Accuracy of the one-point in vivo calibration of "wired" glucose oxidase electrodes implanted in jugular veins of rats in periods of rapid rise and decline of the glucose concentration

Smartphone-Based Electrochemical Systems for Glucose Monitoring in Biofluids: A Review

As a vital biomarker, glucose plays an important role in multiple physiological and pathological processes. Thus, glucose detection has become an important direction in the electrochemical analysis field. In order to realize more convenient, real-time, comfortable and accurate monitoring, smartphone-based portable, wearable and implantable electrochemical glucose monitoring is progressing rapidly. In this review, we firstly introduce technologies integrated in smartphones and the advantages of these technologies in electrochemical glucose detection. Subsequently, this overview illustrates the advances of smartphone-based portable, wearable and implantable electrochemical glucose monitoring systems in diverse biofluids over the last ten years (2012-2022). Specifically, some interesting and innovative technologies are highlighted. In the last section, after discussing the challenges in this field, we offer some future directions, such as application of advanced nanomaterials, novel power sources, simultaneous detection of multiple markers and a closed-loop system.

Electrochemistry in diabetes management

Diabetes devastates lives and burdens society. Hypoglycemic (low glucose) episodes cause blackouts, and severe ones are life-threatening. Periods of hyperglycemia (high glucose) cause circulatory disease, stroke, amputations, blindness, kidney failure and nerve degeneration. In this Account, we describe the founding of TheraSense, now a major part of Abbott Diabetes Care, and the development of two products that have improved the lives of people with diabetes. The first, a virtually painless microcoulometer (300 nL volume), the FreeStyle blood glucose monitoring system, was approved by the FDA and became available in 2000. In 2009, this system was used in more than one billion blood assays. The second, the enzyme-wiring based, subcutaneously-implanted FreeStyle Navigator continuous glucose monitoring system, was approved by the FDA and became available in the United States in 2008. The strips of the FreeStyle blood glucose monitoring system comprise a printed parallel plate coulometer, with a 50 microm gap between two facing printed electrodes, a carbon electrode and a Ag/AgCl electrode. The volume of blood between the facing plates is accurately controlled. The glucose is electrooxidized through catalysis by a glucose dehydrogenase (GDH) and an Os(2+/3+) redox mediator, which is reduced by the glucose-reduced enzyme and is electrooxidized on the carbon electrode. Initially the system used pyrroloquinoline quinone (PQQ)-dependent GDH but now uses flavin adenine dinucleotide (FAD)-dependent GDH. Because the facing electrodes are separated by such a small distance, shuttling of electrons by the redox couple could interfere with the coulometric assay. However, the Os(2+/3+) redox mediator is selected to have a substantially negative formal potential, between 0.0 and -0.2 V, versus that of the facing Ag/AgCl electrode. This makes the flow of a shuttling current between the two electrodes virtually impossible because the oxidized Os(3+) complex cannot be appreciably reduced at the more positively poised Ag/AgCl electrode. The FreeStyle Navigator continuous glucose monitoring system uses a subcutaneously implanted miniature plastic sensor connected to a transmitter to measure glycemia amperometrically and sends the information to a PDA-like device every minute. The sensor consists of a narrow (0.6 mm wide) plastic substrate on which carbon-working, Ag/AgCl reference, and carbon counter electrodes are printed in a stacked geometry. The active wired enzyme sensing layer covers only about 0.1 mm(2) of the working electrode and is overlaid by a flux-limiting membrane. It resides at about 5 mm depth in the subcutaneous adipose tissue and monitors glucose concentrations over the range 20-500 mg/dL. Its core component, a miniature, disposable, amperometric glucose sensor, has an electrooxidation catalyst made from a crosslinked adduct of glucose oxidase (GOx) and a GOx wiring redox hydrogel containing a polymer-bound Os(2+/3+) complex. Because of the selectivity of the catalyst for glucose, very little current flows in the absence of glucose. That feature, either alone or in combination with other features of the sensor, facilitates the one-point calibration of the system. The sensor is implanted subcutaneously and replaced by the patient after 5 days use with minimal pain. The wearer does not feel its presence under the skin.

In vitro, in vivo and post explantation testing of glucose-detecting biosensors: current methods and recommendations

To date, there have been a number of cases where glucose sensors have performed well over long periods of implantation; however, it remains difficult to predict whether a given sensor will perform reliably, will exhibit gradual degradation of performance, or will fail outright soon after implantation. Typically, the literature emphasizes the sensor that performed well, while only briefly (if at all) mentioning the failed devices. This leaves open the question of whether current sensor designs are adequate for the hostile in vivo environment, and whether these sensors have been assessed by the proper regimen of testing protocols. This paper reviews the current in vitro and in vivo testing procedures used to evaluate the functionality and biocompatibility of implantable glucose sensors. An overview of the standards and regulatory bodies that govern biomaterials and end product device testing precedes a discussion of up-to-date invasive and non-invasive technologies for diabetes management. Analysis of current in vitro, in vivo, and then post explantation testing is presented. Given the underlying assumption that the success of the sensor in vitro foreshadows the long-term reliability of the sensor in the human body, the relative merits of these testing methods are evaluated with respect to how representative they are of human models.

Dipolar ruthenium(II) ammine complexes as electron transfer mediators of amperometric glucose sensors

The mediation of dipolar ruthenium(II) ammine complexes containing pyridinium ions [Ru(NH(3))(5)(L(+))](3+)(L(+): pyridinium ions) in glucose oxidation has been investigated by a voltammetric method. These ruthenium(II) complexes had appropriate redox potentials of 0.10-0.18 V vs. Ag/AgCl and high k(s) values of 5.7-17 x 10(6) M(-1) s(-1) which are the second-order rate constants for electron transfer from glucose oxidase in reduced form to [Ru(NH(3))(5)(L(+))](4+). In particular, the k(s) values for [Ru(NH(3))(5)(L(+))](3+) were greater than those of osmium(II)-polypyridine complexes possessing similar redox potentials which are most commonly used. All the dipolar ruthenium(II) complexes used in this study are therefore concluded to be useful for the electron transfer mediators of amperometric glucose sensors.

Enzyme Electrochemistry — Biocatalysis on an Electrode

Oxidoreductase enzymes catalyze single- or multi-electron reduction/oxidation reactions of small molecule inorganic or organic substrates, and they are integral to a wide variety of biological processes including respiration, energy production, biosynthesis, metabolism, and detoxification. All redox enzymes require a natural redox partner such as an electron-transfer protein (e.g. cytochrome, ferredoxin, flavoprotein) or a small molecule cosubstrate (e.g. NAD(P)H, dioxygen) to sustain catalysis, in effect to balance the substrate/product redox half-reaction. In principle, the natural electron-transfer partner may be replaced by an electrochemical working electrode. One of the great strengths of this approach is that the rate of catalysis (equivalent to the observed electrochemical current) may be probed as a function of applied potential through linear sweep and cyclic voltammetry, and insight to the overall catalytic mechanism may be gained by a systematic electrochemical study coupled with theoretical analysis. In this review, the various approaches to enzyme electrochemistry will be discussed, including direct and indirect (mediated) experiments, and a brief coverage of the theory relevant to these techniques will be presented. The importance of immobilizing enzymes on the electrode surface will be presented and the variety of ways that this may be done will be reviewed. The importance of chemical modification of the electrode surface in ensuring an environment conducive to a stable and active enzyme capable of functioning natively will be illustrated. Fundamental research into electrochemically driven enzyme catalysis has led to some remarkable practical applications. The glucose oxidase enzyme electrode is a spectacularly successful application of enzyme electrochemistry. Biosensors based on this technology are used worldwide by sufferers of diabetes to provide rapid and accurate analysis of blood glucose concentrations. Other applications of enzyme electrochemistry are in the sensing of macromolecular complexation events such as antigen–antibody binding and DNA hybridization. The review will include a selection of enzymes that have been successfully investigated by electrochemistry and, where appropriate, discuss their development towards practical biotechnological applications.

An implantable subcutaneous glucose sensor array in ketosis-prone rats: closed loop glycemic control

A closed loop system of diabetes control would minimize hyperglycemia and hypoglycemia. We therefore implanted and tested a subcutaneous amperometric glucose sensor array in alloxan-diabetic rats. Each array employed four sensing units, the outputs of which were processed in real time to yield a unified signal. We utilized a gain-scheduled insulin control algorithm which rapidly reduced insulin delivery as glucose concentration declined. Such a system was generally effective in controlling glycemia and the degree of lag between blood glucose and the sensor signal was usually 3-8 min. After prolonged implantation, this lag was sometimes longer, which led to impairment of sensor accuracy. Using a prospective two-point calibration method, sensor accuracy and closed loop control were good. A revised algorithm yielded better glycemic control than the initial algorithm did. Future research needs to further improve calibration methods and reduce foreign body fibrosis in order to avoid a time-related increase in lag duration.

Biosensors for real-time in vivo measurements

The current status of sensors capable of continuous measurement of analytes in biological media is reviewed. This review containing 173 references deals with devices whose use in single cells, tissue slices, animal models and humans has been demonstrated. In addition to sensors specific for glucose, lactate, glutamate, pyruvate, choline and acetylcholine, insights obtained from monitoring nitric oxide, Na(+), K(+), Ca(2+), and dopamine are presented. Performance criteria for sensor performance are described as is the subject of biosensor calibration. Biocompatibility issues are dealt with in some detail as is the status of continuous blood glucose monitoring in humans.

Glucose sensor for flow injection analysis of serum glucose based on immobilization of glucose oxidase in titania sol-gel membrane

A novel amperometric glucose sensor was constructed by immobilizing glucose oxidase (GOD) in a titania sol-gel film, which was prepared with a vapor deposition method. The sol-gel film was uniform, porous and showed a very low mass transport barrier and a regular dense distribution of GOD. Titania sol-gel matrix retained the native structure and activity of entrapped enzyme and prevented the cracking of conventional sol-gel glasses and the leaking of enzyme out of the film. With ferrocenium as a mediator the glucose sensor exhibited a fast response, a wide linear range from 0.07 to 15 mM. It showed a good accuracy and high sensitivity as 7.2 microA cm(-2) mM(-1). The general interferences coexisted in blood except ascorbic acid did not affect glucose determination, and coating Nafion film on the sol-gel film could eliminate the interference from ascorbic acid. The serum glucose determination results obtained with a flow injection analysis (FIA) system showed an acceptable accuracy, a good reproducibility and stability and indicated the sensor could be used in FIA determination of glucose. The vapor deposition method could fabricate glucose sensor in batches with a very small amount of enzyme.

An amperometric biosensor for glucose based on electrodeposited redox polymer/glucose oxidase film on a gold electrode

In this paper, we described a glucose biosensor based on the co-electrodeposition of a poly(vinylimidazole) complex of [Os(bpy)2Cl](+/2+) (PVI-Os) and glucose oxidase (GOX) on a gold electrode surface. The one-step co-electrodeposition method provided a better control on the sensor construction, especially when it was applied to microsensor construction. The modified electrode exhibited the classical features of a kinetically fast redox couple bound to an electrode surface and the redox potential of the redox polymer/enzyme film was 0.14 V (vs. SCE). For a scan rate of up to 200 mV s(-1), the peak-to-peak potential separation was less than 25 mV. In the presence of glucose, a typical catalytic oxidation current was observed, which reached a plateau at 0.25 V (vs. SCE). Under the optimal experimental conditions, the steady-state electrooxidation current measured at 0.30 V (vs. SCE) was linear to the glucose concentration in the range of 0-30 mM. Successful attempts were made in blood sample analysis.

Fibroblast Cells: A Sensing Bioelement for Glucose Detection by Impedance Spectroscopy

Modifying the electrical properties of fibroblasts against various glucose concentrations can serve as a basis for a new, original sensing device. The aim of the present study is to test a new biosensor based on impedancemetry measurement using eukaryote cells. Fibroblast cells were grown on a small optically transparent indium tin oxide semiconductor electrode. Electrochemical impedance spectroscopy (EIS) was used to measure the effect of D-glucose on the electrical properties of fibroblast cells. Further analyses of the EIS results were performed using equivalent circuits in order to model the electrical flow through the interface. The linear calibration curve was established in the range 0-14 mM. The specification of the biosensors was verified using cytochalasin B as an inhibitor agent of the glucose transporters. The nonreactivity to sugars other than glucose was demonstrated. Such a biosensor could be applied to a more fundamental study of cell metabolism.

Electrochemical catalysis with redox polymer and polyion–protein films

Supramolecular redox-active assemblies on electrodes are of fundamental interest and can be used to create functioning devices such as sensors, biosensors, and bioreactors. The ability of redox-active films to mediate electron transfer reactions in 3-D dramatically increases the sensitivity with which target molecules can be determined. Metallopolyion hydrogel films immobilized on electrode surfaces exhibit many properties that are reminiscent of those shown by redox-active proteins. This review discusses the electrochemical properties and applications of such films, including mediating electron transfer between electrodes and oxidase enzymes. In addition, polyion-protein films grown layer by layer have certain advantages in device fabrication, including facilitating direct electron transfer for many proteins, mechanical stability, use of tiny amounts of protein, and control of film architecture. This review presents examples of iron heme proteins in films grown layer by layer by alternate electrostatic adsorption for catalytic reduction of hydrogen peroxide and trichloroacetic acid and for oxidation of styrene.

A microscopic, continuous, optical monitor for interstitial electrolytes and glucose

Ions, such as hydrogen (pH), sodium, or potassium, as well as metabolites, such as glucose or lactate, diffuse easily between blood in the capillaries and the interstitial fluid (ISF) residing between cells, and tissues. This work represents a synthesis of several unique concepts to achieve accurate, continuous, in vivo monitoring of critical ions and glucose in the ISF under the human skin. Ionic levels are monitored using optode technology that translates the respective concentrations into variable colors of ionophore/dye/polymeric liquid membranes. Glucose is monitored indirectly, by coupling through immobilized glucose oxidase with pH, that is then detected using a similar color scheme. The monitor consists of a tiny plastic bar ("sliver sensor"), 100-300 microns wide and 1-15 mm long, placed just under the skin, with optical spots or stripes for each analyte as well as blanks for calibration. The colors are read and translated into concentration values by a watchlike device placed above the skin. Direct optical coupling between the in vivo sensing bar and the ex vivo detector device requires negligible power, and eliminates the need for wires or optical fibers crossing the skin. The microminiature sliver penetrates the skin easily and painlessly, so that the user could insert it him- or herself. No risk of track infection exists. We are reporting here on the first successful in vitro tests of this approach.

Implantable chemical sensors for real-time clinical monitoring: progress and challenges

Recently, progress has been made in the development of implantable chemical sensors capable of real-time monitoring of clinically important species such as PO(2), PCO(2), pH, glucose and lactate. The need for developing truly biocompatible materials for sensor fabrication remains the most significant challenge for achieving robust and reliable sensors capable of monitoring the real-time physiological status of patients.

Defining the period of recovery of the glucose concentration after its local perturbation by the implantation of a miniature sensor

At the wound caused by implanting a subcutaneous glucose sensor, the concentration of glucose differs transiently from that in the subcutaneous fluid near non-wounded skin. The period of recovery differs for different wounds and is difficult to predict. A diabetic patient implanting a subcutaneous sensor needs to know whether the transient difference subsided sufficiently for the sensor readings and the sensor's in vivo calibration to be valid. The miniature amperometric glucose sensor has a compound membrane including a layer of glucose oxidase the reaction centers of which are electrically connected to the electrode through a redox polymer; layers excluding interferents and poisons of the electrocatalytic oxidation of glucose; and a layer reducing fouling by components of the biological fluids. We show that the sensor maintains its in vitro sensitivity after its implantation for four hours in the jugular vein and in the peritoneal fluid of the rat. For a diabetic patient who implants the sensor the four-hour period is long enough for the insertion-trauma-caused local perturbation of the glucose concentration to subside and to safely rely on the readings of the implanted sensor.

Implanted electrochemical glucose sensors for the management of diabetes

By maintaining a near normal (70-120 mg/dL) glucose concentration, diabetic patients can drastically reduce the likelihood of the occurrence of diabetes complications. In the near future, subcutaneously implanted electrochemical glucose sensors will be available to provide frequent or continuous information on which timely treatment decisions, such as insulin injection or glucose source intake, can be based, as well as timely alarm signals. The currently engineered devices are of three types: (a) innocuous microsensors, with actively mass-transporting areas < 10(-3) cm2, replaced twice a week by the patient; (b) self-contained, surgeon-implanted, transmitter-containing packages of > 1 cm2 area, operating for > 100 days; and (c) devices transporting subcutaneous fluid to an external sensor, based on implanted microfiltration or microdialysis fibers or on iontophoretic transport of the subcutaneous fluid through the skin.

Methods for reducing biosensor membrane biofouling

The deleterious effect that biofouling has on sensor stability is a serious impediment to the development of long term implanted biosensors. This paper reviews the surface modification strategies currently employed to minimize membrane biofouling of in vivo sensors. Nine sensor modifications are discussed herein: hydrogels, phospholipid-based biomimicry, flow-based systems, Nafion, surfactants, naturally derived materials, covalent attachments, diamond-like carbons, and topology.

In situ assembled mass-transport controlling micromembranes and their application in implanted amperometric glucose sensors

Micromembranes were assembled by sequentially chemisorbing polyanions and polycations on miniature (5 x 10(-4) cm2) enzyme electrodes. The sequential chemisorption process allowed the simultaneous tailoring of their sensitivity, dynamic range, drift, and selectivity. When assembled on tips of 250-microm-diameter gold wires coated with redox polymer-"wired" glucose oxidase, they allowed tailoring of the glucose electrodes for > 2 nA/mM sensitivity; 0-30 mM dynamic range; drift of < or =5% per 24 h at 37 degrees C at 15 mM glucose concentration; and < or =5% current increment by the combination of 0.1 mM ascorbate, 0.2 mM acetaminophen, and 0.5 mM urate. The membranes also retained transition metal ions that bound to and damaged the redox polymer "wiring" the enzyme. The electrodes were tested in the jugular veins and in the intrascapular subcutaneous region of anaesthetized and heparinized nondiabetic Sprague-Dawley rats, in which rapid changes of glycemia were forced by intravenous injections of glucose and insulin. After one-point in vivo calibration of the electrodes, all of the 152 data points were clinically accurate when it was assumed that after insulin injection the glycemia in the subcutaneous fluid lags by 9 min behind that of blood withdrawn from the insulin-injected vein.

A glucose oxidase electrode based on electropolymerized conducting polymer with polyanion-enzyme conjugated dopant

An enzyme immobilization method has been developed by electropolymerization chemistry of conducting polymer which results in a more effective and reproducible enzyme electrode. As a model system, in this study, glucose oxidase (GOD) was conjugated with a polyanion, poly(2-acrylamido-2-methylpropane sulfonic acid), via a poly(ethylene oxide) spacer to improve the efficiency of enzyme immobilization into a conducting polymer. GOD was successfully conjugated with a high conjugation yield of more than 90%, and its bioactivity was preserved. The resulting polyanion-GOD conjugate was used as a dopant for the electrochemical polymerization of pyrrole. Polypyrrole was effectively deposited on a Pt wire working electrode with the polyanion-GOD conjugate. The enzyme electrode responded to glucose concentrations of up to 20 mM with a sensitivity of 40 nA/mM at an applied potential of 0.4 V within a response time of 30 s. Although the response signal decreased at the low applied potential of 0.3 V, the enzyme electrode showed sensitive response signals of about 16 nA/mM up to 20 mM in glucose concentration. Under the deoxygenated condition, reduced but clear response current signal was obtained. The results show that the current signal response of the enzyme electrode to glucose concentration may be produced by mixed mechanisms.

Rise in background current over time in a subcutaneous glucose sensor in the rabbit: relevance to calibration and accuracy

In order to calibrate a continuous glucose monitor, accurate determination of the background current (I0) is necessary, in part because I0 could change over time. We compared two methods of I0 measurement: (1), extrapolation of sensor output data (as a function of glucose level) to the intercept at zero glucose and (2) direct measurement of the output of a blank anode with no enzyme coat. We implanted telemetric sensors subcutaneously in rabbits and measured their outputs during tri-level glucose clamps once per week for 5 weeks. The two methods yielded similar results. I0 rose substantially over time and this increase reached significance during week 3 by the direct method but not until week 5 by the extrapolation method. Using the direct method, I0 rose from 3.41 (0.60-8.48 nanoamperes (nA), median and range) during week 1 to 13.42 (9.1-14.3) during week 5. Using the extrapolation method, I0 rose from 0.57 (0-16.7) during week 1 to 15.3 (12.2-21.6) during week 5. We conclude that I0 can rise over time. If this rise went undetected and was assumed to be stable, a one-point calibration procedure would overestimate glycemia in the hypoglycemic range, i.e. fail to appreciate the severity of hypoglycemia. It is recommended that during validation of a chronic glucose sensor, I0 be measured sequentially over time.

A ferric chloride pre-treatment to prevent calcification of Nafion membrane used for implantable biosensors

Since the perfluorosulfonated ionomer Nafion, commonly used for the protection of biosensors, experiences calcification in a biological environment, we evaluated the efficacy of preincubating Nafion membranes in a FeCl3 solution to reduce the number of nucleation sites responsible for the growth of the calcium phosphate crystals. Nafion membranes were prepared and divided into two groups. In the first group, the Nafion membranes were pre-incubated in 0.1 M FeCl3 for a 24 h period. In the second group, no pre-incubation took place. All membranes were placed in a culture medium for a period of up to 4 weeks. All membranes were then examined for changes in: (1) their surface topography (using scanning electron microscopy (SEM)); (2) their near surface chemical properties (using energy dispersive X-ray (EDX)); and (3) their permeability to glucose. The membranes that were not pre-incubated in FeCl3 showed significant cracking of the Nafion surface, extensive calcium phosphate deposits and a resulting decrease in permeability. In contrast, the membranes treated with FeCl3 showed almost no cracking, very little calcium phosphate deposits and no change in permeability to glucose. This study demonstrated that FeCl3 significantly reduces calcification of Nafion and thus should help in preserving the in vivo function of implantable biosensors that utilize Nafion in their design.

Continuous amperometric monitoring of glucose in a brittle diabetic chimpanzee with a miniature subcutaneous electrode

The performance of an amperometric biosensor, consisting of a subcutaneously implanted miniature (0.29 mm diameter, 5 x 10(-4) cm2 mass transporting area), 90 s 10-90% rise/decay time glucose electrode, and an on-the-skin electrocardiogram Ag/AgCl electrode was tested in an unconstrained, naturally diabetic, brittle, type I, insulin-dependent chimpanzee. The chimpanzee was trained to wear on her wrist a small electronic package and to present her heel for capillary blood samples. In five sets of measurements, averaging 5 h each, 82 capillary blood samples were assayed, their concentrations ranging from 35 to 400 mg/dl. The current readings were translated to blood glucose concentration by assaying, at t = 1 h, one blood sample for each implanted sensor. The rms error in the correlation between the sensor-measured glucose concentration and that in capillary blood was 17.2%, 4.9% above the intrinsic 12.3% rms error of the Accu-Chek II reference, through which the illness of the chimpanzee was routinely managed. Linear regression analysis of the data points taken at t>1 h yielded the relationship (Accu-Chek) = 0. 98 x (implanted sensor) + 4.2 mg/dl, r2 = 0.94. The capillary blood and the subcutaneous glucose concentrations were statistically indistinguishable when the rate of change was less than 1 mg/(dl. min). However, when the rate of decline exceeded 1.8 mg/(dl.min) after insulin injection, the subcutaneous glucose concentration was transiently higher.