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Annals of Biomedical Engineering, Vol. 31, pp. 879890, 2003 0090-6964/2003/31~7!/879/12/$20.00Printed in the USA. All rights reserved. Copyright 2003 Biomedical Engineering SocietyCAPSULE FORMATION

When a metal, with or without insulation, is im-planted into a tissue bed, two responses occur: ~1! thetissue fluids try to dissolve it and ~2! an inflammatoryreaction occurs and the implant soon triggers a cascade

an increase in vascularity had also occurred. Necrotictissue and an edematous region encircled the copperwire. The diameter of the capsule was determined par-tially by the extent of the mechanical trauma producedby its introduction into the brain. Fischer and co-workersconcluded that the electrode material of choice for suchstudies is stainless steel.

Collias and Manuelidis,8 implanted bundles of sixstainless steel electrodes ~125 mm in diameter! into theAddress correspondence to L. A. Geddes, Purdue University, De-Criteria for the Selection of Ma

L. A. GEDDESPurdue University, Department of Biomedical E

(Received 23 April 200

AbstractThere are four criteria that must be consideredwhen choosing material for an implanted electrode: ~1! tissueresponse, ~2! allergic response, ~3! electrode-tissue impedance,and ~4! radiographic visibility. This paper discusses these fourcriteria and identifies the materials that are the best candidatesfor such electrodes. For electrodes that make ohmic contactwith tissues: gold, platinum, platinumiridium, tungsten, andtantalum are good candidates. The preferred insulating materi-als are polyimide and glass. The characteristics of stimulatoroutput circuits and the importance of the bidirectional wave-form in relation to electrode decomposition are discussed. Thepaper concludes with an analysis, the design criteria, and thespecial properties and materials for capacitive recording andstimulating electrodes. 2003 Biomedical Engineering Soci-ety. @DOI: 10.1114/1.1581292#

KeywordsImplanted electrodes, Microelectrodes, Stimulatingelectrodes, Electrode arrays, Electrodes.

INTRODUCTION

In these days of nanotechnology techniques, smallelectrodes are implanted to stimulate excitable tissue, todetect bioelectric events, and to measure chemical sub-stances. The choice of electrode metal and an insulationapplied thereto have an important bearing on electrodeperformance and longevity. An ideal implanted electrodeand/or its insulation should not provoke a vigorous localor generalized host response. It should establish a stableimpedance contact with the tissue. The metal and itsinsulation should not produce an allergic response; thislatter important factor has not been considered hitherto.The implanted electrode should be visible radiographi-cally. Capacitive electrodes share the same requirementsexcept for the low-impedance criterion.partment of Biomedical Engineering, 500 Central Drive, West Lafay-ette, IN 47907-2022. Electronic mail: [email protected]

879rials for Implanted Electrodes

d R. ROEDERneering, 500 Central Drive, West Lafayette, IN

cepted 14 April 2003)

of events that results in the implant being encapsulatedby inexcitable fibrous tissue. In the latter case, the cap-sule can be thick or thin, depending on the reactivity ofthe tissue to the implant and the implants surface shapeand condition. If the implant is an electrode, the capsuleseparates it from the tissue that it is designed to stimulateor record from. A capsule surrounding a stimulating elec-trode raises the threshold for stimulation in proportion tothe capsule thickness and the fluid content within thecapsule can also affect the stimulation threshold. A cap-sule surrounding a recording electrode effectively makesthe electrode distant from the active tissue and reducesthe amplitude available for recording.

For diagnostic purposes, Dodge et al.9 implanted elec-trodes in the brain of a patient who died 19 months later.Each electrode consisted of six strands of Formvar-insulated copper wire ~97.5 mm in diameter!. After thepatient died, the brain was examined histologically. Awell-defined capsule was found along the tracks of theelectrodes. In later studies, they used stainless steel elec-trodes which they found to produce less tissue response.

Fischer et al.11 studied the response of the brains ofcats to 1 cm lengths of 24 gauge wires left in situ forperiods up to 4 weeks. The wires employed were ofchlorided silver, bare silver, copper, and stainless steel.Both bare and insulated wires were implanted. After 1week, histological studies showed tissue response. Silverand copper wires proved to be the most toxic to braintissue. After 3 weeks a narrow ring of necrotic tissuesurrounded the silver wire. Around this ring was a cir-cular edematous region, 2 mm in diameter. The reactionto the copper wire after 3 weeks was similar except thatbrains of cats. Describing the histological changes thatoccurred over periods extending up to 6 months, they

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found that an orderly sequence of changes took place in plants were rod-shaped, 5 mm long, and 500750 mm in

880 L. A. GEDDES and R. ROEDERthe tissue surrounding the electrodes. At the end of 24 hthere was a zone of hemorrhage, necrosis, and edemaextending to about 1 mm from the electrode. After 3days there was less hemorrhage and necrotic debris; bythe seventh day a 0.1 mm layer of capillaries occupiedthe necrotic zone. By the 15th day, the capillaries hadalmost completely replaced the necrotic region, and con-nective tissue had started to form. After the passage of amonth, the necrotic debris had disappeared and a well-defined capsule surrounded the electrode track. Capsuleformation was virtually complete after 4 months, atwhich time a thick, dense capsule completely encircledthe electrode.

Robinson and Johnson33 carried out studies similar tothose just described. They implanted wires ~125 mm indiameter! of gold, platinum, silver, stainless steel, tanta-lum, and tungsten into cat brains and studied the tissueresponses at various times over a period extending to 6months. Responses similar to those previously describedwere observed. After about a week the differences be-tween the metals in regard to the reaction produced be-gan to be detectable. Gold and stainless steel evoked theleast tissue response; tantalum, platinum, and tungstenproduced slightly more. Silver precipitated a vigoroustissue reaction. Encapsulation of all electrodes was evi-dent at 15 days, with thicker capsules around the metalsthat provoked the greatest tissue response.

Dymond et al.10 implanted the following materialsinto cat brains and evaluated them histologically after 2months: platinum, platinum8% tungsten, platinum10% rhodium, platinum10% iridium, platinum10%nickel, platinized platinum, a goldnickelchromium al-loy, a goldpalladiumrhodium alloy, a chromiumnickelmolybdenum alloy ~Vitallium!, stainless steel, sil-ver, rhenium, gold, and boron. They found thatplatinum8% tungsten, platinum10% iridium, plati-num, goldnickelchromium alloy, stainless steel, andgoldpalladiumrhodium alloys all had tissue reactionswhich were slightly less than those of platinum10%rhodium, platinum10% nickel, rhenium, and platinizedplatinum. Boron was found to be nontoxic. Interestinglythey found that platinum black produced a denser cap-sule. Chlorided silver and bare silver were found to betoxic. From their study they ranked the materials as can-didates in the following order: gold, platinum8% tung-sten, platinum10% iridium, platinum, goldnickelchromium, stainless steel, titanium, goldpalladiumrhodium, nickelchromiummolybdenum, platinum10% rhodium, platinum10% nickel, platinizedplatinum, tantalum, zirconium, rhenium, and tungsten.The materials found to be unsatisfactory were: silver,silversilver chloride, copper, and iron.

Stensaas and Stensaas39 implanted 27 materials ~met-als and insulators! into the cortices of rabbits. The im-diameter. Histological examination at 30 days revealedno reaction, i.e., no gliosis in response to aluminum,gold, platinum, and tungsten. There was a mild responseto tantalum and Pyrex glass, a severe response to iron,copper, and cobalt, which were highly toxic.

Babb and Kupfer,2 implanted metals and insulators inthe brains of rats and studied the responses at 1163days. They stated that silver and copper were unsuitableas electrodes because of the strong tissue response. Theyrecommended the use of stainless steel and nichrome~80% Ni 20% Cr! as electrodes. Among the insulators,polyimide was better than epoxy, the former producedlittle tissue response.

Agnew and McCreery27 summarized the published lit-erature prior to that time; they stated that the followingwere relative innocuous: aluminum, beryllium, chro-mium, iron, lead, tin, and tungsten. Magnesium and man-ganese produced local necrosis. Bismuth, cadmium, cop-per, cobalt, and nickel produced more severe localnecrosis. Zinc produced prominent lymphocytic cuffing.

INTEGRATED-CIRCUIT TECHNOLOGYELECTRODES

Wise and Starr42 appear to have been the first tofabricate a three-electrode array occupying 50 mm usingsilicon technology. Since then the technology has ad-vanced and it is now possible to produce many micro-electrodes on a silicon chip.

Campbell et al.7 described a fabrication technique tocreate an array of 100 pointed conical needle electrodesof silicon, each 1.5 mm long, and 0.09 mm at the base;the array projected from a 4.234.2 mm monocrystallinesilicon chip. Because silicon oxidizes readily, the elec-trodes were coated with platinum. The electrodes weredesigned for cortical stimulation. In commenting on theirelectrodes, the authors stated, The impedance character-istics of these arrays have been measured and found tobe well suited for stimulation of cortical tissue ~very lowimpedance along the needle, very high impedance be-tween electrodes!. Some drawbacks do exist in the ther-momigration method used to create these electrical char-acteristics. Oftentimes electrodes are shorted together,and the nature of the isolating pn junction pairs is suchthat surface condition is critical to the effectiveness ofthe isolation. Also, the electrode tips should be coatedwith iridium oxide rather than platinum which will en-hance the charge transfer capabilities of each electrode.No data were given on the effective electrode area or themagnitude of the electrode-tissue impedance.

An improved silicon-based 10310 needle-electrodearray, similar to that reported by Campbell et al.7 wasdescribed by Jones et al.20 The technique used to manu-facture these Utah electrode arrays differed from the pre-





vious method in that glass provided electrical isolation old current as the electrode becomes encapsulated with

881Selection of Materials for Implanted Electrodesbetween the individual electrodes in the array. The newelectrode arrays exhibited superior electrical properties.The interelectrode impedances were at least 10 TV, andinterelectrode capacitances of approximately 50 fF. Theauthors stated that their array was adequately strong forcortical insertion. However, they did not measure theeffective electrode area, but provided one data point onelectrode-tissue impedance, namely 10,00020,000 V at1 kHz for a current of 1 mA. Maynard25 described theelectrodes in more detail.

Schmidt et al.36 reported on the biocompatibility ofphosphorous-doped monocrystalline silicon electrodesimplanted in cat brains. Uncoated silicon, silicon coatedwith polyimide, silicon coated with an aluminum-chelating agent, then insulated with polyimide were pre-pared. Fifteen arrays remained implanted for 24 h todetermine early tissue reaction to the implantation pro-cedure, and twelve arrays remained implanted for 6months to determine structural and material biocompat-ibility. Edema and hemorrhage were present around theshort-term implants that involved less than 6% of thetotal area of the tissue covered by the array. With thechronic implants, leukocytes were rarely present andmacrophages were found around roughly one-third of thetracks. Remnants of foreign material from the electrodescould be identified in less than 10% of the tracks. Gliosiswas found around all tracks, forming an annulus between20 and 40 mm thick. A capsule was not always presentand never exceeded a thickness of 9 mm. These resultssuggest that the implantation procedure produced a lim-ited amount of tissue damage and that the arrays arebiocompatible. However, the arrays insulated with poly-imide indicated a reaction due to aluminum oxide in theprimer. These silicon electrodes provoked only a mildtissue response. No data were given on the electrode-tissue interface impedance or effective area. Interestingly,epoxy, which can be used as an insulator, produced atissue response. Pyrex glass produced some response, butpolyimide produced very little tissue response.

At this point, it is possible to identify the metals thatshould not be used for implanted electrodes because ofvigorous tissue reactivity. These pure metals are iron,copper, silver, cobalt, zinc, magnesium, manganese, andaluminum which oxidizes. Agnew and McCreary27 ad-vise against bismuth, cadmium, and nickel.

SIGNIFICANCE OF A TISSUE CAPSULE

The significance of a tissue capsule surrounding astimulating electrode is well known to those who implantcardiac pacemakers. The procedure at the time of im-plantation involves determining the threshold current forstimulation ~capture!. Then the output of the pulse gen-erator is doubled to accommodate the increase in thresh-inexcitable fibrous tissue. Typically platinumiridiumelectrodes are used.

The growth of an inexcitable tissue capsule is due tothe inflammatory response of the tissue. To reduce thisresponse and minimize the increase in pacing currentthreshold that it engenders, Radovsky et al.,28 usedplatinum-iridium pacemaker electrodes that incorporatedan anti-inflammatory steroid ~dexamethasone! as de-scribed by Stokes et al.40 The study was as follows.

A pair of endocardial pacemaker leads, identicalexcept for the presence or absence of dexametha-sone in the distal stimulating electrode, was im-planted into the right ventricle of each of 12 dogsfor either 3 weeks ~n5six pairs! or 6 weeks ~n5sixpairs!. Fibrous connective tissue capsules formedaround all of the distal porous-surfaced stimulatingelectrodes. Connective tissue capsules were com-posed of fibroblasts with an abundant collagen ma-trix and contained scattered macrophages, lympho-cytes, plasma cells, and mast cells. Connectivetissue sheaths around dexamethasone-coated leadswere thinner (p,0.03), less cellular (p,0.10) andhad fewer mast cells (p,0.10) than correspondingnonsteroid leads. Threshold voltages for electricalstimulation of the myocardium were consistentlylower (p50.005) for pacing leads withdexamethasone-eluting electrodes than for leadswithout dexamethasone. The electrode-tissue im-pedance was lower (p50.1) for the steroid-elutingelectrodes. This study gives clear evidence that re-ducing the inflammatory response to an implantedelectrode with an anti-inflammatory agent is highlydesirable.Obviously it is desirable to achieve the thinnest cap-

sule surrounding an implant. In addition to the species ofmetal being important, there is evidence that the size ofthe implant is a factor. For example, when Dodge et al.9used smaller electrodes, they found less tissue response;however, the smaller diameter electrodes were of stain-less steel rather than copper, thereby making it difficultto attribute the thinner capsule to size. However, thestudy by Campbell et al.7 gives evidence that very smalldiameter implants are associated with a thin capsule.

ALLERGENIC CONSIDERATIONS FORIMPLANTED METALS AND ALLOYS

An implanted electrode should not produce an aller-genic response. Hitherto, this factor has been largelyoverlooked when considering an electrode metal or alloyor insulation for implantation. Some metals and alloys









are highly allergenic and are not candidates for an im- could be the cause of her dermatitis and reapplied

882 L. A. GEDDES and R. ROEDERplant. The following discussion will elaborate on thisissue.

An allergic response reflects a hypersensitive stateacquired through prior exposure to a particular substance~allergen! and re-exposure produces an enhanced capa-bility of the immune system to react. An allergen is anysubstance capable of producing a specific type of suscep-tibility. One may view an allergic response as an exag-geration of the bodys defensive mechanisms.

It is not generally known that if an alloy contains anallergenic element, implantation of the alloy will producean allergic response. The following case illustrates thispoint in which a stainless steel screw was implanted.

Barranco and Soloman3 reported a case of nickel der-matitis in a lady who had her ears pierced prior to asurgical implant; they reported: A 20-year-old whitewoman had her ears pierced in 1967. In July 1968, aHauser procedure was done on the left knee for a chroni-cally dislocated patella. A stainless steel screw was usedto secure a transferred tendon in its new location. Thepatient experienced no difficulty with this procedure. Thesame procedure was done on the right knee in June1969; again a stainless steel screw was used to reattachthe transferred tendon. This time the procedure was com-plicated by a wound dehiscence and secondary closure.

In October 1969, an extensive eruption on the chestand back developed. This was a subacute eczematousdermatitis involving the skin of the shoulders, midback,buttocks, abdomen, and breasts. She was treated withboth topical and systemic corticosteroids with only mini-mal improvement. She then disappeared from follow-upuntil November 1970, when the dermatitis was noted tobe widespread as before, but also in areas of contact withjewelry such as the earlobes, neck, and ring fingers. Itwas thought that she had a contact dermatitis, most likelynickel.

Patch testing with nickel sulfate gave a 41 result andBarranco continued: Out of sheer desperation, the stain-less steel screws were considered a possible cause of thedermatitis, and the orthopedic surgeon begrudgingly re-moved them. The day following removal of the screws,the erythema had markedly subsided with very little itch-ing present. 72 h later, she was essentially clear of herdermatitis, with no itching. Her treatment continued to beonly topical corticosteroids.

Five days following removal of the screws,closed patch testing was done with pure nickel,nickel sulfate, pieces of the stainless steel screwrecently removed from the patient and a currentroutine chemical patch-testing tray. All tests werenegative except for a 41 reaction to nickel, nickelsulfate, and the stainless steel screw. The orthope-dist still doubted that the stainless steel screwthe screw to the skin of the back. In a period of 4h, generalized pruritus and erythema again devel-oped.

The foregoing case clearly demonstrates that asubject can respond to a single element in an alloy;stainless steel in this case, which contains nickel,chromium, iron, and other elements. Note that thepatient developed an allergy to only the nickel inthe stainless steel.There is a hierarchy of allergenic metals. Heading the

list are nickel, chromium, and cobalt, followed by beryl-lium, mercury, copper, gold, and silver. Obviously, thesemetals, or alloys containing them, are not candidates forimplanted electrodes in all subjects. If a response resultsfrom their implantation, the implant must be removedeven if it is still functioning. Although allergy has beenreported to result from contact with gold and platinumjewelry, it is important to note that these elements do notappear in pure form in jewelry; other elements are usedto form more durable alloys. If an allergic response issuspected with an electrode, the patch test will give theevidence sought. The addition of a nonallergenic crite-rion for selection of an electrode metal reduces the list ofeligible metals for an implanted electrode. Additionalinformation on allergenic responses to metals and alloyscan be found in Chapter 8 of Medical Device Accidentsauthored by Geddes.16

ELECTRODE IMPEDANCE

The impedance of an electrode-electrolyte interfacedepends on the species of metal, the type of electrolyte itcontacts, the surface area, and the temperature. The im-pedance decreases with increasing area and surfaceroughness. It also decreases with increasing frequencyand increasing current density used to make the measure-ment. The simplest equivalent circuit for the interfaceconsists of a half-cell potential, a series capacitance andresistance ~Warburg model!, in parallel with the Faradicimpedance; the latter accounts for the very low fre-quency and direct-current properties ~see Refs. 14 and15!. The Warburg components account for thealternating-current impedance; all of these componentsare frequency and current-density dependent ~see Refs.37 and 13!; because of this fact they are designatedpolarization elements.

The best single descriptor for comparing the imped-ance of various electrode metals is the Warburg, lowcurrent-density capacitance (Cw) which takes the formB/ f b, where B and b are dependent on the metal speciesand f is the frequency of the current used to make themeasurement. It is the reactance (1/2p f Cw) that is animportant component of the interface impedance; there-fore the Warburg capacitance per unit area, for which







geometric size. Roughening can be achieved by chemicalTABLE 1. Warburg capacitance mFcm2 of metals in contactwith 0.9% saline.

883Selection of Materials for Implanted Electrodesthere are values for some metals, should be considered.The higher the Warburg capacitance, the lower theelectrodetissue impedance. Table 1 presents thecapacitance/cm2 for many metals in contact with 0.9%saline at room temperature. Figure 1 is a loglog plot ofthe low-current density Warburg capacitance versus fre-quency for the materials shown in Table 1. The closer tothe top of the figure, the lower the impedance of theelectrode. Note that platinum black represents the lowestimpedance. Some of the materials shown in Table 1 havenot been qualified for use as implanted electrodes. Inaddition, for many metals that have been used for im-plants, there are no data on the Warburg capacitance.Therefore there are gaps in our knowledge; for example,there are no such data for chromium, tin, tungsten,nichrome, titanium nitride, lead, and gold, some of whichhave been used for implanted electrodes. However, someof these ~lead and chromium! are not good candidates forimplanted electrodes.

Although the Warburg model in which Cw5B/ f b isuseful for comparing electrode materials, it predicts anever increasing impedance as the frequency is decreased.At direct current ~dc!, i.e., f 50, the impedance is infi-nite. However, when the frequency is below about 10Hz, the Faradic impedance starts to dominate and there isa finite directcurrent resistance. The highest impedancethat an electrode-electrolyte interface can attain is thezero-current-density, dc ~Faradic! resistance. A techniquefor making this measurement was reported by Geddesand Roeder.12

ELECTRODE SURFACE CONDITION

In addition to the species of metal and the electrodearea in determining the low currentdensity electrode-tissue impedance, the condition of the electrode surfaceis equally important. Roughening an electrode surfaceincreases its effective surface area without increasing its

Metal type Reference

Platinum black heavy (PtB)(estimated)

149,700f20.366 38

Platinumiridium black (PtIrB) 8619f20.299 30Platinum black medium (PtB) 4950f20.366 38Platinumiridium (PtIr) 2696f20.79 30Copper (Cu) 705f20.518 29Rhodium (Rh) 112f20.210 29Silver (Ag) 103f20.259 29Stainless steel (SS) 161f20.525 13Platinum (Pt) 21.6f20.143 29Stainless steel (SS) 17.2f20.266 29MP35N (Ni Co Cr Mo) 8.4f20.127 29Palladium (Pd) 7.3f20.113 29Aluminum (Al) 2.94f20.126 29etching, electrolytic etching, or mechanical abrasion~sand blasting!. Sputtering a material on an electrode isalso a way of obtaining a large effective area.

The first and the classical example of chemically pre-paring an electrode surface to reduce its impedance isdue to Kohlrausch21 who faced the problem of measuringthe resistivity of electrolytes. To do so required the avail-ability of a low electrode-electrolyte impedance which hecreated by blackening a platinum electrode by electro-lytically depositing finely divided platinum, i.e., platinumblack. Schwan38 optimized the method by the followingtechnique. The platinum electrode to be blackened is firstsand blasted; then it is placed in a solution of 0.025NHCl that contains 3% platinum chloride and 0.025% leadacetate. The platinum electrode to be blackened is madethe cathode with respect to a large-area platinum anodein the solution. A current density of 10 mA/cm2 and adeposit of 30 A-s/cm2 produce the lowest impedance.The resulting electrode has an impedance about 30 timeslower than that of a bare platinum surface. Suchelectrodes are still used for measuring the resistivity ofelectrolytes.

Platinum is often alloyed with iridium to create aplatinumiridium electrode. Such an electrode is widelyused with cardiac pacemakers. It has a lower impedancethan bare platinum but it is higher than that of platinumblack. However, platinumiridium has been blackened toreduce its impedance ~see Fig. 1!. Brummer andRobblee6 state that iridium is better than platinum formaking very small microelectrodes because it is a hardermaterial. They also reported that IrO2 ~an insulator! has100 times the charge transfer capability than iridium.

Another electrode with a large rough surface is due toSchaldach34 who described a method for preparing tita-nium nitride ~TiN! by sputtering. He reported that it hasan impedance that is six-fold lower than that of bareplatinum and used it for cardiac pacing. It provided alower stimulation threshold and larger sensed voltage.

RADIOGRAPHIC VISIBILITY

Implanted electrodes are usually small because theyare designed to stimulate or detect bioelectric signalsfrom small populations of excitable cells. After implan-tation it is highly desirable to be able to view themradiographically over time to monitor their integrity, es-pecially if an abnormal response is encountered. Becauseof their small size it is prudent to select a metal thatabsorbs x rays strongly; this directs the choice of metalsto those that have a high atomic number, being the num-ber of protons in the nucleus. However, if the implant islarge, the atomic number becomes less important.

Table 2 lists the atomic numbers of electrode materi-als that are candidates for implanted electrodes that do










884 L. A. GEDDES and R. ROEDERnot produce a vigorous tissue response. In the case ofalloys, it is possible to estimate an equivalent atomicnumber by scaling according to the percentages of theelements in the alloy; this was the method used for Table2 and is identified by ~eq!.

The materials included in Table 2 were drawn fromthe literature cited. To this list titanium nitride has beenadded because it was shown by Schaldach34 that it hassix-fold lower electrodeelectrolyte impedance than bareplatinum. Tungsten ~W! was added because it was found

FIGURE 1. Warburg capacitance mFcm2 of metals in contact with various electrolytes. Redrawn from Electrodes and theMeasurement of Bioelectric Events by L. A. Geddes, New York: Wiley-Interscience, 1972 see Ref. 18.


TABLE 2. Properties of materials. eqequivalent. TABLE 3. Properties of insulating materials.

885Selection of Materials for Implanted Electrodesby Hubel19 to be an excellent material for corticalrecording.

Although all of the metals and alloys shown in Table2 are eligible for implanted electrodes, only a few havehigh enough atomic number ~or equivalent! to be visu-alized radiographically. Among the most visible are gold,platinum, platinumiridium, tungsten, and tantalum. Al-though the remainder will be less visible radiographi-cally, if this is not a requirement, many of the materialsin Table 2 are candidates. However, nichrome and stain-less steel contain nickel, which is an allergen for somesubjects.

INSULATION EXPANSION-COEFFICIENTMATCHING

When a hard insulation is applied to an electrode,attention must be given to the coefficient of expansion ofthe material that constitutes the electrode and that of theapplied insulation. This is especially important if theinsulation is applied at a higher temperature and thetemperature at which the electrode is used is lower. Forexample, when glass is melted on to an electrode, if theelectrode material has a different thermal coefficient ofexpansion than the insulation, during cooling the insula-tion will crack. Table 3 lists the thermal coefficients ofexpansion of many electrode materials and insulatingmaterials. However, if the insulation is applied at roomtemperature ~20 C!, the increase in temperature to bodytemperature ~37 C! may not strain the insulation, espe-cially if it is a plastic.

Two other factors related to insulators merit consider-ation: ~1! the dielectric constant and ~2! the dielectricstrength; Table 3 presents such data. The former deter-mines the capacitance of the electrode with respect to itselectrolytic environment; the latter is important only forstimulating electrodes. All dielectrics have a finite abilityto withstand a voltage without breakdown. The dielectricstrength of an insulator is expressed in terms of thevoltage that it can sustain per 1/1000th in. ~V/ml!. Table

MaterialAtomic

No.Expansioncoefficient

Meltingpoint

Gold (Au) 79 13.2 1063Platinum (Pt) 78 8.99 1773PlatinumIridium (PtIr) 78 (eq) 7.58.8 18151935Iridium (Ir) 77 4.4-14.7 2716Tungsten (W) 74 4.2 3370Tantalum (Ta) 73 6.5 2996Tin (Sn) 50 2.2 232Rhodium (Rh) 45 8.2 1964Nichrome 27 (eq) 8.5 1395Stainless steel (316) 26 (eq) 1020 14001450Titanium nitride (TiN) 14 (eq) 9.4 29303 lists the dielectric strengths of many insulating mate-rials used to cover electrodes. Exceeding the dielectricstrength makes the electrode conductive. The thinnerthe insulation, the lower the breakdown voltage for anyinsulation.

In situations when an insulating material needs to beheated for application to an electrode, in addition tomatching thermal coefficients of expansion, it is neces-sary that the melting point of the insulation be below thatof the metal that constitutes the electrode. Table 2 liststhe melting points and expansion coefficients of severalmetals. Table 3 lists the melting points and expansioncoefficients for many insulating materials.

An electrically heated circumscribed coil can provideheating to melt an insulator. A flame can also be used.The flame temperature for natural gas and air is 1950 C,that for acetylene and air is 2325 C and that for acety-lene and oxygen is 3000 C.

ELECTRODE-AREA MEASUREMENT

Because electrode impedance depends on the area ofthe electrode exposed to tissues and fluids, it is useful toobtain a measurement of the electrode area. Using a lightmicroscope to make such a measurement is difficult be-cause the insulation is very thin near the electrode tipand the end of the insulation is not visible. Hubel19described a simple method that involves placing a dropof saline on a glass slide that is viewed with a micro-scope. A small wire is then placed in the drop and con-nected to the positive pole of a battery; the other pole isconnected to the microelectrode which is advanced intothe drop, and the tip is viewed in the microscope. Theactive area can be estimated by observation of the areafrom which hydrogen gas bubbles are evolved. Thismethod is very convenient to apply and has been usedsuccessfully to measure the area of a variety of large andsmall needle electrodes.

Material

Dielectricstrength(V/ml)

Dielectricconstant

Expansioncoefficient(31026)

Meltingpoint (C)

Polyimide 550 3.5 3060 400Pyrex glass 335 36 3.3 1245Vycor glass fl 3.8 0.8 1550Soda glass fl 4.8 12.0 1000Quartz crystal fl 4.2 5.21 1410Quartz fused 410 3.78 0.256 1500Aluminum oxide 365 4.8 1.0 2000Formvar fl 8.4 fl flTa Pentoxide fl 1825 fl 1785Epoxy 400 3.6 62 flTitanium dioxide 100210 14110 6.511.0 1843IrO2 fl fl fl 1100



BIMETAL JUNCTION CORROSION waveform will not be the same as the voltage waveform

886 L. A. GEDDES and R. ROEDERIf the conductor that contacts the microelectrode is ofa different metal, this bimetal junction must be fluidproof. If this junction comes into contact with tissuefluids, there exists a short circuited galvanic cell andcurrent will flow in the electrolytic environment with theproduction an unstable potential that will turn up asnoise if the electrode is used for recording ~see Ref. 1!.With the passage of time this electrochemical action willerode the junction and produce an open circuit.

STIMULATING ELECTRODES

When small-area electrodes are used to stimulate ex-citable tissue, despite the fact that the current is low, thecurrent density is high and two new considerations arise:~1! stimulus waveform distortion and ~2! electrochemicaldecomposition. The former is associated with the War-burg capacitive nature of the electrode-electrolyte inter-face and the type of stimulator output circuit ~constantvoltage or constantcurrent!. Electrolytic decompositionresults when there is a direct-current component in thestimulus train.

STIMULATOR OUTPUT CIRCUITS

Basically there are only two types of stimulator outputcircuits: ~1! constantvoltage and ~2! constantcurrent.In both cases they are connected to the stimulating elec-trodes, which have complex resistive and reactive com-ponents, due to the nature of the electrode-electrolyteinterface. With the constantvoltage stimulator, if thestimulus wave form is a rectangular voltage pulse, thecurrent pulse will not be rectangular; it will have a spikeon the rising phase, an exponential decay during thepulse, and an undershoot spike at the end of the pulse.This is the characteristic of a typical functional electricalstimulator in which the peak current depends on thestimulus voltage and inversely with the electrode-tissueimpedance.

With the constantcurrent stimulator, which is typi-cally used for research, the amplitude of the currentpulse is independent of the electrode-tissue impedance. Ifa rectangular current pulse is delivered to stimulate thetissue, the voltage wave form across the electrodes willnot be rectangular; it will have a small step, followed byan exponential rise that is terminated by the end of thepulse.

In practice, stimulators are neither true constantvoltage nor constantcurrent types. The typical stimula-tor has a low output impedance with respect to that ofthe tissue-electrode circuit; therefore it resembles aconstantvoltage source. Consequently, the currentowing to the complex impedance of the electrode-electrolyte interface.

ELECTRODE DECOMPOSITION

When an electrode is implanted for recording a bio-electric signal, the size of the encircling capsule is thefocus of interest. However, when a stimulating electrodeis implanted, capsule thickness and electrode decompo-sition are important factors in choosing the best metalspecies. Of equal importance is the type of stimuluswaveform and the type of stimulator output circuit.These factors will be discussed now.

Faradays law of electrochemical decomposition statesthat for a monovalent element, 1 g equivalent of anelement is removed ~or deposited! by the passage of oneFaraday, i.e., 96,500 C ~A s! of charge transferred. There-fore, if a stimulus waveform has a dc component, elec-trolytic decomposition will occur.

There are two techniques that can be used to mini-mize the presence of a dc component in the stimulus: ~1!use capacitive coupling in the output circuit of the stimu-lator; this method is used in cardiac pacing, ~2! use whatis called the balanced waveform, introduced by Lillyet al.22 in 1955. With this waveform the charge in thefirst phase is equal to the charge in the second, oppo-sitely directed phase. There are two methods of satisfy-ing this requirement: ~1! use a bidirectional pulse inwhich the amplitude and duration of both pulses are thesame, i.e., a reciprocal pulse. ~2! The amplitude of thefirst pulse is higher and the duration is shorter than thatof the second inverted pulse, the areas under both pulsesbeing equal. However, use of the balanced pulse does notguarantee elimination of electrode decomposition due tothe nonlinearity of the electrode-electrolyte interface.Nonetheless, when long-term ~chronic! stimulation is tobe employed, use of the bidirectional ~incorrectly calledbiphasic! wave is the best choice along with use of aconstant-current stimulator.

In 1955, Lilly et al.22 pointed out that in the late1940s, chronic brain stimulation in monkeys and manwas associated with neural damage at the electrodes.They postulated that this damage is due to the use of amonophasic stimulating waveform and they introducedthe bidirectional wave, consisting of two identical 40 msoppositely directed pulses separated by 88 ms. They stud-ied the cortex response to this waveform and stated:

From the results, it is concluded that this form ofelectric current does not detectably injure cellularfunction or structure when it is passed through thecortex near threshold values for 45 h per day for15 weeks.White and Thomas41 carried out an important study to

determine the decomposition of various metal wires THE INSULATED CAPACITIVE ELECTRODE887Selection of Materials for Implanted Electrodes~0.010 in.! used as electrodes in saline. They employed a0.5/0.5 ms bidirectional wave with a frequency of 50/sdelivered from a constantcurrent stimulator. The metalsinvestigated were stainless steel, platinum, iridium, pal-ladium, rhodium, rhenium, gold, tantalum, titanium,tungsten, zirconium, and some conducting oxides. Cur-rent densities ranged from 100 to 2000 mA/cm2 ~thelatter is in excess of stimulation threshold!. The elec-trodes were weighed before and after current flow. Cur-rent was passed for 24 h a day for periods up to 9months. They concluded that iridium, rhodium, platinum,and palladium are very resistant to corrosion. Theystated:

Even microelectrodes 5 mm thick, made of suchmaterials should have lifetimes on the order ofdecades. Gold is somewhat poorer but probablyacceptable; all the other materials tested, includingtungsten and stainless steel, are unacceptable aschronic microelectrode material.An interesting in vitro study on the ability of the

bidirectional balanced wave to reduce the decompositionof 316 stainless steel microelectrodes ~0.08 cm2) wasreported by McHardy et al.26 A bidirectional pulse (230.5 ms! with a frequency of 50/s was delivered to theelectrodes via a 3 mF capacitor ~to block dc! for 13days. They quantitated the electrode decomposition bymeasuring the iron deposited in the saline. They foundthat use of the balanced bidirectional wave permitteddelivery of triple the charge density for the same elec-trode decomposition.

The foregoing study by White and Thomas meritsspecial attention because the method used a balancedbidirectional waveform and a constantcurrent outputcircuit to deliver the pulse of current. Both guarantee thata minimum of dc will flow through the electrodes. Thisbeing so, it is seen that some electrode materials weredecomposed, judged by weight loss, and others wereresistant to decomposition. The method used by Whiteand Thomas is the best for testing the suitability of anelectrode metal for stimulation.

Brummer and Turner5 analyzed the possible electro-chemical reactions, notably pH shift and electrolytic de-composition products that could occur at a platinummicroelectrode-electrolyte interface. They stated thatneural stimulation parameters range from 0.1 to 5 mAapplied to electrodes ranging from 0.0005 to 0.08 cm2.Pulse durations range from 10 to 100 ms with frequen-cies from 10 to 200/s. They conducted in vitro studiesand recommended the use of a balanced bidirectionalwave and to limit the charge density ~C/cm2).It is possible to record bioelectric events and stimulateexcitable tissues with an insulated electrode, i.e., one thatdoes not make ohmic contact with tissues or body fluids.A capacitor consists of two conducting materials sepa-rated by an insulator ~dielectric!. With the insulated elec-trode, one conductor is the metal electrode; the dielectricis the insulation thereon and the other conducting mate-rial is the tissue fluids. The capacitance C5kA/t , wherek is the dielectric constant of the insulator; A is the areaof the electrode, and t is the thickness of the dielectric. Itis desirable to achieve the highest capacitance for agiven area; therefore a thin layer of dielectric is needed.However, the thinner the dielectric, the lower the dielec-tric breakdown voltage. The dielectric breakdown of aninsulator is specified in terms of volts per 1/1000th in.~ml! of thickness.

CAPACITIVE RECORDING ELECTRODES

The first insulated recording electrodes appear to bedue to Richardson et al.31,32 and Lopez et al.24 of the USAir Force. The electrodes consisted of an aluminum plate(2.532.5 cm! that was anodized on the surface placed incontact with the skin. On the back of the electrode wasmounted a field-effect transistor ~FET! with the gate ter-minal connected to the electrode. Surrounding the anod-ized electrode was an insulating block of potting com-pound surrounded by a circular metal ring that acted asan electrostatic shield; the FET was connected as asource follower. To protect the FET from acquiring ahigh electrostatic voltage, a high-resistance leakage pathwas provided by using two diodes ~IN3600! in seriesopposition.

Following Richardsons lead, Wolfson and Neuman43described a small-area insulated electrode for generalpurpose bioelectric recording. The electrode measured636 mm and was fabricated from an 0.01 V cm, 0.23-mm-thick N-type silicon wafer. The circular region, 4.5mm in diameter, was the active detecting portion of thesurface and had an oxide 0.2 mm thick. The surroundingregion, a 1.5-mm-thick oxide, minimized electrical leak-age over the surface of the electrode, and extended downthe sides and over the back of the disk except for a smallregion where an ohmic contact was made to the siliconand photomask; an etch process provided a tenaciousinsulating layer that is highly reproducible and can bemaintained to close tolerances.

Wolfson and Neuman stated that a metaloxidesemiconductor ~FET! source follower was mounted tothe back of the electrode. They reported a low-frequencycutoff of 0.005 Hz and the high-frequency cutoff waswell above that for electrophysiological signals.

The desiderata for a capacitive recording electrode are silver paint that captured a bare stranded wire and con-

888 L. A. GEDDES and R. ROEDERa high dielectric constant and a thin dielectric to achievea high capacitance. However, the breakdown voltage isnot a consideration, but the thin dielectric coat must befree of pinholes. If radiographic visibility is required, ametal with a high atomic number is selected. In addition,the dielectric should not be allergenic.

Although capacitive electrodes are routinely used withdiathermy and sometimes as electrosurgical dispersiveelectrodes, they have seldom been used as stimulatingelectrodes. Because their operation is associated with nonet charge transfer, they merit consideration. In addition,the charge transfer density distribution thereunder ismore uniform than that with electrodes that make ohmiccontact with the subject. However, some dielectrics de-teriorate with time, especially those in contact with con-ducting fluids, and the dielectric becomes conductivethereby changing the character of the electrode to be-come a conductive one.

CAPACITIVE STIMULATING ELECTRODES

An unusual type of capacitive stimulating electrode isdescribed in a patent issued to Batrow and Batrow.4Schaldach35 reported the use of a catheter-tip capacitivecardiac pacing electrode.

The Batrow electrode was designed for transcutaneousphrenic-nerve stimulation to contract the diaphragm toproduce artificial breathing. The electrode consisted of aflattened glass chamber containing argon gas at a lowpressure. When it was placed on the skin and a high-voltage pulse was applied, the gas ionized and emittedlight, becoming one plate of the capacitor electrode; theother plate was the subject. The remarkable feature ofthis capacitor electrode ~and the very-short-durationpulses that were applied to it!, was that it providedmotor-nerve stimulation with very little skin sensation.

The capacitor electrode described by Schaldach,35used for cardiac pacing, consisted of a metaloxideformed on the surface of a catheter-tip titanium electrodeto create the dielectric which was titanium nitride. Schal-dach reported that cardiac pacing could be accomplishedwith two-thirds less energy compared with a conven-tional pacing electrode of the same size. Importantly, hestated that stimulation was the result of charge rearrange-ment, there being no transfer of charge at the electrodesurface.

Geddes et al.17 described a capacitor electrode suit-able for human motor-point stimulation that was con-structed from No. 7740 fused silica tubing ~13 mm outerdiameter, 11 mm inner diameter!. The dielectric constantis 5.1. The tubing was heated in a flame to close the endand blown to form the end chamber, 25 mm in diameterand 1 mm thick. The chamber was filled with conductingstituted one plate of the capacitor; the other plate was thesubject.

The stimulator consisted of an autotransformer of theignition-coil type. Into the primary was discharged an 8mF capacitor. The maximum open-circuit voltage wasvariable to 60 kV. The current wave form was a slightlyunderdamped sine wave, a fraction of a millisecond induration.

Stimulation of the motor points of the forearm pro-duced finger twitches and tetanic contractions with ease,causing little skin sensation. Even less sensation wasperceived when a thin, 25 mm diameter gauze pad,lightly moistened with tap water, was placed between theskin and the capacitive electrode. The combination of acapacitive electrode and a short-duration pulse is idealfor transcutaneous motor-nerve stimulation with verylittle cutaneous sensation.

Loeb and Richmond23 described a tantalum capacitiveelectrode for neural stimulation. The tantalum electrodewas sintered with tantalum powder, then anodized toproduce a small-area tantalum pentoxide electrode with acapacitance of 4 mF. The reference electrode was anelectrically conducting porous oxide of indium; theseconstituted the BION implantable stimulating system.

The capacitive stimulating electrode presents unusualdesign requirements. The dielectric constant of the insu-lation should be high to achieve a high capacitance. Thedielectric should posses a high breakdown voltage ~V/ml!. The thinner the dielectric, the higher the capaci-tance, but the lower the breakdown voltage. The metalon which the insulation is placed should have a highatomic number to be visible radiographically. Finally, theinsulation should be free of pinholes, nonallergenic, andnot deteriorate in the presence of conducting fluids.

IMPEDANCE OF CONDUCTIVE ELECTRODES

Establishing a stable, low-impedance contact with tis-sue is the primary requirement of an implanted electrode.From Fig. 1, the choices are platinum black, platinumiridium black, platinumiridium, copper, rhodium, silver,stainless steel, MP35N, palladium, and aluminum in thatorder. However, platinum black induces a thick tissuecapsule, as do copper and silver. There are no data onplatinumiridium black, but it is likely that it also stimu-lates a thick capsule. Stainless steel contains nickel,which is a potent allergen. MP35N has not been used asan electrode as yet. This leaves only platinum, rhodium,and palladium for which there are impedance data.

Table 2 lists other metals that have been used aselectrodes and contains some that must be rejected onthe basis of provoking a thick tissue capsule. If radio-graphic visibility is a requirement, those with the highestatomic number are the candidates, they are gold, plati-





num, tungsten, tantalum, tin, rhodium, palladium, and CONCLUSION

889Selection of Materials for Implanted Electrodestitanium nitride, the latter having a rough surface toobtain a large surface area; there are no data for a tissueresponse. Tin is a soft metal that is chlorided when usedas a recording electrode. Chloriding will probably stimu-late the growth of a thick tissue capsule. However, baretin is smooth and flexible and is used by one manufac-turer as a defibrillating electrode. Gold, platinum, tung-sten, and titanium have all been used as surgical implantswithout allergenic response. There are no implant data onrhodium and palladium.

Selecting the best candidate from Tables 1 and 2,provides gold, platinum, tungsten, rhodium, palladium,and titanium. All of these metals can be deposited on asilicon substrate and can be used as implants with suit-able insulation. However, White and Thomas41 does notrecommend palladium, rhenium, tantalum, titanium,tungsten, and zirconium as stimulating electrodes due totheir decomposition characteristics.

INSULATING MATERIALS

Table 3 lists the dielectric strengths, dielectric con-stants, expansion coefficients, and melting points ofmany insulating materials used for implanted electrodes.Polyimide appears to produce the least tissue response. Ifglass or fused quartz is used, the coefficient of expansionmust match that of the electrode that it covers; otherwisecracking will result when cooling takes place.

As stated earlier, metals such as aluminum, indium,and tantalum can be anodized to produce a thin oxidecoat that is an insulator. Of the two, tantalum appears toproduce an oxide coat that has the highest capacitanceper unit area.

INSULATORS FOR CAPACTIVE ELECTRODES

Three of the metals ~aluminum, tantalum, and iri-dium! have been anodized to produce an oxide coatwhich is a dielectric; these oxides have a reasonabledielectric constant. The magnitude of the capacitance de-pends on the electrode area and inversely with the thick-ness of the oxide layer. The capacitance can be increasedby first etching the surface before anodizing. The break-down voltage depends on the thickness of the oxidelayer. With a moderately thick layer, breakdown voltagesof several hundred are attainable.

Both aluminum oxide (Al2O3) and tantalum pentox-ide (Ta2O5) are used in commercially available capaci-tors. If radiographic visualization is a requirement, tan-talum is the choice because of its higher atomic number~73!, compared to that of aluminum ~13!. An implantableTa2O5 capacitive electrode was reported by Loeb andRichmond ~2001!.23 Neither Al2O3or Ta2O5 have beenreported to be allergenic.Based on the four criteria ~tissue response, allergenic-ity, impedance, and radiographic visibility!, it is neces-sary to distinguish between stimulating and recordingelectrodes. From the study by White and Thomas~1974!,41 the best metals are platinum, iridium, andrhodium, the latter being less visible radiographically.Gold is also a candidate for recording ~noncurrent-carrying! electrodes. The choice for stimulating elec-trodes are platinum, platinumiridium, gold, tungsten,and rhodium, the latter being a little less visible radio-graphically. For capacitive stimulating electrodes, tanta-lum pentoxide has the highest dielectric constant, fol-lowed by iridium oxide. Aluminum oxide is a candidatewith a lower dielectric constant.

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