according to the present invention;  
         [0038]      FIG. 6B  is an enlarged perspective view in elevation with greater resolution of a portion of the embodiment illustrated in  FIG. 6A ;  
         [0039]      FIG. 6C  is a perspective view in elevation of the embodiment of  FIG. 6A  in a partially assembled configuration; and  
         [0040]      FIG. 6D  is a perspective view in elevation of the embodiment of  FIG. 6A  in an assembled configuration. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0041]      FIG. 1  illustrates a preferred embodiment in which an implantable biosensor, generally  10 , and associated sensor cable  20  are provided. Miniaturized and highly flexible, the biosensor  10  may be placed into a subject subcutaneously through a cannula such as an introducer catheter  30 . The biosensor  10  as illustrated is associated percutaneously through the sensor cable  20  with a sensor module  40 , which in turn is associated via a module cable  50  with a Sensor Display Unit (“SDU”)  60 . The SDU  60  can be structured to be interactive across SDU cable  70  with computer hardware and other software, generally  80 .  
         [0042]     The foregoing biosensor system may include a single-use portion and a reusable portion. The single-use portion includes the introducer catheter  30 , the biosensor  10 , the sensor cable  20 , the sensor module  40  and the module cable  50 . The introducer catheter  30  can generally be regarded as a separate component, although certain embodiments may incorporate the catheter to carry a portion of a biosensor probe. The biosensor  10 , sensor cable  20 , sensor module  40  and module cable  50  desirably are all permanently affixed to each other. Module cable  50  typically is removably attached at disconnect  55  to SDU  60 . The SDU  60  and SDU cable  70  may be reused. When attached to the SDU  60 , the SDU cable  70  allows the glucose information to be downloaded to a personal computer  80  that is loaded with the sensor download software.  
         [0043]     To install a preferred embodiment of a biosensor  10 , introducer catheter  30  can be inserted into the subcutaneous tissue of a subject on a supporting needle (not illustrated). The supporting needle is removed to leave an opening through the cannula and, typically, a short path extension into the subject&#39;s tissue. Then, the biosensor  10  maybe placed into the introducer catheter  30  such that a portion of the biosensor  10  protrudes beyond the introducer catheter  30 . The working electrode  100  and reference electrode  110  of the presently preferred embodiment are designed to be deployed 3-10 mm into the subcutaneous fatty tissue of a subject to monitor glucose concentration in the interstitial fluids. The introducer catheter  30 /biosensor  10  assembly, as well as the sensor module  40 , is then generally affixed to the skin (not shown) via an adhesive patch.  
         [0044]     The biosensor  10  produces a small electrical current that is proportional to the glucose concentration. This current is amplified and conditioned by the sensor module  40 . The sensor module  40  also provides a polarization voltage to the working electrode of the biosensor  10 . The amplified signal typically is interpreted by the SDU  60 , which generally calibrates, displays and stores the glucose data.  
         [0045]     The biosensor  10 , as set forth in  FIGS. 2A-2D , includes a sensor shaft  90  with sensor cable  20  extending therefrom, a working electrode  100 , a reference electrode  110  and a hub  120  for attaching the biosensor  10  to the introducer catheter  30 . With reference to  FIG. 2B , the working electrode  100  and reference electrode  110  are adjacent a first dielectric spacer  130 . The reference electrode  110  and sensor shaft  90  are adjacent a second dielectric spacer  140 . A filament core  150  is visible in  FIG. 2B  through a polymer cap  160 . The dielectric spacers  130 ,  140  provide one arrangement of structure operable to prevent the two electrodes from shorting together through a direct physical contact between the electrodes.  
         [0046]     The filament core  150  may include any of a variety of suitable materials, such as polymeric, ceramic, or flexible metallic materials, that can sometimes be insulated. One currently preferred filament core  150 , as illustrated in  FIG. 3  at an intermediate stage of construction of a biosensor  10 , includes a plurality of filamentous fibers  170  of a polymeric material bundled in substantially axial alignment with respect to each other. Fibers  170  forming an exemplary filament core  150  may be formed from natural or synthetic fibers, and may have round, rectangular, uniform, or even irregular cross-sections. A desirable core material will have sufficient tensile strength to aid in extraction of biosensor elements entrained thereon.  
         [0047]     A desirable flexible filament core  150  forms a biosensor  10  having enhanced transverse flexibility operable to reduce irritation at the installation location in a subject compared to rigid needle-type biosensors. A filament core  150  desirably is structured and arranged in a multistrand configuration to increase transverse flexibility of biosensor  10 . A multistrand core provides a plurality of strands, each strand having a significantly reduced cross-section and bending stiffness compared to a solid cross-section replaced by that core. A plurality of such strands  170  in combination can form a transversely flexible biosensor  10 . For the purpose of this disclosure, a solid copper needle having a diameter of about 25 gauge is regarded as being transversely rigid.  
         [0048]     With reference to  FIGS. 3 and 4 , working wire lead  180  provides structure that forms a conductive path that extends from the working electrode  100  for electric communication through the sensor cable  20  (see,  FIG. 1 ). The conductive path can be disposed among the fibers  170  and extends axially along the sensor shaft  90 . The working electrode  100  of biosensor  10  is typically formed of platinum, or a platinum compound, and desirably circumscribes the filament core  150  in the form of continuous working coils, generally  190 . One operable conductive path is formed by a proximally directed axial extension of a wire formed at its distal end into working electrode  100 .  
         [0049]     The reference electrode  110  in  FIG. 4  preferably is formed from a chlorided silver substrate. A reference electrode  110  typically extends axially along a portion of the filament core  150  and desirably circumscribes the filament core  150  in the form of continuous reference coils, generally  200 . A reference wire lead  210  forms a conductive path that extends from the coils  200  of reference electrode  110  for electric communication through the sensor cable  20 . An exemplary conductive path can be formed from a proximally extending portion of reference wire lead  210  forming the reference electrode  110 . The conductive path can be insulated and/or disposed among the fibers  170 . Both the working wire lead  180  and the reference wire lead  210  are typically available for termination to a distal end of the sensor cable  20  at a hub  120 . An extension to leads  180  and  210  may effectively continue from electrical contacts, generally located in association with the hub, to extend along sensor cable  20  and provide electrical contacts at a proximal end of sensor cable  20 .  
         [0050]     The sensor shaft  90 , in certain embodiments, is formed as a cylinder about the filament core  150 . One workable cylinder may, at least in part, be formed of small-diameter stainless steel wire. A sensor shaft  90  may be arranged, as illustrated, to circumferentially circumscribe the filament core  150  in the form of continuous body coils, generally  220 . Generally, wire used to form coils  190 ,  200 , and  220  has a diameter between about 0.001 and 0.005 inch, with about 0.002 inch being currently preferred. The configuration of coils  190 ,  200 , and  220  desirably lends additional axial compressive load-carrying capability to the fibers  170  of the biosensor  10  while maintaining the lateral flexibility of the highly flexible, sometimes even flaccid, fibers  170 , thereby reducing a tendency toward scarring in surrounding tissue when implanted.  
         [0051]     While the illustrations generally depict electrodes and sensor shafts that are substantially cylindrical, such is not a strict requirement. For instance, a workable core can be formed having a triangular, square, rectangular, or even other alternatively shaped cross-section. An electrode or shaft reinforcement can be wound around such core to form a tube with a cross-section substantially conforming to that of the core. In another example, a reinforcing electrode can be applied to a core having such a noncircular cross-section by way of a coating, printing, vapor deposition, or other procedure to form a tubular electrode that may be characterized as providing some “effective” inner and outer diameters. Furthermore, in some cases, a shaft reinforcement can be formed from a shrink-fit tubing that substantially conforms to an underlying core profile.  
         [0052]     The coils  190 ,  200 , and  220  may be relatively less closely wound (with respect to an axial spacing, or pitch, between centerlines of adjacent coils) about the fibers  170  in certain configurations other than embodiments illustrated in this disclosure. However, an increase in the relative closeness of the coils  190  and  200  results in an increase in reactive surface area for the respective electrodes  100 ,  110 , thus enhancing sensitivity and accuracy of readings obtained from a biosensor  10 . Adjacent coils  220  can be placed abutting one another (with an axial spacing, or pitch, between centerlines of adjacent coils of one coil-wire diameter) to maximize axial load-carrying capabilities of a sensor shaft  90 , while still retaining a significant increase in transverse flexibility, compared to a rigid solid shaft.  
         [0053]     Construction of a biosensor  10 , including coils  190 ,  200 ,  220  as illustrated, generally enhances the sensor&#39;s flexibility and resistance to damage. Transverse flexibility is greatly increased over a comparable solid cross-section because the load path is changed. Both a solid shaft and a cylinder have a cross-section that carries a bending-induced load along an uninterrupted, axially directed load path as axial tension and axial compression stress. Coils provide an axially interrupted load path along a length of the electrode (or sensor shaft  90 ). Coil structures cannot carry bending loads in the same way an uninterrupted surface can. Under transverse bending of an illustrated biosensor  10 , the coils displace in a shear mode and carry loads as torsion and bending loading in the coil elements, but the bending load path and effective displacements are entirely different than those in a solid shaft. For example, the bending of a coil element is essentially orthogonal to the bending in the equivalent uninterrupted surface. The stress induced in the coil element is, therefore, significantly lower (potentially by orders of magnitude) than the stress induced in the comparable solid cross-section. A coil arrangement therefore resists breaking-off of electrode portions inside a subject and reduces irritation at the implantation interface.  
         [0054]     An axially interrupted electrode can be formed other than as a coil structure. For example, a cylinder can be made to provide circumferential relief, or radially directed cuts, in an overlapping finger pattern. Such relief can be laser etched from a continuous cylinder. Alternatively, such pattern can be printed or etched. The relief also provides a radial component to the electrode surface, thereby potentially increasing the available reactive surface area of the electrode.  
         [0055]     Filament core  150  and its associated cap  160  work in harmony to further resist leaving any broken-off portions of electrode, such as working electrode  100 , behind in a subject when a biosensor  10  is removed from the subject&#39;s tissue. Cap  160  desirably is operable as a stopper forming an interference to resist extraction of filament core  150  from within an electrode. That is, the stopper functions to hold an electrode (such as working electrode  100 ) at a distal tip end  310  ( FIG. 5 ), placing the working electrode  100  into compression during withdrawal of a biosensor  10 . A cap  160  desirably provides structure sized larger than an inside diameter of an electrode. Therefore, the cap  160  forms an interference with the electrode to resist separation of the electrode from the filament core  150 . Certain embodiments of cap  160  may adhere an electrode, or a portion of an electrode, directly to a filament core  150 . It is within contemplation for a cap  160  to be formed by melting a distal portion of a filament core  150 . The filament core  150  desirably provides a strand of material having sufficient tensile strength to overcome resistance due to adhesion between body tissue and portions of a biosensor  10 . Therefore, filament core  150  and cap  160  are relied upon for extraction of the biosensor  10 .  
         [0056]     The electrodes  100  and  110  of the biosensor  10 , in a preferred embodiment, are illustrated in an enlarged view in  FIG. 5  to illustrate three layers of membranes. An inner exclusion membrane  230  is depicted as surrounding and being adjacent to the working electrode  100 . The inner exclusion membrane  230 , preferably formed of polysulfone or sulfonated polyethersulfone, serves to reduce the sensor artifact that is caused by non-endogenous electroactive molecules, thus excluding interfering compounds such as ascorbic acid and acetaminophen. A middle enzymatic membrane  240  surrounds the inner exclusion membrane  230 . The middle enzymatic membrane  240  includes immobilized glucose oxidase enzyme that converts glucose to hydrogen peroxide to generate a current. An outer polymer membrane  250  surrounds the middle enzymatic membrane  240 , as well as the reference electrode  110 , to restrict diffusion of glucose while allowing the free passage of oxygen. This outer polymer membrane  250  may be formed of various polymers. One preferred embodiment of an outer polymer membrane  250  is formed of polyurethane. A careful approach to material selection for the membrane layers  230 ,  240 , and  250  facilitates correction of the nonlinear diffusion of glucose and reduces errors resulting from interfering electroactive species.  
         [0057]     It can be appreciated that the introducer catheter  30 , typically used in conjunction with a preferred embodiment biosensor  10 , provides access from outside the body (not shown) to the tissue just under the skin layer (not shown). With reference to  FIG. 2A , the biosensor  10  is inserted into and through a lumen  260  of the introducer catheter  30  to a point at which the polymer cap  160 , working electrode  100  and reference electrode  110  of the biosensor  10  protrude beyond and outside the introducer catheter lumen  260 . Such placement allows the working electrode  100  and reference electrode  110  to be in communication with the surrounding tissue (not illustrated).  
         [0058]     With reference to  FIGS. 2B and 5 , polymer cap  160 , located at a head portion  270  of the biosensor  10 , provides a conformal material that coats the fibers  170  extending beyond the working electrode  100  and adheres the fibers  170  into the unified filament cap  160 . The working electrode  100 , as illustrated, extends along a leading portion, generally  280 , of the biosensor  10 . As further illustrated, the reference electrode  110  extends along the trailing portion  290 . The leading portion  280  and trailing portion  290 , as best illustrated in  FIG. 2D , extend beyond the lumen  260  of the introducer catheter  30  when introduced into a subject. The sensor shaft  90  may include a tail portion  300  along which the body coils  220  may be located (see,  FIG. 4 ).  
         [0059]     As illustrated in  FIG. 5 , working electrode  100  includes a distal tip end, generally  310 , and a proximal tip end, generally  320 . The distal tip end  310 , as illustrated, is associated with the filament core  150  at or near the head portion  270 . The proximal tip end  320  is associated with the filament core  150  at or near the trailing portion  290  or tail portion  300  depending upon the configuration. In one configuration, a reference electrode  110  is separate from a needle-probe portion of a biosensor. In another configuration and as illustrated in  FIG. 5 , a reference electrode  110  is included on the biosensor  10  probe.  
         [0060]     The preferred embodiment  10  illustrates the working electrode  100  as being structured in the form of coils. However, it is only necessary that the working electrode  100  be in length substantially not less than the leading portion  280  when the leading portion  280  is laterally deflected to a maximum extent. Such a limitation is operable to resist separation of an electrically conductive path from the electrode due to bending of the biosensor. Correspondingly, whereas in a preferred embodiment the reference electrode  110  is illustrated as being in the form of coils, in essence a working electrode  110  may be in length substantially not less than the trailing portion  290  when the trailing portion  290  is laterally deflected to a maximum extent.  
         [0061]      FIGS. 6A-6D  illustrate an alternative preferred embodiment of a biosensor, generally indicated at  330 , including an introducer catheter, generally indicated at  340 . The biosensor  330  includes a working electrode, generally  350 , typically corresponding in function, materials, location and other general characteristics with the working electrode  100 . The biosensor  330  further includes a polymer cap  360 , filament core  370 , working coils  380 , dielectric spacer  390 , head portion, generally  400 , leading portion  410 , tail portion  420 , hub  430 , working electrode lead  440 , and body coils  445 . Biosensor  330  generally includes membranes and is structured to provide characteristics and features that in turn generally correspond to those of the biosensor embodiment  10 .  
         [0062]     The introducer catheter  340 , like the introducer catheter  30 , includes a lumen that may be thought of as an interior cannula lumen  450 . Furthermore, the illustrated introducer catheter  340  presents an advanced end  460  designed for subcutaneous or other intra-tissue placement, an opposite end, generally  470 , and a cannula wall  480  defining the interior cannula lumen  450  and comprising an exterior surface  490 . The exterior surface  490  and the interior cannula lumen  450  extend between the advanced end  460  and the opposite end  470 . In the biosensor embodiment  330 , a reference electrode  500  is associated with the exterior surface  490  of catheter  340  in the vicinity of the advanced end  460 . Electrode  500  may take other forms, such as a film, band, etching, printed or imprinted layer, or a shell or coating. The interior cannula lumen  450  is of sufficient cross-sectional diameter to pass the biosensor  330 . The advanced end  460  and exterior surface  490  of the introducer catheter  340  are structured and arranged to enable access of the advanced end  460  into and through subcutaneous or other subject tissue.  
         [0063]     The opposite end  470  generally includes one or more surfaces  510  useful for adhesively fixing the introducer catheter  340  to the skin. The hub  430  may be anchored to the opposite end  470  of the introducer catheter  340 . The opposite end  470  may be further structured and arranged to engage the hub  430 , upon advancement of the biosensor  330  through the interior cannula lumen  450  sufficiently far, so that the alternative working electrode  350  reaches a position extending beyond the advanced end  460 . Upon achievement of such a position, a reference wire lead  520  associated with the hub  430  may be brought into register with the reference electrode  500  carried by the catheter  340 .  
         [0064]     Glucose (“Glu”), in a somewhat restricted manner, and Oxygen (“O 2 ”), comparatively freely, diffuse from the interstitial tissues of the subject through the outer polymer membrane  250  (see  FIG. 5 ) and, in the presence of the glucose oxidase (“GO x ”) of the middle enzymatic membrane  240 , produce gluconic acid “GluA”) and hydrogen peroxide (“H 2 O 2 ”). The H 2 O 2 , upon interaction with the platinum (“Pt”) working electrode  100 , which is typically polarized at approximately 0.7 volts, creates a current which travels up the working wire lead  180  for processing through the sensor module  40 . A differential signal is generally measured between the working electrode  100  and the reference electrode  110  at the sensor module  40 , and successively transmitted to the SDU  60  and ultimately the computer  80 .  
         [0065]     In the manufacture of a biosensor  10 , a plurality of filamentous fibers  170  of the filament core  150  are axially aligned in a bundle and bonded to form the polymer cap  160 . The wire material of a working electrode  100  can be manually or mechanically wrapped around the filament core  150  beginning at the head portion  270  and continuing proximally across the leading portion  280  to form the working coils  190  (see  FIGS. 4 and 5 ). An exemplary working electrode  100  is somewhat cylindrical, about 0.60 inch in axial length and about 0.015 inch in maximum outside diameter. It is currently preferred to form an electrode, such as a working electrode  100 , from a wire wound on a spiral path.  
         [0066]     If the biosensor includes a reference electrode  110  adjacent to, but apart from, the working electrode  100 , the reference electrode  110  can likewise be manually or mechanically wrapped around the filament core  150  and working wire lead  180 . Reference electrode  110  is structured to occupy a desired axial distance and desirably forms reference coils  200  electrically communicating with the reference wire lead  210 . Reference wire lead  210  and the working wire lead  180  extend proximally among the fibers  170 . Body coils  220  are then similarly wrapped around the filament core  150  and leads  180  and  210 , terminating at a proximal end, generally indicated at  305  ( FIG. 4 ).  
         [0067]     A core may also be threaded through preformed electrodes and dielectric spacers. Certain preferred polymer cores can be heated and drawn slightly at a distal portion to form an operable needle to assist in threading the electrodes. In embodiments manufactured by threading one or more premanufactured electrodes, a conductive path from the respective electrode(s) is generally insulated prior to the threading assembly step. The conductive path typically includes a proximally protruding portion of the wire forming a coiled electrode. Such proximally directed wire desirably is disposed among strands of a core for additional insulation.  
         [0068]     The working electrode  100  is next manually or mechanically dipped in a vertical orientation into at least one coating of 5% polyethersulfone in the solvent DMAC to form the inner exclusion membrane  230  and dried to ensure solidification of the coating. Of course, while reference is made in this disclosure to dipping, it is to be realized that other procedures operable to apply a coating (e.g., brushing, spraying, vapor deposition, and the like) are intended to be encompassed by such language. Successive coatings may be desirable and accomplished by repeating the application process.  
         [0069]     The working electrode  100  and filament core  150  are then manually or mechanically dipped in a vertical orientation into at least one coating of 1% glucose oxidase, 0.6% albumin and 0.5% gluteraldehyde in water to form the middle enzymatic membrane  240 , and dried to ensure solidification ofthe coating. As with the inner exclusion membrane  230 , successive coatings may be desirable and accomplished by repeating the foregoing process. In certain embodiments, the electrode is assembled onto a filament core  150  before the step of applying glucose oxidase. In that case, the glucose oxidase can fill in any spaces between the core fibers  170  to increase the volume of glucose oxidase associated with the electrode. The increased volume of glucose oxidase provides enhanced sensor stability and shelf life.  
         [0070]     Next, in biosensor configurations such as embodiment  10 , having the working electrode  100  and reference electrode  110  positioned adjacent but separate from each other on the filament core  150 , both the working and reference electrodes  100  and  110  are manually or mechanically dipped in a vertical orientation into at least one coating of 5% polyurethane in the solvent tetrahydrofuran to form the outer polymer membrane  250 , and dried to ensure solidification of the coating. Again, successive coatings may be desirable and accomplished by repeating the foregoing process. Successive coatings contemplate use of an approximately 5% solution in a solvent such as, for example, tetrahydrofuran or methylene chloride, to allow for solvent drying from liquid to gel to jelly to a tightly bound conformal coating. The coating materials and respective number of layers are selected to balance response time, electrical insulation, biocompatibility and diffusive properties. For example, a thicker layer increases response time but provides better insulation. To enhance biocompatibility, the outermost surface of the final layer can be made microscopically rough by phase inversion polymerization, i. e., by immediately dipping the last layer in water to allow the miscible solvent to be largely rinsed away soon after the first of the fibers comprising the 5% solution have begun to bond with the second to last layer. Such a procedure typically results in a surface including projecting particles that are sized between about 5 and 50 microns.  
         [0071]     If the biosensor is structured to include coterminous wrapping of both the working and reference electrodes  100 ,  110 , then the sequence of the foregoing method of manufacturing would be altered by, prior to coiling, applying the inner exclusion membrane  230 , the middle enzymatic membrane  240  and a preliminary outer polymer membrane  250  coating to the portion of the working electrode  100  to be coiled, then coiling both the coated working electrode  100  and the reference electrode  110  over a portion of the filament core  150  comparable in length to both working and reference coils  190 ,  200  when adjacent but separate, and finally coating both coterminous coiled electrodes  100 ,  110  as desired.  
         [0072]     The system, apparatus and method of the present invention provide distinct advantages over prior implantable biosensors. Thus, reference herein to specific details of the illustrated or other preferred embodiments is by way of example and not intended to limit the scope of the appended claims. It will be apparent to those skilled in the art that modifications of the basic illustrated embodiments may be made without departing from the spirit and scope of the invention as recited by the claims.