ISSN 1318-0010 KZLTET 32(3-533)165(1998) BEHAVIOR OF PLATINUM STIMULATING ELECTRODES IN PHYSIOLOGICAL MEDIA OBNA[ANJE PLATINASTIH STIMULACIJSKIH ELEKTROD V FIZIOLO[KEM MEDIJU JANEZ ROZMAN1, B. PIHLAR2, M. JENKO3 1ITIS d.o.o. Ljubljana, Centre for Implantable Technology and Sensors, Lepi pot 11, 1001 Ljubljana, University of Ljubljana 2Faculty of Chemistry and Chemical Technology, A{ker~eva 5, 1001 Ljubljana 3Institute of Materials and Technology, Lepi pot 11, 1001 Ljubljana, Republic of Slovenia Prejem rokopisa - received: 1998-12-04; sprejem za objavo - accepted for publication: 1998-12-14 Electrical stimulation of neuro-muscular system generates a response in excitable cells by an electrical field between two electrodes produced by the flow of ions, i.e., ionic current, in the biological fluid. The compatibility of a chronic neural prostheses requires that the electrical charge must be delivered without producing toxic reactions products or degrading the electrode. For a given electrode there is a limit to the quantity of charge that can be injected in either the anodic or cathodic direction with reversible surface processes. This limit will depend upon the size of the electrode, its geometry, and the parametres of the stimulating waveform. To determine this limit experimentally, knowledge of the potential range over which they can occur is required. The electrochemical technique of cyclic voltammetry can delineate an operational potential window between hydrogen and oxygen evolution. Keeping the electrode potential within this window during pulsing guarantees that water electrolysis reaction will not occur. The study reported here seeks to characterize platinum electrode behavior of the multielectrode spiral cuff system for selective stimulation of different superficial regions of peripheral nerves in a protein containing solution. Each platinum electrode of the cuff system had a flat geometric surface of 2 mm2. In a typical cyclic voltammetry experiment, the potential of the tested electrode versus a Saturated Calomel Electrode (SCE) was cycled at an appropriate rate between two potential limits. All measurement have been carried out using a specially designed electrochemical cell at 37°C, and the Potentiostat/Galvanostat (Princeton Applied Research). Besides, investigations of surfaces of electrodes using high resolution AES method were performed. Key words: platinum stimulating electrodes, peripheral nerves, voltammetry, electrical charge Elektri~na stimulacija živ~no-mi{i~nega sistema povzro~i z elektri~nim poljem med dvema elektrodama tok ionov skozi živ~no tkivo in s tem odgovor vzdražljivih celic. Biokompatibilnost kroni~nih živ~nih protez pa zahteva, da mora biti elektri~ni naboj doveden tako, da ne povzro~a nastanka toksi~nih produktov ali razkroja elektrod. Za dano elektrodo obstaja zgornja meja naboja, ki ga lahko vnesemo v anodnem ali katodnem delu stumulacijskega impulza ob reverzibilnih povr{inskih procesih. Ta je odvisna od velikosti elektrode, njene geometrije in parametrov ter oblike stimulacijskih impulzov. Za experimentalno dolo~itev te meje je potrebno poznavanje območja elektrokemijskih potencialov, ki se lahko pojavijo na elektrodah. S tehniko cikli~ne voltametrije lahko določimo potencialno okno med izločanjem vodika in kisika. Z zadrževanjem potenciala elektrod znotraj tega okna ob stimulaciji lahko zagotovimo, da ne bo pri{lo do elektrolize vode. Cilj te {tudije je bil dolo~iti obna{anje platinastih elektrod znotraj večelektrodnega sistema v obliki spiralne objemke za selektivno stimulacijo posameznih povr{inskih področij perifernih živcev v fiziolo{kem mediju. Geometrijska povr{ina posamezna elektroda znotraj spiralnega sistema je bila 2 mm2. Pri eksperimentu s ciklično voltamerijo smo potencial testne stimulacijske elektrode glede na nasičeno Calomelovo elektrodo (SCE) ciklično in s primerno hitrostjo spreminjali med dvema zgornjima mejama potencialov. Vse meritve smo izvajali v posebej izdelani elektrokemijski celici pri temperaturi 37°C in s Potentiostat-om/Galvanostat-om (Princeton Applied Research). Narejene so bile tudi raziskave povr{in elektrod z visoko ločljivim Augerjevim spektroskopom. Ključne besede: platinaste stimulacijske elektrode, periferni živci, voltametrija, električni naboj 1 INTRODUCTION The electrical activation of nervous tissue provides a means to exert external control over body systems that are normally under control of the nervous system1. For example, the activation of paralyzed muscles by electrical stimulation of intact lower motor neurons allows restoration of movements to persons with spinal cord injury, head injury, or stroke2,3. In most of applications, at least one electrode is used to activate each muscle4. The aim of controlling larger numbers of muscles requires the implantation and maintennance of a large number of electrodes. To reduce the number of implanted electrodes in advanced motor prostheses, electrodes are required that can stimulate independently several muscles5,6,7. However, the long-term use of electrical stimulation in this way requires that the stimulation be applied selectively and without causing tissue injury. Tissue damage and stimulating electrode corrosion are both associated with high charge density stimulation8,9. Long-term stimulation of the nervous system implies the absence of irreversible electrochemical reactions such as electrolysis of water, evolution of chlorine gas by oxidation of chloride, or the formation of metal oxides1,10. During the application of the electrical pulse, the potential limits at which a significant amount of oxygen and hydrogen are formed should not be exceeded11,12. Namely, as the application of an external potential on the electrode result in their po-larization1,13,14, the potential of the anode and cathode is shifted in the anodic and cathodic direction, respectively. In the presence of electrochemically active substances in the vicinity of the electrodes, oxidative processes on the anode and reductive reactions on the cathode occur1. For a given electrode there is a limit to the quantity of charge KOVINE, ZLITINE, TEHNOLOGIJE 32 (1998) 3-4 16 5 J. ROZMAN ET AL.: BEHAVIOUR OF PLATINUM STIMULATING... that can be injected in either the anodic or cathodic direction with reversible surface processes15. This limit will depend upon the size of the electrode, its geometry, and the parametres of the stimulating waveform. The safe limits for injection have been found to be much higher for balanced charge biphasic pulses than for mo-nophasic pulses10,11. Presumably, the charge delivered during secondary pulse reverses potentially toxic products generated during primary pulse1. When long pulse widths are used or when a delay is introduced between the primary and the secondary pulse, there is more time for electrochemical reactions to occur. The unreversed products can lead to tissue damage, and in the case of balanced charge biphasic pulses can lead to excess anodic drift of the interpulse electrode during the anodic phase. This drift could possibly cause corrosion. To determine safe limitis experimentally knowledge is required on the potential range over which reversible surface processes can occur9,13. The electrochemical technique of cyclic voltammetry can delineate an operational potential window between hydrogen and oxygen evolu-tion3. Keeping the electrode potential within this window during pulsing guarantees, that water electrolysis reaction will not occur. If the stimulating charge density exceeds this limit then the potential reached by the electrode will induce ionic flow by the faradaic reactions. The faradaic processes available on Pt have been classified as reversible or irreversible14,15. Reversible charge injection limits for cathodic pulses range from 0.25 |iCb/mm2 for some platinum electrodes to 35 |iCb/mm2 for Ir oxide electrodes. Reversible reactions are those that can be quantitatively reversed by passing a current in the opposite direction, and do not produce new chemical species in the bulk of the solution. Irreversible faradaic reactions are those that involve soluble species in the tissue fluid and will lead to the production of new chemical species3,11,15. The charge required for electrical stimulation with miniature stimulating electrodes often exceeds the limits for reversible charge injection and a small fraction of the charge is transferred by faradaic re-action3,14,16. Accordingly, an important component in the design of electrical stimulation is the stimulating electrode itself; its properties determine the nature and kinetics of charge transfer between electron conduction in the external circuit and ionic conduction through electrolytes within the tissue. The study reported here seeks to characterize platinum electrode behavior of the 45-electrode spiral cuff system developed in our laboratory in physiological media for selective stimulation of superficial regions of peripheral nerves. 2 METHODS Multielectrode spiral cuff A 45-electrode cuff system with a spiral transverse cross section for selective stimulation of superficial regions of peripheral nerves was designed to be expand- able so that it could be sized to fit around a nerve trunk5,6,7. It was manufactured by bonding two 0.1 mm thick flexible silicone sheets together. One sheet stretched and fixed in that position was covered by a layer of adhesive. A second unstretched sheet was placed on top of the adhesive and the composite was compressed to a thickness of 0.3 mm. When released, the cuff curled into a spiral tube as the stretched sheet contracted to its natural length. The diameter of the cuff was related to the amount of stretch: the greater the stretch, the smaller the diameter. 45 rectangular electrodes with a width of 0.6 mm and length of 1.5 mm made of cold rolled and annealed 50 | m thick platinum ribbon (99.99% purity) were mounted on the third silicone sheet with a thickness of 0.1 mm. Then Teflon insulated, mul-tistranded lead wires were connected to the electrodes. For experimental purposes, the junctions between platinum electrodes and lead wires were implemented using a special tin alloy. Electrodes were arranged in three parallel groups each containing fifteen electrodes with a distance of 0.5 mm between them, while the distance between the groups was of 6 mm. An electrode with a certain number within each of the three parallel groups had the same position, and accordingly, fifteen groups of three electrodes in the same line in a longitudinal direction were formed. The electrodes arranged on the sili-cone sheet were then bonded on the inner side of the mechanically opened spiral cuff. The completed spiral cuff with an inner diameter of 2.5 mm was then trimmed to a length of 20 mm as shown in Figure 1. Evaluation of impedance and galvanometric behavior of stimulating electrodes The impedance of single electrode within the spiral cuff was measured "in vivo" 20 days after implantation on the sciatic nerve of a Beagle dog17. In these measure- Figure 1: 45-electrode system for the selective stimulation of superficial regions of peripheral nerves Slika 1: 45-elektrodni sistem za selektivno stimulacijo površinskih področij perifernih živcev 534 KOVINE, ZLITINE, TEHNOLOGIJE 32 (1998) 6 J. ROZMAN ET AL.: BEHAVIOUR OF PLATINUM STIMULATING. ments one stimulating electrode of the spiral cuff was connected to the impedance meter (Hewlett Packard) as the tested electrode while the large surface electrode, representing the common electrode when the stimulating system is implanted, was connected to the aforementioned instrument to complete the electrical circuit. Small electrodes of the spiral cuff are needed to affect as selective as possible the activation of small groups of nerve fibres18,19. Moreover, it is necessary to depolarize axons at some distances from the electrode. Therefore, it is desirable to be able to inject enough charge. Platinum is capable of delivering the desired charge density solely by reversible processes. Each electrode of the spiral cuff had a flat geometric surface of 2 mm2. The proposed biphasic rectangular and quasitrapezoidal cathodic first stimulating waveforms required for stimulation should result in low charge density. As shown below (equation 1), in the calculation of maximal charge that could be required in selective stimulation of superficial region of the dog in stimulating, cathodic part of rectangular stimulating pulse pair we proposed for curent amplitude to be 1 mA, and for width to be 200 ms. The time delay between biphasic phases was settled to be 50 ms as shown in Figure 2, where the aforementioned and experimentally used current pulse pair is presented in upper trace and corresponding voltage waveform appeared between the stimulating electrodes in physiological solution (0.9% NaCl) in lower trace. As the quantity of the reaction product generated by an electrochemical reaction is directly proportional to the absolute charge injected, we supposed that the proposed electrode delivering 100 nCb per pulse will not generate much product. In the evaluation of galvanometric behavior of single stimulating electrode within the spiral cuff the electrochemical technique of cyclic voltammetry was used. The main goal was to delineate an operational po- 20 uS/d 1998/08/19 l*t