Can Continuous Glucose Monitoring Be Used for the Treatment of

Can Continuous Glucose Monitoring Be Used for the Treatment of

Diabetes Gérard Reach INSERM U341 (Biomedical Engineering and Diabetes) Hôtel-Dieu Hospital Paris, France

George S. Wilson Department of Chemistry University of Kansas Lawrence, KS 66045

During the past 10 years intense effort has been directed toward the development of biosensors. No goal has been more important than the realization of continuous blood glucose monitoring as an aid to diabetes therapy. A t t a i n m e n t of this goal, however, is made elusive by the need to consider the specific analytical requirements of this special application. In this REPORT our aim is to describe the nature of the analytical problem from chemical and clinical perspectives and to explore possible solutions. Glucose-sensing function of pancreatic beta cells I n s u l i n is a v i t a l h o r m o n e t h a t makes it possible to assimilate nutri0003 - 2700/92/0364 -381 A/$02.50/0 © 1992 American Chemical Society

? REPORT ents. It is secreted continuously by the beta cells of the pancreas, and peak secretions coincide with meals. By allowing the passage of glucose into the cells and turning off hepatic production of glucose during glucose intake, insulin prevents hyperglycemia after a meal. A unique feature of insulin secretion is that it is regulated mainly by

glucose concentration in the blood. This regulation is an intrinsic property of beta cells. Insulin secretion in r e s p o n s e to glucose is e x t r e m e l y rapid; it occurs within 1 min and can be triggered by a glucose concentration as low as ~2 mM. Once insulin has produced its effect (i.e., brought the glucose concentration back to its basal level), peak insulin secretion stops, thus avoiding a swing in the opposite direction that would lead to hypoglycemia. This overall mechanism, known as glucose-controlled i n s u l i n secretion, keeps the blood glucose concentration within very narrow limits (80120 mg/dL or 4.4-6.6 mM) in normal persons in spite of the fact that various amounts of carbohydrates are consumed both during and between meals. Figure 1 compares blood glucose fluctuations in normal and diabetic patients. Note that even when insulin is administered, blood glucose levels vary widely. Glucose monitoring for diabetes mellitus In Type I diabetes mellitus, a disease observed in three to seven individuals per 1000, this feedback homeo-

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REPORT stasis of blood glucose concentration is lost. The destruction of the pancreatic beta cells leads to severe insulin deficiency, which (in the absence of insulin replacement) would lead to death because of the inability to assimilate nutrients. Fortunately, the discovery of i n s u l i n in t h e early 1920s transformed the prognosis of what had been a rapid and fatal disease. Patients were able to survive by injecting themselves with insulin two to four times a day. Unfortunately, there is a major difference between the normal secretion of insulin by the pancreas and external methods of insulin administration. Because there is no automatic regulation of insulin administration, an inadequate amount of insulin accompanying a meal leads to hyperglycemia and an excessive dose induces hypoglycemia. The p a t i e n t must therefore adjust the insulin dose daily according to estimations of the effect of insulin injected on the previous day. Patient test kits were developed to help make such estimations. Initially the kits consisted of chemical strips for the detection of glucose in urine. (The presence of abnormal amounts of sugar in the urine indicates that in the past few hours blood glucose was higher than the normal threshold for urinary excretion of glucose, which is -170 mg/dL). Since the early 1980s blood glucose levels have been selfmonitored by placing a drop of blood on dry chemical strips. The resulting

color development is related to glucose concentration and can be evaluated visually or by using a reflectance-based glucose meter. P a t i e n t s are t a u g h t to perform such measurements and are encouraged to make them as often as possible. Highly motivated patients (e.g., pregnant women with diabetes) may m a k e m e a s u r e m e n t s u p to seven times a day—before and after each meal and at bedtime. Most patients, however, are reluctant to perform self-monitoring more t h a n two or three times a day, and many patients simply refuse to perform this boring task. The advent of self-monitoring is considered a major advance in diabetes management, although it is a discontinuous process t h a t tells t h e patient about blood glucose concentration only at the moment it is performed. If blood glucose determinations are widely spaced, there is no indication of t h e fluctuations t h a t can occur between two m e a s u r e ments. These fluctuations can be so rapid, particularly after a meal or after insulin administration, that hypoglycemic episodes can occur in spite of frequent monitoring. Hypoglycemic episodes are, at best, unpleasant and can, at worst, lead to u n c o n s c i o u s n e s s . To avoid such risks, patients often set their glycémie (blood glucose) objectives at a level well above the normal range because it is very difficult to predict the balance between insulin injection

and glucose consumption. This discrepancy p a r t i a l l y e x p l a i n s why, even with t h e most sophisticated regimens for insulin administration, it is impossible to maintain normal blood glucose values regularly. For example, among the thousands of patients enrolled in the large Diabetes Control and Complication Trial c u r r e n t l y r u n n i n g in t h e U n i t e d States, interim results indicate that those in t h e e x p e r i m e n t a l group (those undergoing intensive insulin therapy) have improved their levels of glycosylated hemoglobin (a marker of metabolic control), but they have not reached the normal range {!). This result is of concern because inadequate glycémie control is at least partially responsible for the development of diabetes complications later in life t h a t can lead to disabilities such as blindness or end-stage renal failure (2). For example, in Europe between 1976 and 1985, diabetes was found to be the cause of -10% of the new patients accepted for renal replacement therapy (3). Great expectations The previous example points to a great urgency for the development of a system for continuous glucose monitoring. The ultimate goal is a closedloop insulin delivery system, essentially an artificial beta cell consisting of an insulin pump. The flow rate of the pump would be controlled by the continuous estimation of blood glucose concentration by the sensor. A less ambitious but equally important application is the use of a sensor as part of an alarm system to inform the patient about blood glucose concentrations outside the normal range, especially in the direction of hypoglycemia. Such a safeguard would encourage t i g h t e r glycémie control and provide reassurance to those patients exhibiting "hypoglycemic u n a w a r e n e s s , " a condition in which the patient is completely unaware of an impending hypoglycemic episode and therefore does not ingest glucose. Glucose sensor for in vivo monitoring

Figure 1. Time-dependent blood glucose levels in normal and diabetic patients. 382 A · ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15, 1992

Clark suggested a promising a p proach to glucose m o n i t o r i n g (4) nearly 30 years ago in the form of an enzyme electrode that uses the oxidation of glucose by the enzyme glucose oxidase. This approach has been incorporated into several clinical analyzers used routinely for in vitro blood glucose estimation. The general approaches used for glucose detection are shown in Figure 2.

In the first approach, oxygen consumption can be measured. However, a reference, nonenzymatic electrode is also required to provide an amperometric difference signal. The inner membrane can be made permeable to oxygen while eliminating electroactive interferences (Figure 2a). The second approach, based on detecting hydrogen peroxide, requires an applied p o t e n t i a l of 650 mV, which is sufficiently high to oxidize other endogenous, interfering species such as ascorbate (Figure 2b). Elimination of these interferences requires a permselective inner membrane or a scavenger enzyme (5). A third approach (Figure 2c) takes advantage of the fact that the enzymatic reaction requires two steps: reduction of the enzyme by glucose and oxidation of the reduced enzyme by an electron acceptor (normally oxygen). Direct electron transfer between the enzyme and the electrode does not occur at a significant rate; therefore, an electron acceptor mediator is required to make this process proceed rapidly and efficiently. This result has been accomplished by using redox polymers (6, 7) and by adsorbing or trapping a small molecule, such as ferrocene, within the enzyme layer (8). For stable, long-term use, the mediator must not diffuse out of the enzyme layer and must effectively compete with oxygen for the reduced enzyme. Optical sensing detectors have been suggested (9, 10) but, because of its simplicity, the amperometric approach seems to be the most promising method for continuous in vivo monitoring. In the early 1970s macroscopic systems were designed for automatic control of blood glucose in diabetic patients. The best known system, the Biostator, consists of a flow-through chamber where glucose is detected in diluted blood by the use of a sensor configured similarly to the one in Figure 2b. Blood is drawn by a catheter connected to a double-lumen needle in the patient's vein. The current generated during glucose oxidation is processed by a computer that controls the infusion of insulin into the patient by using a pump. This "artificial pancreas" is useful as a research tool. Unfortunately, this system is inappropriate for daily treatment of diabetes because of its size; the need for catheterization, heparinization, and dilution of the blood; and the fouling of the sensor's outer membrane by blood and cells, necessitating frequent calibration. It is evident that the glucose sensor is the weak link.

"If somebody says that a task is mechanical, this does not mean that people are incapable of doing the task, it only means that only a machine could do it over and over without ever complaining or feeling bored." (Douglas Hofstadter, BUdel, Escher, Bach: An Eternal Golien Broie

Figure 2. Detection approaches for the glucose enzyme electrode. Detection based on (a) oxygen, (b) peroxide, and (c) a mediator. Red and Ox stand tor the reduced and oxidized forms, respectively, of the mediator. FAD and FADH2 are the coenzyme flavin adenine dinucleotide and its reduced form, respectively.

The development of portable, lightweight pumps for insulin delivery (11) and the need to achieve better metabolic control to avoid diabetic complications (2) also have been demonstrated. Despite two decades of research, patients have yet to benefit from continuous in vivo monitoring. Clearly there is a big difference between a short-term, in vitro functioning system and an implantable one. In the field of artificial organs, the development of an implantable artificial organ is often based on the hope that a system initially designed for in vitro application can be used in vivo (e.g., the artificial kidney as an in vivo application of dialysis) and that a system first designed for short-term use can be implanted and will function on a long-term basis. Developing an implantable artificial organ thus is very difficult. To take the same example, the implantable artificial kidney is today but a dream. Another point should be considered in the development of an artificial pancreas. Although diabetes can result in life-threatening complications, many individuals lead long and productive lives without developing kidney failure or blindness. Diabetes does not immediately threaten life as do end-stage renal, cardiac, or hepatic failures; it can be treated, imperfectly, but (almost) safely by external injections of insulin. The major r e q u i r e m e n t for a glucosesensing system is safety. Safety must be considered from two points of view. First, the implanting of the sensor must not be a source of any danger to the patient. For example, implanting a sensor in the vascular bed of a h u m a n on a long-term basis presents substantial risk of clot formation and infection. Thus, even though such a system can function satisfactorily for several months in a limited number of dogs (12), the risk may not justify the potential advantages. Second, the consequences of glucose sensing must not be a source of danger to the patient (e.g., an inappropriate delivery of insulin). If the sensor is considered part of a closed-loop system, it must be extremely reliable. These factors were recognized in the early 1980s and led to the concept of glucose sensing in alternative, extravascular sites in the body, using a needleshaped glucose sensor (13). Where should glucose be sensed? When considering a site for glucose sensing, one must evaluate the theo-

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REPORT retical and the technical feasibility. Theoretically, the two places where glucose concentration is identical to plasma glucose concentration, under stationary and dynamic conditions, are the peritoneal cavity and the subcutaneous tissue. We have shown t h a t glucose concentration in the peritoneal cavity of rats changes rap­ idly during both insulin-induced hy­ poglycemia and glucose-induced hy­ perglycemia (14). Glucose concentration in the sub­ cutaneous fluid and in the blood of dogs and humans is initially identi­ cal. Changes in blood glucose concen­ tration are transmitted to the subcu­ taneous tissue within 5 min. This result was demonstrated by compar­ ing p l a s m a glucose concentration and subcutaneous glucose concentra­ tion using the wick technique (a cot­ ton thread is implanted subcutaneously, removed, and then centrifuged to collect the fluid) in dogs (15) and using microdialysis in humans (16). It has also been demonstrated that blood glucose homeostasis can be controlled while feeding glucose orally to diabetic dogs by using an artificial pancreas whose sensing el­ ement is placed in the subcutaneous tissue (17). Technically, implantation in the peritoneal cavity requires t h a t the lifespan of the sensor be long enough to avoid frequent sensor replace­ ment. In this context, the implant­ able insulin pump would be expected to work for at least five years. Unfor­ tunately, a sensor with such a life span has not yet been developed. On the other hand, the subcutaneous site demands a sensor of an appro­ priate size and shape so that it can be readily replaced by the patient. Experience indicates t h a t diabetic patients inject themselves with insu­ lin using needles of 2 6 - 2 9 gauge (-0.35 mm o.d.). Patients who wear external pumps change the needle at the tip of the catheter about every two days. How­ ever, such frequent replacement is not realistic for a glucose sensor if one takes into account the time re­ quired to get a stable signal after re­ implantation. Most of the glucose sensors we have described have sta­ bilization times of - 3 - 4 h. Shorten­ ing this time is a priority in sensor development. Other important con­ siderations are the hazards (e.g., in­ fection) associated with transcutane­ ous implantation of any device. With these factors in mind, a glu­ cose sensor that uses a needle simi­ lar in size to those commonly used by diabetic patients, which is changed

approximately every five to seven days, would represent a reasonable objective for glucose monitoring in subcutaneous tissue—currently the most realistic site for glucose sens­ ing. The long-range goal is a nonin­ vasive sensor t h a t could accomplish the same objectives. General requirements and design implications In addition to the size and shape re­ quirements, the sensor must have certain characteristics dictated by the expected fluctuations in glucose levels: a linear range up to 15-20 mM (nonlinear response can be used, but calibration becomes complicat­ ed); response independent of ambient p02 levels (these first two character­ istics can be defined by proper con­ trol of the external membrane [Fig­ ure 2, Reference 18]); and < 5 min response time. Mathematical model­ ing indicates t h a t the overall r e ­ sponse time for a closed-loop insulin delivery system must be < 10-15 min to avoid changes in blood glucose concentrations outside normal ranges following orally ingested glu­ cose and subsequent overadministration of insulin (19). Another desirable characteristic is sensitivity high enough to produce an acceptable signal-to-noise ratio. This level can be achieved with sen­ sitivities on the order of 0.3-3 nA/ mM. In the past few years various groups have described several sys­ tems that meet these requirements, and the in vitro characteristics of such a sensor designed by our group have also been described (18). When such a sensor is implanted in the subcutaneous tissue of a rat, the in­ t r a p e r i t o n e a l injection of glucose produces an extremely rapid increase in the signal generated by the sensor. Special problems in subcutaneous glucose monitoring Calibration. Calibration of a sensor placed in the vascular bed is trivial because blood glucose values are ob­ tained directly. In the case of a subcutaneously implanted glucose sen­ sor, establishment of the correlation with blood glucose values is less straightforward. Using the in vitro parameters (the background current in the absence of glucose and the sensor's sensitivity) would be accept­ able only if it were verified that these parameters are always identical in vitro and in vivo. However, this situ­ ation is not the case. Several groups have confirmed that the in vivo sensitivity is lower when the glucose sensor is implanted

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Figure 3. In vivo sensor calibration. (a) Definition of calibration parameters and (b) estimation of subcutaneous glucose concentration (SGC).

in the subcutaneous tissue of r a t s (20), dogs (15), and humans (21) than it is when evaluated in vitro. The reasons for this phenomenon are not clear, but it cannot be explained sim­ ply as blockage of the sensor surface by protein. The sensor can eventually reach a stable value; therefore, it is both useful and essential to calibrate it in vivo. Thus we have proposed a two-point in vivo calibration (22) (Figure 3a). The blood glucose level is changed by administering glucose and mea­ suring two blood glucose levels, Cx and C2. By comparing them with cor­ responding values of the current, it is possible, by linear extrapolation, to determine an in vivo sensitivity in nA/mM and an extrapolated back­ ground current t h a t would be ob­ served in vivo in the absence of glu­ cose (I0). Finally, the current at any time (Γ) can be used to estimate the subcutaneous glucose concentration (SGC) by using the formula SGC = (/-/„) / (Δ/ / ΔΟ where AC is the change in blood glu­ cose concentration and AI is the cor­ responding variation in sensor out­ put. As shown in Figure 3b, it is possible to obtain good correspon­ dence between the blood glucose and subcutaneous glucose levels. Figure 4 represents the evaluation of SGC in a dog submitted to two consecutive infusions of glucose (23).

Figure 4. In vivo sensor response in a dog. The dog is fed glucose for 20 min and again for 45 min, as indicated by the bars, (a) Time-dependent blood glucose levels. P1 - 5 represent plateaus in blood glucose levels before and after glucose infusion, (b) Sensor output, (c) Comparison of calibrated sensor with blood glucose levels, with no correction for time lag. (Adapted with permission from Reference 23.)

Figure 4a shows the time - dependent variation in blood glucose level, Figure 4b illustrates the concomitant variation in sensor output, and Figure 4c is a comparison of the two responses on the same scale. Several plateaus are created at high or low glucose levels, during which time the subcutaneous and blood glucose values equilibrate. The upper plateau is created by glucose infusion. Similar results have been obtained in rats (22, 24). For patients, calibration could be done by measuring glycemia with test strips before and after meals.

This information would then be entered into a microprocessor-based monitoring unit. This approach has been demonstrated to be feasible in dogs (25). D e p e n d e n c e on o x y g e n t e n s i o n in the s u b c u t a n e o u s tissue. It has been claimed that glucose detection could be flawed by variations in the oxygen tension at the sensing site. This claim was one rationale for developing sensors based on oxygen detection (12) or sensors using mediators such as ferrocene (26). As noted above, this obstacle can be overcome

if the outer membrane t h a t limits glucose diffusion is much more restrictive to glucose diffusion than to oxygen diffusion. We have verified that such sensors are essentially independent of oxygen tension, both in vitro and in vivo in rats (27), as long as the oxygen tension is higher t h a n 8 mm Hg. Normal tissue levels should be in the range 2 0 - 2 5 mm Hg. Fischer et al. (28) made the same observation with a nonminiaturized sensor. Interferences. A system based on electrochemical detection of hydrogen peroxide generated during glucose oxidation can be biased by the presence of electroactive substances susceptible to oxidation at the operating potential. Usually the enzyme is separated from the working electrode by a membrane that acts as a filter for substances (such as ascorbate and urate), and the interferences produced can be readily eliminated. However, acetaminophen—a substance widely known for therapeutic use—cannot be easily eliminated. For example, a concentration of 0.2 mM (commonly observed in h u m a n plasma after the ingestion of 1 g of acetaminophen) would produce an in vitro current of the same order of magnitude as that produced by 5 mM glucose (29). Similar results were observed with a glucose sensor implanted in subcutaneous rat tissue. Preliminary experiments in our laboratory suggest that this problem can be solved. Implantability, biocompatibility, and biostability. Wide experience indicates that it is impossible to implant an object without invoking a system response. Early research on this problem led to the pessimistic conclusion that it might be difficult to develop a sensor with a life span longer than two or three days (13, 30). An inflammatory reaction is observed around the sensor after this period of time, and it was suggested that the reaction could be responsible for the decrease in the sensor's sensitivity by limiting access of the analyte to the sensor. Factors contributing to inflammatory response, which are extremely complex, include the size and shape of t h e i m p l a n t e d object and t h e chemical and physical morphology of its surface. Experience has shown that the size, shape, and rigidity of the sensor may play important roles in the system response. We have designed a miniaturized, flexible sensor capable of working up to at least 10 days following implan-

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REPORT for Artificial Internal Organs 1991, 37, M298-M300. (24) Moatti, D.; Capron, F.; Poitout, V; Reach, G.; Bindra, D. S.; Zhang, Y; Wilson, G. S.; Thévenot, D. R. Diabetologia, in press. (25) Poitout, V.; Moatti-Sirat, D.; Reach, G., submitted for publication in Biosensors and Bioelectronics. This work is supported in part by the National (26) Claremont, D. J.; Sambrook, I. E.; Institutes of Health (DK30718), the Institut Penton, C; Pickup, J. C. Diabetologia National de la Santé et de Recherche Médicale 1986, 29, 817-21. (INSERMX899014), and L'Aide aux Jeunes Di(27) Zhang, Y., unpublished results, abétiques. 1991. (28) Fischer, U.; Hidde, Α.; Hermann, S.; von Woedtke, T.; Rebrin, K.; Abel, P. References Biomed. Biochim Acta 1989, 48, 965-71. (1) Diabetes Control and Complication (29) Moatti-Sirat, D.; Velho, G.; Reach, Trial Research Group. Diabetes Care G. Biosensors and Bioelectronics, in press. 1987, 10, 1-19. (30) Rebrin, K.; Fischer, U.; Hahn von (2) Tchobroutsky, G. /. Diabetic ComplicaDorsche, H.; von Woedtke, T.; Abel, P.; tions 1989, 3, 1-5. Brunstein, E. Journal of Biomedical Engi­ (3) Brunner, F. P.; Brynger, H.; Fassneering 1992, 14, 33-40. binder, W. et al. Nephrol. Dial. Transplant. (31) Williams, D. F. Enjeux 1986, 74, 441988, 3, 585-95. 45. (4) Clark, L. C, Jr.; Lyons, C. Ann. N.Y. (32) Zhang, Y.; Bindra, D. S.; Barrau, Acad. Sci. 1962, 102, 29-45. M-B.; Wilson, G. S. Biosensors and Bio­ (5) Maidan, R.; Heller, A. /. Am. Chem. electronics 1991, 6, 653-61. Soc, in press. (6) Gregg, Β. Α.; Heller, A. /. Phys. Chem. 1991, 95, 5976-80. (7) Foulds, N. C; Lowe, C. R. Anal. Chem. 1988, 60, 2473-78. (8) Cass, A.E.G.; Davis, G.; Francis, G. D.; Hill, H.A.O.; Aston, W. J.; Higgins, I. J.; Plotkin, E. V.; Scott, L.D.L.; Turner, A.P.F. Anal. Chem. 1984, 56, 667-71. (9) Arnold, Μ. Α.; Small, G. W. Anal. Chem. 1990, 62, 1457-64. (10) Srinivasan, K. R.; Mansouri, S.; Schultz, J. S. Biotechnol. Bioeng. 1986, 2, 233-39. (ll)Slama, G; Hautecouverture, M.; Assau, R.; Tchobroutsky, G. Diabetes 1974, 23, 732-38. (12) Armour, J. C; Lucisano, J. V.; McGerard Reach (right) became director of Kean, B. D.; Gough, D. A. Diabetes 1990, the INSERM Research Unit No. 341, Bio­ 39 1519—26 medical Engineering and Diabetes Melli(13) Shichiri, M.; Y a r n a s a k i , Y.; Kawamori, R.; Hakui, N.; Abe, H. Lancet tus, in 1991. He received his M.D. degree 1982 2 1129—31 from the University of Paris in 1977 and (14) vèlho, G.; Froguel, P.; Reach, G. Diadid postdoctoral work at the Mayo Clinic. betologia 1989, 32, 331-36. His research interests include the develop­ (15) Fischer, U.; Ertle, R.; Abel, P.; Rement of in vivo glucose sensors and the use brin, K.; Brunstein, E.; Hahn von Dorsche, H.; Freyse, E. J. Diabetologia 1987, of isolated islets of Langerhans for the bio30, 940-45. artificial pancreas. In addition, he prac­ (16) Jansson, P. Α.; Fowelin, J.; Smith, U.; Lbnnroth, P. Am. ]. Physiol. 1988,255, tices at the Hôtel-Dieu Hospital in Paris, E218-20. where he cares for diabetic patients in(17) Rebrin, K.; Fischer, U.; Woedtke, cluding children. He is also the current edT. V.; Abel, P.; Brunstein, E. Diabetologia itor-in-chief of Diabète et Métabo1989, 32, 573-76. lisme. (18) Bindra, D. S.; Zhang, Y.; Wilson, G. S.; Sternberg, R.; Thévenot, D. R.; Moatti, D.; Reach, G. Anal. Chem. 1991, George S. Wilson is Higuchi Distin63, 1692-96. guished Professor of Chemistry and Phar(19) Sorensen, J.; Colton, C. K.; Hillman, R. S.; Soeldner, J. S. Diabetes Care 1982, maceutical Chemistry at the University of 5, 148-56. Kansas. He received his Ph.D. in chemis(20) Velho, G.; Froguel, P.; Thévenot, try from the University of Illinois in 1965 D. R.; Reach, G. Biomed. Biochim. Acta and remained there for two years as a 1989, 48, 957-64. postdoctoral fellow in biochemistry. After (21) Kerner, W.; Keck, F. S.; Bruckel, J.; 20 years at the University of Arizona, he Zier, H.; Pfeiffer, E. F. Presented at the AIDSPIT Study Group Meeting, Igls, assumed his current position. His reAustria, Jan. 28-30, 1990. search interests are in the areas of biologi(22) Velho, G.; Froguel, P.; Thévenot, cal electron transfer involving proteins, D. R.; Reach, G. Diabetes Nutrition and biochemical reactions of sulfur-containMetabolism, Clinical and Experimental 1988 1 227—33 ing species, and the application of biologi(23) POit'out, V.; Moatti, D.; Velho, G.; cal recognition (antibodies and enzymes) Reach, G.; Sternberg, R.; Thévenot, to analysis. He is also chairman of the D. R.; Bindra, D. S.; Zhang, Y.; Wilson, G. S. Transactions of the American Society IUPAC Commission on Electrochemistry. pump systems. The glucose sensor must operate reliably in an in vivo environment, provide the clinical information needed, and be easy to operate and manufacture.

Figure 5. Schematic of implant response. (a) Initial implant and (b) stabilized implant.

tation (24). Histological examination of the tissue surrounding the sensor revealed the presence of regenerated proximal capillaries (24). Thus the inflammatory reaction could be considered beneficial for long-term sensor function. This conclusion leads to careful consideration of the concept of biocompatibility, defined by the European Society for Biomaterials as "the ability of a material to perform with an appropriate host response in a specific application" (31). It is possible that, after implantation, a biostable environment will be rapidly formed and that it will be compatible with the sensor's function (Figure 5). In this case, the capillaries damaged by implantation are regenerated and the sensor is encapsulated by a layer of collagen permeable to both glucose and oxygen. Again, the real life span of the sensor would then be determined by the tissue acceptance of a transcutaneous implant. The toxicity of the sensor with respect to the materials used in its construction and the products generated by the enzymatic reaction must also be considered. Methods have been developed for evaluating toxicity and for sterilizing the sensor without loss of enzymatic activity (32). Summary In the case of the glucose sensor, clinicians and chemists must cooperate in interdisciplinary research to carefully define the analytical problem. Although not specifically discussed in this article, another group t h a t must participate in this effort is engineers. Their expertise is needed to design the monitoring and control unit that contains the alarm and

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