Minimizing iridium oxide electrodes for high visual acuity subretinal stimulation.

Vision loss from diseases of the outer retina, such as Age-Related Macular Degeneration (AMD), are among the leading causes of irreversible blindness in the world today. The goal of retinal prosthetics is to replace the photo-sensing function of photoreceptors lost in these diseases with optoelectronic hardware to electrically stimulate patterns of retinal activity corresponding to vision. To enable high-resolution retinal prosthetics, the scale of stimulating electrodes must be significantly decreased from current designs; however, this reduces the amount of stimulating current that can be delivered. The efficacy of subretinal stimulation at electrode sizes suitable for high visual acuity retinal prosthesis are not well understood, particularly within the safe charge injection limits of electrode materials. Here, we measure retinal ganglion cell responses in a mouse model of blindness to evaluate the stimulation efficacy of 10, 20, and 30 µm diameter iridium oxide electrodes within the electrode charge injection limits, focusing on measures of charge threshold and dynamic range. Stimulation thresholds were lower for smaller electrodes, but larger electrodes could elicit a greater dynamic range of spikes and recruited more ganglion cells within charge injection limits. These findings suggest a practical lower limit for planar electrode size and indicate strategies for maximizing stimulation thresholds and dynamic range.SIGNIFICANCE STATEMENTNeural prosthetics offer hope to cure intractable neurological disorders. To enable fine control over patterns of neural activity, stimulating electrode size must be decreased to the scale of neurons. We examined how electrode size at this scale influences stimulation threshold and the range of possible responses, by fabricating planar iridium oxide electrodes between 10 and 30 µm in diameter, and examined neural stimulation in a mouse model of retinal degeneration. This work provides new insights into how small stimulation electrodes translate charge into neural activity and the physical factors that contribute to differences between responses over this range of electrode sizes. This has important implications for the design of high-acuity retinal prosthetics, as well as next-generation neural stimulators.