Electrical stimulation of isolated retina with microwire glass electrodes

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 0.0256cm2 microwire electrode. The current and pulse duration were varied from 498 to 1660μA and 0.1 to 3ms, 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 0.7–1ms 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±4.3μC/cm2. We also examined the response of retinas (n=6) to stimulation with a smaller microwire electrode, 0.0002cm2. We found that less current was required (15μA versus 756μA) for a 1ms pulse, but at the expense of greater charge density (75μC/cm2 versus 29.5μC/cm2). Nonetheless, a 128-fold reduction in area resulted in only a 2.7-fold increase in charge density required for a 1ms 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.