WO2001020019A9 - Implantable glucose sensor - Google Patents
Implantable glucose sensorInfo
- Publication number
- WO2001020019A9 WO2001020019A9 PCT/US2000/040888 US0040888W WO0120019A9 WO 2001020019 A9 WO2001020019 A9 WO 2001020019A9 US 0040888 W US0040888 W US 0040888W WO 0120019 A9 WO0120019 A9 WO 0120019A9
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- enzyme
- emulsion
- oxygen
- glucose
- sensor
- Prior art date
Links
Classifications
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/145—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue
- A61B5/1486—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue using enzyme electrodes, e.g. with immobilised oxidase
- A61B5/14865—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue using enzyme electrodes, e.g. with immobilised oxidase invasive, e.g. introduced into the body by a catheter or needle or using implanted sensors
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- C—CHEMISTRY; METALLURGY
- C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
- C12Q—MEASURING OR TESTING PROCESSES INVOLVING ENZYMES, NUCLEIC ACIDS OR MICROORGANISMS; COMPOSITIONS OR TEST PAPERS THEREFOR; PROCESSES OF PREPARING SUCH COMPOSITIONS; CONDITION-RESPONSIVE CONTROL IN MICROBIOLOGICAL OR ENZYMOLOGICAL PROCESSES
- C12Q1/00—Measuring or testing processes involving enzymes, nucleic acids or microorganisms; Compositions therefor; Processes of preparing such compositions
- C12Q1/001—Enzyme electrodes
- C12Q1/002—Electrode membranes
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- C—CHEMISTRY; METALLURGY
- C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
- C12Q—MEASURING OR TESTING PROCESSES INVOLVING ENZYMES, NUCLEIC ACIDS OR MICROORGANISMS; COMPOSITIONS OR TEST PAPERS THEREFOR; PROCESSES OF PREPARING SUCH COMPOSITIONS; CONDITION-RESPONSIVE CONTROL IN MICROBIOLOGICAL OR ENZYMOLOGICAL PROCESSES
- C12Q1/00—Measuring or testing processes involving enzymes, nucleic acids or microorganisms; Compositions therefor; Processes of preparing such compositions
- C12Q1/001—Enzyme electrodes
- C12Q1/005—Enzyme electrodes involving specific analytes or enzymes
- C12Q1/006—Enzyme electrodes involving specific analytes or enzymes for glucose
Definitions
- the present invention concerns the field of electrochemical devices for detection and measurement purposes and more specifically an enzyme emulsion for use in an implantable miniature polarographic glucose sensor.
- Occasional insulin injections (up to several per day) are unable to duplicate the strict control of blood glucose afforded by a properly functioning pancreas which continually meters out just enough insulin to maintain a stable and relatively normal blood glucose level. Extremes in blood glucose level need be avoided. Yet despite avoiding extremes in blood glucose level insulin-dependent diabetics suffer a host of other maladies, mentioned above, that decrease both the quality and length of life. Diabetics experience frequent vascular disease that often results in amputation of limbs as impaired circulation prevents adequate blood flow. Abnormal vascular growth within the eye may result in intraocular bleeding and retinal damage with progressive loss of vision. Nerve degeneration may lead to loss of sensation and other related problems.
- pancreatic islet cells The ultimate goal of diabetes treatment is a replacement for the patient's non- functioning pancreatic islet cells.
- Some scientists are seeking ways to transplant functioning islet cells into diabetic patients to provide a naturally controlled source of insulin.
- Other scientists are working on automatic insulin injection systems that deliver exogenously supplied insulin as needed to maintain precise blood glucose control. Most probably both of these "cures" will be needed.
- transplanted islet cells would seem to be the optimal solution, at this time anti-rejection drugs required for transplants have almost as many negative side effects as diabetes itself.
- a self-regulating artificial insulin source is needed to limit the damage caused by diabetes until islet transplantation is perfected.
- a self regulating insulin source will be needed for patient maintenance prior to transplantation, and, perhaps, for some post-transplantation support.
- the Clark glucose sensor consisted of a platinum anode, a layer of glucose oxygen oxioreductase (glucose oxidase) and a cellophane or cellulose acetate membrane. A silver "reference" electrode was also incorporated into the sensor. Only 0.01 ml of blood was required and the final analysis was complete in about one minute. Since then literally billions of blood samples have been analyzed by this type of instrument.
- the glucose detection process is dependent upon the measurement of electrons removed from hydrogen peroxide in equation (2).
- the electrode is normally formed from a noble metal such as gold or platinum. The latter preferred metal although carbon, pyrolytic or glassy, graphite and other electrically conducting materials are sometimes used.
- Glucose is extremely soluble in biological fluids whereas oxygen is poorly soluble in these same fluids and must be carried by specialized biomolecules such as hemoglobin.
- Many tissues ofthe human body have an oxygen tension equivalent to between about 2-5% oxygen in mtrogen or lower.
- oxygen tension equivalent equivalent to between about 2-5% oxygen in mtrogen or lower.
- there may be a ratio of glucose to oxygen sometimes as high as 100 to 1 in subcutaneous interstitial and peritoneal fluids. This means that at the electrode surface there may be only 1% of the oxygen required for glucose oxidase to quantitatively oxidize the available glucose for measurement purposes.
- glucose oxidase in a glucose sensor must be protected from proteases and other macromolecules which might destroy or inhibit the glucose oxidase, from enzymes such as catalase which destroy hydrogen peroxide (catalase, dehydrogenases, etc.), from microbes which digest the enzymes and from soluble compounds, such as ascorbate and acetoaminophen, which interfere with the either the enzymatic or electrochemical reactions.
- This protection can be achieved by separating the glucose oxidase from biological fluids by a semipermeable membrane.
- the best known membranes that are capable of selectively excluding proteins such as catalase while allowing the entry of glucose are so-called dialysis membranes.
- membranes are generally hydrophilic membranes containing "pores" that readily admit neutral molecules with molecular weights below about 5,000 Daltons.
- Common examples of these membranes are prepared from various regenerated celluloses such as cellophane, Spectrapore® or Cuprophan® (brands of regenerated cellulose), cellulose esters, and membranes of polycarbonate or polysulfone. While such semipermeable membranes do a good job of excluding undesirable proteins as well as retaining the essential glucose oxidase, they also impede oxygen diffusion.
- an implanted glucose sensor will tend to be oxygen limited and, thus, effectively measure oxygen instead of, or together with, glucose. That is, under ideal conditions when the glucose concentration is low, oxygen would be adequate so that an increase in glucose concentration would result in a concomitant and proportional increase in hydrogen peroxide and, therefore, measured current at the electrode.
- oxygen tension concentration
- glucose to oxygen ratio of human tissues.
- the level of glucose can be reduced either by providing a permeability barrier to glucose or by providing additional, non-peroxide generating enzyme systems, such as dehydrogenases, besides glucose oxidase, to consume excess glucose.
- the polyurethane membrane mentioned above is an example of glucose restriction.
- the second approach involves an attempt to increase the level of available oxygen or to maximize' the availability of oxygen to the oxygen-requiring enzymes.
- the present inventor has previously disclosed methods for increasing the oxygen level in U.S. Patent Nos. 4,680,268 and 4,721,677, which are hereby incorporated by reference.
- These patents teach the use of an oxygen-collecting chamber made of an oxygen permeable material such as silicone rubber. This chamber is separated from the oxygen-requiring enzyme by an oxygen permeable membrane. The chamber collects oxygen and delivers it by diffusion to the enzyme mixture near the measuring electrode.
- These patents also disclose filling the oxygen-collecting chamber with an oxygen-dissolving compound such as a perfluorocarbon liquid for speeding diffusion of oxygen to the oxygen-requiring enzyme.
- an emulsion of a perfluorocarbon liquid and the enzyme solution could be used to fill the chamber with the device configured so that the emulsion flows slowly onto the electrode, supplying oxygen and replenishing the enzyme.
- a recent publication Wang and Lu, J. American Chem. Soc. 120:1048-50(1998) adopts this liquid emulsion strategy but add graphite or carbon powder so the emulsion also functions directly as an electrode. This could cause difficulties with an implantable electrode since macrophages might react to the carbon powder.
- oxygen sensitivity of enzyme- based polarographic electrodes can be significantly reduced or eliminated by providing an oxygen-reservoir in intimate contact with the oxidative enzyme. This is achieved by making a stabilized emulsion of the enzyme and a compound in which oxygen is extremely soluble.
- an aqueous glucose oxidase solution can be emulsified with a perfluorocarbon liquid and the resulting emulsion stabilized by chemically crosslinking the mixture to form a gel.
- Thin layers of the emulsion ideal for placing into contact with a noble metal electrode can be fabricated by spreading a layer of the emulsion prior to crosslinking. Additional carrier proteins such as albumin can be added to the oxidase prior to crosslinking to protect enzymatic activity from the crosslinking reagent.
- Figure 1 illustrates diagrammatic view of a glucose sensor of the current invention
- Figure 2 illustrates an cross-sectional view of a working electrode of the device of Figure 1;
- Figure 3 shows the response of a stabilized enzyme mixture to varying glucose concentrations with ambient or with 5% oxygen
- Figure 4 shows the response of a stabilized enzyme emulsion containing about 7.5% Krytox brand of liquid perfluorocarbon to varying glucose concentrations with ambient or with 5% oxygen
- Figure 5 shows the response of a stabilized enzyme emulsion containing about 15% AP200 fluorocarbon to varying glucose concentrations with ambient or with 5% oxygen;
- Figure 6 shows the response of a stabilized enzyme emulsion containing about 7% AP200 fluorocarbon to varying glucose concentrations with ambient or with 5% oxygen;
- Figure 7 shows the response of a stabilized enzyme emulsion containing about 37% AP215 fluorocarbon to varying glucose concentrations with ambient or with 5% oxygen
- Figure 8 shows the response of a stabilized enzyme emulsion containing about 15% AP240 fluorocarbon to varying glucose concentrations with ambient or with 5% oxygen;
- Figure 9 shows the response of a stabilized enzyme emulsion containing 0.4% ferrocene to varying glucose concentrations at ambient oxygen concentrations, at 5% oxygen and at 0% oxygen (nitrogen atmosphere);
- Figure 10 shows the response of a stabilized enzyme emulsion containing a trace of ferrocene dissolved in 15% AP215 to varying glucose concentrations at ambient oxygen concentrations, at 5% oxygen at 2% oxygen, and at 0% oxygen (nitrogen atmosphere).
- the present invention is directed towards a fluorocarbon-containing enzyme suspension or emulsion.
- a fluorocarbon-containing enzyme suspension or emulsion can be advantageously used in a variety of implantable electrodes, it is especially useful in a miniature implantable device described in copending applications 08/769,863 and 08/779,304, which are incorporated herein by reference.
- a working implantable sensor of the glucose oxidase polarographic type can be readily constructed having a volume not much greater than a United States quarter.
- the overall shape of such an implantable sensor 18, as shown in Fig. 1, may be disc shaped although many other configurations are also possible.
- a case 22 contains a cavity 23 holding a printed circuit board 24 and is closed by a top 26 sealed by an O- ring 25.
- the case 22 also contains a first openings 28 for a reference electrode 30 and a second opening 20 for a working electrode 10.
- the working electrode 10 of the device comprises an outer shell 13 with an opening 12 through which an enzyme mixture 14 along with an underlying electrode 16 contacts the body fluids.
- the electrode 16 can conveniently be made from platinum although a variety of other conductive materials are also useable.
- a conductor 46 connects the electrode 16 with the circuit board 24.
- Most of the shell 13 is filled with insulating glass or plastic 15 through which the conductor 46 passes.
- the enzyme mixture 14 is covered by a semipermeable membrane 19 to protect the enzymes from proteases, interfering substances, and attack by microbes and/or their oxidase destroying enzymes or other products.
- This membrane 19 is selected to be permeable both to glucose and to oxygen.
- the actual working tip of this electrode could be as small al 10 ⁇ m in diameter.
- the area occupied by the opening 12 is small as compared to the surface area ofthe device 18 constituting as little as 1% of the total surface area.
- the device is implanted beneath the surface ofthe skin with the opening 12 facing towards the underlying layer of muscle. This position allows ready access to the unit for repair or replacement.
- the device can also be implanted so that the opening 12 contacts the peritoneal cavity.
- the device is not directly in contact with the circulatory system so that formation of blood clots does not interfere with operation. All of the body tissues come into glucose equilibrium with the blood fairly rapidly so that placement of the device in contact with the blood is not really required.
- the implanted sensor is measuring a body compartment that is in equilibrium with the blood
- blood glucose measurements can be used to effect calibration. If the patient or technician takes a series of blood glucose measurements over time, these can be plotted against sensor output to develop a time constant for sensor response. Thereafter, manual blood glucose measurements can be used to automatically calibrate or adjust the implanted sensor.
- the present inventor has also disclosed methods to use a single electrode to measure both oxygen and hydrogen peroxide (see U.S. Patent No. 5,030,333 which is incorporated herein by reference). This provides a way to automatically adjust the sensors output where the enzyme mixture response shows a varying slope depending on oxygen concentration as well as providing knowledge of oxygen availability at the electrode.
- oxygen is relatively poorly soluble in biological fluids, and the membrane 19 that covers the opening 12 of the glucose sensor 10 is generally not very permeable to oxygen.
- most of the cells of the human body require oxygen to function and in health receive an adequate supply.
- oxygen is not very soluble in biological fluids, it is highly "soluble” in the red blood cells by forming weak bonds with hemoglobin.
- These oxygen rich cells circulate in close proximity of virtually all of the body's cells so that the necessary oxygen can diffuse across the oxygen barrier (biological fluids and cell membranes) between the red blood cell's hemoglobin and an oxygen- requiring site such as a tissue cell.
- the speed of oxygen diffusion through a barrier is controlled by the thickness of the barrier and by the amount of oxygen that can dissolve in a unit thickness of the barrier. That is, making the barrier thinner, or making the barrier dissolve more oxygen will increase the rate of oxygen diffusion. Therefore, the enzyme mixture 14 and the membrane 18 should be made as thin as feasible to maximize the rate of oxygen movement into the glucose sensor 10.
- the present inventor has taken a novel approach to increasing the solubility of oxygen in the "barrier" of the glucose sensor 10.
- Various perfluorochemical liquids are widely known to dissolve relatively large amounts of oxygen.
- the present inventor's patents (U.S. Patent Nos. 4,105,798; 4,110,474; 4,187,252; 4,289,499; 4,443,480; RE33,451; 5,514,720; 5,635,539; 5,684,050; 5,674,913; 5,824,703; and 5,840,767) on the use of perfluorocarbon chemicals as emulsions and blood substitutes are incorporated herein by reference. There are a very large number of suitable perfluorocarbon liquids including those described in experiments below.
- the list comprises, but is not limited to, perfluorooctyl bromide, perfluorodichlorooctanes, perfluorodecalin, perfluoroindane, perfluorophenanthrene, perfluorotetramethylcyclohexane, perfluoropolyalkylether oil, perfluoromethyldecalin, perfluorodimethylethylcyclohexane, perfluoro- dimethyldecalin, perfluorotrimethyldecalin, perfluoroisopropyldecalin, perfluoropentamethyldecalin, perfluorodiisopropyldecalin, perfluorodiethyldecalin, perfluoromethyladamantane, perfluorodimethyladamantane, perfluoro-di-xylethane (mixed isomers), and perfluoro6,7 H-undec-6-ene.
- Perfluorocarbon liquids are virtually insoluble in aqueous solutions, and proteins such as glucose oxidase are completely insoluble in perfluorocarbon liquids.
- hydrocarbon drugs e.g., cortical steroids
- silicones, silanes, cyclic silanes, siloxanes, fluorinated silicones and other similar organo-silicon compounds are excellent oxygen solvents and are useful in the present invention.
- the most preferred compounds do not dissolve hydrogen peroxide.
- the particles of these compounds act as stepping stones for oxygen to reach the electrode surface.
- emulsions can be produced using perfluorocarbon liquids and aqueous solutions.
- glucose oxidase, or other hydrogen peroxide-forming enzymes are incorporated into the aqueous phase of such an emulsion, the perfluorocarbon serves as a pathway for oxygen to reach the enzyme as well as a reservoir of available oxygen.
- a cross-linking agent such as an aldehyde similar to glutaraldehyde to chemically cross-link proteins into a gel. Tiny perfluorocarbon droplets are then enmeshed permanently by a cross- linked protein gel.
- aldehyde-based crosslinking agents such as glutaraldehyde
- a number of other effective protein crosslinking agents are well known in the art including carbodiimides, pyrocarbonates (i.e., diethyl pyrocarbonate), imidoesters, N-hydroxysuccinimid esters and multifunctional epoxides (i.e., polyethylene glycol diglycidyl ether).
- the present invention provides a greatly improved sensor by producing a cross-linked gel containing glucose oxidase or similar hydrogen peroxide-producing enzymes, an emulsified oxygen binding/permeable material, such as a perfluorocarbon, a silicone oil, a fluorosilicone oil, an aliphatic oil or organic compound such as a steroid, to carry oxygen to the enzyme, additional gelling agents, buffers and optional additives such as other enzymes and/or preservatives.
- a cross-linked gel containing glucose oxidase or similar hydrogen peroxide-producing enzymes, an emulsified oxygen binding/permeable material, such as a perfluorocarbon, a silicone oil, a fluorosilicone oil, an aliphatic oil or organic compound such as a steroid.
- an oxygen binding/permeable material such as a perfluorocarbon, a silicone oil, a fluorosilicone oil, an aliphatic oil or organic compound such as a
- this emulsified oxygen carrier is two-fold. On one hand it holds oxygen and brings it into intimate contact with the enzyme to accept electrons from the enzyme. Because this substance is oxygen permeable it necessarily raises the effective oxygen concentration at the electrode and allows for more rapid diffusion of oxygen from a source such as the human circulatory system. At the same time the oxygen carrier effectively lowers the glucose level because it replaces a significant aqueous volume in which glucose is very soluble with an oxygen carrying volume in which glucose is extremely poorly soluble. That is, the glucose/oxygen ratios can be adjusted by increasing the hydrophobic oxygen carrier phase at the expense of the hydrophilic glucose-dissolving phase.
- the oxygen permeable particles could comprise tiny gas bubbles (trapped bubbles as in a foam) produced by incorporating relatively high vapor pressure perfluorocarbon liquid into a protein-containing gel emulsion. Over time the perfluorocarbon would vaporize to form gas bubbles which remain trapped within the gel. These bubbles would hold a considerable supply of oxygen, and gaseous diffusion within the bubbles would be more rapid than diffusion within a liquid particle ofthe same size.
- a suitable soluble carrier protein such as an albumin, i.e., bovine serum albumin (BSA), or human serum albumin (HSA), or gelatin
- BSA bovine serum albumin
- HSA human serum albumin
- a suitable buffer such as 0.2M sodium acetate buffer (pH 5.0)
- a hydrogen peroxide producing enzyme such as glucose oxidase is dissolved in the mixture at about 1% to 5% by weight final concentration.
- concentrations of 70% or greater glucose oxidase are also effective. At such high levels it is generally unnecessary to add albumin or other proteins to aid in gel formation.
- Sufficient purified glutaraldehyde as an aqueous 2.5% solution is added to dilute the protein solution to the correct final concentration.
- the final glutaraldehyde concentration following dilution is preferably between 0.1 and 1% and more preferably about 0.6%.
- This mixture is swirled briefly to mix and is then poured onto a glass plate and spread with a glass rod.
- Aldehyde vapors can also be used to induce crosslinking.
- a uniform layer of enzyme gel is formed. This gel can be stored at 4°C in a humidified atmosphere to prevent dehydration of the gel.
- an oxygen dissolving substance such as a perfluorocarbon liquid
- a suitable amount ofthe oxygen dissolving liquid (usually between about 5% and 20% by volume) is added to the protein mixture and sonicated for two 15 second intervals while being maintained on ice. After the sonication, glutaraldehyde is added and the material is treated as above. The resulting gel may be stored in an atmosphere saturated with water and perfluorocarbon vapors to prevent evaporation of the perfluorocarbon.
- An alternate method of preparation is to add the active glucose utilizing enzymes to the sonicated BSA-perfluorocarbon emulsion prior to the glutaraldehyde addition to avoid possible denaturation of the enzyme during sonication.
- a small piece is placed over a platinum electrode and covered with a piece of a Cuprophan® (brand regenerated cellulose) membrane.
- the gel can be divided into numerous small particles, and a slurry of these particles can be placed on the electrode surface and covered by the membrane.
- An additional variation is to paint the fluorocarbon-enzyme emulsion onto a membrane before the crosslinking agent has caused the mixture to "set.”
- Fig. 3 shows the response of an ordinary stabilized enzyme mixture electrode to a range of glucose concentrations in either ambient (about 20%) or in 5% oxygen.
- An enzyme mixture was prepared according to the above method and contained about 2% glucose oxidase in an about 4% BSA gel stabilized with about 0.6% glutaraldehyde. Note that an ambient oxygen trace 32 shows an approximately linear response to at least a glucose concentration of 400 mg% (0.4%). On the other hand, a 5% oxygen trace 34 plateaus above about 50 mg% glucose indicating that oxygen is limiting the reaction.
- the concentration of BSA relative to glucose oxidase is may be important for producing higher and/or more stable signals.
- these factors do not appear to greatly affect the glucose concentration at which a sensor plateaus because of oxygen limitation.
- the goal is to produce electrode response that is largely oxygen independent, or that at least produces a near linear response at low oxygen tensions.
- Fig. 4 illustrates the effect of adding an emulsified perfluorocarbon liquid to the stabilized enzyme mixture.
- the mixture in this case contains about 13% BSA, about 3% glucose oxidase and about 7.5% of emulsified Krytox ® (brand of perfluoropolyalkylether synthetic oil, product of du Pont de Nemours) perfluorocarbon crosslmked with about 0.6% glutaraldehyde.
- Krytox ® brand of perfluoropolyalkylether synthetic oil, product of du Pont de Nemours
- the intent is for the perfluorocarbon to act as an oxygen source for the enzyme reaction. Because the perfluorocarbon is emulsified into tiny particles, there is an intimate association between the oxygen carrying perfluorocarbon and the oxygen-requiring enzyme. This limits the distance that oxygen must diffuse through a poor oxygen carrier such as water. With the perfluorocarbon acting as an oxygen source adequate enzyme response can occur even at low oxygen tensions.
- the ambient oxygen trace 32 is linear to at least 400 mg% glucose.
- the 5% oxygen trace 34 is now linear to at least 100 mg% glucose. Above this glucose concentration the slope ofthe response changes, but the electrode continues to show increasing response to over 350 mg% glucose. The electrode even shows some response at a very low oxygen concentration of 2% oxygen as is shown in a third trace 36.
- Fig. 5 shows the results of a stabilized enzyme emulsion containing about 12% BSA, 4% glucose oxidase, 15% AP200 perfluorocarbon (mixed trimethyl and/or isopropyl perfluoro-decalins) (boiling point approximately 200°C) crosslmked with 0.6% glutaraldehyde.
- both the ambient oxygen trace 32 and the 5% oxygen trace 34 show relatively linear responses to above 350 mg% glucose although the slopes ofthe responses are somewhat different.
- the 2% oxygen trace 36 shows a very shallow response.
- Fig. 7 illustrates the results obtained from an emulsion similar to that of Fig. 5 except that 37% perfluorophenanthrene (AP215) (boiling point approximately 215°C) is used in place ofthe AP200.
- AP215 roofing point approximately 215°C
- the results are very similar to those of Fig. 5 indicating that there is probably little benefit to greatly increasing the quantity of perfluorocarbon beyond about 15%. As the amount of perfluorocarbon is increased, the overall signal decreases.
- Fig. 8 shows the results of a stabilized enzyme emulsion containing about 15% AP240 (mixed pentamethyl and/or diisopropyl perfluoro-decalins) perfluorocarbon (boiling point approximately 240°C), 10% BSA, 3% glucose oxidase and 0.6% glutaraldehyde.
- the AP240 was emulsified with the aid of 0.75% (final concentration) Pluronics F-68 brand emulsifying agent.
- emulsifying agents suitable for producing stable perfluorocarbon emulsions are well known in the art and can readily is used in the present invention.
- 0.75% glucose was added to protect the active site of glucose oxidase during cross-linking reaction.
- the comparison of the ambient oxygen trace 32 with the 5% oxygen trace 34 shows that this preparation is somewhat more active than the preparation illustrated in Fig. 5.
- Equations (1) and (2) show that the sensor indicates the rate of glucose oxidation (proportional to glucose concentration) by measuring removal of electrons from hydrogen peroxide at the electrode.
- the enzyme oxidizes a molecule of glucose, an electron is removed from the glucose and accepted by a cofactor within the enzyme. Reacting with oxygen, which accepts an electron from the enzyme cofactor to produce hydrogen peroxide, regenerates the enzyme. At the surface of the electrode the electron is removed from the hydrogen peroxide thus regenerating oxygen. The electron flows through the circuit and is measured as a current.
- the hydrogen peroxide merely acts as an electron carrier to move electrons from glucose (by way of glucose oxidase) to the electrode.
- any of a number of artificial electron carriers with the correct redox (reduction/oxidation) potential can perform the role of oxygen and hydrogen peroxide, thus rendering the entire reaction oxygen insensitive.
- Possible electron carriers include indophenol dyes, methyl viologen dyes, and various organometallic compounds.
- a presently preferred electron carrier for use with glucose oxidase is ferrocene (dicyclopentadienyliron) and its derivatives in which iron acts as the actual electron carrier.
- ferrocene can carry enough electrons from glucose oxidase to the electrode surface that the glucose oxidase reaction becomes independent of oxygen (i.e., can occur anaerobically).Ferrocene is virtually insoluble in aqueous solutions, and while ferrocene is soluble in some organic solvents, these solvents are generally not suitable for use in an implantable electrode. However, ferrocene is somewhat soluble in certain perfluorocarbons.
- Fig. 9 shows the results of adding 0.4% by weight ferrocene to a stabilized enzyme mixture containing about 15% by weight BSA, 3.3% by weight glucose oxidase and about 0.6% glutaraldehyde.
- the ferrocene is dispersed into the buffer used to dissolve the glucose oxidase but does not appreciably dissolve therein. However, the ferrocene does affect the electrode response.
- Both the ambient oxygen trace 32 and the 5% oxygen trace 34 show a reasonably linear response to increasing glucose concentration, albeit at different slopes.
- a 0% oxygen trace 38 (experiment performed under nitrogen) also shows some response to glucose.
- the reaction is more pronounced in the presence of oxygen, the ferrocene is able to carry at least some electrons from the glucose oxidase to the electrode otherwise there would be no response at 0% oxygen.
- ferrocene (as well as certain ferrocene derivatives) is slightly soluble in perfluorocarbon liquids.
- Fig. 10 shows the results of incorporating ferrocene-containing perfluorocarbon into a stabilized enzyme emulsion.
- the enzyme mixture contained about 15% AP215, 12% BSA, 4% glucose oxidase and about 0.6% glutaraldehyde as a crosslinking agent.
- a quantity of ferrocene (less than 1% by weight) was added to the perfluorocarbon and allowed to dissolve overnight prior to sonicating the perfluorocarbon into the remaining ingredients. Enough ferrocene dissolves into the AP215 to color the liquid a light yellow. The intent is to saturate the perfluorocarbon with ferrocene.
- the ambient oxygen trace 32, the 5% oxygen trace 34 and the 2% oxygen trace 36 all show a linear response with surprising similar slopes.
- the 0% (mtrogen) trace 38 shows a much flatter response slope. This indicates that while the trace amount of ferrocene incorporated in the mixture does not carry as many electrons as does hydrogen peroxide, the perfluorocarbon plus ferrocene shows an unexpected synergistic activity superior to either ferrocene or perfluorocarbon alone. In some unknown way the ferrocene potentiates the effect of the emulsified perfluorocarbon particles.
- the perfluorocarbon-insoluble enzyme mixtures ofthe present invention also permit other modifications that enhance the long-term stability and useful life of the implanted sensors.
- the device of the present invention is preferably implanted at a site away from direct blood circulation to avoid problems caused by formation of blood clots, leukocytes can migrate out of the circulatory system to congregate around any "foreign" body. This leukocyte accumulation may damage the membrane and/or compromise the accuracy of the glucose readings.
- an anti- inflammatory, anti-leukocyte compound into the enzyme mixture.
- hydrocortisone or similar cortical steroids such as cortisone and prednisone, at about 0.1 to 1.0% by weight.
- steroids gradually dissolve in the aqueous phase of the enzyme mixture and very slowly diffuse out through the membrane 18 to keep the surrounding area free from attack by leukocytes (especially by macrophages).
- An advantage is that steroids, like perfluorocarbons, are much better at dissolving oxygen than is water.
- non-steroidal anti-inflammatory drugs i.e., aspirin, ibuprofen, naproxyn, ketoprofen and the like
- anti-inflammatory lymphokines or "anti- rejection” drugs e.g., cyclosporine
- drugs that impede cell replication e.g., "antineoplastic” agents
- useful agents include vinca alkaloids (vincristine and vinblastine), taxol derivatives and other well-known anti-tumor drugs.
- microbes may directly destroy the glucose-metabolizing enzyme, it is also likely for them to disrupt the glucose measurement by consuming glucose and oxygen or by producing catalase or peroxidase or other enzymes that consume the hydrogen peroxide before it can react with the electrode surface.
- antifungals or wide spectrum antibiotics into the enzyme mixture largely prevents microbial interference.
- gentamycin and/or penicillin, and/or other broad-spectrum antibiotics and antifungals can be incorporated into the enzyme mixture to prevent microbial growth.
- a relatively large concentration of antibiotic can be added so that sterility of the enzyme mixture is guaranteed for a long period of time. Slow diffusion of the antibiotic through the membrane keeps the entire area around the implanted sensor free of infection. Further, the electrode constantly produces hydrogen peroxide which is a powerful anti-infective agent.
- the semipermeable membrane 19 is generally believed to protect the glucose oxidase from various proteases.
- stabilized glucose oxidase is not readily attacked by a common proteolytic enzyme, trypsin.
- trypsin a common proteolytic enzyme that stabilizes the enzyme destroys the trypsin sensitive sites. Therefore, trypsin may be incorporated as an anti-proteolytic enzyme to help destroy other proteolytic enzymes that might be produced by microorganisms, etc.
- Stability of the enzyme mixture of the present invention can also be improved by the addition of antioxidants and/or free radical trapping agents.
- Vitamin E, lycopene, and carotene which are also oxygen solvents, can be incorporated into the enzyme mixture as can any of a number of "preservatives" such as various parabens, BHT (butylated hydroxy-toluene) and its analogs, and/or superoxide dismutases. Further angiogenic factors can be added to ensure capillary growth and blood circulation near the sensor. Addition of an enzyme system to generate nitric oxide from arginine can be used to monitor microcirculation near the sensor.
Abstract
Description
Claims
Priority Applications (6)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP2001523790A JP4981225B2 (en) | 1999-09-14 | 2000-09-13 | Implantable glucose sensor |
AU21120/01A AU2112001A (en) | 1999-09-14 | 2000-09-13 | Implantable glucose sensor |
CA002383435A CA2383435C (en) | 1999-09-14 | 2000-09-13 | Implantable glucose sensor |
AT00984516T ATE462132T1 (en) | 1999-09-14 | 2000-09-13 | IMPLANTABLE GLUCOSE SENSOR |
DE60044064T DE60044064D1 (en) | 1999-09-14 | 2000-09-13 | IMPLANTABLE GLUCOSE SENSOR |
EP00984516A EP1214586B1 (en) | 1999-09-14 | 2000-09-13 | Implantable glucose sensor |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US09/395,466 | 1999-09-14 | ||
US09/395,466 US6343225B1 (en) | 1999-09-14 | 1999-09-14 | Implantable glucose sensor |
Publications (3)
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EP1214586A2 (en) | 2002-06-19 |
US6815186B2 (en) | 2004-11-09 |
AU2112001A (en) | 2001-04-17 |
EP1214586B1 (en) | 2010-03-24 |
US20020068860A1 (en) | 2002-06-06 |
CA2383435C (en) | 2009-11-17 |
US6343225B1 (en) | 2002-01-29 |
WO2001020019A2 (en) | 2001-03-22 |
JP2003513230A (en) | 2003-04-08 |
CA2383435A1 (en) | 2001-03-22 |
JP4981225B2 (en) | 2012-07-18 |
ATE462132T1 (en) | 2010-04-15 |
WO2001020019A3 (en) | 2002-01-17 |
DE60044064D1 (en) | 2010-05-06 |
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