In vivo Electrochemical Characterization of Activated Iridium Oxide Stimulation Electrodes Implanted Sub–Retinally in Rabbit

Abstract

Abstract: : Purpose: To measure the electrochemical response of retinal stimulation electrodes implanted sub–retinally in rabbit and to compare the in vivo response with that obtained in vitro in an inorganic interstitial fluid model. Methods: Activated iridium oxide electrodes on polyimide arrays were implanted in the sub–retinal space of Dutch–Belted rabbits. The iridium oxide was characterized electrochemically by open–circuit potential, cyclic voltammetry (CV) and chronopotentiometry using a Ag|AgCl reference electrode connected through a salt bridge to the rabbit tongue. A gold–coated eye coil acted as the counterelectrode. Measurements were made within one hour of implantation and after eight hours. Voltage transients in response to biphasic current pulses and potential–biased, monophasic current pulses were recorded using either the Ag|AgCl reference electrode or the eye coil as a pseudo–reference. The in vitro electrochemical responses were recorded in a three–electrode cell in an inorganic model interstitial fluid (ISF) electrolyte. Results: CVs obtained sub–retinally over a potential range of –0.6 V to 0.8 V (vs. Ag|AgCl) were similar to those obtained in vitro, although the in vivo CVs exhibited a higher overall charge storage capacity. No damage to the retina at the electrode site from potential cycling during the voltammetry was evident from histologic inspection using light microscopy. Electrodes pulsed for 8 hr at charge densities that did not result in the iridium oxide being polarized outside the potential window for electrolysis of water preserved normal CV behavior and were undamaged when inspected by scanning electron microscopy. Features in the cyclic voltammograms that are characteristic of electrode degradation were observed in electrodes that were pulsed beyond the iridium oxide charge–injection limits. Steady–state currents observed by chronopotentiometry between 0.1 V and 0.8 V (Ag|AgCl) were consistently higher in vivo than in vitro by a factor of about three, with the in vivo current densities ranging from 20–50 µA/cm² at 0.8 V. Conclusions: In this acute study, the electrochemical response of iridium oxide electrodes in the sub–retinal space was similar to that observed in vitro using the inorganic ISF electrolyte. The condition of the iridium oxide could be assessed by cyclic voltammetry, which provides a means of monitoring the stability of electrodes during acute or chronic studies. Electrodes could be pulsed for eight hours at charge densities of 0.64 mC/cm² (45 Hz) without damage

Keywords: retina