Electrochemical cell sensor for continuous, short- term use in tissues and blood

lZl373~ I~:Lk:C'l'l~OCllllMICAL Cl:l.L S~:NSOl~ FOI~ ~:ONTINUOUS, S~IORT--TERM USE It~ TISSUE:S AND BLOOD BACKGROUNV OF TEIE INVENTION A variety of biomedical sensors are routinely used by physicians or clinicians to monitor physiological variables such as respiratory rate, blood pressure, and temperature.

A relatively new addition to the repertoire of biomedical sensors is the enzyme electrode.

This is a sensor that combines certain analytical enzymatic techniques with commonly used chemical-selective electrodes. Enzy~ne electrodes enable the user to determine the concentration of certain biochemicals ra~idly and with considerable accuracy.

Currently there are enzyme electrodes that can detect urea, uric acid, glucose, various alcohols, and a number of amino acids when used in certain well-defined situations.

of the available enzyme electrodes, perhaps the one that is rnost widely used is the glucose electrode, of which there exist several variations. The first repor-t that enzymes co~1ld be used to measure glucose was that of Clark in U.S.

Patent No. 3,539,455. They proposed that glucose could be detected amperometrically using the enzyme glucose oxidase held between two membranes surrounding an oxygen or ~ hydrogen peroxide electrode.

As glucose and oxygen diffuse ; 25 through the membrane, there was a reduction in ox~gen concentration proportional to the concentration of glucose in the sample fluid as a result o~ the enzymatic process described below.

Glucose oxidase ose t 2 -t ~J2 ~ E12O2 t Gluconic Acid [UOCGl-P~.C26~ U.C. CASE NO. 83-20~-1 ' , ~2873~ --2- The electrodc~ can be ~olarized cathodically -to detect resi~ual oxygen not consumed by the enzymatic process or polarized anodically to detect the product of the enzyme reaction, hydrogen peroxide.

~he glucose enz~me electrode was apparently first put into practice by ~licks et al. as describecl in U.S.

Patent No. 3,542,662.

These inventors employed two oxygen electrodes, unlike ~he single electrode design of Clark, and immobilized glucose oxidase on one of them. A dual enzyme electrode configuration, where one electrode had immobilized enzyme, was intende~ to be insensitive to changes in oxygen levels not mediated through glucose oxidase. Glucose oxidase was immobilized by entrapment in a polyacrylamide gel matrix over one o~ the oxygen electrodes. Since this electrode was still sensitive to changes in oxyqen tensisn, the difference between the output of the two oxyyen electrodes was recorded to reflect glucose concentrations that were relatively independent of fluctuations in background oxygen concentration.

Additional changes in the overall desiyn o~ the basic oxygen sensor as they relate to modifications in the enzyme membrane surroundiny the sensor or to mo~ifications in the electrodes are described in U.S.

Patent Nos. 4,356,074;

4,073,713; 1,442,303; 3,9~8,745; and 3,~47,777, respectively. None of these modi~ied enzyme oxygen sensin~ electrodes can be useæ to monitor m v vo levels of various enzyme substrates or their byproducts.

It is desirable to h~ve enzyme electrodes that can be implanted in patients to continuously monitor blood or tissue fluid concentrations. ~or instance, it is [UOCGl-PA.C26] U.C~ CASE NO. 83-208-1 ~2~3~7381~ - 3 - 66128-172 particularly desirable to have an implantable enzyme electrode sensor -for use in diabetics, to continuously monitor glucose concentrations. While there exist a number o~ oxidase-based enzyme electrodes capable of detecting glucose or other substances such as alcohol and uric acid ln vitro because of design fea-tures associated with these sensors, -they are not suitable for use to detect these substances ln vivo.

SUMMARY OF THE INVENTION An enzyme electrode sensor is described for determining directly in the body the concentration of certain biochemicals, particularly glucose, alcohol or uric acld, comprising a fine needle that can be implanted subcutaneously in the blood stream or in other bod~ compartments. The active region of the sensor is situated on the side of the needle rather than at the tip, thus permitting the construction of a small diameter needle sensor suitable for implantation and providing for sufficient sensitive area to produce an easily measured signal.

According to one embodiment of the invention, there is disclosed an electrochemical cell sensor capable o~ being implan 0 ted into an animal body comprising:a housing with an opening or openings in the wall of said housing, said housing comprising a hollow needle composed of platinum or stainless steel, and said housing and said opening or openings covered with a layer of porous biocompatible material;

electrode means situated in said housing and in fluid ~2~7~ - 3a - 66128~172 communication via said opening or openings with fluids present in said animal body and responsive to enæyme substrates or products present in said fluid, said electrode means comprising:

an enzyme substrate or product dependent sensing electrode in communication with an oxidase enzyme, an enzyme substrate independent oxygen-sensing electrode, a reference electrode, and a common counter electrode;

and means for relating said response of said electrode means to the concentration of said enzyme substrates or products present in said fluids.

DESCRIPTION OF THE DRAWINGS FIGURE 1 is a perspective view of one configuration of the sensor;

FIGUR~ 2 is a top plan view of the sensor, FIGURE 3 is a sectional view taken on Line 3-3 of Figure 2, FIGURE 4 is an enlarged sectional view taken on Line 4-4 of Figure 2;

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373~ -4- 66128-172 I;I~URE 5 is an enlarge~lse~tional view ~akerlo~l Line 5-5 o~ ure 2;

EIGUl~E 6 is a top plan view of an alternative configLlration of th~ sensor;

FIGUR~ 7 is a sectional view taken on Line 7-7 of Figure 6;

FIGURE 8 is an enlarged sectional view taken on Line 8 8 of Figure 7; and FIGURE 9 is an enlarged sectional view taken on Line 9-9 of Figure 7.

DETAILED DESCRIP~ION OF T~E INVENTION It is the purpose of this invention to provide an electrochemical cell sensor for determining ~n situ the presence of certain bioloyical molecules in bodily fluids ~here these molecules are substrates for or products produced by oxidase enzyMes. Virtually any substrate that is capable of undergoing enzymatic oxidation with molecular oxygen and which involves an enzyme catalyst can be detected. For the purpose of description only, the invention will be elucidated as to its use in measuring glucose, but it will be understood to those skilled in the art that it is not so limited.

The electrochemical cell sensor shown in Figure 1 comprises a housing 10 covered with biocompatible material, particularly useful 1s a fine hollow needle suitable for pierciny the skin.

In the preferred embodiment of the invention shown in Fisure 1, there are two elongated openings, 12 a~ld 14, in the wall of the housing that pro~ide a means whereby the interior of the housing can be in fluid [~OCGl-PA.C26~ U.C. C~SE NO. 83-208-1 73~ ~5~ 66128-172 ,.~ communication with the external environment. Figure ~ shows a top view of the openings 12 and 14. Situated in the ho~siny shown in Figure 3 at the respective openings are two oxygen sensors 16 and 18.

An oxidase enzyme is situated ~hysically near the surface of the sensor 18 by embedding it in a porous ~el 20 that surrounds the sensor.

This can be accomplished, for example, as described by G.

Guilbault et al. in U.S.

Patent ~lo. 3,94B,745 by chemically binding an enzyme to a gel matrix, particularly useful are matrices composed of acrylamide or acrylic. The gel-forming material can be introduced into the space between the electrodes and the housing and crosslinked or polymerized in place.

Alternatively, the sensor 18 can be covered by a porous membrane containing oxidase enzyme as described by Wingard et al in ~Ln~l Q~ ~iomedic~l ~5Ç~iDl~ Resea~çh (1979, 13:921-935)~ The second oxygen sensor 16 is utilize~ to monitor the oxygen concentration of the environment and, hence, is devoid of enzyme.

Figures 4 and 5 show cross-sectional views through the ensor at two different locations alony the sensor.

Figure 4 shows both oxygen-sensing electrodes and the silver/silver chloride reference electrode, while Figure 5 reveals only the oxygen sensing electrode embedded in an enzyme gel matrix and the reference electrode.

In the design in which the electrode that is in contact with the enzyme gel is polari~ed as an anoae for detection of the enzymatic product, hydrogen peroxide, the second ~lectrode is still polarized cathodically to determine oxygen concentration In this case, glucose is determined directly from the signal of the anode, and the oxygen sensor [UOCGl-PA.C26] U.C. CASE NO. 83-208-1 .... .. ..

~L2~3~3~ -6- 66128-172 is used for the determination of oxy~en to assure that sufficient oxygen is present to not limit the enzyme reaction.

In those instances where the first sensor 18 is embedded in a gel matrix containing enzyme, the second oxygen sensor is similarly embedded in a matrix 22 but lacking enzyme. The openings in the housing beneath which the sensors are situated may be covered with a thin porous material 24 in those instances where the electrochemical cell sensor is implanted in oxygen poor tissue. To avoid low oxygen concentrations ~rom being rate limiting in the enzymatic reaction, the material chosen should permit the electrochemical cell structure to remain sensitive to glucose over a useful range of concentrations in the presence of relatively low oxygen concentrations. This is accomplished by selecting a material that restricts the diffusion or partitioning of glucose while remaining relatively permeable ~o oxygen. Typically such membranes are made either of porous or perforated polydimethylsiloxane (Silastic~*. Alternat~vely, the dif~usion of gLucose can be controlled by incorporating domains of hydrophobic material in the gel on which the enzyme is embedded or attached, as suggested by D.

Gough in U.S.

Patent No. 4,484,987.

Each chemical sensor is situated in the electrochemical cell structure housing at a di~ferent opening, and preferably near the center of the housing. The two sensors are insulated ~rom each other with sui~able insulating material 26 and 27, such as ~used glass or epoxy. The sensor wires extend down the housing and emanate from its *Trade-mark [UOCGl-PA.C26] U.C. CASE NO. 83 208-1 ~/, : ` , ; . i.,. i .

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:~2~373~30 -7 hub allowing for connection to instrumentation routinely utilized in electrochemical monitoring procedures.

Additionally containecl in the electroche~ical cell struct~re housing is a ref~rence electrode 28 made of material well know to those in the art, a common example being chlorided silver.

The referenee eleetrode is preferably situated close to a unshieldecl segment of the two eleetrode sensors, and also exten~s out o~ the hub of the housing and is connected to the instrumentation. Lastly, the housing serves as a fourth eleetrode, a common eounter eleetrode to ~hich eurrent of the two eleetrode sensors flow. The housing is similarl~ eonnected to instrumentation by attaehment to a wire 30 at the hub.

When the electrochemical cell strueture is implanted into biological tissues or fluids containing glucose and oxygen, these substances communicate with the respective sensors by ingress through the openings of the housing.

Upon applying the characteristic potential between the respective sensors and the referenee electrode, eurrent passes between the sensors and the housinq counter electrode, resulting in the immediate consumption of oxygen at the sensor surfaces The sensor 18 that eontacts the oxidase enzyme eontaininy membrane experiences a reduction in oxygen flux or oxygen produeed current compared to the second sensor 16 due to prior consumption of oxygen by the enzymatie proeess. This relative decrease in current is a ~unetion o~ the glucose concentration present. Thus, the amount o~ glueose is determined by the differential eurrent output ~rom the two sensors. The output ean be quantified using operational amplifier eircuitry.

UOCGl-P~.C26~ U~C. CASE NO. R3-20B-l lll the desigrl incorporating the hydroyen peroxide anode, the current from that ~ensor is proportional to glucose concentration, provided that sufficient oxygen is present in the tissue to not limit the enzyme reactions.

This limit is determined by the second oxygen sensor. When the oxygen siynal is lower than the glucose signal r the latter is disreqarded.

A second embodiment of the invention is shown in Eigure~ 6 and 7. The electrochemical cell structure again comprises a housiny 10' with an opening 32 in a side of the housing, and again a fine hollow needle capable of piercing the skin being preferred for use as the housing. Figure 7 shows that within the housing is a single oxyyen-sells:ing noble-metal electrode 34 embedded in a gel matrix 20' containing enzyme as described earlier. Alternatively~ the electrode can be covered with an oxidase enzyme containing membrane. Additionally, the electrochemical cell sensor contains a silver/silver chloride reference electrode 28' and a counter electrode 30', the needle housing acting as the counter electrode. All three electrodes are connected to appropriate recording instrumentation by wire leads that contact the electrodes at the position where they emanate ~ from the housiny.

The entire housing is covered with bio; compatible material 36 that is permeable to small molecular weight substance, which pernlits the diffusion of oxygen in the bodily fluids into the interior of the housing.

In those instances when the electrochemical cell structure is implanted in regions of the body where there are low oxyyen concen~rations, it is desirable to fabricate the yel matrix or its outer layer of a material that permits [UOCGl-PA.C26] U.C. CASE NO. 83-208-l ,:

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~Z~3~38~ -9- 66128-172 the electrochemical cell sensor to remain sensitive to glucose over a useul range of concentrations in the presence of such low oxygen levels. This is accomplished by selecting a material that restricts the diffusion or partitioning of glucose while remaining relatively permeable to oxygen. As described above, the diffusion of glucose can be controlled by incorporating domains of hydrophobic material in a gel matrix in which the oxidase enzyme is embedded and which contacts the oxygen sensor.

Alternatively, a membrane of Silastic or other such hydrophilic rnaterial can be positioned between the gel matrix and the external environment. The nonworkiny regions of the oxygen electrode and the silver/silver chloride elec~rode are insulated with suitable insulating material 27~. Particularly useful is epox~ or ~u~ed glass.

The working regions of the electrodes are situated near the ; opening or openin~s in the electrochemical housing wall and are not insulated.

In order to assure the accessibility of oxidase enzyme associated with the oxygen sensing electrode to oxygen, the preferred position o~ the electrode is near the opening in the housing. ~dditionally, a tunnel 38 is desirable that connects the oxygen-sensing regions with the external environment to allow ~or ingress of oxygen from the outside.

2S By providing atmospheric oxygen to the enzyme electrode sensing region, the oxidase enzyme reaction is limited mainly by glucose and not by oxygen. Consequently, it is possible to monitor glucose or other enzyme ~ubstrates without a oxygen reference electrode.

[UOCGl-PA.C26] V.C. CAS E NO . 83-208-1 -, . .

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, ~L2~7380 --10- Figures 8 and 9 show cross-sectional views at different positions along the length of the sensor. Figure 8 depicts the tunnel 38 and the oxygen sensing 34 and reference electrodes 28', while Figure 9 shows the opening 32 and the oxygen sensing 34 and reference electrodes 28'.

It will be apparent to those skilled in the art that there are a variety of means available for supplying oxygen to the oxygen sensing region of the oxygen sensor. An alternative means is to charge a reservoir that cornmunicates with the oxygen-sensing reyion with oxygen prior to implanting the electrode into the body. In this situation the oxygen would eventually be consumed, but the sensing lifetime would, nevertheless, be adequate for determining the concentrations of oxidase substrates.

The following example is described for illustrative purposes and should not be construed as narrowing the scope of the invention.

It will be apparent to those skilled in the art that there exist many variations as to which particular steps of the invention may be practiced.

EXAMPL~ Determination o~ ylucose in bodily fluids can be carried out by measuring the oxidation of glucose in the presence of oxygen by the enzyme glucose oxidase. It is possible using an enzyme electrode (shown in Pigure 1) to measure the concentration of glucose after implanting an electrochemical cell sensor containing two oxyyen sensors, one o~ W}liC}I .iS in communication with glucose oxidase. The latter is positioned over the sensor by embedding it in a crosslinked collagen matrix as described by D.~.

Gough, J.K.

~UOCGl-PA.C26] U.C. CASE NO. 83-208-1 . . , : : . , .: . . . .

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~ ;~13'73~ --11- Leypoldt, and J.C. ~rmour in Viabetes Care (1982, 5:190198). The electrochemical cell sensor containiny the sensors situated in a housing were inserted subcutaneously near the surface in the leg region of an anesthetized dog.

Glucose was infused intravenously through a venous catheter, ~nd at various time intervals thereafter the resulting blood glucose concentration monitored by analyzing the differential signal comirly from the sensors. Table 1 shows that after the sensors have stabilized, that there is a rapid and signiEicant response by the sensors to the in~used ylucose.

In order to rel~te the levels of ylucose present to the ylucose dependent current changes, glucose levels were measured by standard laboratory methods using a blood glucose analyzer.

~ TABLE 1 ;D~etection of Blood ~luco~e ~evels ~i~h the ~lectxochemic~l Cell Sensor Time after ylucose injection (minutes) 0.5 1.0 3.0 ~0.0 30.0 40.0 50.0 60.0 Blood glucose (mg/deciliter~450200140115 100 80 75 70 Glucose-dependent difference current (nano amps) 0 0 0 5 15 20 25 30 ~VOCGl-P~.C26] U.C~ CA~E NO. ~3-208-1 - ~