Electrical stimulation of isolated retina with microwire glass electrodes

Abstract

The development of high-resolution retinal prostheses fabricated from silicon wafers presents an interesting problem: how to electrically bridge the space between the flat silicon wafer and the curved retinal surface. One potential “bridge” is a microwire glass electrode. In this paper we present our results in evaluating microwire glass electrodes. We stimulated isolated rabbit retina ( n=5) with a 0.0256 cm 2 microwire electrode. The current and pulse duration were varied from 498 to 1660 μA and 0.1 to 3 ms, respectively. We found that short pulses produced more spikes per coulomb and longer pulses produced more spikes per milliamp. The optimal pulse duration range of 0.7–1 ms was identified as a compromise between the advantages of short and long pulses. Stimulation of isolated rabbit retina with microwire glass results in consistent neuronal spike formation at safe charge density, 20.7±4.3 μC/cm 2. We also examined the response of retinas ( n=6) to stimulation with a smaller microwire electrode, 0.0002 cm 2. We found that less current was required (15 μA versus 756 μA) for a 1 ms pulse, but at the expense of greater charge density (75 μC/cm 2 versus 29.5 μC/cm 2). Nonetheless, a 128-fold reduction in area resulted in only a 2.7-fold increase in charge density required for a 1 ms pulse duration. The results presented here indicate that microwire glass can be used as a neural stimulating electrode to bridge the gap between flat microelectronic stimulator chips and curved neuronal tissue.