In vitro measurement and characterization of current density profiles produced by nonrecessed,...

IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING. VOL 38. NO. S. MAY l Y Y l 40 I

In Vitro Measurement and Characterization of Current Density Profiles Produced by Nonrecessed, Simple

Recessed, and Radially Varying Recessed Stimulating Electrodes

Michael F. Suesserman, Student Member, IEEE, Francis A. Spelman, Member, IEEE, and Jay T . Rubinstein, Member, IEEE

Abstract-Potential fields induced by nonrecessed, simple recessed, and radially varying recessed electrode designs were measured in vitro. Comparison of experimental results with theoretical analyses substantiated the experimental measurement technique and emphasized the importance of considering both nonuniform charge injection and surface electrochemistry when designing implantable stimulating electrodes. Radially varying recesses produced uniform charge injection at the electrode surface and at the aperture-tissue interface. In general, the radially varying recessed electrodes provided a combina- tion of uniform charge injection and flexibility in design and fabrication that warrants their incorporation into all appropriate planar stimulating electrode designs.

I. INTRODUCTION MPLANTABLE stimulating electrodes must provide I safe transmission of externally controlled electrical sig-

nals to specific cells or tissues. In practice, the degree to which an electrode design allows safe stimulation depends on many factors. Stimulus levels must remain low enough to ensure that no irreversible electrochemical reactions occur at the electrode surface [ 11; the high charge densities necessary to produce desired physiologic effects in auditory prostheses are difficult to attain safely with planar electrode designs. Since this paper pertains to planar electrodes, emphasis is placed on the effects of high peak current density normally associated with electrode edges. Specifically, high charge injection produces two undesirable results: direct tissue pathology [2], [3] and irreversible dissolution of the electrode surface [4]. Re- cent theoretical analyses [ 5 ] , [6] suggest that the estab-

Manuscript received April 25. 1988; revised June 29. 1990. This work was supported by NIH under Grants GM07266, RR00166. and NS13056. by a subcontract to the University of Washington from the University of Michigan, and by the PROPHET Network of the NIH.

M. F. Suesserman is with the Regional Primate Research Center and the Department o f Electrical Engineering, University of Washington, Seattle, WA 98195.

F. A. Spelman is with the Regional Primate Research Center and the Center for Bioengineering, University of Washington, Seattle, WA 98 195.

J . T. Rubinstein is with the Regional Primate Research Center, the Cen- ter for Bioengineering. and the School of Medicine. University of Wash- ington, Seattle, WA 98195.

IEEE Log Number 9144690.

lished limits for safe electrical stimulation are inaccurate because interpretation of experimental results ignored effects of both the nonuniform current density profile produced by a given planar electrode design and the electrochemical reactions that occur at an electrode surface.

The goal of these studies was to characterize the charge injection profiles produced by various planar stimulating electrode designs. In vitro measurements of induced potential fields in conjunction with theoretical analyses provided a basis for developing a thorough understanding of electrostatic and electrochemical phenomena that affect an electrode-tissue interface. In addition, comparisons of charge injection profiles generated by planar stimulating electrodes of widely varying geometries were used to identify designs with improved current injection properties. Both theoretical and experimental results revealed straightforward design changes that minimized nonideal behavior normally associated with planar stimulating electrodes. Primarily, all suitable planar electrodes should be recessed into their insulating carriers. Recessed electrodes produce uniform charge injection across the electrode surface, and appropriately shaped recesses result in similar desirable current density profiles across the aperture-tissue interface.

11. METHODS Quantitative, in vitro characterization of fields pro-

duced by stimulating electrodes with widely varying sizes and geometries requires precise spatial measurement of potentials induced by the electrodes within an aqueous en- vironment. Details of the experimental apparatus used in these studies are given in Fig. 1. Electrode-dependent fields result when charge-balanced sinusoidal current, driven with a controlled current source, passes from a test electrode through a saline-filled (0.154M NaCl (0.9%) solution) tank to a distant, low-impedance ground. Mea- surements involved three general types of planar electrode designs: nonrecessed, simple recessed, and radially varying recessed. Test electrode dimensions ranged in size from 0.50 to 35 mm in diameter; recessed disk electrodes

001 8-9294/9 1 /05oO-0401$01 .OO 0 199 1 IEEE

402 IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING. VOL. 38. NO. 5. MAY 1Y91

3-axis

Solution

Fig. I . Experimental setup used to measure potential fields produced by various stimulating electrodes when sinusoidal current ( I C ) passes from the electrode surface through saline solution to a distant ground reference.

ranged in depth from 10 to 125% of the electrode diameter. All measurements were conducted within a Pyrex cylinder measuring 150 mm in diameter by 150 mm tall. Using a tank with dimensions at least four times larger than those of the test electrode ensures that induced potential fields are not distorted by the measurement tank

Experimental error can be separated into two cate- gories: 1) inaccuracies in spatial measurement of the actual induced potential fields and 2) measurement of unwanted potentials in the system. Errors in the former category depend primarily on the physical and electrical properties of the measurement system. We used a single- wire microelectrode measurement system, consisting of a voltage-sensing microelectrode and a very high-impedance buffer amplifier, to quantitate potential fields induced by stimulating electrodes. The buffer amplifier remained securely attached to a Narashige three- dimensional stereotaxic device, allowing precise spatial control ( f 5 pm) over placement of a measuring microelectrode. The buffer amplifier output was connected to a standard high-gain instrumentation amplifier before pass- ing to an oscilloscope for analysis.

The voltage-sensing microelectrode was constructed by threading 6 cm of 2-mil (50.8 pm) diameter platinumlir- idium (90% / 10%) wire, coated with Teflon to a total di- meter of 3 mil (76.2 pm), through needle stock that was 4 mil (101.6 pm) in ID, 8 mil (203.2 pm) in OD, and 5 cm long. Approximately 2 mm of wire extended out of the measuring end of the needle stock. The entire microelectrode was sealed with epoxy. It was connected via a 2-pin microconnector directly to the input of the high- impedance unity-gain buffer amplifier [8]. The buffer amplifier had an input impedance > 300 MQ in parallel with 0.2 pF, an input bias current < 2 PA, and a driven shield. In addition to protecting sensitive buffer amplifier input circuitry, the driven shield was connected to the needle

[71.

stock, resulting in a driven shield that surrounded a majority of the measuring microelectrode. The driven microelectrode shield was necessary to minimize coupling of stray capacitive signals to the high-impedance measuring microelectrode. Although the platinumhridium measuring electrodes represent large, complex source imped- ances (typically 1.66 MQ in parallel with 0.176 nF at 1 kHz), the buffer amplifier input impedance ensured negligible loss of signal across the measuring electrode. In addition, the low-bias current drawn by the buffer amplifier input resulted in negligible polarization of the microelectrode during potential field measurements.

Test electrodes were constructed by etching copper circuit boards into the desired electrode geometries. This method allowed precise control of an electrode size and shape while insuring that the thin metallic surface remained tightly bound to a large insulating carrier. A small, epoxy-coated lead etched between an electrode and the PC board edge supplied current to the test electrode. The test electrodes were made of copper because of its sus- ceptibility to corrosion, which increases the observable effects of electrochemical reactions on fields generated above an electrode. Measurements with platinum test electrodes produced results similar to those found for copper electrodes except that platinum showed less evidence of electrochemical surface reactions. Simple recessed electrodes were simulated by attaching Plexiglas plates, each with an electrode-shaped hole in the center, to a surface-mounted electrode (see Fig. 1 ) . In a similar fashion, radially varying recessed electrodes were constructed by adding a Plexiglas plate with an appropriately shaped aperture to the surface-mounted test electrode. Point source electrodes, which were used for in vivo calibration of the measurement system, were made by drilling a small hole through a PC board, placing a wire in the hole, gluing it in place with epoxy, and sanding the epoxy and wire flush with the board.

In vitro measurements of induced potential fields were used to quantitate spatial charge injection characteristics of a stimulating electrode by determining potential differences induced adjacent to the test electrode. The initial measurement of the potential difference involved direct measurement using a two-wire differential measurement system similar to the single-wire setup described previously. In all of the cases studied, the measuring electrode nearest the surface of the test electrode interfered with fields sensed by the further measuring electrode, and the interference created significant errors in direct differential measurement of potential fields. Therefore, we deter- mined potential difference by subtracting a spatially separated single-wire measurement of induced potential fields. All in vitro calibrations (discussed in the following) confirmed that the single-wire microelectrode measurement system accurately measured induced potential fields without distorting the inherent fields.

Errors arising in the second general category, a measurement of unwanted potentials in the system, occur as a result of improperly generating or incorrectly interpret-

SI!P,SSFRMAN c / < I / MEASUREMENT A N D CHAKACTERI%ATIOK OF DENSITY I’KOFll.kS 403

ing mcasured potential fields. Since the signal-to-noise rd- tio was typically greater than 30 dB it was not necessary to average the output signal to reduce asynchronous noise. To ensure that induced fields depend solely on the stimulating electrode being tested. we calibrated the measurement system systematically by comparing in Liitrn measurements with results of theoretical analyses. In vitro measurement of fields generated by a simple point source electrode allowed rapid assessment of the accuracy with which the system measured theoretically predicted potential fields. Point source measurements permitted simulta- neous itz tliri-o testing of several critical system properties, including buffer amplifier input impedance and bias current, amplification of buff‘er amplifier signals, controlled current source stability, accuracy of saline resistivity measurements. and distortions due to the presence of the measurement tank or measuring microelectrodes. Close agreement between the experimental and theoretical results depended on the proper operation of all of the com- ponents of the measurement system and validated that the measurement system functioned in a predictable and re- producible fashion.

RESULTS A N D DISCUSSION

The goal of these studies was to measure current injection profiles produced by various designs of stimulating planar electrodes both to characterize the electrical and electrochemical processes associated with charge transfer across an electrode surface and to identify electrode designs with improved charge injection characteristics. In tlirro measurements were conducted on planar electrodes ranging in size from 0.5 to 35 mm in diameter. The current magnitudes and frequency ranges used for disk electrodes between 5 and 35 mm in diameter covered ranges of average current densities from 20 nC/cm2 to 20 pC/cm’. and each electrode design generated the same results over the entire range of sizes and current densities tested 161. Although the measurements presented in this paper were obtained with disk-shaped planar electrode designs. the same general results can be obtained with other common geometries such as rectangular and banded electrodes.

The potential field produced by a point current source located on the surface of an infinite, planar insulator in a semi-infinite, isotropic conductive medium is described by

where Vis the potential. p is the resistivity of the semiin- finite space, I is the current, and r is the radial distance from the electrode. Direct resistivity measurement of 0.9% NaCl, using standard conductivity cell techniques 191, produced a value of 63.62 R-cm at 25°C. The results of typical measurements involving a point current source test electrode are summarized in Fig. 2. In general, ex- perimentally measured potential fields generated by point

0.0 0.5 1 .O 1.5 2.0

lmistance From Electrode (”-1)

Fig. 2. Point curve source measurements used to calibrate system param- eters. Experimental regression curves are compared with theoretical cal- culations. Saline resistivity equals 63.62 0-cm. The theoretically predicted slopes of potential difference versus reciprocal distance from an electrode are 2.531 and 25.31 mV-mm for electrode currents of 25 and 250 PA, respectively. Regression of experimental data yields regression coefficients greater than 0.99 uith slopes of 2.54 and 25.33 mV-mm for electrode current\ o f 25 and 250 PA. respectively.

current sources deviate from theory by < 1 %, provided that the current remains within the linear operating region of the electrode. See Fig. 2 for additional comments. All in vitro measurements presented in this paper involved current levels within the linear operating region of the test electrode, and typically the maximum current used was 75% of that found to cause nonlinear behavior. The precise agreement between theoretical and experimental results observed during systematic in vitro characterization of potential fields produced by point current sources ensured that the system was reliably generating and measuring fields induced by the test electrode.

Although point current source test electrodes provide a convenient way of verifying the accuracy with which the system measures theoretically predicted potential fields, the measurements produce little useful information concerning stimulating electrodes. We obtained in vitro measurements with various nonrecessed and recessed stimulating electrode designs, and compared them with theoretically predicted fields both to verify the theoretical models and to quantitate the electrical and electrochemical properties of planar stimulating electrodes. All theoretical analyses involved solving Laplace’s equation for the quasi-static fields of a test electrode. For a complete description of the methods used to obtain theoretical so- lutions, see Rubinstein ef ul. 141 and Rubinstein [ 5 ] . In general, current density characteristics of a stimulating electrode depend on two competing charge-transfer processes that occur at the electrode-electrolyte interface. The primary current distribution results when the impedance of the electrode double layer approaches a short circuit, which most readily occurs at frequencies high enough to shunt a majority of the current through the double-layer capacitance. As current levels increase above the limiting current. electrochemical reaction and diffusion kinetics dominate charge transfer across the electrode surface, resulting in the secondary current distribution. Since theo-

404 Ikkk rKANSACTlONS O N BIOMEDICAL ENGINEERING. VOL. 38. NO. 5. MAY 1991

retical analyses solve for the primary current distribution of a test electrode, deviations between experimental measurements and theory indicate the occurrence of irreversible electrochemical charge transfer processes at the stimulating electrode surface.

We used potential difference measurements to quantitate charge injection profiles of nonrecessed and recessed planar stimulating electrodes. Since current flows out of the test electrode to a distant ground, the potential difference measured adjacent to an electrode is proportional to current density flowing out of the stimulating electrode. Measurement of a stimulating electrode's true current density profile must take into account the divergence of current as it leaves the electrode surface; however, we used a slightly modified approach without any loss in accuracy. Specifically, all potential difference measurements were made normal to and at a finite distance above the surface of the planar test electrode, resulting in a current density profile that differed from the true profile by the degree to which current flow diverged from a direction normal to the electrode surface. Instead of attempting to correct the in vitro measurements to account for divergence of fields, we compared our experimental results directly with corresponding theoretical simulations of the measured potential differences. Since all in vitro measurements were performed in the same fashion, comparisons between charge injection profiles from different electrodes permitted straightforward identification of the designs that best reduced or eliminated undesirable charge injection characteristics normally associated with planar stimulating electrodes.

A . In Vitro Churucterization of Non recessed Disk Stimulating Electrodes

For in vitro measurements of nonrecessed disk electrodes we used both polished and corroded electrodes. which improved our understanding of charge transfer mechanisms at the surface of stimulating electrodes. Re- sults obtained for a 35-mm diameter polished disk electrode are plotted in Fig. 3 . Measurements gave potential difference, induced between 1 and 3 mm from the electrode surface as a function of both frequency and distance in an outward direction from the disk's surface. The amplitude of electrode current was 1.5 mA. Notice that high driving frequencies (f 2 3 kHz) produced fields that agreed to within 10% with theoretically predicted fields; however, the measured fields for low-frequency currents uniformly redistributed over the electrode surface, producing large deviations from theory. These results agree with measurements performed by Webster [ IO] .

The actual charge transfer distribution across an electrode surface depends on both amplitude and frequency of the electrode current. Since ac current was used, measured charge injection profiles could be attained to either quasi-static fields or electrode skin effects. Because measurements involving recessed electrodes at various stimulation frequencies and magnitudes produced uniform current density profiles along the electrode surface [ 71,

1.8 - 5 w z -0- 3 w z - 1 kHz 1.6

1.4

o . x ! . , , , , , , , . , . , , I 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5

Distance From Center (mm)

Fig. 3 . Potential ditfercnce rneaaurcd above a polished nonrecessed disk electrode at various lrequencie\. Electrode radius equals 17.5 iiiiii and elcc- trode current equals I . S rnA.

the induced potential fields depended primarily on quasi- static fields. Close agreement between theoretical and experimental results at high frequencies indicates that, except for the region of high current density near the electrode edge, the primary current distribution applies and a majority of the injected charge is transferred across the double-layer capacitance. At low stimulating frequencies the large deviations from theory indicate that the primary current distribution cannot account for fields produced by the electrode. The quantity of charge injected during each half-cycle of the stimulus increases with decreasing frequency, causing favorable electrochemical reactions to become rate limited. The observed current profile approaches a uniform distribution across the electrode surface, because irreversible electrochemical reactions. such as electrolysis of water or various oxidation reactions [ 1 I ] , are recruited to transfer injected charge. Since the charge injection profile depends on rate-limited reaction and diffusion kinetics, the secondary current distribution domi- nates at low frequencies.

In vitro measurements of potential fields induced by a 35-mm diameter corroded disk electrode are shown in Fig. 4. As with the polished electrode, measurements gave potential difference, induced between l and 3 mm from the electrode surface: the magnitude of electrode current was 1.5 niA. Surface corrosion was produced by soaking a copper disk electrode in 0.9% NaCl for 2 days. resulting in a visibly rough, oxidized surface. Unlike the results obtained with a polished disk electrode (Fig. 3), experimental plots in Fig. 4 show little deviation from theory for stimulating frequencies as low as 500 Hz. Since the corrosion product (Cu/CuO,) prevents both reversible faradic reactions and corrosive electrochemical reactions at the electrode surface, other electrochemical charge transfer mechanisms must have carried injected-current density. Measurements provide insight into both the electrical properties of the corrosive coating covering the copper electrode and the actual electrochemical charge injection processes.

The corrosive coating must be an ionic, dielectric or electronic conductor. An ionic coating causes the charge

SUESSERMAN cf U / . : MEASUREMENT A N D CHARACTERIZATION OF DENSITY PROFILES

0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5

Distance From Center (mm)

Fig. 4. Potential difference measured above a corroded nonreceased disk electrode at various frequencies. Electrode radius equals 17.5 nim and electrode current equals I .5 mA.

transfer at the electrode-coating interface to be diffusion limited, resulting in deviation from the primary current distribution even at relatively high stimulating frequencies. A dielectric coating produces capacitive charge transfer, resulting in no deviation from the primary current distribution as frequency is lowered. For an electron- ically conducting coating, the charge transfer depends on electrochemical reactions that occur at the coating-electrolyte interface, and rate and diffusion limited-reaction kinetics would prevail at low frequencies as observed for the polished electrode. Therefore, the corrosive coating primarily represents an electronic conductor. As with the polished electrode, the reversible reactions become rate limited as stimulating frequency decreases, and additional irreversible electrochemical reactions carry injected charge. The significant decrease in the frequency at which measurements begin to deviate from theory for the corroded electrode suggests that the corrosion increases surface area to the extent that a substantially larger quantity of unimpeded irreversible charge passes across the electrode surface.

The field measurements obtained for surface-mounted disk electrodes emphasize the importance of considering both quasi-static and electrochemical phenomena when designing implantable stimulating devices. In particular, undesirable electrochemical reactions dominate the fields produced by stimulating electrodes both during low-frequency stimulation and in the regions of high peak current density normally found at electrode edges. Theoretical analyses provide a reference against which to compare in vitro measurements; low frequency and high current limits are readily identified for a given electrode design. In addition, measurements provide information concerning the general electrochemical mechanisms responsible for injecting charge across an electrode surface.

B. In Vitro Characterization of Simple Recessed Disk Stimulating Electrodes

Recent theoretical [ 5 ] , [6] and experimental [12] analyses suggest that recessed stimulating electrodes could

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significantly decrease tissue pathology by protecting the electrode surface from irreversible electrochemical reactions and by decreasing the area of tissue exposed to high peak current density. Both theoretically derived and ex- perimentally measured fields produced by an 18-mm diameter disk electrode that was recessed either 6 or 9 mm are plotted in Fig. 5 . Both upper plots represent potential difference between 0.5 and 1 mm above the electrode surface, and the lower plots represent potential difference between 0.5 mm below the aperture and at the aperture. In vitro measurements of recessed electrodes were conducted at a stimulation frequency of 1 kHz, and the amplitude of the sinusoidal electrode current was 1 mA. No- tice that a recessed planar electrode produced nearly uniform current injection over an electrode surface. Elim- inating the high peak current density normally associated with planar electrode edges results in numerous desirable effects. A true uniform current distribution allows accu- rate estimation of injected charge density, which reduces the likelihood of releasing potentially unsafe levels of current density into the stimulated tissues. In addition, since the injected current distributes equally across the electrode surface, dissolution of electrode material at the edges, which releases potentially toxic chemicals into adjacent tissues, is minimized.

Charge injection across a simple recessed electrode aperture behaves similarly to that across a nonrecessed electrode. The aperture edges cause regions of high current density (see Fig. 5 ) that are less extreme than in a nonrecessed electrode. Theoretically as the depth of the recess increases, the current density singularity moves closer to the electrode edge, exposing a smaller region of tissue to high current levels [4]. The observed difference between measured and theoretical results at the aperture is due to a <0.3-mm placement error in positioning the measuring microelectrode at the electrode aperture. In general, the results suggest that recessed electrodes produce safer stimulation than nonrecessed electrodes.

C. In Vitro Characterization of Radially Varying Recessed Disk Stimulating Electrodes

A primary goal of this paper was to identify improved planar stimulating electrode designs that can inject high charge densities safely into biological tissues. As discussed above, both theoretical and experimental results indicated that simple recessed electrodes improved the undesirable current injection distributions normally associated with planar stimulating electrodes. Recessed electrodes uniformly couple charge across the electrode-aperture interface, but the transfer of charge across the aperture-tissue interface results in undesirable edge effects similar to those observed for nonrecessed electrodes. The results suggest using appropriately shaped apertures to further improve the charge injection characteristics of recessed electrodes. To test this hypothesis, we measured potential fields induced by three radially varying recessed electrode designs: an exponentially recessed disk electrode, a conically recessed disk electrode, and a stepwise

406 IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING. VOL. 38, NO. 5. MAY 1991

3 ] I ' I I 9 mm recess *at surface I 6 mm recm

*at surface

-9.0 4 . 5 0.0 4.5 9.0 -9.0 4 . 5 0.0 4 5 9.0

Distance From Center (mm) Distance From Center (mm)

3 - 1 'I 9 mm recess 6 mm r e m *at aperture *at aperture >

g n 2 u 3 1

0 Model 9 Measured I

0 4 . , . , . , . 4 0 4 . , . , . , . I -9.0 4 . 5 0.0 4.5 9.0 -9.0 4 5 0.0 4.5 9 0

Distance From Center (nun) Distance From Center (mm)

Fig. 5 . Potential differences measured for 18-mm diameter recessed electrodes. The upper two graphs plot potential fields just above the electrode surface: the lower two graphs plot potentials at the aperture of the recess. Electrode current equals 1 mA at I kHz. (Reprinted from [12]).

approximation of a conically recessed disk electrode. In all cases presented, a recess depth of one electrode radius (radius = 9 mm) was used to ensure uniform charge injection across the electrode surface; the in vitro measurements involved a sinusoidal electrode current at 1 kHz with a magnitude of 1 mA. Specific geometries of the tested electrodes are given in Fig. 6 .

Results obtained for an exponentially recessed electrode [Fig. 6(a)] are plotted in Fig. 7. The upper graph, representing potential difference measured between 0.5 and 1.0 mm from the electrode surface, reveals the uniform charge injection characteristic produced by all recessed electrodes. The < 5 % asymmetry observed for potential difference measured across the electrode surface resulted from both surface imperfections and a small systematic inaccuracy in spatial positioning of the measuring microelectrode. The lower graph shows the potential difference measured between 0.5 mm below the aperture and the aperture. Induced potential difference was maximum at the center and minimum at the aperture's edge; the charge injection profile across the aperture approximated that created by a point source electrode located near the center of the electrode surface. Of all recessed electrode designs studied, an exponentially recessed disk electrode produced the most ideal charge injection profile. Not only was current released uniformly across the electrode surface, thereby minimizing electrode damage as discussed previously, but also the maximum amount of charge was injected across the center of the aperture into adjacent tissues, minimizing direct tissue damage normally caused by edge-induced regions of extremely high current density.

The difficulties associated with constructing an expo- nential recess using common electrode fabrication techniques suggest that simpler aperture geometries should be

(a)

y(x) = 2x

(C) Fig. 6 . Geometries of radially varying recessed electrodes. Electrode ra-' dius equals 9 mm for all presented results. (a) Exponentially recessed electrode. (b) Conically recessed electrode. (c) Stepwise sampling of a conically recessed electrode.

-9 -6 -3 0 3 6 9 Distance From Center (mm)

At Aperture


Fig. 7. Potential difference measured for an exponentially recessed electrode. Electrode radius equals 9 mm and electrode current equals I mA at I kHz.

tested. A practical stimulating electrode design must op- timize tradeoffs between the ideal charge injection profile produced by an exponentially recessed electrode and the ease of fabrication associated with planar or simple recessed electrodes. Conically recessed electrodes provide the best balance between these tradeoffs by producing uniform charge injection across the aperture and by being geometrically simple enough to construct on very small scales by using common planar multielectrode fabrication techniques. Results obtained for a conically recessed electrode with a recess slope of 2 [Fig. 6(b)] are plotted in Fig. 8. The charge injection remained uniform across the electrode surface, and there was a slight asymmetry as with the exponentially recessed electrode. More significantly, the conically recessed electrode also injected charge uniformly across the aperture-tissue interface,

SUESSERMAN er ( I / . : MEASUREMENT A N D CHARACTERIZATION OF DENSITY PROFILES 407

At Surface

e

E


3

E

8 2 r - !!i 3 1 d 2 3

0


Fig. 8. Potential ditference measured for a conically recessed electrode. Electrode radius equal\ 9 mm and electrode current equals 1 mA at I k H z .

making it equivalent to a surface-mounted electrode that injects uniform charge. Like the exponentially recessed electrode, the conically recessed electrode produced a uniform current density profile that minimized the possi- bility of damage to both the electrode surface and adjacent biological tissues.

The simple geometry of a conically recessed electrode allows straightforward construction using a variety of electrode fabrication techniques. Some planar fabrication processes, such as those used for making thin film electrodes, create sharp edges for any given layer and cannot easily produce gradually varying layers. The third electrode design measured (Fig. 6(c)) approximated this sit- uation by sampling the geometry of a conically recessed electrode in three stepwise increments. The results are plotted in Fig. 9. Notice that the induced fields roughly approximate those measured for the conically recessed electrode, with differences related primarily to “smooth- ness” of the uniform charge injected across both the electrode surface and the aperture-tissue interface. Irregular- ities in potential fields induced across the aperture result from high charge injection produced by edges in the stepwise varying recessed electrode; high current density generated at the aperture of a simple recessed electrode gets broken up into smaller packets and spread out in space along the stepwise recess. In vitro measurements showed that with as few as three steps in the recess the electrode closely simulates a conically varying recessed electrode and uniformly injects charge across the aperture into adjacent tissues.

Charge injection profiles produced by all the electrode designs described in this paper are summarized in Fig. 10. Arrows represent direction of current flow, with the size of the arrows roughly proportional to magnitude of injected current density. Nonrecessed planar electrodes ex-

- 3 I

- 4: e 6’ - s I E

1



Fig. 9. Potential difference measured for a stepwise sampling of a conically recessed electrode. Electrode radius equals 9 mm and electrode current equals l mA at l k H z .

4 t t t t t t t t t t t t r

r t t t t t t t t t t t r

. . (e)

Fig. I O . Summary of charge-injection profiles produced by the stimulating electrode geometries discussed in this paper. (a) Surface-mounted electrode. (b) Simple recessed electrode. (c) Exponentially recessed electrode. (d) Conically recessed electrode. (e) Stepwise sampling of a conically recessed electrode.

pose both the electrode and adjacent tissues to regions of extremely high edge-induced current densities, which reduces the likelihood of stimulating tissues safely. Recess- ing a planar electrode into an insulating carrier constrains fields so that charge is uniformly injected across the metallic electrode surface. Notice in the last three drawings how radially varying the recess of an electrode maintains

408 IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 38, NO. 5 , MAY 1991

the uniform charge injection at the electrode surface while allowing control over charge injection profiles across the aperture and into adjacent biological tissues.

IV. CONCLUSIONS Stimulating electrodes in cochlear or other neural

prostheses must achieve a proper balance between high charge injection and long-term biocompatibility . Unfor- tunately, present tests of biocompatibility disregard the importance of considering both nonuniform charge injection and degree of surface electrochemistry when char- acterizing implantable stimulating electrodes. Since comparisons between experimental measurements and theoretical analyses reveal both current density profiles and the degree of irreversible electrochemical reactions occurring at an electrode surface, in vitro measurements are used to identify improved electrode designs. The results suggest a number of methods for protecting an electrode surface from irreversible electrochemical reactions as well as for protecting biological tissues from potentially damaging levels of current density. Primarily, recessed electrodes produce virtually ideal uniform fields at an electrode surface. More specifically, radially varying recessed electrodes provide a great deal of flexibility for designing and fabricating implantable stimulating electrode arrays while ensuring optimal charge injection both at the electrode surface and into adjacent tissues. Exper- iments are currently under way to confirm the results in vivo. Although discussions have centered on implantable microelectrode stimulating devices, results can be gener- alized to virtually all electrical stimulating devices and warrant incorporating radially varying recesses into all appropriate planar stimulating electrode designs.

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F. T. Hambrccht, ”Neural prostheses,” Aiit i . R r i , . Biopliys. B i o r t i g . ,

P. A. Leakc-Jones, S. M. Walsh. and M. M . Merzenich, “Cochlear pathology following chronic intracochlear electrical stimulation.“ Ann. Orol. Rhitiol . Laryrigol., vol. 90, (wppl. 82). pp. 6-8. 1981. U . K . Shepherd, G. M. Clark, and R. C. Black. “Chronic electrical stimulation of the auditory nerve in cats.” Acru Orolurwigol, suppl.

S. B. Brummer, 1. McHardy, and M . J . Turner, “Electrical stimulation with Pt electrodes: Trace analysis for dissolved platinum and other dissolved electrochemical products.” Bruiri Brhoi.. E i d . . vol 14. pp. 10-22. 1977. J . T. Rubinstein, F. A. Spelman. M. Soma, and M. F. Suesscrnman. “Current density profiles of surface mounted and recessed electrodes for neural prostheses.“ / € E E Trum. Biomrd. O i g . , vol. BME-34, pp. 864-874, Nov. 1987. I. T . Rubinstein, “Quasi-static analytical models for electrical stimulation of the auditory ncrvous system.” Ph.D. dissertation. U n i v . Washington, Dec. 1987. M. F. Suessernian, J . T . Rubinstein. and F. A. Spelman, unpublished data. R. P. Scobey. D. L. Howard. and A. J . Gabor, ”A simple circuit to reduce the input capacitance of inicroclectronic amplifiers,” lEEE Trutis. Biomed. Et ig . , vol. 28, Apr. 1981.

vol. 8 , pp. 239-267, 1979.

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191 L . A. Geddes, Elecrrodes u d /he Meusuremeiir of Bioelrctric, Elserlts. New York: Wiley-Interscience, 1972.

1 I O ] J . G. Webster, “Minimizing cutaneous pain during electrical stimulation.” in Proc. Ninth Anti. Cotif. / € E € Enx. M d . B i d . Soc.., Bos- ton, MA, 1987, pp. 986-987.

[ I I ] S. B. Brummer and M. J . Turner. ”Electrical stimulation of the ner- vous system: The principles of safe charge injection with noble metal electrodes,” Bioelecrrochern. Bioenerg.. vol. 2. pp. 13-25. 1975.

1121 J . T . Rubinstein, M. F. Suesserman. and F. A. Spelman. “Mcasure- ments and models of potential fields above a recessed electrode,” i n Proc. Nitirh Ami. Cotif. / € € E Etig. M r d . B i d . Sei,. . Boston, MA. 1987, pp. 913-914.

Michael F. Suesserman (S’87) received B.S. degrees both in biochemistry and in cellular and de- velopment biology from the University of Arizona in 1983, and the M.S. degree in electrical engineering from the University of Washington in 1988. He is presently working toward the Ph.D. degree in electrical engineering at the University of Washington.

Currently, he is a Research Assistant with the Regional Primate Research Center at the Univer- sity of Washington. His major research interests

are simulation, analysis, and control of electrodeitissue interactions in cochlear and neural prostheses.

Francis A. Spelman (S’55-M’60) was born in San Francisco, CA, in 1937 He received the B.S.E.E. degree from Stanford University in 1959, the M.S.E E degree from the University of Washington, Seattle, in 1968, and the Ph D degree from the University of Washington in 1975

He has headed the Bioengineering Division of the Regional Primate Research Center at the Uni- versity of Washington since 1965 He joined the faculty of the Center for Bioengineering at the University of Washington in 1977, and the facul-

ties of the Department of Otolaryngology, Head and Neck Surgery and Electrical Engineering in 1981 He was a Vislting Researcher at the De- partment of Bioengineering, Linkoping University, Linkoping, Sweden. from 1985 to 1986. He is a member of a U S /U S S R joint research study of hypertension in baboons. He is investigating the electrical properties of the inner ear, electrical current flow in the inner ear, and electrode design for cochlear iniplants He also studies local and neural control of peripheral blood flow

Dr Spelman is a member of Tau Beta Pi, the American Society of Pn- matologists. the Association for Research in Otolaryngology. and the American Association for the Advancement of Science

Jay T. Rubinstein (S’82-M‘88) was born in New York City in 1960. He received the Sc.B. and Sc.M. degrees in engineering from Brown Uni- versity, Providence, RI, in 1981 and 1983, and the M.D. and Ph.D. degrees in bioengineering from the University of Washington School of Medicine’s Medical Scientist Training Program. Seattle, WA, in 1987 and 1988, respectively.

He completed a surgical internship at Beth Is- rael Hospital in 1989 and a research fellowship in otolarvngologv at the Massachusetts Eve and Ear

~ - -_ Infirmary, Boston, MA, in 1990. He is currently a resident in otolaryngology at the Massachusetts Eye and Ear Infirmary. His research interests include volume conduction, speech processing, auditory physiology, and functional electrical stimulation.

Dr. Rubinstein is a member of Alpha Omega Alpha, Sigma X i , and the Association for Research in Otolaryngology.

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