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Flux limiting membrane for intravenous amperometric biosensorFlux limiting membrane for intravenous amperometric biosensor description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070202562, Flux limiting membrane for intravenous amperometric biosensor. Brief Patent Description - Full Patent Description - Patent Application Claims
CLAIM OF PRIORITY UNDER 35 U.S.C. .sctn.119 [0001]The present Application for Patent claims priority to Provisional Application No. 60/777,139 filed Feb. 27, 2006, and assigned to the assignee hereof and hereby expressly incorporated by reference herein. FIELD OF THE INVENTION [0002]The invention relates to amperometric biosensors for measuring blood chemistry. In particular, the invention relates to an intravenous sensor for measuring a biological parameter such as blood glucose concentration. BACKGROUND [0003]Amperometric biosensors are known in the medical industry for analyzing blood chemistry. Early biosensors, also known as enzyme electrodes, were first proposed by Clark and Lyons and implemented by Updike and Hicks. Enzyme electrodes typically include an oxidase enzyme, such as glucose oxidase, that is immobilized behind a dialysis membrane at the surface of an electrode. In the presence of blood, the membrane selectively passes an analyte of interest, e.g. glucose, to the oxidase enzyme where it undergoes oxidation or reduction, e.g. the reduction of oxygen to hydrogen peroxide. Amperometric biosensors function by producing an electric current when a potential sufficient to sustain the reaction is applied between two electrodes in the presence of the reactants. For example, in the reaction of glucose and glucose oxidase, the hydrogen peroxide reaction product may be subsequently oxidized by electron transfer to an electrode. The resulting flow of electrical current in the electrode is indicative of the concentration of the analyte of interest. [0004]Applications for amperometric biosensors include measuring analytes in blood-borne gases, electrolyte levels in blood and in particular, blood glucose concentration. For measuring glucose, subcutaneous methods have been proposed. For example, see Renard, "Implantable Glucose Sensors for Diabetes Monitoring," Minim Invasive Ther Allied Technol, Vol. 13, No. 2, pp. 78-86 (2004). While these minimally invasive glucose monitoring systems properly display trends in plasma glucose concentration, they do not track glucose accurately enough to be used for intensive insulin therapy, for example, where inaccuracy at conditions of hypoglycemia could pose a very high risk to the patient. In addition, sensors based upon the enzyme glucose oxidase need to have access to adequate oxygen to provide a linear glucose response. Sensor systems optimized for subcutaneous tissue would not necessarily function well in venous blood, where oxygen tension can be 20 mm Hg or less. [0005]At the present time, the most reliable way to obtain a highly accurate blood glucose measurement in an ICU patient is by a direct time-point method, which involves drawing a blood sample and sending it off for laboratory analysis. This is a time-consuming method that is often incapable of producing needed results in a timely manner. Despite ongoing research in this field, many improvements in glucose monitoring are still needed. [0006]One of the difficulties impeding the development of an intravenous amperometric sensor is that the sensor must be small enough to be suspended within a blood vessel, but robust enough to immobilize an enzyme so that a reaction may be sustained for a sufficient length of time. An intravenous sensor must also be biocompatible, such that it does not release any toxins into a patient, and when implanted, e.g. through a catheter in a femoral vein, discourages clotting of blood at the membrane surface that would prevent plasma from diffusing to the enzyme layer. SUMMARY [0007]The invention discloses a biocompatible flux limiting membrane for an amperometric biosensor designed for intravenous use and continuous analyte monitoring. The flux limiting membrane may be formed on a sensor electrode that is at least partially coated with a reagent selected to react with a substance found in blood. The flux limiting membrane limits a rate at which the substance diffuses through the flux limiting membrane to react with the reagent. The flux limiting membrane may include an ethylene vinylacetate (EVA) polymer selected for its biocompatibility, adhesion, physical, and diffusion properties. In one embodiment, the membrane may include one or more cured layers of EVA that are applied by spraying a solution having a percentage of EVA dissolved in paraxylene. [0008]An intravenous amperometric biosensor may be formed using the EVA membrane as a flux limiting layer to at least partially cover the surface of an enzyme electrode. The biosensor may be formed on a flex circuit substrate having reference, counter, and working electrodes mounted thereon, wherein one working electrode may be the enzyme-bearing electrode. In one embodiment, the biosensor may be a glucose sensor, the working electrode may be at least partially coated with glucose oxidase, and an EVA membrane may be formed on the working electrode to provide a flux limiting barrier that selectively allows diffusion of oxygen and glucose from blood to the glucose oxidase. Adhesive properties of EVA mechanically seal the glucose oxidase to the electrode and the electrode to the substrate to improve mechanical integrity during intravenous insertion. The composition of the EVA membrane may be optimized such that, when the biosensor is located intravenously with the working electrode energized, the current output of the working electrode is a linear function of blood glucose concentration. [0009]A related method is also disclosed for forming a flux limiting layer on an enzyme electrode that is bonded to a substrate of an amperometric biosensor. The method may include dissolving EVA in a solvent such as paraxylene, applying a layer of the dissolved EVA to an area of the substrate that includes at least a portion of the enzyme electrode, and curing the applied layer. The EVA may be dissolved in paraxylene to facilitate application by spray-coating, and the thickness and concentration of the EVA membrane may be optimized to promote a linear output of electrode current as a function of blood glucose concentration. BRIEF DESCRIPTION OF THE DRAWINGS [0010]The features, objects, and advantages of the invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, wherein: [0011]FIG. 1 shows an amperometric biosensor in the form of a flex circuit having a working electrode coated with a flux limiting membrane according to an embodiment of the invention. [0012]FIG. 2 is a magnified side cross-sectional view of the working electrode portion of the biosensor of FIG. 1, shown prior to application of a flux limiting membrane according to an embodiment of the invention. [0013]FIG. 3 is a magnified cross-sectional view of the working electrode portion of the biosensor of FIG. 1, shown after application of the flux limiting membrane according to an embodiment of the invention. [0014]FIG. 4 is a process flow chart illustrating steps for forming a flux limiting membrane on a biosensor substrate according to an embodiment of the invention. [0015]FIG. 5 is a graph of current output vs. glucose concentration for biosensors formed with flux limiting membranes according to an embodiment of the invention. [0016]FIG. 6 is a graph of glucose assay results of the current output over time covering multiple step changes in glucose concentration, for a biosensor formed with flux limiting membranes according to an embodiment of the invention. [0017]FIG. 7 shows results of an acute in vivo swine test for response of a glucose sensor having a flux limiting membrane according to an embodiment of the invention. DETAILED DESCRIPTION [0018]The invention discloses an adhesive biocompatible polymer for forming a flux-limiting membrane on an enzyme-bearing electrode in an intravenous amperometric biosensor. When the sensor is installed in a patient to measure blood chemistry, the membrane improves sensor accuracy by allowing oxygen to pass from the blood to the sensor while limiting the passage of larger molecules. The biocompatibility of the membrane limits the number of toxins that may be introduced into the bloodstream, and the adhesive properties enhance the mechanical integrity of the sensor during installation and operation. Continue reading about Flux limiting membrane for intravenous amperometric biosensor... 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