US20080287760A1 - method of making an electrochemical sensor - Google Patents

Abstract

A process for the manufacture of small sensors with reproducible surfaces, including electrochemical sensors. One process includes forming channels in the surface of a substrate and disposing a conductive material in the channels to form an electrode. The conductive material can also be formed on the substrate by other impact and non-impact methods. In a preferred embodiment, the method includes cutting the substrate to form a sensor having a connector portion and a transcutaneous portion, the two portions having edges that define one continuous straight line.

Description

• This application is a continuation of application Ser. No. 11/204,551 filed Aug. 16, 2005, which is a continuation of application Ser. No. 10/405,765 filed Mar. 31, 2003, now U.S. Pat. No. 6,973,706, which is a continuation of application Ser. No. 09/598,776, filed Jun. 16, 2000, abandoned, which is a continuation of application Ser. No. 09/034,422, filed Mar. 4, 1998, now U.S. Pat. No. 6,103,033, all of which are incorporated herein by reference.
FIELD OF THE INVENTION
• The present invention relates to a process for the manufacture of small sensors, including small electrochemical sensors. More particularly, the process of the invention includes disposing a conductive material on a substrate, preferably in channels formed on the surface of the substrate, thereby forming conductive traces and electrodes in a rapid, efficient manner, with reproducible surface areas and conductivities, and particularly forming very small conductive traces.
BACKGROUND OF THE INVENTION
• The monitoring of the level of glucose or other biochemicals, such as lactate, in individuals is often important. High or low levels of glucose or other biochemicals may be detrimental to an individual's health. The monitoring of glucose is particularly important to individuals with diabetes as they must determine when insulin is needed to reduce glucose levels in their bloodstream or when additional glucose is needed to raise the level of glucose in the bloodstream.
• Conventional techniques for monitoring blood glucose levels currently include the periodic drawing of blood, the application of that blood to a test strip, and the determination of the blood glucose concentration using electrochemical, calorimetric, or photometric methods. This technique does not allow for continuous monitoring of blood glucose levels, but must be performed on a periodic basis.
• A variety of other devices have also been developed for continuous monitoring of analytes in the blood stream or subcutaneous tissue. Many of these devices use electrochemical sensors which are directly implanted in a blood vessel or in the subcutaneous tissue of a user. However, these devices are often large, bulky, and/or inflexible and many can not be used effectively outside of a controlled medical facility, such as a hospital or a doctor's office, unless the user is restricted in his activities.
• The user's comfort and the range of activities that can be performed while the sensor is implanted are important considerations in designing extended-use sensors for continuous in vivo monitoring of the level of an analyte, such as glucose. There is a need for a small, comfortable device which can continuously monitor the level of an analyte, such as glucose, while still permitting the user to engage in normal activities outside the boundaries of a controlled medical facility. There is also a need for methods that allow such small, comfortable devices to be relatively inexpensively, efficiently, reproducibly and precisely manufactured.
• A significant problem in the manufacture of in vitro electrochemical sensors has been the inability to manufacture small electrodes with reproducible surfaces. Present techniques for printing or silk screening carbon electrodes onto substrates yield electrodes with poorly defined or irreproducible surface areas and conductivities, particularly at trace widths below 250 μm (10 mils).
• Small sized non-electrochemical sensors including, for example, temperature probes, would also be useful if they could be reliably and reproducibly manufactured. A process for the manufacture of small sensors with reproducible surfaces is needed.
SUMMARY OF THE INVENTION
• The present invention provides a process for the manufacture of small sensors which is efficient, reliable, and provides reproducible surfaces. The process of the invention includes forming one or more channels on a surface of a substrate and disposing a conductive material within the formed channels to form an electrode. Various embodiments of the process include the manufacture of electrochemical sensors by disposing a sensing layer on the conductive material within the formed channels; the manufacture of a sensor having one or more working electrodes; counter/reference electrodes, temperature sensors and the like formed in a plurality of channels on one or more surfaces of the substrate; and sensors having a plurality of electrode traces separated by very small distances to form a small electrochemical sensor.
• One aspect of the present invention relates to a process for the manufacture of an electrochemical sensor using a web process, which may be continuous or non-continuous. The process includes the steps of providing a substrate web, and disposing a pattern of a conductive material on the continuous substrate web to form an electrode, including one or more working electrodes and counter electrodes. The method also includes the step of disposing a sensing layer on the working electrode disposed on the web. Such a continuous web process is adapted for relatively inexpensively, efficiently, reproducibly and precisely manufacturing electrochemical sensors.
• Another aspect of the present invention includes a process for the manufacture of an electrochemical sensor having one or more working and/or counter electrodes disposed on a sensor substrate. The method includes the steps of providing a substrate and disposing a conductive material on the substrate to form one or more working electrodes and/or counter electrodes, and optionally disposing a sensing layer on the working electrode.
• A further aspect of the present invention relates to process for the manufacture of an electrochemical sensor having electrodes and conductive traces disposed within channels defined by a sensor substrate. The process includes the steps of providing a substrate, and forming first and second channels in the substrate. The process also includes the step of disposing a conductive material within the channels to form a working electrode located at the first channel, and a counter electrode located at the second channel. The process further includes the optional step of disposing a sensing layer on the working electrode.
• The invention includes a continuous process for multi-step preparation of sensors including the efficient and precise deposition of small electrode tracings; sensing layers; counter electrodes, temperature sensors, and like constituents to efficiently produce electrochemical and non-electrochemical biosensors.
• The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The Figures and the detailed description which follow more particularly exemplify these embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
• The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:
• FIG. 1 is a block diagram of one embodiment of an analyte monitor using an analyte sensor, according to the invention;
• FIG. 2 is a top view of one embodiment of an analyte sensor, according to the invention;
• FIG. 3A is a cross-sectional view of the analyte sensor ofFIG. 2;
• FIG. 3B is a cross-sectional view of another embodiment of an analyte sensor, according to the invention;
• FIG. 4A is a cross-sectional view of yet another embodiment of an analyte sensor, according to the invention;
• FIG. 4B is a cross-sectional view of a fourth embodiment of an analyte sensor, according to the invention;
• FIG. 5 is an expanded top view of a tip portion of the analyte sensor ofFIG. 2;
• FIG. 6 is a cross-sectional view of a fifth embodiment of an analyte sensor, according to the invention;
• FIG. 7 is an expanded top view of a tip-portion of the analyte sensor ofFIG. 6;
• FIG. 8 is an expanded bottom view of a tip-portion of the analyte sensor ofFIG. 6;
• FIG. 9 is a side view of the analyte sensor ofFIG. 2;
• FIG. 10 is a top view of the analyte sensor ofFIG. 6; and
• FIG. 11 is a bottom view of the analyte sensor ofFIG. 6.
• FIG. 12 is a schematic illustration of an exemplary method or system for manufacturing the sensor ofFIG. 2;
• FIG. 13 is a perspective view of an exemplary embossing roller suitable for use in the system ofFIG. 12;
• FIG. 14 is a perspective of an alternative embossing roller;
• FIG. 15A is cross sectional view taken along section line15a-15aofFIG. 12;
• FIG. 15B is a cross sectional view taken alongsection line15b-15bofFIG. 12;
• FIG. 15C is a cross sectional view taken alongsection line15c-15cofFIG. 12;
• FIG. 15D is a cross sectional view taken alongsection line15d-15dofFIG. 12;
• FIG. 16 illustrates a system in accordance with the principles of the present invention for making the sensor ofFIGS. 10 and 11; and
• FIG. 17 is a top view of another embodiment of an analyte sensor, according to the invention.
• While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
• The process of the present invention is applicable to the manufacture of an analyte sensor for the in vivo and/or in vitro determination of an analyte, such as glucose or lactate, in a fluid. The process is also applicable to the production of other sensors, including, for example biosensors relaying a chemical signal through a conductive tracing.
• The analyte sensors of the present invention can be utilized in a variety of contexts. For example, one embodiment of the analyte sensor is subcutaneously implanted in the interstitial tissue of a patient for the continuous or periodic monitoring of a level of an analyte in a patient's interstitial fluid. This can then be used to infer the analyte level in the patient's bloodstream. Other in vivo analyte sensors can be made, according to the invention, for insertion into a vein, artery, or other portion of the body containing fluid in order to measure a bioanalyte. The in vivo analyte sensors may be configured for obtaining a single measurement and/or for monitoring the level of the analyte over a time period which may range from hours to days or longer.
• Another embodiment of the analyte sensor is used for the in vitro determination of the presence and/or level of an analyte in a sample, and, particularly, in a small volume sample (e.g., 1 to 10 microliters or less). While the present invention is not so limited, an appreciation of various aspects of the invention will be gained through a discussion of the examples provided below.
• The following definitions are provided for terms used herein. A “counter electrode” refers to an electrode paired with the working electrode, through which passes a current equal in magnitude and opposite in sign to the current passing through the working electrode. In the context of the invention, the term “counter electrode” is meant to include counter electrodes which also function as reference electrodes (i.e., a counter/reference electrode).
• An “electrochemical sensor” is a device configured to detect the presence and/or measure the level of an analyte in a sample via electrochemical oxidation and reduction reactions on the sensor. These reactions are transduced to an electrical signal that can be correlated to an amount, concentration, or level of an analyte in the sample.
• “Electrolysis” is the electrooxidation or electroreduction of a compound either directly at an electrode or via one or more electron transfer agents.
• A compound is “immobilized” on a surface when it is entrapped on or chemically bound to the surface.
• A “non-leachable” or “non-releasable” compound or a compound that is “non-leachably disposed” is meant to define a compound that is affixed on the sensor such that it does not substantially diffuse away from the working surface of the working electrode for the period in which the sensor is used (e.g., the period in which the sensor is implanted in a patient or measuring a sample).
• Components are “immobilized” within a sensor, for example, when the components are covalently, ionically, or coordinatively bound to constituents of the sensor and/or are entrapped in a polymeric or sol-gel matrix or membrane which precludes mobility.
• An “electron transfer agent” is a compound that carries electrons between the analyte and the working electrode, either directly, or in cooperation with other electron transfer agents. One example of an electron transfer agent is a redox mediator.
• A “working electrode” is an electrode at which the analyte (or a second compound whose level depends on the level of the analyte) is electrooxidized or electroreduced with or without the agency of an electron transfer agent.
• A “working surface” is that portion of the working electrode which is coated with or is accessible to the electron transfer agent and configured for exposure to an analyte-containing fluid.
• A “sensing layer” is a component of the sensor which includes constituents that facilitate the electrolysis of the analyte. The sensing layer may include constituents such as an electron transfer agent, a catalyst which catalyzes a reaction of the analyte to produce a response at the electrode, or both. In some embodiments of the sensor, the sensing layer is non-leachably disposed in proximity to or on the working electrode.
• A “non-corroding” conductive material includes non-metallic materials, such as carbon and conductive polymers.
Analyte Sensor Systems
• The sensors of the present invention can be utilized in a variety of devices and under a variety of conditions. The particular configuration of a sensor may depend on the use for which the sensor is intended and the conditions under which the sensor will operate (e.g., in vivo or in vitro). One embodiment of the analyte sensor is configured for implantation into a patient or user for in vivo operation. For example, implantation of the sensor may be made in the arterial or venous systems for direct testing of analyte levels in blood. Alternatively, a sensor may be implanted in the interstitial tissue for determining the analyte level in interstitial fluid. This level may be correlated and/or converted to analyte levels in blood or other fluids. The site and depth of implantation may affect the particular shape, components, and configuration of the sensor. Subcutaneous implantation may be preferred, in some cases, to limit the depth of implantation of the sensor. Sensors may also be implanted in other regions of the body to determine analyte levels in other fluids. Particularly useful sensors are described in U.S. Pat. No. 6,134,461, incorporated herein by reference.
• An implantable analyte sensor may be used as part of an analyte monitoring system to continuously and/or periodically monitor the level of an analyte in a body fluid of a patient. In addition to thesensor42, theanalyte monitoring system40 also typically includes acontrol unit44 for operating the sensor42 (e.g., providing a potential to the electrodes and obtaining measurements from the electrodes) and aprocessing unit45 for analyzing the measurements from thesensor42. Thecontrol unit44 andprocessing unit45 may be combined in a single unit or may be separate.
• Another embodiment of the sensor may be used for in vitro measurement of a level of an analyte. The in vitro sensor is coupled to a control unit and/or a processing unit to form an analyte monitoring system. In some embodiments, an in vitro analyte monitoring system is also configured to provide a sample to the sensor. For example, the analyte monitoring system may be configured to draw a sample from, for example, a lanced wound using a wicking and/or capillary action. The sample may then be drawn into contact with the sensor. Examples of such sensors may be found in U.S. patent application Ser. No. 08/795,767 and PCT Patent Application Publication No. WO 98/35225, incorporated herein by reference.
• Other methods for providing a sample to the sensor include using a pump, syringe, or other mechanism to draw a sample from a patient through tubing or the like either directly to the sensor or into a storage unit from which a sample is obtained for the sensor. The pump, syringe, or other mechanism may operate continuously, periodically, or when desired to obtain a sample for testing. Other useful devices for providing an analyte-containing fluid to the sensor include microfiltration and/or microdialysis devices. In some embodiments, particularly those using a microdialysis device, the analyte may be drawn from the body fluid through a microporous membrane, for example, by osmotic pressure, into a carrier fluid which is then conveyed to the sensor for analysis. Other useful devices for acquiring a sample are those that collect body fluids transported across the skin using techniques, such as reverse iontophoresis, to enhance the transport of fluid containing analyte across the skin.
The Sensor
• Asensor42, according to the invention, includes at least one workingelectrode58 formed on asubstrate50, as shown inFIG. 2. Thesensor42 may also include at least one counter electrode60 (or counter/reference electrode) and/or at least one reference electrode62 (seeFIG. 8). Thecounter electrode60 and/orreference electrode62 may be formed on thesubstrate50 or may be separate units. For example, the counter electrode and/or reference electrode may be formed on a second substrate which is also implanted in the patient or, for some embodiments of the implantable sensors, the counter electrode and/or reference electrode may be placed on the skin of the patient with the working electrode or electrodes being implanted into the patient. The use of an on-the-skin counter and/or reference electrode with an implantable working electrode is described in U.S. Pat. No. 5,593,852, incorporated herein by reference.
• The working electrode orelectrodes58 are formed usingconductive traces52 disposed on thesubstrate50. Thecounter electrode60 and/orreference electrode62, as well as other optional portions of thesensor42, such as a temperature probe66 (seeFIG. 8), may also be formed usingconductive traces52 disposed on thesubstrate50. These conductive traces52 may be formed over a smooth surface of thesubstrate50 or withinchannels54 formed by, for example, embossing, indenting or otherwise creating a depression in thesubstrate50.
• A sensing layer64 (seeFIGS. 3A and 3B) is often formed proximate to or on at least one of the workingelectrodes58 to facilitate the electrochemical detection of the analyte and the determination of its level in the sample fluid, particularly if the analyte can not be electrolyzed at a desired rate and/or with a desired specificity on a bare electrode. Thesensing layer64 may include an electron transfer agent to transfer electrons directly or indirectly between the analyte and the workingelectrode58. Thesensing layer64 may also contain a catalyst to catalyze a reaction of the analyte. The components of the sensing layer may be in a fluid or gel that is proximate to or in contact with the workingelectrode58. Alternatively, the components of thesensing layer64 may be disposed in a polymeric or sol-gel matrix that is proximate to or on the workingelectrode58. Preferably, the components of thesensing layer64 are non-leachably disposed within thesensor42. More preferably, the components of thesensor42 are immobilized within thesensor42.
• In addition to theelectrodes58,60,62 and thesensing layer64, thesensor42 may also include a temperature probe66 (seeFIGS. 6 and 8), a mass transport limiting layer74 (seeFIG. 9), a biocompatible layer75 (seeFIG. 9), and/or other optional components, as described below. Each of these items enhances the functioning of and/or results from thesensor42, as discussed below.
The Substrate
• Thesubstrate50 may be formed using a variety of non-conducting materials, including, for example, polymeric or plastic materials and ceramic materials. Suitable materials for aparticular sensor42 may be determined, at least in part, based on the desired use of thesensor42 and properties of the materials.
• In some embodiments, the substrate is flexible. For example, if thesensor42 is configured for implantation into a patient, then thesensor42 may be made flexible (although rigid sensors may also be used for implantable sensors) to reduce pain to the patient and damage to the tissue caused by the implantation of and/or the wearing of thesensor42. Aflexible substrate50 often increases the patient's comfort and allows a wider range of activities. Aflexible substrate50 is also useful for an invitro sensor42, particularly for ease of manufacturing. Suitable materials for aflexible substrate50 include, for example, non-conducting plastic or polymeric materials and other non-conducting, flexible, deformable materials. Examples of useful plastic or polymeric materials include thermoplastics such as polycarbonates, polyesters (e.g., Mylar™ and polyethylene terephthalate (PET)), polyvinyl chloride (PVC), polyurethanes, polyethers, polyamides, polyimides, or copolymers of these thermoplastics, such as PETG (glycol-modified polyethylene terephthalate).
• In other embodiments, thesensors42 are made using a relativelyrigid substrate50 to, for example, provide structural support against bending or breaking. Examples of rigid materials that may be used as thesubstrate50 include poorly conducting ceramics, such as aluminum oxide and silicon dioxide. One advantage of animplantable sensor42 having a rigid substrate is that thesensor42 may have a sharp point and/or a sharp edge to aid in implantation of asensor42 without an additional insertion device. In addition,rigid substrates50 may also be used in sensors for in vitro analyte monitors.
• It will be appreciated that formany sensors42 and sensor applications, both rigid and flexible sensors will operate adequately. The flexibility of thesensor42 may also be controlled and varied along a continuum by changing, for example, the composition and/or thickness of thesubstrate50.
• In addition to considerations regarding flexibility, it is often desirable thatimplantable sensors42, as well as in vitro sensors which contact a fluid that is returned to a patient's body, should have asubstrate50 which is non-toxic. Preferably, thesubstrate50 is approved by one or more appropriate governmental agencies or private groups for in vivo use.
• Thesensor42 may include optional features to facilitate insertion of animplantable sensor42, as shown inFIG. 17. For example, thesensor42 may be pointed at thetip123 to ease insertion. In addition, thesensor42 may include abarb125 which assists in anchoring thesensor42 within the tissue of the patient during operation of thesensor42. However, thebarb125 is typically small enough that little damage is caused to the subcutaneous tissue when thesensor42 is removed for replacement.
• Although thesubstrate50 in at least some embodiments has uniform dimensions along the entire length of thesensor42, in other embodiments, thesubstrate50 has adistal end67 at afirst portion67aofsensor42 and aproximal end65 at asecond portion65aofsensor42.First portion67aandsecond portion65ahavedifferent widths53,55, respectively, as illustrated inFIG. 2.Width53 is measured betweenedge67′ and edge67″ offirst portion67a, andwidth55 is measured betweenedge65′ and edge65″ ofsecond portion65a. In these embodiments, thefirst portion67ahavingdistal end67 of thesubstrate50 may have a relativelynarrow width53. Forsensors42 which are implantable into the subcutaneous tissue or another portion of a patient's body, thenarrow width53 of thefirst portion67ahavingdistal end67 of thesubstrate50 may facilitate the implantation of thesensor42. Often, the narrower the width of thesensor42, the less pain the patient will feel during implantation of the sensor and afterwards.
• For subcutaneouslyimplantable sensors42 which are designed for continuous or periodic monitoring of the analyte during normal activities of the patient, thefirst portion67ahavingdistal end67 of thesensor42 which is to be implanted into the patient has awidth53 of 2 mm or less, preferably 1 mm or less, and more preferably 0.5 mm or less. If thesensor42 does not have regions of different widths, then thesensor42 will typically have an overall width of, for example, 2 mm, 1.5 mm, 1 mm, 0.5 mm, 0.25 mm, or less. However, wider or narrower sensors may be used. In particular, wider implantable sensors may be used for insertion into veins or arteries or when the movement of the patient is limited, for example, when the patient is confined in bed or in a hospital.
• Forsensors42 which are designed for measuring small volume in vitro samples, thenarrow width53 may reduce the volume of sample needed for an accurate reading. Thenarrow width53 of thesensor42 results in all of the electrodes of thesensor42 being closely congregated, thereby requiring less sample volume to cover all of the electrodes. The width of an invitro sensor42 may vary depending, at least in part, on the volume of sample available to thesensor42 and the dimensions of the sample chamber in which thesensor42 is disposed.
• Returning toFIG. 2, theproximal end65 of thesensor42 may have awidth55 larger than thedistal end67 to facilitate the connection betweencontact pads49 of the electrodes and contacts on a control unit. The wider thesensor42 at this point, the larger thecontact pads49 can be made. This may reduce the precision needed to properly connect thesensor42 to contacts on the control unit (e.g.,sensor control unit44 ofFIG. 1). However, the maximum width of thesensor42 may be constrained so that thesensor42 remains small for the convenience and comfort of the patient and/or to fit the desired size of the analyte monitor. For example, theproximal end65 of a subcutaneouslyimplantable sensor42, such as thesensor42 illustrated inFIG. 1, may have awidth55 ranging from 0.5 mm to 15 mm, preferably from 1 mm to 10 mm, and more preferably from 3 mm to 7 mm. However, wider or narrower sensors may be used in this and other in vivo and in vitro applications.
• The thickness of thesubstrate50 may be determined by the mechanical properties of the substrate material (e.g., the strength, modulus, and/or flexibility of the material), the desired use of thesensor42 including stresses on thesubstrate50 arising from that use, as well as the depth of any channels or indentations formed in thesubstrate50, as discussed below. Typically, thesubstrate50 of a subcutaneouslyimplantable sensor42 for continuous or periodic monitoring of the level of an analyte while the patient engages in normal activities has a thickness of 50 to 500 μm and preferably 100 to 300 μm. However, thicker andthinner substrates50 may be used, particularly in other types of in vivo and in vitrosensors42.
• The length of thesensor42 may have a wide range of values depending on a variety of factors. Factors which influence the length of animplantable sensor42 may include the depth of implantation into the patient and the ability of the patient to manipulate a smallflexible sensor42 and make connections between thesensor42 and thesensor control unit44. A subcutaneouslyimplantable sensor42 for the analyte monitor illustrated inFIG. 1 may have a length ranging from 0.3 to 5 cm, however, longer or shorter sensors may be used. The length of the narrow portion of the sensor42 (e.g., the portion which is subcutaneously inserted into the patient), if thesensor42 has narrow and wide portions, is typically about 0.25 to 2 cm in length. However, longer and shorter portions may be used. All or only a part of this narrow portion may be subcutaneously implanted into the patient.
• The lengths of otherimplantable sensors42 will vary depending, at least in part, on the portion of the patient into which thesensor42 is to be implanted or inserted. The length of in vitro sensors may vary over a wide range depending on the particular configuration of the analyte monitoring system and, in particular, the distance between the contacts of the control unit and the sample.
Conductive Traces
• At least oneconductive trace52 is formed on the substrate for use in constructing a workingelectrode58. In addition, otherconductive traces52 may be formed on thesubstrate50 for use as electrodes (e.g., additional working electrodes, as well as counter, counter/reference, and/or reference electrodes) and other components, such as a temperature probe. The conductive traces52 may extend most of the distance along alength57 of thesensor50, as illustrated inFIG. 2, although this is not necessary. The placement of the conductive traces52 may depend on the particular configuration of the analyte monitoring system (e.g., the placement of control unit contacts and/or the sample chamber in relation to the sensor42). For implantable sensors, particularly subcutaneously implantable sensors, the conductive traces typically extend close to the tip of thesensor42 to minimize the amount of the sensor that must be implanted.
• The conductive traces52 may be formed on thesubstrate50 by a variety of techniques, including, for example, photolithography, screen printing, or other impact or non-impact printing techniques. The conductive traces52 may also be formed by carbonizing conductive traces52 in an organic (e.g., polymeric or plastic)substrate50 using a laser.
• Another method for disposing the conductive traces52 on thesubstrate50 includes the formation of recessedchannels54 in one or more surfaces of thesubstrate50 and the subsequent filling of these recessedchannels54 with aconductive material56, as shown inFIG. 3A. The recessedchannels54 may be formed by indenting, embossing, or otherwise creating a depression in the surface of thesubstrate50. The depth of the channels is typically related to the thickness of thesubstrate50. In one embodiment, the channels have depths in the range of about 12.5 to 75 μm (0.5 to 3 mils), and preferably about 25 to 50 μm (1 to 2 mils).
• The conductive traces are typically formed using aconductive material56 such as carbon (e.g., graphite), a conductive polymer, a metal or alloy (e.g., gold or gold alloy), or a metallic compound (e.g., ruthenium dioxide or titanium dioxide). The formation of films of carbon, conductive polymer, metal, alloy, or metallic compound are well-known and include, for example, chemical vapor deposition (CVD), physical vapor deposition, sputtering, reactive sputtering, printing, coating, and painting. Theconductive material56 which fills thechannels54 is often formed using a precursor material, such as a conductive ink or paste. In these embodiments, theconductive material56 is deposited on thesubstrate50 using methods such as coating, painting, or applying the material using a spreading instrument, such as a coating blade. Excess conductive material between thechannels54 is then removed by, for example, running a blade along the substrate surface.
• In one embodiment, theconductive material56 is a part of a precursor material, such as a conductive ink, obtainable, for example, from Ercon, Inc. (Wareham, Mass.), Metech, Inc. (Elverson, Pa.), E.I. du Pont de Nemours and Co. (Wilmington, Del.), Emca-Remex Products (Montgomeryville, Pa.), or MCA Services (Melbourn, Great Britain). The conductive ink is typically applied as a semiliquid or paste which contains particles of the carbon, metal, alloy, or metallic compound and a solvent or dispersant. After application of the conductive ink on the substrate50 (e.g., in the channels54), the solvent or dispersant evaporates to leave behind a solid mass ofconductive material56.
• In addition to the particles of carbon, metal, alloy, or metallic compound, the conductive ink may also contain a binder. The binder may optionally be cured to further bind theconductive material56 within thechannel54 and/or on thesubstrate50. Curing the binder increases the conductivity of theconductive material56. However, this is typically not necessary as the currents carried by theconductive material56 within the conductive traces52 are often relatively low (usually less than 1 μA and often less than 100 nA). Typical binders include, for example, polyurethane resins, cellulose derivatives, elastomers, and highly fluorinated polymers. Examples of elastomers include silicones, polymeric dienes, and acrylonitrile-butadiene-styrene (ABS) resins. One example of a fluorinated polymer binder is Teflon® (DuPont, Wilmington, Del.). These binders are cured using, for example, heat or light, including ultraviolet (UV) light. The appropriate curing method typically depends on the particular binder which is used.
• Often, when a liquid or semiliquid precursor of the conductive material56 (e.g., a conductive ink) is deposited in thechannel54, the precursor fills thechannel54. However, when the solvent or dispersant evaporates, theconductive material56 which remains may lose volume such that theconductive material56 may or may not continue to fill thechannel54. Preferredconductive materials56 do not pull away from thesubstrate50 as they lose volume, but rather decrease in height within thechannel54. Theseconductive materials56 typically adhere well to thesubstrate50 and therefore do not pull away from thesubstrate50 during evaporation of the solvent or dispersant. Other suitableconductive materials56 either adhere to at least a portion of thesubstrate50 and/or contain another additive, such as a binder, which adheres theconductive material56 to thesubstrate50. Preferably, theconductive material56 in thechannels54 is non-leachable, and more preferably immobilized on thesubstrate50. In some embodiments, theconductive material56 may be formed by multiple applications of a liquid or semiliquid precursor interspersed with removal of the solvent or dispersant.
• In another embodiment, thechannels54 are formed using a laser. The laser carbonizes the polymer or plastic material. The carbon formed in this process is used as theconductive material56. Additionalconductive material56, such as a conductive carbon ink, may be used to supplement the carbon formed by the laser.
• In a further embodiment, the conductive traces52 are formed by pad printing techniques. For example, a film of conductive material is formed either as a continuous film or as a coating layer deposited on a carrier film. This film of conductive material is brought between a print head and thesubstrate50. A pattern on the surface of thesubstrate50 is made using the print head according to a desired pattern of conductive traces52. The conductive material is transferred by pressure and/or heat from the film of conductive material to thesubstrate50. This technique often produces channels (e.g., depressions caused by the print head) in thesubstrate50. Alternatively, the conductive material is deposited on the surface of thesubstrate50 without forming substantial depressions.
• In other embodiments, the conductive traces52 are formed by non-impact printing techniques. Such techniques include electrophotography and magnetography. In these processes, an image of the conductive traces52 is electrically or magnetically formed on a drum. A laser or LED may be used to electrically form an image. A magnetic recording head may be used to magnetically form an image. A toner material (e.g., a conductive material, such as a conductive ink) is then attracted to portions of the drum according to the image. The toner material is then applied to the substrate by contact between the drum and the substrate. For example, the substrate may be rolled over the drum. The toner material may then be dried and/or a binder in the toner material may be cured to adhere the toner material to the substrate.
• Another non-impact printing technique includes ejecting droplets of conductive material onto the substrate in a desired pattern. Examples of this technique include ink jet printing and piezo jet printing. An image is sent to the printer which then ejects the conductive material (e.g., a conductive ink) according to the pattern. The printer may provide a continuous stream of conductive material or the printer may eject the conductive material in discrete amounts at the desired points.
• Yet another non-impact printing embodiment of forming the conductive traces includes an ionographic process. In this process, a curable, liquid precursor, such as a photopolymerizable acrylic resin (e.g., Solimer 7501 from Cubital, Bad Kreuznach, Germany) is deposited over a surface of asubstrate50. A photomask having a positive or negative image of the conductive traces52 is then used to cure the liquid precursor. Light (e.g., visible or ultraviolet light) is directed through the photomask to cure the liquid precursor and form a solid layer over the substrate according to the image on the photomask. Uncured liquid precursor is removed leaving behindchannels54 in the solid layer. Thesechannels54 can then be filled withconductive material56 to form conductive traces52.
• Conductive traces52 (andchannels54, if used) can be formed with relatively narrow widths, for example, in the range of 25 to 250 μm, and including widths of, for example, 250 μm, 150 μm, 100 μm, 75 μm, 50 μm, 25 μm or less by the methods described above. In embodiments with two or moreconductive traces52 on the same side of thesubstrate50, the conductive traces52 are separated by distances sufficient to prevent conduction between the conductive traces52. The edge-to-edge distance between the conductive traces is preferably in the range of 25 to 250 μm and may be, for example, 150 μm, 100 μm, 75 μm, 50 μm, or less. The density of the conductive traces52 on thesubstrate50 is preferably in the range of about 150 to 700 μm/trace and may be as small as 667 μm/trace or less, 333 μm/trace or less, or even 167 μm/trace or less.
• The workingelectrode58 and the counter electrode60 (if a separate reference electrode is used) are often made using aconductive material56, such as carbon. Suitable carbon conductive inks are available from Ercon, Inc. (Wareham, Mass.), Metech, Inc. (Elverson, Pa.), E.I. du Pont de Nemours and Co. (Wilmington, Del.), Emca-Remex Products (Montgomeryville, Pa.), or MCA Services (Melbourn, Great Britain). Typically, the workingsurface51 of the workingelectrode58 is at least a portion of theconductive trace52 that is in contact with the analyte-containing fluid (e.g., implanted in the patient or in the sample chamber of an in vitro analyte monitor).
• Thereference electrode62 and/or counter/reference electrode are typically formed usingconductive material56 that is a suitable reference material, for example silver/silver chloride or a non-leachable redox couple bound to a conductive material, for example, a carbon-bound redox couple. Suitable silver/silver chloride conductive inks are available from Ercon, Inc. (Wareham, Mass.), Metech, Inc. (Elverson, Pa.), E.I. du Pont de Nemours and Co. (Wilmington, Del.), Emca-Remex Products (Montgomeryville, Pa.), or MCA Services (Melbourn, Great Britain). Silver/silver chloride electrodes illustrate a type of reference electrode that involves the reaction of a metal electrode with a constituent of the sample or body fluid, in this case, Cl⁻.
• Suitable redox couples for binding to the conductive material of the reference electrode include, for example, redox polymers (e.g., polymers having multiple redox centers.) It is preferred that the reference electrode surface be non-corroding so that an erroneous potential is not measured. Preferred conductive materials include less corrosive metals, such as gold and palladium. Most preferred are non-corrosive materials including non-metallic conductors, such as carbon and conducting polymers. A redox polymer can be adsorbed on or covalently bound to the conductive material of the reference electrode, such as a carbon surface of aconductive trace52. Non-polymeric redox couples can be similarly bound to carbon or gold surfaces.
• A variety of methods may be used to immobilize a redox polymer on an electrode surface. One method is adsorptive immobilization. This method is particularly useful for redox polymers with relatively high molecular weights. The molecular weight of a polymer may be increased, for example, by cross-linking.
• Another method for immobilizing the redox polymer includes the functionalization of the electrode surface and then the chemical bonding, often covalently, of the redox polymer to the functional groups on the electrode surface. One example of this type of immobilization begins with a poly(4-vinylpyridine). The polymer's pyridine rings are, in part, complexed with a reducible/oxidizable species, such as [Os(bpy)₂Cl]^+/2+ where bpy is 2,2′-bipyridine. Part of the pyridine rings are quaternized by reaction with 2-bromoethylamine. The polymer is then crosslinked, for example, using a diepoxide, such as polyethylene glycol diglycidyl ether.
• Carbon surfaces can be modified for attachment of a redox species or polymer, for example, by electroreduction of a diazonium salt. As an illustration, reduction of a diazonium salt formed upon diazotization of p-aminobenzoic acid modifies a carbon surface with phenylcarboxylic acid functional groups. These functional groups can then be activated by a carbodiimide, such as 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride. The activated functional groups are then bound with a amine-functionalized redox couple, such as the quaternized osmium-containing redox polymer described above or 2-aminoethylferrocene, to form the redox couple.
• Similarly, gold can be functionalized by an amine, such as cystamine. A redox couple such as [Os(bpy)₂(pyridine-4-carboxylate)Cl]^0/+ is activated by 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride to form a reactive O-acylisourea which reacts with the gold-bound amine to form an amide.
• In one embodiment, in addition to using the conductive traces52 as electrodes or probe leads, two or more of the conductive traces52 on thesubstrate50 are used to give the patient a mild electrical shock when, for example, the analyte level exceeds a threshold level. This shock may act as a warning or alarm to the patient to initiate some action to restore the appropriate level of the analyte.
• The mild electrical shock is produced by applying a potential between any twoconductive traces52 that are not otherwise connected by a conductive path. For example, two of theelectrodes58,60,62 or oneelectrode58,60,62 and thetemperature probe66 may be used to provide the mild shock. Preferably, the workingelectrode58 and thereference electrode62 are not used for this purpose as this may cause some damage to the chemical components on or proximate to the particular electrode (e.g., the sensing layer on the working electrode or the redox couple on the reference electrode).
• The current used to produce the mild shock is typically 0.1 to 1 mA. Higher or lower currents may be used, although care should be taken to avoid harm to the patient. The potential between the conductive traces is typically 1 to 10 volts. However, higher or lower voltages may be used depending, for example, on the resistance of the conductive traces52, the distance between theconductive traces52 and the desired amount of current. When the mild shock is delivered, potentials at the workingelectrode58 and across thetemperature probe66 may be removed to prevent harm to those components caused by unwanted conduction between the working electrode58 (and/ortemperature probe66, if used) and the conductive traces52 which provide the mild shock.
Contact Pads
• Typically, each of the conductive traces52 includes acontact pad49. Thecontact pad49 may simply be a portion of theconductive trace52 that is indistinguishable from the rest of thetrace52 except that thecontact pad49 is brought into contact with the conductive contacts of a control unit (e.g., thesensor control unit44 ofFIG. 1). More commonly, however, thecontact pad49 is a region of theconductive trace52 that has a larger width than other regions of thetrace52 to facilitate a connection with the contacts on the control unit. By making thecontact pads49 relatively large as compared with the width of the conductive traces52, the need for precise registration between thecontact pads49 and the contacts on the control unit is less critical than with small contact pads.
• Thecontact pads49 are typically made using the same material as theconductive material56 of the conductive traces52. However, this is not necessary. Although metal, alloys, and metallic compounds may be used to form thecontact pads49, in some embodiments, it is desirable to make thecontact pads49 from a carbon or other non-metallic material, such as a conducting polymer. In contrast to metal or alloy contact pads, carbon and other non-metallic contact pads are not easily corroded if thecontact pads49 are in a wet, moist, or humid environment. Metals and alloys may corrode under these conditions, particularly if thecontact pads49 and contacts of the control unit are made using different metals or alloys. However, carbon andnon-metallic contact pads49 do not significantly corrode, even if the contacts of the control device are metal or alloy.
• One embodiment of the invention includes asensor42 havingcontact pads49 and acontrol unit44 having conductive contacts (not shown). During operation of thesensor42, thecontact pads49 and conductive contacts are in contact with each other. In this embodiment, either thecontact pads49 or the conductive contacts are made using a non-corroding, conductive material. Such materials include, for example, carbon and conducting polymers. Preferred non-corroding materials include graphite and vitreous carbon. The opposing contact pad or conductive contact is made using carbon, a conducting polymer, a metal, such as gold, palladium, or platinum group metal, or a metallic compound, such as ruthenium dioxide. This configuration of contact pads and conductive contacts typically reduces corrosion. Preferably, when the sensor is placed in a 3 mM, and more preferably, in a 100 mM, NaCl solution, the signal arising due to the corrosion of the contact pads and/or conductive contacts is less than 3% of the signal generated by the sensor when exposed to concentration of analyte in the normal physiological range. For at least some subcutaneous glucose sensors, the current generated by analyte in a normal physiological range ranges from 3 to 500 nA.
• Each of theelectrodes58,60,62, as well as the two probe leads68,70 of the temperature probe66 (described below), are connected to contactpads49 as shown inFIGS. 10 and 11. In one embodiment (not shown), thecontact pads49 are on the same side of thesubstrate50 as the respective electrodes or temperature probe leads to which thecontact pads49 are attached.
• In other embodiments, the conductive traces52 on at least one side are connected through vias in the substrate to contactpads49aon the opposite surface of thesubstrate50, as shown inFIGS. 10 and 11. An advantage of this configuration is that contact between the contacts on the control unit and each of theelectrodes58,60,62 and the probe leads68,70 of thetemperature probe66 can be made from a single side of thesubstrate50.
• In yet other embodiments (not shown), vias through the substrate are used to provide contact pads on both sides of thesubstrate50 for eachconductive trace52. The vias connecting the conductive traces52 with thecontact pads49acan be formed by making holes through thesubstrate50 at the appropriate points and then filling the holes withconductive material56.
Exemplary Electrode Configurations
• A number of exemplary electrode configurations are described below, however, it will be understood that other configurations may also be used. In one embodiment, illustrated inFIG. 3A, thesensor42 includes two workingelectrodes58a,58band onecounter electrode60, which also functions as a reference electrode. In another embodiment, the sensor includes one workingelectrode58a, onecounter electrode60, and onereference electrode62, as shown inFIG. 3B. Each of these embodiments is illustrated with all of the electrodes formed on the same side of thesubstrate50.
• Alternatively, one or more of the electrodes may be formed on an opposing side of thesubstrate50. This may be convenient if the electrodes are formed using two different types of conductive material56 (e.g., carbon and silver/silver chloride). Then, at least in some embodiments, only one type ofconductive material56 needs to be applied to each side of thesubstrate50, thereby reducing the number of steps in the manufacturing process and/or easing the registration constraints in the process. For example, if the workingelectrode58 is formed using a carbon-basedconductive material56 and the reference or counter/reference electrode is formed using a silver/silver chlorideconductive material56, then the working electrode and reference or counter/reference electrode may be formed on opposing sides of thesubstrate50 for ease of manufacture.
• In another embodiment, two workingelectrodes58 and onecounter electrode60 are formed on one side of thesubstrate50 and onereference electrode62 and atemperature probe66 are formed on an opposing side of thesubstrate50, as illustrated inFIG. 6. The opposing sides of the tip of this embodiment of thesensor42 are illustrated inFIGS. 7 and 8.
Sensing Layer
• Some analytes, such as oxygen, can be directly electrooxidized or electroreduced on the workingelectrode58. Other analytes, such as glucose and lactate, require the presence of at least one electron transfer agent and/or at least one catalyst to facilitate the electrooxidation or electroreduction of the analyte. Catalysts may also be used for those analyte, such as oxygen, that can be directly electrooxidized or electroreduced on the workingelectrode58. For these analytes, each workingelectrode58 has asensing layer64 formed proximate to or on a working surface of the workingelectrode58. Typically, thesensing layer64 is formed near or on only a small portion of the workingelectrode58, often near a tip of thesensor42. This limits the amount of material needed to form thesensor42 and places thesensing layer64 in the best position for contact with the analyte-containing fluid (e.g., a body fluid, sample fluid, or carrier fluid).
• Thesensing layer64 includes one or more components designed to facilitate the electrolysis of the analyte. Thesensing layer64 may include, for example, a catalyst to catalyze a reaction of the analyte and produce a response at the workingelectrode58, an electron transfer agent to indirectly or directly transfer electrons between the analyte and the workingelectrode58, or both.
• Thesensing layer64 may be formed as a solid composition of the desired components (e.g., an electron transfer agent and/or a catalyst). These components are preferably non-leachable from thesensor42 and more preferably are immobilized on thesensor42. For example, the components may be immobilized on a workingelectrode58. Alternatively, the components of thesensing layer64 may be immobilized within or between one or more membranes or films disposed over the workingelectrode58 or the components may be immobilized in a polymeric or sol-gel matrix. Examples of immobilized sensing layers are described in U.S. Pat. Nos. 5,262,035, 5,264,104, 5,264,105, 5,320,725, 5,593,852, and 5,665,222, and PCT Patent Application Publication No. WO 98/35053, incorporated herein by reference.
• In some embodiments, one or more of the components of thesensing layer64 may be solvated, dispersed, or suspended in a fluid within thesensing layer64, instead of forming a solid composition. The fluid may be provided with thesensor42 or may be absorbed by thesensor42 from the analyte-containing fluid. Preferably, the components which are solvated, dispersed, or suspended in this type ofsensing layer64 are non-leachable from the sensing layer. Non-leachability may be accomplished, for example, by providing barriers (e.g., the electrode, substrate, membranes, and/or films) around the sensing layer which prevent the leaching of the components of thesensing layer64. One example of such a barrier is a microporous membrane or film which allows diffusion of the analyte into thesensing layer64 to make contact with the components of thesensing layer64, but reduces or eliminates the diffusion of the sensing layer components (e.g., a electron transfer agent and/or a catalyst) out of thesensing layer64.
• A variety of different sensing layer configurations can be used. In one embodiment, thesensing layer64 is deposited on theconductive material56 of a workingelectrode58a, as illustrated inFIGS. 3A and 3B. Thesensing layer64 may extend beyond theconductive material56 of the workingelectrode58a. In some cases, thesensing layer64 may also extend over thecounter electrode60 orreference electrode62 without degrading the performance of the glucose sensor. For thosesensors42 which utilizechannels54 within which theconductive material56 is deposited, a portion of thesensing layer64 may be formed within thechannel54 if theconductive material56 does not fill thechannel54.
• Asensing layer64 in direct contact with the workingelectrode58amay contain an electron transfer agent to transfer electrons directly or indirectly between the analyte and the working electrode, as well as a catalyst to facilitate a reaction of the analyte. For example, a glucose, lactate, or oxygen electrode may be formed having a sensing layer which contains a catalyst, such as glucose oxidase, lactate oxidase, or laccase, respectively, and an electron transfer agent that facilitates the electrooxidation of the glucose, lactate, or oxygen, respectively.
• In another embodiment, thesensing layer64 is not deposited directly on the workingelectrode58a. Instead, thesensing layer64 is spaced apart from the workingelectrode58a, as illustrated inFIG. 4A, and separated from the workingelectrode58aby aseparation layer61. Theseparation layer61 typically includes one or more membranes or films. In addition to separating the workingelectrode58afrom thesensing layer64, theseparation layer61 may also act as a mass transport limiting layer or an interferent eliminating layer, as described below.
• Typically, asensing layer64, which is not in direct contact with the workingelectrode58a, includes a catalyst that facilitates a reaction of the analyte. However, thissensing layer64 typically does not include an electron transfer agent that transfers electrons directly from the workingelectrode58ato the analyte, as thesensing layer64 is spaced apart from the workingelectrode58a. One example of this type of sensor is a glucose or lactate sensor which includes an enzyme (e.g., glucose oxidase or lactate oxidase, respectively) in thesensing layer64. The glucose or lactate reacts with a second compound (e.g., oxygen) in the presence of the enzyme. The second compound is then electrooxidized or electroreduced at the electrode. Changes in the signal at the electrode indicate changes in the level of the second compound in the fluid and are proportional to changes in glucose or lactate level and, thus, correlate to the analyte level.
• In another embodiment, two sensinglayers63,64 are used, as shown inFIG. 4B. Each of the two sensinglayers63,64 may be independently formed on the workingelectrode58aor in proximity to the workingelectrode58a. Onesensing layer64 is typically, although not necessarily, spaced apart from the workingelectrode58a. For example, thissensing layer64 may include a catalyst which catalyzes a reaction of the analyte to form a product compound. The product compound is then electrolyzed in thesecond sensing layer63 which may include an electron transfer agent to transfer electrons between the workingelectrode58aand the product compound and/or a second catalyst to catalyze a reaction of the product compound to generate a signal at the workingelectrode58a.
• For example, a glucose or lactate sensor may include afirst sensing layer64 which is spaced apart from the working electrode and contains an enzyme, for example, glucose oxidase or lactate oxidase. The reaction of glucose or lactate in the presence of the appropriate enzyme forms hydrogen peroxide. Asecond sensing layer63 is provided directly on the workingelectrode58aand contains a peroxidase enzyme and an electron transfer agent to generate a signal at the electrode in response to the hydrogen peroxide. The level of hydrogen peroxide indicated by the sensor then correlates to the level of glucose or lactate. Another sensor which operates similarly can be made using a single sensing layer with both the glucose or lactate oxidase and the peroxidase being deposited in the single sensing layer. Examples of such sensors are described in U.S. Pat. No. 5,593,852, U.S. Pat. No. 5,665,222 and PCT Patent Application Publication No. WO 98/35053, incorporated herein by reference.
• In some embodiments, one or more of the workingelectrodes58bdo not have acorresponding sensing layer64, as shown inFIGS. 3A and 4A, or have a sensing layer (not shown) which does not contain one or more components (e.g., an electron transfer agent or catalyst) needed to electrolyze the analyte. The signal generated at this workingelectrode58btypically arises from interferents and other sources, such as ions, in the fluid, and not in response to the analyte (because the analyte is not electrooxidized or electroreduced). Thus, the signal at this workingelectrode58bcorresponds to a background signal. The background signal can be removed from the analyte signal obtained from other workingelectrodes58athat are associated with fully-functional sensing layers64 by, for example, subtracting the signal at workingelectrode58bfrom the signal at workingelectrode58a.
• Sensors having multiple workingelectrodes58amay also be used to obtain more precise results by averaging the signals or measurements generated at these workingelectrodes58a. In addition, multiple readings at a single workingelectrode58aor at multiple working electrodes may be averaged to obtain more precise data.
Electron Transfer Agent
• In many embodiments, thesensing layer64 contains one or more electron transfer agents in contact with theconductive material56 of the workingelectrode58, as shown inFIGS. 3A and 3B. In some embodiments, it is acceptable for the electron transfer agent to diffuse or leach away from the working electrode, particularly for in vitrosensors42 that are used only once. Other in vitro sensors may utilize a carrier fluid which contains the electron transfer agent. The analyte is transferred to the carrier fluid from the original sample fluid by, for example, osmotic flow through a microporous membrane or the like.
• In yet other embodiments of the invention, there is little or no leaching of the electron transfer agent away from the workingelectrode58 during the period in which thesensor42 is implanted in the patient or measuring an in vitro analyte-containing sample. A diffusing or leachable (i.e., releasable) electron transfer agent often diffuses into the analyte-containing fluid, thereby reducing the effectiveness of the electrode by reducing the sensitivity of the sensor over time. In addition, a diffusing or leaching electron transfer agent in animplantable sensor42 may also cause damage to the patient. In these embodiments, preferably, at least 90%, more preferably, at least 95%, and, most preferably, at least 99%, of the electron transfer agent remains disposed on the sensor after immersion in the analyte-containing fluid for 24 hours, and, more preferably, for 72 hours. In particular, for an implantable sensor, preferably, at least 90%, more preferably, at least 95%, and most preferably, at least 99%, of the electron transfer agent remains disposed on the sensor after immersion in the body fluid at 37° C. for 24 hours, and, more preferably, for 72 hours.
• In some embodiments of the invention, to prevent leaching, the electron transfer agents are bound or otherwise immobilized on the workingelectrode58 or between or within one or more membranes or films disposed over the workingelectrode58. The electron transfer agent may be immobilized on the workingelectrode58 using, for example, a polymeric or sol-gel immobilization technique. Alternatively, the electron transfer agent may be chemically (e.g., ionically, covalently, or coordinatively) bound to the workingelectrode58, either directly or indirectly through another molecule, such as a polymer, that is in turn bound to the workingelectrode58.
• Application of thesensing layer64 on a workingelectrode58ais one method for creating a working surface for the workingelectrode58a, as shown inFIGS. 3A and 3B. The electron transfer agent mediates the transfer of electrons to electrooxidize or electroreduce an analyte and thereby permits a current flow between the workingelectrode58 and thecounter electrode60 via the analyte. The mediation of the electron transfer agent facilitates the electrochemical analysis of analytes which are not suited for direct electrochemical reaction on an electrode.
• In general, the preferred electron transfer agents are electroreducible and electrooxidizable ions or molecules having redox potentials that are a few hundred millivolts above or below the redox potential of the standard calomel electrode (SCE). Preferably, the electron transfer agents are not more reducing than about −150 mV and not more oxidizing than about +400 mV versus SCE.
• The electron transfer agent may be organic, organometallic, or inorganic. Examples of organic redox species are quinones and species that in their oxidized state have quinoid structures, such as Nile blue and indophenol. Some quinones and partially oxidized quinhydrones react with functional groups of proteins such as the thiol groups of cysteine, the amine groups of lysine and arginine, and the phenolic groups of tyrosine which may render those redox species unsuitable for some of the sensors of the present invention because of the presence of the interfering proteins in an analyte-containing fluid. Usually substituted quinones and molecules with quinoid structure are less reactive with proteins and are preferred. A preferred tetrasubstituted quinone usually has carbon atoms in positions1,2,3, and4.
• In general, electron transfer agents suitable for use in the invention have structures or charges which prevent or substantially reduce the diffusional loss of the electron transfer agent during the period of time that the sample is being analyzed. The preferred electron transfer agents include a redox species bound to a polymer which can in turn be immobilized on the working electrode. The bond between the redox species and the polymer may be covalent, coordinative, or ionic. Useful electron transfer agents and methods for producing them are described in U.S. Pat. Nos. 5,264,104; 5,356,786; 5,262,035; and 5,320,725, incorporated herein by reference. Although any organic or organometallic redox species can be bound to a polymer and used as an electron transfer agent, the preferred redox species is a transition metal compound or complex. The preferred transition metal compounds or complexes include osmium, ruthenium, iron, and cobalt compounds or complexes. The most preferred are osmium compounds and complexes. It will be recognized that many of the redox species described below may also be used, typically without a polymeric component, as electron transfer agents in a carrier fluid or in a sensing layer of a sensor where leaching of the electron transfer agent is acceptable.
• One type of non-releasable polymeric electron transfer agent contains a redox species covalently bound in a polymeric composition. An example of this type of mediator is poly(vinylferrocene).
• Another type of non-releasable electron transfer agent contains an ionically-bound redox species. Typically, this type of mediator includes a charged polymer coupled to an oppositely charged redox species. Examples of this type of mediator include a negatively charged polymer such as Nafion® (DuPont) coupled to a positively charged redox species such as an osmium or ruthenium polypyridyl cation. Another example of an ionically-bound mediator is a positively charged polymer such as quaternized poly(4-vinyl pyridine) or poly(1-vinyl imidazole) coupled to a negatively charged redox species such as ferricyanide or ferrocyanide. The preferred ionically-bound redox species is a highly charged redox species bound within an oppositely charged redox polymer.
• In another embodiment of the invention, suitable non-releasable electron transfer agents include a redox species coordinatively bound to a polymer. For example, the mediator may be formed by coordination of an osmium or cobalt 2,2′-bipyridyl complex to poly(1-vinyl imidazole) or poly(4-vinyl pyridine).
• The preferred electron transfer agents are osmium transition metal complexes with one or more ligands, each ligand having a nitrogen-containing heterocycle such as 2,2′-bipyridine, 1,10-phenanthroline, or derivatives thereof. Furthermore, the preferred electron transfer agents also have one or more ligands covalently bound in a polymer, each ligand having at least one nitrogen-containing heterocycle, such as pyridine, imidazole, or derivatives thereof. These preferred electron transfer agents exchange electrons rapidly between each other and the workingelectrodes58 so that the complex can be rapidly oxidized and reduced.
• One example of a particularly useful electron transfer agent includes (a) a polymer or copolymer having pyridine or imidazole functional groups and (b) osmium cations complexed with two ligands, each ligand containing 2,2′-bipyridine, 1,10-phenanthroline, or derivatives thereof, the two ligands not necessarily being the same. Preferred derivatives of 2,2′-bipyridine for complexation with the osmium cation are 4,4′-dimethyl-2,2′-bipyridine and mono-, di-, and polyalkoxy-2,2′-bipyridines, such as 4,4′-dimethoxy-2,2′-bipyridine. Preferred derivatives of 1,10-phenanthroline for complexation with the osmium cation are 4,7-dimethyl-1,10-phenanthroline and mono, di-, and polyalkoxy-1,10-phenanthrolines, such as 4,7-dimethoxy-1,10-phenanthroline. Preferred polymers for complexation with the osmium cation include polymers and copolymers of poly(1-vinyl imidazole) (referred to as “PVI”) and poly(4-vinyl pyridine) (referred to as “PVP”). Suitable copolymer substituents of poly(1-vinyl imidazole) include acrylonitrile, acrylamide, and substituted or quaternized N-vinyl imidazole. Most preferred are electron transfer agents with osmium complexed to a polymer or copolymer of poly(1-vinyl imidazole).
• The preferred electron transfer agents have a redox potential ranging from −100 mV to about +150 mV versus the standard calomel electrode (SCE). Preferably, the potential of the electron transfer agent ranges from −100 mV to +150 mV and more preferably, the potential ranges from −50 mV to +50 mV. The most preferred electron transfer agents have osmium redox centers and a redox potential ranging from +50 mV to −150 mV versus SCE.
Catalyst
• Thesensing layer64 may also include a catalyst which is capable of catalyzing a reaction of the analyte. The catalyst may also, in some embodiments, act as an electron transfer agent. One example of a suitable catalyst is an enzyme which catalyzes a reaction of the analyte. For example, a catalyst, such as a glucose oxidase, glucose dehydrogenase (e.g., pyrroloquinoline quinone glucose dehydrogenase (PQQ)), or oligosaccharide dehydrogenase, may be used when the analyte is glucose. A lactate oxidase or lactate dehydrogenase may be used when the analyte is lactate. Laccase may be used when the analyte is oxygen or when oxygen is generated or consumed in response to a reaction of the analyte.
• Preferably, the catalyst is non-leachably disposed on the sensor, whether the catalyst is part of a solid sensing layer in the sensor or solvated in a fluid within the sensing layer. More preferably, the catalyst is immobilized within the sensor (e.g., on the electrode and/or within or between a membrane or film) to prevent unwanted leaching of the catalyst away from the workingelectrode58 and into the patient. This may be accomplished, for example, by attaching the catalyst to a polymer, cross linking the catalyst with another electron transfer agent (which, as described above, can be polymeric), and/or providing one or more barrier membranes or films with pore sizes smaller than the catalyst.
• As described above, a second catalyst may also be used. This second catalyst is often used to catalyze a reaction of a product compound resulting from the catalyzed reaction of the analyte. The second catalyst typically operates with an electron transfer agent to electrolyze the product compound to generate a signal at the working electrode. Alternatively, the second catalyst may be provided in an interferent-eliminating layer to catalyze reactions that remove interferents, as described below.
• One embodiment of the invention is an electrochemical sensor in which the catalyst is mixed or dispersed in theconductive material56 which forms theconductive trace52 of a workingelectrode58. This may be accomplished, for example, by mixing a catalyst, such as an enzyme, in a carbon ink and applying the mixture into achannel54 on the surface of thesubstrate50. Preferably, the catalyst is immobilized in thechannel53 so that it cannot leach away from the workingelectrode58. This may be accomplished, for example, by curing a binder in the carbon ink using a curing technique appropriate to the binder. Curing techniques include, for example, evaporation of a solvent or dispersant, exposure to ultraviolet light, or exposure to heat. Typically, the mixture is applied under conditions that do not substantially degrade the catalyst. For example, the catalyst may be an enzyme that is heat-sensitive. The enzyme and conductive material mixture should be applied and cured, preferably, without sustained periods of heating. The mixture may be cured using evaporation or UV curing techniques or by the exposure to heat that is sufficiently short that the catalyst is not substantially degraded.
• Another consideration for in vivo analyte sensors is the thermostability of the catalyst. Many enzymes have only limited stability at biological temperatures. Thus, it may be necessary to use large amounts of the catalyst and/or use a catalyst that is thermostable at the necessary temperature (e.g., 37° C. or higher for normal body temperature). A thermostable catalyst may be defined as a catalyst which loses less than 5% of its activity when held at 37° C. for at least one hour, preferably, at least one day, and more preferably at least three days. One example of a thermostable catalyst is soybean peroxidase. This particular thermostable catalyst may be used in a glucose or lactate sensor when combined either in the same or separate sensing layers with glucose or lactate oxidase or dehydrogenase. A further description of thermostable catalysts and their use in electrochemical inventions is found in U.S. Pat. No. 5,665,222 and in PCT Patent Application Publication No. WO 98/35053.
Electrolysis of the Analyte
• To electrolyze the analyte, a potential (versus a reference potential) is applied across the working andcounter electrodes58,60. The minimum magnitude of the applied potential is often dependent on the particular electron transfer agent, analyte (if the analyte is directly electrolyzed at the electrode), or second compound (if a second compound, such as oxygen or hydrogen peroxide, whose level is dependent on the analyte level, is directly electrolyzed at the electrode). The applied potential usually equals or is more oxidizing or reducing, depending on the desired electrochemical reaction, than the redox potential of the electron transfer agent, analyte, or second compound, whichever is directly electrolyzed at the electrode. The potential at the working electrode is typically large enough to drive the electrochemical reaction to or near completion.
• The magnitude of the potential may optionally be limited to prevent significant (as determined by the current generated in response to the analyte) electrochemical reaction of interferents, such as urate, ascorbate, and acetaminophen. The limitation of the potential may be obviated if these interferents have been removed in another way, such as by providing an interferent-limiting barrier, as described below, or by including a workingelectrode58b(seeFIG. 3A) from which a background signal may be obtained.
• When a potential is applied between the workingelectrode58 and thecounter electrode60, an electrical current will flow. The current is a result of the electrolysis of the analyte or a second compound whose level is affected by the analyte. In one embodiment, the electrochemical reaction occurs via an electron transfer agent and the optional catalyst. Many analytes B are oxidized (or reduced) to products C by an electron transfer agent species A in the presence of an appropriate catalyst (e.g., an enzyme). The electron transfer agent A is then oxidized (or reduced) at the electrode. Electrons are collected by (or removed from) the electrode and the resulting current is measured. This process is illustrated by reaction equations (1) and (2) (similar equations may be written for the reduction of the analyte B by a redox mediator A in the presence of a catalyst):
• 
• As an example, an electrochemical sensor may be based on the reaction of a glucose molecule with two non-leachable ferricyanide anions in the presence of glucose oxidase to produce two non-leachable ferrocyanide anions, two hydrogen ions, and gluconolactone. The amount of glucose present is assayed by electrooxidizing the non-leachable ferrocyanide anions to non-leachable ferricyanide anions and measuring the current.
• In another embodiment, a second compound whose level is affected by the analyte is electrolyzed at the working electrode. In some cases, the analyte D and the second compound, in this case, a reactant compound E, such as oxygen, react in the presence of the catalyst, as shown in reaction equation (3).
• 
• The reactant compound E is then directly oxidized (or reduced) at the working electrode, as shown in reaction equation (4)
• 
• Alternatively, the reactant compound E is indirectly oxidized (or reduced) using an electron transfer agent H (optionally in the presence of a catalyst), that is subsequently reduced or oxidized at the electrode, as shown in reaction equations (5) and (6).
• 
• In either case, changes in the concentration of the reactant compound, as indicated by the signal at the working electrode, correspond inversely to changes in the analyte (i.e., as the level of analyte increase then the level of reactant compound and the signal at the electrode decreases.)
• In other embodiments, the relevant second compound is a product compound F, as shown in reaction equation (3). The product compound F is formed by the catalyzed reaction of analyte D and then be directly electrolyzed at the electrode or indirectly electrolyzed using an electron transfer agent and, optionally, a catalyst. In these embodiments, the signal arising from the direct or indirect electrolysis of the product compound F at the working electrode corresponds directly to the level of the analyte (unless there are other sources of the product compound). As the level of analyte increases, the level of the product compound and signal at the working electrode increases.
• Those skilled in the art will recognize that there are many different reactions that will achieve the same result; namely the electrolysis of an analyte or a compound whose level depends on the level of the analyte. Reaction equations (1) through (6) illustrate non-limiting examples of such reactions.
Temperature Probe
• A variety of optional items may be included in the sensor. One optional item is a temperature probe66 (FIGS. 8 and 11). Thetemperature probe66 may be made using a variety of known designs and materials. Oneexemplary temperature probe66 is formed using two probe leads68,70 connected to each other through a temperature-dependent element72 that is formed using a material with a temperature-dependent characteristic. An example of a suitable temperature-dependent characteristic is the resistance of the temperature-dependent element72.
• The two probe leads68,70 are typically formed using a metal, an alloy, a semimetal, such as graphite, a degenerate or highly doped semiconductor, or a small-band gap semiconductor. Examples of suitable materials include gold, silver, ruthenium oxide, titanium nitride, titanium dioxide, indium doped tin oxide, tin doped indium oxide, or graphite. The temperature-dependent element72 is typically made using a fine trace (e.g., a conductive trace that has a smaller cross-section than that of the probe leads68,70) of the same conductive material as the probe leads, or another material such as a carbon ink, a carbon fiber, or platinum, which has a temperature-dependent characteristic, such as resistance, that provides a temperature-dependent signal when a voltage source is attached to the two probe leads68,70 of thetemperature probe66. The temperature-dependent characteristic of the temperature-dependent element72 may either increase or decrease with temperature. Preferably, the temperature dependence of the characteristic of the temperature-dependent element72 is approximately linear with temperature over the expected range of biological temperatures (about 25 to 45° C.), although this is not required.
• Typically, a signal (e.g., a current) having an amplitude or other property that is a function of the temperature can be obtained by providing a potential across the two probe leads68,70 of thetemperature probe66. As the temperature changes, the temperature-dependent characteristic of the temperature-dependent element72 increases or decreases with a corresponding change in the signal amplitude. The signal from the temperature probe66 (e.g., the amount of current flowing through the probe) may be combined with the signal obtained from the workingelectrode58 by, for example, scaling the temperature probe signal and then adding or subtracting the scaled temperature probe signal from the signal at the workingelectrode58. In this manner, thetemperature probe66 can provide a temperature adjustment for the output from the workingelectrode58 to offset the temperature dependence of the workingelectrode58.
• One embodiment of the temperature probe includes probe leads68,70 formed as two spaced-apart channels with a temperature-dependent element72 formed as a cross-channel connecting the two spaced-apart channels, as illustrated inFIG. 8. The two spaced-apart channels contain a conductive material, such as a metal, alloy, semimetal, degenerate semiconductor, or metallic compound. The cross-channel may contain the same material (provided the cross-channel has a smaller cross-section than the two spaced-apart channels) as the probe leads68,70. In other embodiments, the material in the cross-channel is different than the material of the probe leads68,70.
• One exemplary method for forming this particular temperature probe includes forming the two spaced-apart channels and then filling them with the metallic or alloyed conductive material. Next, the cross-channel is formed and then filled with the desired material. The material in the cross-channel overlaps with the conductive material in each of the two spaced-apart channels to form an electrical connection.
• For proper operation of thetemperature probe66, the temperature-dependent element72 of thetemperature probe66 cannot be shorted by conductive material formed between the two probe leads68,70. In addition, to prevent conduction between the two probe leads68,70 by ionic species within the body or sample fluid, a covering may be provided over the temperature-dependent element72, and preferably over the portion of the probe leads68,70 that is implanted in the patient. The covering may be, for example, a non-conducting film disposed over the temperature-dependent element72 and probe leads68,70 to prevent the ionic conduction. Suitable non-conducting films include, for example, Kapton™ polyimide films (DuPont, Wilmington, Del.).
• Another method for eliminating or reducing conduction by ionic species in the body or sample fluid is to use an ac voltage source connected to the probe leads68,70. In this way, the positive and negative ionic species are alternately attracted and repelled during each half cycle of the ac voltage. This results in no net attraction of the ions in the body or sample fluid to thetemperature probe66. The maximum amplitude of the ac current through the temperature-dependent element72 may then be used to correct the measurements from the workingelectrodes58.
• The temperature probe can be placed on the same substrate as the electrodes. Alternatively, a temperature probe may be placed on a separate substrate. In addition, the temperature probe may be used by itself or in conjunction with other devices.
Biocompatible Layer
• Anoptional film layer75 is formed over at least that portion of thesensor42 which is subcutaneously inserted into the patient, as shown inFIG. 9. Thisoptional film layer74 may serve one or more functions. Thefilm layer74 prevents the penetration of large biomolecules into the electrodes. This is accomplished by using afilm layer74 having a pore size that is smaller than the biomolecules that are to be excluded. Such biomolecules may foul the electrodes and/or thesensing layer64 thereby reducing the effectiveness of thesensor42 and altering the expected signal amplitude for a given analyte concentration. The fouling of the workingelectrodes58 may also decrease the effective life of thesensor42. Thebiocompatible layer74 may also prevent protein adhesion to thesensor42, formation of blood clots, and other undesirable interactions between thesensor42 and body.
• For example, the sensor may be completely or partially coated on its exterior with a biocompatible coating. A preferred biocompatible coating is a hydrogel which contains at least 20 wt. % fluid when in equilibrium with the analyte-containing fluid. Examples of suitable hydrogels are described in U.S. Pat. No. 5,593,852, incorporated herein by reference, and include crosslinked polyethylene oxides, such as polyethylene oxide tetraacrylate.
Interferent-Eliminating Layer
• An interferent-eliminating layer (not shown) may be included in thesensor42. The interferent-eliminating layer may be incorporated in thebiocompatible layer75 or in the mass transport limiting layer74 (described below) or may be a separate layer. Interferents are molecules or other species that are electroreduced or electrooxidized at the electrode, either directly or via an electron transfer agent, to produce a false signal. In one embodiment, a film or membrane prevents the penetration of one or more interferents into the region around the workingelectrodes58. Preferably, this type of interferent-eliminating layer is much less permeable to one or more of the interferents than to the analyte.
• The interferent-eliminating layer may include ionic components, such as Nafion®, incorporated into a polymeric matrix to reduce the permeability of the interferent-eliminating layer to ionic interferents having the same charge as the ionic components. For example, negatively charged compounds or compounds that form negative ions may be incorporated in the interferent-eliminating layer to reduce the permeation of negative species in the body or sample fluid.
• Another example of an interferent-eliminating layer includes a catalyst for catalyzing a reaction which removes interferents. One example of such a catalyst is a peroxidase. Hydrogen peroxide reacts with interferents, such as acetaminophen, urate, and ascorbate. The hydrogen peroxide may be added to the analyte-containing fluid or may be generated in situ, by, for example, the reaction of glucose or lactate in the presence of glucose oxidase or lactate oxidase, respectively. Examples of interferent eliminating layers include a peroxidase enzyme crosslinked (a) using gluteraldehyde as a crosslinking agent or (b) oxidation of oligosaccharide groups in the peroxidase glycoenzyme with NaIO₄, followed by coupling of the aldehydes formed to hydrazide groups in a polyacrylamide matrix to form hydrazones are describe in U.S. Pat. Nos. 5,262,305 and 5,356,786, incorporated herein by reference.
Mass Transport Limiting Layer
• A masstransport limiting layer74 may be included with the sensor to act as a diffusion-limiting barrier to reduce the rate of mass transport of the analyte, for example, glucose or lactate, into the region around the workingelectrodes58. By limiting the diffusion of the analyte, the steady state concentration of the analyte in the proximity of the working electrode58 (which is proportional to the concentration of the analyte in the body or sample fluid) can be reduced. This extends the upper range of analyte concentrations that can still be accurately measured and may also expand the range in which the current increases approximately linearly with the level of the analyte.
• It is preferred that the permeability of the analyte through thefilm layer74 vary little or not at all with temperature, so as to reduce or eliminate the variation of current with temperature. For this reason, it is preferred that in the biologically relevant temperature range from about 25° C. to about 45° C., and most importantly from 30° C. to 40° C., neither the size of the pores in the film nor its hydration or swelling change excessively. Preferably, the mass transport limiting layer is made using a film that absorbs less than 5 wt. % of fluid over 24 hours. This may reduce or obviate any need for a temperature probe. For implantable sensors, it is preferable that the mass transport limiting layer is made using a film that absorbs less than 5 wt. % of fluid over 24 hours at 37° C.
• Particularly useful materials for thefilm layer74 are membranes that do not swell in the analyte-containing fluid that the sensor tests. Suitable membranes include 3 to 20,000 nm diameter pores. Membranes having 5 to 500 nm diameter pores with well-defined, uniform pore sizes and high aspect ratios are preferred. In one embodiment, the aspect ratio of the pores is preferably two or greater and more preferably five or greater.
• Well-defined and uniform pores can be made by track etching a polymeric membrane using accelerated electrons, ions, or particles emitted by radioactive nuclei. Most preferred are anisotropic, polymeric, track etched membranes that expand less in the direction perpendicular to the pores than in the direction of the pores when heated. Suitable polymeric membranes included polycarbonate membranes from Poretics (Livermore, Calif., catalog number 19401, 0.01 μm pore size polycarbonate membrane) and Corning Costar Corp. (Cambridge, Mass., Nucleopore™ brand membranes with 0.015 μm pore size). Other polyolefin and polyester films may be used. It is preferred that the permeability of the mass transport limiting membrane changes no more than 4%, preferably, no more than 3%, and, more preferably, no more than 2%, per ° C. in the range from 30° C. to 40° C. when the membranes resides in the subcutaneous interstitial fluid.
• In some embodiments of the invention, the masstransport limiting layer74 may also limit the flow of oxygen into thesensor42. This can improve the stability ofsensors42 that are used in situations where variation in the partial pressure of oxygen causes non-linearity in sensor response. In these embodiments, the masstransport limiting layer74 restricts oxygen transport by at least 40%, preferably at least 60%, and more preferably at least 80%, than the membrane restricts transport of the analyte. For a given type of polymer, films having a greater density (e.g., a density closer to that of the crystalline polymer) are preferred. Polyesters, such as polyethylene terephthalate, are typically less permeable to oxygen and are, therefore, preferred over polycarbonate membranes.
Anticlotting Agent
• An implantable sensor may also, optionally, have an anticlotting agent disposed on a portion the substrate which is implanted into a patient. This anticlotting agent may reduce or eliminate the clotting of blood or other body fluid around the sensor, particularly after insertion of the sensor. Blood clots may foul the sensor or irreproducibly reduce the amount of analyte which diffuses into the sensor. Examples of useful anticlotting agents include heparin and tissue plasminogen activator (TPA), as well as other known anticlotting agents.
• The anticlotting agent may be applied to at least a portion of that part of thesensor42 that is to be implanted. The anticlotting agent may be applied, for example, by bath, spraying, brushing, or dipping. The anticlotting agent is allowed to dry on thesensor42. The anticlotting agent may be immobilized on the surface of the sensor or it may be allowed to diffuse away from the sensor surface. Typically, the quantities of anticlotting agent disposed on the sensor are far below the amounts typically used for treatment of medical conditions involving blood clots and, therefore, have only a limited, localized effect.
Sensor Lifetime
• Thesensor42 may be designed to be a replaceable component in an in vivo or in vitro analyte monitor, and particularly in an implantable analyte monitor. Typically, thesensor42 is capable of operation over a period of days. Preferably, the period of operation is at least one day, more preferably at least three days, and most preferably at least one week. Thesensor42 can then be removed and replaced with a new sensor. The lifetime of thesensor42 may be reduced by the fouling of the electrodes or by the leaching of the electron transfer agent or catalyst. These limitations on the longevity of thesensor42 can be overcome by the use of abiocompatible layer75 or non-leachable electron transfer agent and catalyst, respectively, as described above.
• Another primary limitation on the lifetime of thesensor42 is the temperature stability of the catalyst. Many catalysts are enzymes, which are very sensitive to the ambient temperature and may degrade at temperatures of the patient's body (e.g., approximately 37° C. for the human body). Thus, robust enzymes should be used where available. Thesensor42 should be replaced when a sufficient amount of the enzyme has been deactivated to introduce an unacceptable amount of error in the measurements.
Manufacturing Process—Substrate and Channel Formation
• FIG. 12 is a schematic illustration of anexemplary system200, in accordance with the principles of the present invention, for manufacturing thesensor42. Thesystem200 utilizes a continuous film orsubstrate web202 that is guided along a serpentine pathway by a series ofrollers206. Along the pathway, theweb202 is processed at the various processing stations or zones. For example, at one station channels can be formed in theweb202. At subsequent stations, conductive material can be placed in the channels, sensor chemistry can be deposited over portions of the conductive material corresponding with working electrodes, and a protective film or micro-porous membrane can be affixed to theweb202. At a final step, thesensor42 can be cut, stamped or otherwise removed from thecontinuous web202. A more detailed description of the various steps is provided in the following paragraphs.
• Thecontinuous substrate web202 ultimately forms thesubstrate50 of thesensor42. Consequently, for certain applications, theweb202 is made of nonconducting plastic or polymeric materials such as those previously identified in the specification with respect to thesubstrate50. In one particular embodiment, theweb202 comprises a continuous plastic or polymeric film having a thickness in the range of 50 to 500 μm (2-20 mil), and preferably in the range of 100 to 300 μm (4-12 mil).
• To initiate the manufacturing process, theweb202 is pulled from asource reel203 and passed through aheater204. As shown inFIG. 12, theheater204 includes two heated platens arranged and configured to allow theweb202 to pass between parallel heated surfaces at a predetermined feed rate and distance. For many applications, theweb202 is heated to a sufficient temperature, for example, to a glass transition temperature of thesubstrate web202 to soften theweb202 in preparation for subsequent embossing or stamping steps.
• With respect to the heating step, it will be appreciated that certain web materials may have sufficient deformability to allow channels to be pressed therein without requiring a heating step. Similarly, if no channels are desired to be formed in theweb202, or channels are to be formed through non-mechanical techniques such as laser or chemical etching, the initial heating step can also be eliminated from the process. Furthermore, if it is desired to soften theweb202 via heat, it will be appreciated that any number of known heating sources/configurations, such as radiant or convection heaters, can be utilized. Alternatively, the forming tool may be heated and not the web.
• After theweb202 has been heated to a desired temperature by theheater204, theweb202 is preferably conveyed to a channel formation station/zone205 where thechannels54 are preferably mechanically pressed into theweb202 by a continuous embossing process. For example, as shown inFIG. 12, thechannels54 of thesensor42 are formed in theweb202 by pressing theweb202 between aflat roller207 and anembossing roller208 having a desired embossing pattern formed on its outer surface. As theweb202 passes between therollers207 and208, a desired channel pattern is stamped, embossed, formed or otherwise pressed into one side of theweb202. During the embossing step, an outline or planform of thesensor42, as shown inFIG. 2, can optionally be pressed into theweb202 to generate perforations that extend partially through theweb202. In one particular embodiment, theweb202 is perforated to a depth of about 70% of the thickness of theweb202. Alternatively, about 70% of the perimeter of the planform is completely perforated. Perforating theweb202 facilitates subsequently removing thesensor42 and provides the advantage of lessening registration constraints at later stages of the manufacturing process.
• FIG. 15A is a cross-sectional view taken through theweb202 immediately after thesensor channels54 have been formed within theweb202. As shown inFIG. 15A, thechannels54 are generally uniformly spaced across the width of theweb202 and have generally rectangular cross-sectional profiles. The width of the channels may be in the range of about 25 to about 250 μm. In one particular embodiment of the present invention, the channels have individual widths of 250 μm (about 8 mils), 150 μm, 100 μm, 75 μm, 50 μm, 25 μm or less. The depth of the channels is typically related to the thickness of theweb202. In one embodiment, the channels have depths in the range of about 12.5 to 75 μm (0.5 to 3 mils), and preferably about 25 to 50 μm (1 to 2 mils). The distance between the conductive traces may be in the range of about 25 to 150 μm, and may be, for example, 150 μm, 100 μm, 75 μm, 50 μm, or less. The density of the conductive traces52 on thesubstrate50 may be in the range of about 150 to 700 ™ and may be as small as 667 μm/trace or less, 333 μm/trace or less, or even 167 μm/trace or less.
• It will be appreciated that embossing rollers suitable for use with the present invention can be designed to form a wide range of different channel patterns. For example,FIG. 13 provides a perspective view of oneembossing roller208 that is adapted for forming the channel configuration of thesensor42. As shown inFIG. 13, the embossing stamp orroller208 includes a pattern of raised members orportions210 that project radially outward from the outer surface of theroller208. The raisedportions210 extend about the circumference of theroller208 and are arranged in a configuration that corresponds to the desired channel configuration shown inFIG. 2. Specifically, the raisedportions210 include generally parallel, relatively closely spaced raisedlines211 corresponding to the channel pattern desired to be formed along thenarrow portion67aof thesensor42. The raisedportions210 also include angled or diverging/converging raisedlines213 corresponding to the channel pattern desired to be formed along thewider portion65aof thesensor42. In certain embodiments, the raisedlines211 and213 have widths less than about 150 microns, preferably less than about 100 microns, and most preferably less than about 50 microns.
• The raisedportions210 further include tabs or punchmembers215 adapted for forming contact pad depressions in which conductive material can be disposed to form thecontact pads49 of thesensor42. When theweb202 is pressed against the outer surface of theroller208, the raisedportions210 project or extend into theweb202 causing theweb202 to deform or indent such that thechannels54 and contact pad depressions are formed within theweb202. In other words, the raisedportions210 of theroller208 form a pattern of depressions in theweb202 that includes such features as thechannels54 and the contact pad depressions.
• As shown inFIG. 13, a single embossing pattern is disposed on the outer surface of theroller208. However, it will be appreciated that by enlarging the diameter of theroller208, multiple identical patterns can be arranged about the circumference of the roller. Furthermore, multiple different patterns can be arranged about the circumference of the roller to allow different sensor configurations to be manufactured with a single embossing roller.
• Referring now toFIG. 14, analternative roller208′ is illustrated. Thealternative roller208′ includes a plurality of raisedannular rings210′ that extend about the circumference of theroller208′. Eachring210′ can extend continuously about the entire circumference of theroller208′, or can be separated into discrete segments by gaps located at predetermined intervals about theroller208′. Theroller208′ is adapted to form a plurality of substantially parallel, straight channels in theweb202. One use of such aroller208′ relates to the manufacture of sensors having substantially constant widths.
• It will be appreciated that embossing tools suitable for use with the present invention, such as rollers, presses or stamps, can be manufactured using a variety of techniques. For example, such tools can be molded, formed or cast using conventional techniques. Exemplary materials for making such embossing tools include steel and other metals, minerals such as sapphire and silicon, epoxides, ceramics, and appropriate polymers.
• In one particular embodiment of the present invention, silicon is used to make an embossing tool such as an embossing roller or stamp. Preferably, a desired pattern of raised portions is formed on the embossing surface of the tool using photolithographic and etching techniques to remove selected portions of the tool. It has been determined that such a process can yield an embossing tool having a desirable surface finish, precisely shaped features at small sizes, no burrs, and sharp features (e.g., small radii between intersecting features).
• Silicon is preferred for a flat (non-cylindrical) tool, and may be etched using techniques common to the integrated circuit industry to create profiles in the wafer surface. Such profiles may be either positive in relief above the surface or negative below the wafer surface. Positive profiles may be used directly as tools to create indentations in a softer substrate. Negative profiles may be used as a master to create a series of second generation positives that are used as the final tool. The second generation positives may be made from any castable material with the appropriate mechanical properties.
Manufacturing Process—Formation of Conductive Traces
• Referring back toFIG. 12, after thechannels54 of thesensor42 have been formed in theweb202, theweb202 is conveyed to a channel filling station/zone210 where conductive material is placed, flowed, applied, filled, flooded or otherwise disposed within thechannels54. For certain applications, the conductive material can be applied as a precursor conductive material having a liquid form. An exemplary precursor conductive material includes conductive material dissolved or suspended in a solvent or dispersant. A preferred precursor conductive material is a carbon based ink that can be flooded in liquid form into thechannels54. Other conductive inks or pastes that include carbon or metal, such as, for example, gold, copper, or silver, may be used. Other techniques for applying the conductive material or precursor conductive material include spraying, coating, flooding, applying with a saturated roller, pumping, as well as electrostatic, ionographic, magnetographic, and other impact and non-impact printing methods.
• After thechannels54 have been substantially filled with conductive material or precursor conductive material, theweb202 is preferably passed through an arrangement/device for scraping or wiping excess conductive material/precursor conductive material from the surface of theweb202. For example, as shown inFIG. 12, acoating blade212 androller214 are used to remove excess material from theweb202. After theweb202 has passed by thecoating blade212 androller214, the conductive material/precursor conductive material substantially fills thechannels54 such that the web and conductive material/precursor conductive material together form a substantially flat or planar surface.
• FIG. 15B shows a cross section through theweb202 after the excess conductive material/precursor conductive material has been wiped from theweb202. While it is preferred for thechannels54 to be substantially filled with the conductive material/precursor conductive material, it will be appreciated that in certain embodiments it may be desirable to only partially fill thechannels54, or to slightly overfill thechannels54 with conductive material/precursor conductive material.
• As shown inFIG. 12, a single series of channel forming, filling and wiping steps are used to fill thechannels54. It will be appreciated that in alternative embodiments, multiple channel formation, filling and wiping steps can be utilized to fill channels formed in thesubstrate50. For example, to manufacture thesensor42 ofFIG. 2, it may be desirable to utilize two separate channel formation steps, and two separate filling and wiping steps. In such a process, the reference electrode channel could initially be formed in the substrate, and then filled with a suitable conductive material such as silver/silver chloride. Subsequently, the working electrode channels of thesensor42 could be formed in the substrate and filled with a conductive material such as carbon. Separating the various channel formation, filling and wiping steps can assist in inhibiting cross contamination of conductive materials between the various electrodes. Of course, the particular sequence of processing steps identified herein are strictly exemplary and should not be construed as a limitation upon the scope of the present invention.
Manufacturing Process—Other Methods for Forming Conductive Traces
• In addition to the above identified mechanical techniques for forming thechannels54 in theweb202, other techniques can also be utilized. For example, the channels can be formed by removing or carbonizing a portion of thesubstrate50 orweb202 using a laser, or photolithographic patterning and etching of thesubstrate50 orweb202. Furthermore, for certain applications, channels may not be formed in thesubstrate50 orweb202 at all. For example, as discussed above, the conductive traces52 can be formed on thesubstrate50 by a variety of techniques, including photolithography, screen printing, other printing techniques, stamping traces into the substrate orweb202, or using a laser to micro-machine traces into thesubstrate50 orweb202. Each of these techniques has corresponding limits on the reproducibility, precision, and cost of producing the conductive traces.
• Another method for forming the conductive traces uses techniques common to pad printing or hot stamping methods, whereby a film of conductive material is formed, for example, as a continuous sheet or as a coating layer deposited on a carrier film. The film of conductive material is brought between a print head and the substrate500. A pattern ofconductive traces52 is formed on thesubstrate50 using the print head. The conductive material is transferred by pressure and/or heat from the conductive film to thesubstrate50. This technique may produce channels (e.g., depressions caused by impact of the print head on the substrate50). Alternatively, the conductive material is deposited directly without forming substantial depressions in the surface of thesubstrate50.
• In other embodiments, the conductive traces52 are formed by non-impact printing techniques. These methods do not require the formation of channels in the substrate. Instead, conductive traces may be formed directly on a planer substrate. Such techniques include electrophotography and magnetography. In these processes, an image of the conductive traces52 is electrically or magnetically formed on a drum. A laser or LED may be used to electrically form the image or a magnetic recording head may be used to magnetically form the image. A toner material (e.g., a conductive material, such as a conductive ink) is then attracted to portions of the drum according to the image. The toner material is then applied to the substrate by contact between the drum and the substrate. For example, the substrate may be rolled over the drum. The toner material may then be dried and/or a binder in the toner material may be cured to adhere the toner material to the substrate.
• Another non-impact printing technique includes ejecting droplets of conductive material onto the substrate in a desired pattern. Examples of this technique include ink jet printing and piezo jet printing. An image is sent to the printer which then ejects the conductive material (e.g., a conductive ink) according to the pattern. The printer may provide a continuous stream of conductive material or the printer may eject the conductive material in discrete amounts at the desired points.
• Yet another embodiment of forming the conductive traces includes an ionographic process. In this process, a curable, liquid precursor, such as a photopolymerizable acrylic resin (e.g., Solimer 7501 from Cubital, Bad Kreuznach, Germany), is deposited over a surface of asubstrate50. A photomask having a positive or negative image of the conductive traces52 is then used to cure the liquid precursor. Light (e.g., visible or ultraviolet light) is directed through the photomask to cure the liquid precursor and form a solid layer over the substrate according to the image on the photomask. Uncured liquid precursor is removed leaving behindchannels54 in the solid layer. Thesechannels54 can then be filled withconductive material56 to form conductive traces52.
Manufacturing Process—Drying and Curing
• Once theweb202 has been wiped by thecoating blade212 androller mechanism214, theweb202 is moved through a dryingchamber216. The dryingchamber216 preferably provides sufficient heat to drive off or evaporate solvents or dispersants that may be contained in precursor conductive material within thechannels54. After heating, conductive material is preferably left as a residue in thechannels54. In certain cases, the dryingchamber216 exposes theweb202 to sufficient temperatures to cure optional binders that may be present with the conductive material. It will be appreciated that ultraviolet light could also be used to cure optional binders interspersed with the conductive material.
Manufacturing Process—Sensor Chemistry Deposition
• After theweb202 has been heated in theheating chamber216, theweb202 is directed to a sensor chemistry deposition station/zone218 at which sensor chemistry is deposited, placed, or otherwise disposed over portions of the conductive material within thechannels54 so as to form the sensing layers64 over the workingelectrodes58.FIG. 15C is a cross-sectional view cut through theweb202 after the sensor chemistry has been deposited on theweb202. As shown inFIG. 15C, sensor chemistry is only deposited over the conductive material corresponding to the workingelectrodes58, which in one embodiment, as illustrate inFIG. 4A, are formed at the twoouter channels54. Consequently, a relatively precise application technique is preferably used to inhibit sensor chemistry from being applied to both the workingelectrodes58 and electrodes that should not be coated. It is acceptable, in some situations, for the sensing layer to also coat thecounter electrode60.
• It will be appreciated that a variety of techniques can be used to apply or deposit the sensor chemistry on theweb202. In one particular embodiment of the present invention, piezo jet technology or the like is used to deposit the chemistry upon theweb202 to form the sensing layers64. A solenoid valve can be rapidly shuttered and when supplied with liquid under a precisely controlled over-pressure condition, a droplet of controlled size will be ejected from the valve. Resolutions to 500 picoliters can be achieved. Conventional ink jet printers can also be used.
• To enhance adhesion of the sensor chemistry to theweb202, the surface of theweb202 can optionally be roughened by techniques such as abrasion or plasma treatment prior to applying the sensor chemistry. For example, by pre-treating the surface of theweb202, for example, by a corona discharge, free radicals are generated on the web surface to enhance adhesion of the sensor chemistry to theweb202 and workingelectrodes58.
• Once the sensor chemistry has been applied to theweb202, theweb202 is preferably conveyed through anotherheating chamber220. Theheating chamber220 preferably provides sufficient temperature/heating to release solvents from the deposited sensor chemistry. Theheating chamber220 can also heat theweb202 to sufficient temperatures to cause potential polymerization reactions such as cross link reactions between polymers and the redox mediator and/or redox enzyme.
Manufacturing Process—Membrane Layer
• Upon exiting theheating station220, thesubstrate web202 is brought into alignment with amembrane web222 adapted for forming a membrane layer, that may include one or more individual membranes, such as a masstransport limiting layer74 or abiocompatible layer75, over at least some portions of the electrodes. The membrane layer may be applied to only one or two or more surfaces of the substrate. For certain embodiments, solvents such as methyl ethyl ketone and acetone can be applied, for example, sprayed, on theweb202 to soften theweb202 and solvent bond it to themembrane web222. By heating the solvent after theweb202 has been brought in contact with themembrane web222, the twowebs202 and222 can be bonded together such that theweb222 covers and protects portions of the sensor adapted to be implanted. Alternatively, the twowebs202 and222 can be bonded or fused together at awelding station224 such as a sonic or laser welding station. The resultant combination of thesubstrate web202 and themembrane web222 results in a laminated structure in which theprotective membrane74 is selectively fused to thepolymer substrate50. In some embodiments,individual membrane webs222 are bonded to two or more surfaces of theweb202.
• The membrane layer may include one or membranes that individually or in combination serve a number of functions. These include protection of the electrode surface, prevention of leaching of components in the sensing layer, mass transport limitation of the analyte, exclusion of interfering substances, reduction or enhancement of oxygen mass transport, and/or biocompatibility. In one embodiment, a membrane is selected which has mass transport limiting pores that do not change appreciably in size over a physiologically relevant temperature range (e.g., 30° C. to 40° C.). This may reduce the temperature dependence of the sensor output.
Manufacturing Process—Cutting
• As a final step in thesequence200, thelaminated webs202 and222 enter a cutting station/zone226 in which thesensor42 planform, as shown inFIG. 2, is cut from thecontinuous webs202 and222. For example, the cuttingstation226 can include a die stamper, embosser, embossing roller, laser cutter or any other mechanism for cutting, pressing or otherwise removing thesensors42 from thewebs202 and204. This cutting step may result in discrete sensor components or the sensors may be partially cut out and retained on the webs for secondary operations such as surface mounting of electronic components or packaging. A take-upreel230 accumulates the web material remaining after thesensors42 have been cut from the web.
Multiple Traces/Multiple Surfaces
• FIG. 16 is a schematic illustration of anexemplary system300, in accordance with the principles of the present invention, for manufacturing thesensor42 ofFIGS. 6-8 and10-11. Thesystem300 utilizes a continuous film orweb302 that is guided along a serpentine pathway by a series ofrollers305. To provide channels on opposite sides of theweb302, the system utilizes a series of embossing steps. For example, thesystem300 includes afirst embossing roller308 configured for forming the channels for the working andcounter electrode58,60, respectively, in a first side of theweb302, asecond embossing roller310 configured for forming the channel for the temperature probe/sensor66 and thereference electrode62 in a second opposite side of theweb302, and athird embossing roller312 configured for forming the channel for the temperature-dependent element72 extending between the channels for the two temperature probe leads68,70. In a preferred embodiment, opposing embossing rollers are used to emboss both sides simultaneously in a single step.
• In basic operation of the system, theweb302 is first pulled from a spool or reel301 and preferably heated. Next, the channels for the working electrode andcounter electrodes58,60 are formed in the first side of theweb302 by thefirst embossing roller308. It will be appreciated that thefirst embossing roller308 preferably includes a pattern of raised portions having a configuration that corresponds to the channel configuration depicted inFIG. 7. Thereafter, the channels of the working andcounter electrodes58,60 are filled with conductive material/precursor conductive material, such as a flowable conductive carbon ink, at a firstchannel filling station314. Subsequently, excess conductive material/precursor conductive material is wiped from theweb302 by a firstweb wiping arrangement316.
• Once the channels for the working andcounter electrodes58,60 have been filled with conductive material/precursor conductive material and wiped, the opposite second side of theweb302 is embossed by thesecond embossing roller310 such that the channels for the temperature probe leads68,70 and thereference electrode62 are formed in the opposite side of theweb302. It will be appreciated that thesecond embossing roller310 preferably includes a pattern of raised portions having a configuration that corresponds to the channel configuration depicted inFIG. 8 (except for channel for the temperature-dependent element72). It will also be appreciated that theembossing roller310 can be equipped with projections or punch members for forming vias through theweb302 at desiredpad49 locations of thesensor42.
• After the channels for the temperature probe leads68,70 andreference electrode72 have been formed in theweb202, such channels are filled with suitable conductive material/precursor conductive material at a secondchannel filling station318 and excess conductive material/precursor conductive material is wiped from theweb302 at wipingmechanism320. While onefilling station318 is shown for filling both channels for the temperature probe leads68,70 and thereference electrode62, it will be appreciated that the fillingstation318 may include multiple separate filling steps for individually or separately filling each channel.
• Once the channels for the temperature probe leads68,70 andreference electrode62 have been filled with conductive material/precursor conductive material and wiped, the channel for the temperature-dependent element72 of thetemperature probe66 is formed between the channels for the temperature probe leads68,70 by thethird embossing roller312. Subsequently, the channel for the temperature-dependent element72 is filled with appropriate material atchannel filling station322, and excess material is wiped from theweb302 by wipingmechanism324.
• Once both sides of theweb302 have been filled with the appropriate conductive and/or resistive material, sensor chemistry is applied to the workingelectrodes58 at a sensorchemical application station326. The sensor chemistry can be applied at the sensorchemical application station326 by a variety of techniques. Exemplary techniques include piezo jet printing, ink jet printing, spraying, flowing the sensor chemistry onto the electrodes, coating chemistry on the electrodes, or any other technique suitable for applying chemistry to a relatively precise location. As shown inFIG. 7, to reduce the required printing precision, the workingelectrodes58 optionally have ends that are staggered with respect to the end of thecounter electrode60. Such a configuration assists in inhibiting the sensor chemistry from unintentionally being applied to thecounter electrode60.
• As a next step in the process, aprotective membrane web328 is then bought into contact with thesubstrate web302 such that at least portions of the working andcounter electrodes58 and60 are covered by themembrane328. Atmembrane bonding station330, theprotective membrane328 and thesubstrate web302 are bonded or fused together by techniques such as solvent bonding, adhesive bonding, laser bonding, laser welding, and/or sonic welding. In the case of solvent bonding, the solvent is applied before the protective membrane is brought into contact with the substrate web. A second membrane may optionally be laminated onto the opposing side of the substrate web to protect the reference electrode and temperature probe. The resulting laminate structure that exits themembrane bonding station330 is conveyed to a cuttingstation332 in which individual discrete planforms of thesensor42″ are cut, pressed, stamped or otherwise separated from thecontinuous web302. For certain applications, it may be desirable to only partially cut the individual sensor planforms from theweb302 such that the sensors are retained on the web for secondary operations. Remaining web material is taken up by take-upreel334.
• It will be appreciated that the particular operating sequence illustrated inFIG. 16 is strictly exemplary and that variations can be made in the number of steps and the sequence of steps without departing from the principles of the present invention. Additionally, although not shown inFIG. 16, various heating or energy dispersive stations can be placed at locations along the web pathway to heat theweb302 for such purposes as plasticizing thesubstrate web302 prior to embossing, curing binders contained within conductive material deposited within the channels of the sensors, and evaporating solvents or dispersants. Furthermore, althoughFIGS. 12 and 16 each relate to continuous web processes, it will be appreciated that the present invention is not limited to continuous web processes. For example, the various process steps disclosed herein can be performed with respect to discrete or individual sensors completely separate from a web. Sheet fed processing may also be employed as an alternative to a continuous web. Moreover, while in certain embodiments of the present invention the web can be moved continuously through various processing steps at a substantially constant speed, in other embodiments the web can be intermittently stopped and started, or the speed of the web can be varied.
• The process of the invention for the manufacture of sensors is rapid and efficient. The process of the invention can produce approximately 5000 conductive traces per hour. Within batch variation of the sensors will be less than between batch variation, thus it is desirable to produce the sensors in large batches. For example, batches of 100 or more or of 1000 or more sensors may be produced.
• The sensor may be provided with a code, for example a batch code, during processing. The code may be applied to the sensor, for example by printing the code on the substrate. The sensor code may include information such as the batch number, the type and quantity of chemistry applied to the sensor, and/or calibration data.
• The present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the instant specification. The claims are intended to cover such modifications and devices.

Claims (2)

1. An in vivo glucose sensor, the sensor comprising:

a working electrode having an end; and

a counter electrode having an end that is staggered with respect to the end of the working electrode; and

sensor chemistry comprising a catalyst,

wherein the sensor is capable of operation for at least three days.

2. The method ofclaim 1, wherein each produced sensor is capable of operation for a period of at least one week.

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