US20110091817A1 - Sensor with layered electrodes - Google Patents

Abstract

A thin film sensor, such as a glucose sensor, is provided for transcutaneous placement at a selected site within the body of a patient. The sensor includes several sensor layers that include conductive layers and includes a proximal segment defining conductive contacts adapted for electrical connection to a suitable monitor, and a distal segment with sensor electrodes for transcutaneous placement. The sensor electrode layers are disposed generally above each other, for example with the reference electrode above the working electrode and the working electrode above the counter electrode. The electrode layers are separated by dielectric layer.

Description

RELATED APPLICATION DATA
• This is a continuation of U.S. patent application Ser. No. 12/229,206, filed Aug. 20, 2008, now U.S. Pat. No. ______, which is a divisional of U.S. patent application Ser. No. 11/234,523, filed Sep. 23, 2005, now U.S. Pat. No. 7,725,148, all of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• This invention relates generally to a sensor and methods for manufacturing a sensor for placement at a selected site within the body of a patient. More specifically, this invention relates to the positioning of electrodes in an improved flexible thin film sensor of the type used, for example, to obtain periodic blood glucose (BG) readings.
• 2. Description of Related Art
• Thin film electrochemical sensors are generally known in the art for use in a variety of specialized sensor applications. Such thin film sensors generally comprise one or more thin conductors applied by photolithography mask and etch techniques between thin layers of a nonconductive film material, such as polyimide film. The conductors are shaped to define distal segment ends having an appropriate electrode material thereon, in combination with proximal end contact pads adapted for conductive connection with appropriate electronic monitoring equipment. In recent years, thin film sensors of this general type have been proposed for use as a transcutaneous sensor in medical applications. As one example, thin film sensors have been designed for use in obtaining an indication of BG levels and monitoring BG levels in a diabetic patient, with the distal segment portion of the electrodes positioned subcutaneously in direct contact with patient blood. Such readings can be especially useful in adjusting a treatment regimen which typically includes regular administration of insulin to the patient. In this regard, BG readings are particularly useful in conjunction with semiautomated medication infusion pumps of the external type, as generally described in U.S. Pat. Nos. 4,562,751; 4,678,408; and 4,685,903; or automated implantable medication infusion pumps, as generally described in U.S. Pat. No. 4,573,994.
• Relatively small and flexible electrochemical sensors have been developed for subcutaneous placement of sensor electrodes in direct contact with patient blood or other extracellular fluid, wherein such sensors can be used to obtain periodic readings over an extended period of time. In one form, flexible transcutaneous sensors are constructed in accordance with thin film mask techniques wherein an elongated sensor includes thin film conductive elements encased between flexible insulative layers of polyimide sheet or similar material. Such thin film sensors typically include exposed electrodes at a distal segment for transcutaneous placement in direct contact with patient blood or the like, and exposed conductive contacts at an externally located proximal segment end for convenient electrical connection with a suitable monitor device. Such thin film sensors hold significant promise in patient monitoring applications, but unfortunately have been difficult to place transcutaneously with the sensor electrodes in direct contact with patient blood or other extracellular fluid. Improved thin film sensors and related insertion sets are described in commonly assigned U.S. Pat. Nos. 5,299,571, 5,390,671; 5,391,250; 5,482,473; 5,568,806; and 5,586,553 and International Publication No. WO 2004/036183, which are incorporated by reference herein.
BRIEF SUMMARY OF THE INVENTION
• The present invention relates specifically to an improved sensor adapted to have a thin configuration for quick and easy placement of the film sensor on a patient with sensor electrodes in direct contact with patient blood or other extracellular fluid.
• In accordance with embodiments of the invention, a sensor, such as a flexible thin film electrochemical sensor, is provided that may be placed at a selected site within the body of the patient. In certain embodiments, the sensor includes several electrodes, configured so that the overall size of the sensor is thinner than traditional sensors. In an embodiment of the present invention, the sensor includes electrodes in electrode layers positioned generally above each other. The electrodes and traces from the electrodes to contact pads, which are adapted to connect to sensor electronics, may be horizontally displaced from each other with other materials layered in between. The electrodes themselves may be in a staggered configuration so that the lower electrodes extend further, allowing portions of the electrodes to be exposed. Each of the electrodes may also be of the same size or different sizes. In the layered configuration, the electrode layers are staggered to expose a part of each electrode to contact the patient fluid.
• In further embodiments of the invention, the electrodes may include a working electrode and a counter electrode and may further include a reference electrode. Alternatively, the electrodes may include more or fewer electrodes, depending on the desired use. The electrodes may comprise gold and chrome and/or other adhesive/conductive layers, such as titanium, platinum, tungsten, etc. The working and counter electrodes may be plated with platinum black and the reference electrode may be plated with silver and silver chloride. For glucose sensing, the sensor may include a layer of glucose oxidase, which may be mixed with albumin. Over the glucose oxidase may be a glucose limiting membrane, such as one that includes a polyamine, such as polyoxypropylene-diamine sold under the trademark JEFFAMINE®, and polydimethylsiloxane. There may be a hydrophilic membrane over the glucose limiting membrane.
• In an embodiment of the invention, a subcutaneous insertion set is provided for placing the sensor at a selected site within the body of a patient. The insertion set comprises the sensor and further comprises a slotted insertion needle extending through a mounting base adapted for seated mounting onto the patient's skin. The flexible thin film sensor includes a proximal segment carried by the mounting base, and a distal segment protruding from the mounting base and having one or more sensor electrodes thereon. The distal segment of the sensor is carried within a protective cannula which extends from the mounting base with a portion of the cannula being slidably received within the insertion needle. One or more apertures formed in the cannula are positioned in general alignment with the staggered sensor electrodes on the sensor distal segment.
• In embodiments of the invention, when the mounting base is pressed onto the patient's skin, the insertion needle pierces the skin to transcutaneously place the cannula with the sensor distal segment therein. The insertion needle can be withdrawn from the mounting base, leaving the cannula and sensor distal segment within the patient, with the sensors electrodes thereon exposed through the aperture or apertures for direct contact with to patient fluid at the selected position within the patient, such as a subcutaneous, intravascular, intramuscular, or intravenous site. Other sites may include intraorgan and interperitoneal sites. Conductive contacts on the sensor proximal segment end can be electrically connected to a suitable monitor device so that appropriate blood chemistry readings can be taken.
• In further embodiments of the invention, during insertion, the insertion needle and the protective cannula cooperatively protect and guide the sensor to the desired transcutaneous placement position. The insertion needle can then be withdrawn, whereupon the slotted needle geometry permits the insertion needle to slide over and longitudinally separate from the second portion of the cannula, thereby leaving the cannula and sensor therein at the selected insertion site.
• Other features and advantages of the present invention will become more apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
• A detailed description of embodiments of the invention will be made with reference to the accompanying drawings, wherein like numerals designate corresponding parts in the figures.
• FIG. 1ais an enlarged fragmented sectional view of a sensor according to an embodiment of the invention;
• FIG. 1bis an enlarged fragmented sectional view corresponding generally with a first and second electrode layer of a sensor according to an embodiment of the invention;
• FIG. 1cis an enlarged fragmented sectional view corresponding generally with a first, second, and third electrode layer of a sensor according to an embodiment of the invention;
• FIG. 2 is an enlarged side view of a sensor according to an embodiment of the invention;
• FIG. 3 is an enlarged cross-sectional view taken generally on the line2-2 ofFIG. 1c;
• FIG. 4 is an enlarged cross-sectional view taken generally on the line3-3 ofFIG. 1c;
• FIG. 5 is a perspective view illustrating a transcutaneous sensor insertion set according to an embodiment of the invention;
• FIG. 6 is an enlarged longitudinal vertical section taken generally on the line2-2 ofFIG. 5 according to an embodiment of the invention;
• FIG. 7 is an exploded perspective view illustrating a plurality of thin film electrochemical sensors formed on a rigid flat substrate according to an embodiment of the invention.
DETAILED DESCRIPTION
• In the following description, reference is made to the accompanying drawings which form a part hereof and which illustrate several embodiments of the present inventions. It is understood that other embodiments may be utilized and structural and operational changes may be made without departing from the scope of the present inventions.
• In embodiments of the present invention, a method is provided for producing electrochemical sensors of the type used, for example, in subcutaneous or transcutaneous monitoring of analytes in a patient. For example, they may be used for monitoring of blood glucose levels in a diabetic patient. The sensors of the invention may also be used for sensing other analytes, such as lactate. While certain embodiments of the invention pertain to glucose sensors, the structure of the sensor disclosed and methods of creating the sensor can be adapted for use with any one of the wide variety of sensors known in the art. A number of enzyme sensors (e.g., glucose sensors that use the enzyme glucose oxidase to effect a reaction of glucose and oxygen) are known in the art. See, for example, U.S. Pat. Nos. 5,165,407, 4,890,620, 5,390,671 and 5,391,250, and International Publication No. WO 2004/036183, which are herein incorporated by reference. Sensors for monitoring glucose concentration of diabetics are further described in Schichiri, et al., “In Vivo Characteristics of Needle-Type Glucose Sensor-Measurements of Subcutaneous Glucose Concentrations in Human Volunteers,” Horm. Metab. Res., Suppl. Ser. 20:17-20 (1988); Bruckel, et al., “In Vivo Measurement of Subcutaneous Glucose Concentrations with an Enzymatic Glucose Sensor and a Wick Method,” Klin. Wochenschr. 67:491-495 (1989); and Pickup, et al., “In Vivo Molecular Sensing in Diabetes Mellitus: An Implantable Glucose Sensor with direct Electron Transfer,” Diabetologia 32:213-217 (1989), which are herein incorporated by reference. Other sensors are described, for example, in Reach, et al., ADVANCES IN IMPLANTABLE DEVICES, A. Turner (ed.), JAI Press, London, Chap. 1, (1993), which is herein incorporated by reference.
• The electrochemical sensors of embodiments of the invention are film sensors that include several electrodes, configured so that the overall size of the sensor is thinner than traditional sensors. In further embodiments of the present invention, the sensor includes three electrodes that are each positioned generally above the other, although the term “above” is intended to mean generally disposed in a plane vertically on top of each other, not necessarily directly over or disposed on one another. For example, the electrodes may be above and adjacent to another electrode, such as offset horizontally. The sensor of the invention may have only two electrodes or more than three electrodes. In an embodiment with three electrodes, each of the three electrodes may be of the same size or different sizes. In particular embodiments, the electrode layers are staggered to expose at least a portion of each electrode to contact the patient fluid. The three sensor electrodes may all serve different functions. For example, there may be a working electrode, a counter electrode, and a reference electrode. The reference electrode facilitates the filtering out of background chemical reactions that could detract from a correct reading of the BG level. In between layers of the sensor electrodes, a layer of insulation or dielectric material may be spread so that there is no communication between the individual electrodes.
• The exposed portions of the electrodes are coated with a thin layer of material having an appropriate chemistry. For example, an enzyme such as glucose oxidase, glucose dehydrogenase, or hexokinase can be disposed on the exposed portion of the sensor element within an opening or aperture defined in a cover layer.
• FIGS. 1a-cillustrate longitudinal cross-sections of an embodiment of the invention where the various layers of thedistal end16 of the sensor12 (shown inFIG. 2), specifically the sensor layers64a,64b, and64c, are shown. In the illustrated embodiment, there are three metallization steps taken to form theelectrodes18a,18b, and18c. The metallization process forms one or more conductive layers/electrode layers54a,54b, and54con top of abase layer42. Thebase layer42 is generally an electrically insulating layer such as a polyimide substrate, which may be self-supporting or further supported by another material. In one embodiment, thebase layer42 comprises a polyimide tape, dispensed from a reel. Providing thebase layer42 in this form can facilitate clean, high density mass production. Further, in some production processes using such a polyimide tape, sensors can be produced on both sides of a tape.
• FIG. 2 illustrates an enlarged fragmented sectional view of a sensor according to an embodiment of the invention. Theelectrodes18a,18b, and18cat thedistal end16 of thesensor12 lead throughtraces48a,48b, and48ctoconductive pads21a,21b, and21cat aproximal end20 of the sensor. Although the figures and the description below describes the formation and structure of a sensor with three electrodes, the method and structure can be used in a sensor of fewer or more electrodes.
• The first metallization step, shown inFIG. 1a, applies theconductive layer54aonto theinsulative base layer42. The conductive layer may be provided as a plurality of thin film conductive layers, such as an initial chrome-based layer suitable for chemical adhesion to the base layer, followed by subsequent formation of a thin film gold-based layer. Optionally, a chrome-based top layers may be formed on top of the thin film gold-based layer. The conductive layer may also be formed of gold and/or chrome in different ratios and/or other adhesive/conductive layers, such as titanium, platinum, tungsten, etc. In alternative embodiments, other electrode layer conformations or materials can be used. Theconductive layer54acan be applied using electrode deposition, surface sputtering or another suitable process step. The electrical circuit of each conductive layer typically comprises one or more conductive paths with regions at a proximal end to form contact pads and regions at a distal end to form sensor electrodes. Generally, etching is performed to define the electrical circuit of each layer. Alternatively, “lift off” may be used, in which the photoresist defines a pattern prior to metal sputtering, after which the photoresist is dissolved away (along with the unwanted metal), and the metal pattern is left behind. In further embodiments, photoresisting is performed to protect the metallized trace and electrode and photoimaging is performed to cure specified areas. For example, the conductive layer is covered with a selected photoresist coating, followed by an etch step resulting in one or more conductive paths. An electrically insulative cover layer (or dielectric layer)44a, such as a polymer coating, is then applied over at least portions of theconductive layer54a. Acceptable polymer coatings for use as theinsulative cover layer44ainclude, for example, non-toxic biocompatible polymers such as polyimide, biocompatible solder masks, epoxy acrylate copolymers, and the like. Further, these coatings can be photoimageable to facilitate photolithographic forming of apertures through to theconductive layer54ato expose theelectrode18a. This first metallization step is finished by developing and rinsing the producedelectrode18a. In an embodiment thiselectrode18ais the counter electrode. Alternatively, theelectrode18amay be a working or reference electrode.
• The second metallization step, shown inFIG. 1b, applies a secondconductive layer54bover the firstinsulative cover layer44aand covering and repeats the process of covering the secondconductive layer54bwith anotherinsulative cover layer44b. This produces anotherelectrode18b, positioned generally above thefirst electrode18a. In an embodiment,electrode18bis the working electrode. Alternatively, theelectrode18bmay be a counter or working electrode. The third metallization step, shown inFIG. 1c, repeats all of the previous steps to form athird electrode18c. In an embodiment,electrode18cis the reference electrode. Alternatively, theelectrode18cmay be a working or counter electrode. As shown inFIG. 1c, the electrodes are in a staggered configuration, so that at least a portion of each electrode may be exposed. Theconductive layers54a,54band54c, may be directly above each other or horizontally displaced from each other (into and out of the page). The electrodes may further be configured in any way that allows the electrodes to contact fluid when inserted into a body of a patient.
• Thesensor12 is thus shown with the subsequentconductive layers54a,54b, and54calternating with the insulative layers44a,44b, and44c. In between every two conductive layers there is an insulative layer that serves to isolate each conductive layer so that there is no trace communication between the layers. Apertures19a,19b, and19care formed in the topinsulative cover layer44c. Although theelectrodes18a,18b, and18care shown as lying on top of each other, it is also possible to have them generally above each other, but spaced sideways so that they are not directly on top of each other (e.g., horizontally displaced). This is also true for the traces that lead to conductive contacts, which electrically connect to the sensor electronics, at the opposite end of the sensor from the electrodes. The apertures can be made through photolithographic development, laser ablation, chemical milling, etching, or the like. The exposed electrodes and/or contact pads can also undergo secondary processing through the apertures, such as additional plating processing, to prepare the surfaces, and/or strengthen the conductive regions.
• As shown inFIG. 4, typically, asensor chemistry layer72 is disposed on one or more of the exposed electrodes (e.g.,18a) of the conductive layers. In certain embodiments, thesensor chemistry layer72 is an enzyme layer, for example, glucose oxidase. If the enzyme layer is glucose oxidase, it reacts with glucose to produce hydrogen peroxide, which modulates a current to the electrode which can be monitored to measure an amount of glucose present. Thesensor chemistry layer72 can be applied over portions of thesensor12 or over the entire sensor, including the protective layer (e.g.,44a). Thesensor chemistry layer72 is generally disposed on at least portions of a working electrode. In further embodiments, the sensory chemistry layer may be disposed on at least portions of other electrodes, such as a counter electrode. For example, if theelectrode18ainFIG. 4 is the counter electrode, then thesensor chemistry layer72 is disposed on at least portions of theelectrode18a. Example methods for generating the sensor chemistry layer include spin coating processes, dip and dry processes, low shear spraying processes, ink-jet printing processes, silk screen processes, casting process, and the like.
• In certain embodiments, thesensor chemistry layer72 comprises glucose oxidase and a carrier protein. The glucose oxidase and carrier protein may be in a substantially fixed ratio. In further embodiments, the glucose oxidase and the carrier protein are distributed in a substantially uniform manner throughout the disposed enzyme layer. Typically, the carrier protein comprises albumin, generally in an amount of about 2-10% by weight, preferably about 5% by weight. As used herein, “albumin” refers to those albumin proteins typically used by artisans to stabilize polypeptide compositions, such as human serum albumin, bovine serum albumin, and the like. The application of the glucose oxidase and albumin mixture may be made, for example, by a spin coating process, a casting process, a screen printing process or a doctor blading process. Optionally, the glucose oxidase layer that is formed on the sensor is less than 2 microns in thickness. In further embodiments, the glucose oxidase layer may be less than 1, 0.5, 0.25 or 0.1 microns in thickness. The choice of the glucose oxidase layer thickness may be made to balance fast response and fast hydration verses a sensor lifetime. Generally, thin layers hydrate and respond more quickly, but do not last as long. Thick layers last a long time, but hydrate more slowly and respond to glucose more slowly.
• Thesensor chemistry layer72 may be coated with one or more cover layers. In certain embodiments, as shown inFIG. 4, thecover layer74 comprises a membrane that can regulate the amount of analyte that can contact the enzyme of thesensor chemistry layer72. For example, thecover layer74 can comprise a glucose limiting membrane, which regulates the amount of glucose that contacts the glucose oxidase enzyme layer on an electrode. Such glucose limiting membranes can be made from a wide variety of materials suitable for such purposes, such as, for example, silicone, polyurethane, cellulose acetate, Nafion, polyester sulfonic acid (Kodak AQ), hydrogels, etc. In further embodiments the glucose limiting membrane includes a polyamine, such as polyoxypropylene-diamine sold under the trademark JEFFAMINE®, and polydimethylsiloxane (Structure I) and/or a polysilane, such as polydimethylsiloxane (PDMS) (Structure II). In still further embodiments, the glucose limiting membrane is a random block copolymer made from JEFFAMINE® and PDMS. In further embodiments, the glucose limiting membrane may also, or alternatively, be a mechanically limiting membrane. For example, a glucose limiting membrane may be used that is an oxygen passing/glucose limiting polymer, such as silicone, and a small window with the correct ratio size may be cut into the polymer to meter glucose directly to the sensor surface.
• 
• In still further embodiments, the JEFFAMINE®, which is glucose permeable, and the PDMS, which is non-glucose permeable but oxygen permeable, are linked together with diisocyanide. By using this random block copolymer, an excess of oxygen by the glucose oxidase and the electrodes can be ensured along with a restricted amount of glucose.
• In further embodiments, an adhesion promoter (not shown) is provided between theglucose limiting membrane74 and thesensor chemistry layer72 to facilitate contact and/or adhesion. The adhesion promoter layer can be made from any one of a wide variety of materials that facilitates bonding, for example materials comprising a silane compound, such as an aminopropyltriethoxy silane. Alternatively, protein or like molecules in thesensor chemistry layer72 can be sufficiently crosslinked or otherwise prepared to allow theglucose limiting membrane74 to be disposed in direct contact with the sensor chemistry layer in absence of an adhesion promoter layer. The adhesion promoter layer can be spin coated, sprayed, cast, etc. onto the enzyme layer. It may be exposed to heat and humidity to create silanol (sticky) groups. In further embodiments, the layer may be repeated. Although not necessary, the coating may help adhesion oxygen buffering. The time for exposure to heat and humidity is time sufficient to create silanol (sticky) groups, for example about two hours.
• Ahydrophilic membrane76, which may be non-toxic and biocompatible, may be positioned above theglucose limiting membrane74. Thehydrophilic membrane76 promotes tolerance of the sensor in the body.
• Typically, the electrodes are formed by one of the variety of methods known in the art such as photoresist, etching and rinsing to define the geometry of the active electrodes. The electrodes can then be made electrochemically active, for example by electrodeposition of platinum black for the working and counter electrode, and silver followed by silver chloride on the reference electrode. The sensor chemistry layer is then disposed on the conductive layer by a method other than electrochemical deposition, usually followed by vapor crosslinking, for example with a dialdehyde, such as glutaraldehyde, or a carbodi-imide.
• The electrodes and conductive layers are generally composed of conductive materials. However, they are not limited to conductive elements. Other useful sensor elements can be formed from any material that is capable of producing a detectable signal after interacting with a preselected analyte whose presence is to be detected. The detectable signal can be, for example, an optically detectable change, such as a color change or visible accumulation of the desired analyte (e.g., cells). Exemplary such materials include polymers that bind specific types of cells, single-strand DNA, antigens, antibodies and reactive fragments thereof, etc. Sensor elements can also be formed from materials that are essentially non-reactive (i.e., controls). The foregoing alternative sensor elements are beneficially included, for example, in sensors for use in cell-sorting assays and assays for the presence of pathogenic organisms, such as viruses (HIV, hepatitis-C, etc.), bacteria, protozoa, and the like.
• As shown inFIGS. 2 and 3, in one embodiment of the present invention asensor12 has the threeelectrodes18a,18b, and18cpositioned generally one above the other. The three electrode layers comprise a plurality of elongated conductive traces48a,48b, and48cconnected to theelectrodes18a,18b, and18con one end and connected toconductive pads21a,21b, and21con the opposingproximal segment20. Each electrode layer is formed between an underlyinginsulative base layer42 and an overlyinginsulative cover layer44. Apertures (not shown) may be formed on theinsulative cover layer44 to expose thedistal segment16 and theproximal segment20 of the electrodes. In a glucose monitoring application, theflexible sensor12 is placed transcutaneously so that thedistal segment16 is in direct contact with patient blood or extracellular fluid, and wherein theproximal segment20 is disposed externally for convenient connection, either by wired or wireless communication, to a monitoring device (not shown).
• One or more sensors are formed on a rigid flat substrate, such as a glass plate or a ceramic. When finished, the sensors may be removed from the rigid flat substrate by a suitable method, such as laser cutting. Other materials that can be used for the substrate include, but are not limited to, stainless steel, aluminum, and plastic materials. As seen inFIG. 7, theflexible sensors12a,12b, and12care formed in a manner which is compatible with photolithographic mask and etch techniques, but where thesensors12a,12b, and12care not physically adhered or attached directly to thesubstrate52. Each sensor comprises a plurality ofthin film electrodes18a,18b, and18cformed between an underlyinginsulative base layer42 and aninsulative cover layer44. A plurality of elongated conductive traces48a,48b, and48cmay connect theproximal segment end20 to thedistal segment end16. At theproximal segment end20,contact pads21a,21b, and21care formed. Apertures (not shown) formed in the insulative cover layer expose thedistal end16 portion of theelectrodes18a,18b, and18cso that they are in direct contact with patient blood or extracellular fluid.
• In one embodiment of a sensor set, shown inFIGS. 5 and 6, a flexibleelectrochemical sensor12 is constructed according to so-called thin film mask techniques to include elongated thin film conductors embedded or encased between layers of a selected insulative material such as polyimide film or sheet. The sensor electrodes18 (shown in exaggerated form in the drawings) at a tip end of the sensordistal segment16 are exposed through one of the insulative layers for direct contact with patient fluids, such as blood and/or interstitial fluids, when the sensor is transcutaneously placed.FIG. 6 shows how thedistal segment16 is joined to aproximal segment20, the end of which terminates in suitable conductive contact pads or the like which are also exposed through one of the insulative layers. As illustrated schematically inFIG. 6, theproximal segment20 and the contact pads thereon are adapted for electrical connection to asuitable monitor22 for monitoring patient condition in response to signals derived from thesensor electrodes18. The sensor electronics may be separated from the sensor by wire or be attached directly on the sensor. For example, the sensor may be housed in a sensor device including a housing that contains all of the sensor electronics, including any transmitter necessary to transmit data to a monitor or other device. The sensor device alternatively may include two portions, one portion housing the sensor and the other portion housing the sensor electronics. The sensor electronics portion could attach to the sensor portion in a side-to-side or top-to-bottom configuration, or any other configuration that would connect the two portions together. If the sensor electronics are in a housing separated by a wire from the sensor, the sensor electronics housing may be adapted to be placed onto the user's skin or placed on the user's clothing in a convenient manner. The connection to themonitor22 may be wired or wireless. In a wired connection, the sensor electronics may essentially be included in the monitor instead of in a housing with the sensor. Alternatively, sensor electronics may be included with the sensor as described above. A wire could connect the sensor electronics to the monitor. Examples of wireless connection include, but are not limited to, radio frequency, infrared, WiFi, ZigBee and Bluetooth. Additional wireless connections further include single frequency communication, spread spectrum communication, adaptive frequency selection and frequency hopping communication. In further embodiments, some of the electronics may be housed on the sensor and other portions may be in a detachable device. For example, the electronics that process and digitize the sensor signal may be with the sensor, while data storage, telemetry electronics, and any transmission antenna may be housed separately. Other distributions of electronics are also possible, and it is further possible to have duplicates of electronics in each portion. Additionally, a battery may be in one or both portion. In further embodiments, the sensor electronics may include a minimal antenna to allow transmission of sensor data over a short distance to a separately located transmitter, which would transmit the data over greater distances. For example, the antenna could have a range of up to 6 inches, while the transmitter sends the information to the display, which could be over 10 feet away.
• Further description of flexible thin film sensors of this general type may be found in U.S. Pat. No. 5,482,473, which is incorporated by reference herein. Theproximal segment20 may be conveniently connected electrically to themonitor22 by means of aconnector block24 as shown and described in U.S. Pat. No. 5,482,473, which is also incorporated by reference herein.
• The overall sensor height of the sensor12 (from base to top insulative layer) may be about 0.001 inches or 25 microns. The base layer is about 12 microns and each insulative layer is about 5 microns. The conductive/electrode layers are each in the range of several thousand angstroms. Any of these layers could be thicker if desired. The overall width of the sensor is as small as about 150 microns. It could be slightly larger, about 250 microns or 0.010 inches. The width could also larger if desired. The length of the sensor is dependant on how deep the tissue is at the placement site. For example, for subcutaneous sensing, the sensor length may be about 0.50 inches to about 1.5 inches, for example, about 1 inch.
• Thesensor12 is carried by a mountingbase26 adapted for placement onto the skin of a patient. As shown, the mountingbase26 comprises an enlarged and generally rectangular pad having an underside surface coated with a suitable pressure sensitive adhesive layer, with a peel-offpaper strip28 normally provided to cover and protect the adhesive layer, until the insertion set10 is ready for use. As shown inFIGS. 5 and 6, the mounting base comprises upper andlower layers30 and32, with theproximal segment20 of theflexible sensor12 sandwiched between. Theproximal sensor segment20 has a forwardmost end joined to thedistal segment16 which is folded angularly to extend downwardly through aslot34 formed in thelower base layer32.
• Theinsertion needle14 is adapted for slide-fit reception through aneedle port36 formed in theupper base layer30 and further through thelower slot34 in thelower base layer32. As shown inFIG. 5, theinsertion needle14 has a sharpenedtip38 and anopen slot40 which extends longitudinally from thetip38 at the underside of the needle to a position at least within theslot34 in thelower base layer32. Above the mountingbase26, theinsertion needle14 may have a full round cross sectional shape and is desirably closed at a rear end. In the preferred embodiment, the slottedneedle14 has a part-circular cross sectional shape, with an arcuate dimension or span greater than 180 degrees, such as on arcuate dimension of about 210 degrees. This leaves a longitudinal slot in the needle with an arcuate dimension of about 150 degrees.
• Thecannula15 is shown inFIG. 6 and comprises a part circular cross section fitted within theinsertion needle14 to extend downwardly from the mountingbase26. Thiscannula15 is constructed from a suitable medical grade plastic or elastomer, such as polytetrafluoroethylene, silicone, or the like. Thecannula15 has one end fitted into theslot34 formed in thelower layer32 of the mountingbase26, wherein thecannula15 is desirably secured to the mounting base by a suitable adhesive or other selected attachment means. From the mountingbase26, the cannula extends angularly downwardly with its first portion nested within theinsertion needle14, terminating slightly before theneedle tip38. One ormore apertures19 are formed in thecannula15 near thedistal segment end16, in general alignment with thesensor electrodes18, to permit direct electrode exposure to patient body fluid when the sensor is transcutaneously placed.
• In use, the insertion set10 permits quick and easy transcutaneous placement of the sensordistal segment16 at a selected site within the body of the patient. More specifically, the peel-off strip28 is removed from the mountingbase26, at which time the mountingbase26 can be pressed onto and seated upon the patient's skin. During this step, theinsertion needle14 pierces the patient's skin and carries theprotective cannula15 with the sensordistal segment16 therein to the appropriate transcutaneous placement site. During insertion, thecannula15 provides a stable support and guide structure to carry the flexible sensor to the desired insertion site.
• When thesensor12 is transcutaneously placed, with the mountingbase26 seated upon the patient's skin, theinsertion needle14 can be slidably withdrawn from the patient. The slotted needle geometry permits theinsertion needle14 to slide over and longitudinally separate from the second portion of thecannula15, thereby leaving thecannula15 as well as the sensordistal segment16 withelectrodes18 at the selected insertion site. Theseelectrodes18 are directly exposed to patient body fluid via theapertures19. The sensorproximal segment20 is appropriately coupled to themonitor22, so that thesensor12 can then be used over a prolonged period of time for taking blood chemistry readings, such as BG readings in a diabetic patient. In an embodiment, when the insertion needle is withdrawn, a protective sheath (not shown) contained in the mounting base is dislodged and covers the needle tip as the needle is separated from the mounting base. If desired, thecannula15 can also be used to deliver medication and/or sensor calibration fluid to the vicinity of theelectrodes18, or alternately to withdraw patient fluid such as blood for analysis.
• While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention.
• The presently disclosed embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description. All changes that come within the meaning of and range of equivalency of the claims are intended to be embraced therein.
EXAMPLES
• The examples set forth below are illustrative of different compositions and conditions that can be used in practicing the invention. All proportions are by weight unless otherwise indicated. It will be apparent, however, that the invention can be practiced with many types of compositions and can have many different uses in accordance with the disclosure above and as pointed out hereinafter.
Example IStep 1: Counter Electrode Metallization Step
• A KAPTON® polyimide base is used as a base layer and spin coated on a rigid substrate, such as glass. The first metallization comprised sputter-based chrome, gold and top-chrome. The top chrome pattern was photoresisted using photomask to protect the metallized trace and counter electrode. The uncovered areas of the base-chrome and gold were etched. The etching was performed at 50° C. for the gold and at ambient temperature for the chrome. The strip photoresisting was performed in isopropyl alcohol (“IPA”) at ambient temperature, and then again on all areas except the counter electrode, bond pad, and plating pad, at ambient temperature. A second etching was then performed on the top-chrome of all uncovered areas at ambient temperature. Strip photoresisting was performed again in IPA at ambient temperature. Polyimide was spin coated on as an insulative layer. Photoimaging was then performed to cure specific areas. The resulting electrode layer was developed and rinsed.
Step 2: Working Electrode Metallization Step
• The next electrode layer is formed generally above the counter electrode layer. This second metallization step involves the same sputter base-chrome and gold as a base layer. Photoresisting is performed using photomask to protect the working metallized trace and working electrode being formed. Etch is performed on the uncovered areas at 50° C. for the gold and at ambient temperature for the chrome. Strip photoresisting is again performed in IPA at ambient temperature. Next, photoresisting is performed on all areas except the counter electrode, bond pad, and plating pad. Etching is performed on all uncovered areas of the top-chrome at ambient temperature. Strip photoresisting is performed in IPA at ambient temperature. Polyimide was spin coated on as an insulative layer. Photoimaging was then performed to cure specific areas. The resulting electrode layer was developed and rinsed.
Step 3: Reference Electrode Metallization Step
• The next electrode layer is formed generally above the counter electrode layer. This third metallization step also involves the sputter base-chrome and gold combination as a base layer. Photoresisting is performed using photomask to protect the working metallized trace and working electrode being formed. Etch is performed on the uncovered areas at 50° C. for the gold and at ambient temperature for the chrome. Strip photoresisting is again performed in IPA at ambient temperature. Next, photoresisting is performed on all areas except the counter electrode, bond pad, and plating pad. Etching is performed on all uncovered areas of the top-chrome at ambient temperature. Strip photoresisting is performed in IPA at ambient temperature. Polyimide was spin coated on as an insulative layer. Photoimaging was then performed to cure specific areas. The resulting electrode layer was developed and rinsed. After all three electrode layers are formed, the layers are subjected to a final bake at a high temperature, such as 325° C.

Claims (29)

1. A method of making a flexible analyte sensor, the method comprising:

providing a base layer;

forming a first electrode layer on the base layer, wherein the first electrode layer includes a first electrode;

forming a first insulative layer on the first electrode layer;

forming a second electrode layer on the first insulative layer, wherein the second electrode layer includes a second electrode; and

forming a second insulative layer on the second electrode layer,

wherein the maximum height of the sensor is about 25 microns.

2. The method ofclaim 1, wherein the height of each electrode layer is less than about 5000 angstroms.

3. The method ofclaim 1, wherein the height of each insulative layer is about 5 microns.

4. The method ofclaim 1, wherein the first and second electrodes are in a staggered configuration.

5. The method ofclaim 4, wherein the second electrode layer is disposed directly above the first electrode layer.

6. The method ofclaim 4, wherein the second electrode layer is laterally offset from the first electrode layer.

7. The method ofclaim 1, wherein the first electrode layer is disposed directly on the base layer without any intervening layers.

8. The method ofclaim 1, wherein the first insulative layer is disposed directly on the first electrode layer without any intervening layers.

9. The method ofclaim 1, wherein the second electrode layer is disposed directly on the first insulative layer without any intervening layers.

10. The method ofclaim 1, wherein the second insulative layer is disposed directly on the second electrode layer without any intervening layers.

11. The method ofclaim 1, wherein the providing the base layer includes spin-coating polyimide onto a rigid substrate.

12. The method ofclaim 1, wherein the forming the first electrode layer includes surface sputtering a chrome-based layer on the base layer.

13. The method ofclaim 12, wherein the forming the first electrode layer further includes surface sputtering a gold-based layer on the chrome-based layer.

14. The method ofclaim 1, wherein the forming the first insulative layer includes spin-coating polyimide onto the first electrode layer.

15. The method ofclaim 1, further including forming a first aperture in the first insulative layer to expose at least a portion of the first electrode and forming a second aperture in the second insulative layer to expose at least a portion of the second electrode.

16. The method ofclaim 15, wherein said first and second apertures are formed by a photoresist process.

17. The method ofclaim 1, further comprising depositing platinum black on one or more of the first and second electrodes.

18. The method ofclaim 1, further comprising:

forming a third electrode layer on the second insulative layer, wherein the third electrode layer includes a third electrode; and

forming a third insulative layer on the third electrode layer.

19. The method ofclaim 18, wherein the first, second and third electrodes are in a staggered configuration.

20. The method ofclaim 19, wherein the third electrode layer is disposed directly above the second electrode layer, and the second electrode layer is disposed directly above the first electrode layer.

21. The method ofclaim 19, wherein the third electrode layer is laterally offset from the second electrode layer, and the second electrode layer is laterally offset from the first electrode layer.

22. The method ofclaim 18, wherein one of the electrodes is a counter electrode, one of the electrodes is a reference electrode, and one of the electrodes is a working electrode.

23. The method ofclaim 18, further comprising depositing one or more compounds selected from the group consisting of silver and silver chloride on the third electrode.

24. The method ofclaim 23, further comprising coating at least a portion of the first electrode and the second electrode with an enzyme layer including glucose oxidase.

25. The method ofclaim 24, further comprising forming an adhesion promoter layer above the enzyme layer.

26. The method ofclaim 24, further comprising forming a glucose limiting membrane over the enzyme layer.

27. The method ofclaim 26, further comprising forming a hydrophilic layer over the glucose limiting membrane.

28. The method ofclaim 18, wherein the forming the first electrode layer further includes forming a first conductive trace leading from the first electrode to a first conductive pad configured to electrically connect to sensor electronics, the forming the second electrode layer further includes forming a second conductive trace leading from the second electrode to a second conductive pad configured to electrically connect to sensor electronics, and the forming the third electrode layer further includes forming a third conductive trace leading from the third electrode to a third conductive pad configured to electrically connect to sensor electronics.

29. The method ofclaim 28, wherein the sensor electronics communicate wirelessly with a monitor.

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