Charge–injection Coatings For Flexible Electrode Arrays Used In Retinal Prostheses

Abstract

Abstract: : Purpose:Retinal prostheses are being developed for restoration of vision in patients blinded by retinitis pigmentosa. The prosthesis interfaces with the retina through an array of electrodes on a flexible substrate that is placed on the retina to electrically stimulate remaining viable cells. Charge–injection requirements for stimulation can exceed established limits for noble metal electrodes, requiring the use of high charge–injection capacity coatings. Our objective is to evaluate candidate charge–injection coatings on flexible, polymer substrates that would be used for electrode arrays. Methods:Flexible polyimide substrates were metallized by thin–film processing techniques to provide arrays with multiple electrodes of varying surface areas. Charge–injection coatings of iridium oxide (Ir–oxide) and fractal titanium nitride (TiN) were formed on the charge–injection sites by either activation of iridium metal, electrodeposition of Ir–oxide, or sputtering. The coatings were characterized electrochemically by impedance spectroscopy and cyclic voltammetry, and then challenged by aggressive current pulsing with biphasic, charge–balanced waveforms. Results:Ir–oxide coatings formed by electrodeposition of the oxide or by activation of sputtered iridium metal exhibited charge–injection capabilities similar to those obtained with Ir–oxide formed on rigid substrates. Using an 0.6 V (Ag|AgCl) anodic bias, charge–injection capacities of 4 mC/cm² and 8 mC/cm² were obtained with 0.4 ms and 1 ms pulse widths, respectively. Combining anodic biasing with charge–balanced, but asymmetric waveforms, was effective in maximizing the deliverable charge with Ir–oxide. Sputtered Ir–oxide and TiN coatings also exhibited high levels of charge–injection, but typically required much thicker coatings, >2 µm compared with <0.5 µm, to inject levels of charge similar to the activated and electrodeposited oxides. Conclusions:A 0.6 V bias with an asymmetric waveform is optimal for delivering charge–balanced, biphasic pulses with Ir–oxide. No significant differences in the charge–injection properties of Ir–oxide on flexible and rigid substrates are observed. The charge–injection capabilities of all coatings are appropriate for retinal stimulation, however, long–term stability and the impact of the in vivo environment on electrode stability is yet to be adequately determined.

Keywords: retina • retinitis • signal transduction