Photovoltaic Restoration of Sight with High Visual Acuity
Patients with retinal degeneration lose sight due to gradual demise of photoreceptors. Electrical stimulation of the surviving retinal neurons provides an alternative route for delivery of visual information. Subretinal photovoltaic arrays with hexagonal 70mm pixels are used to convert pulsed light into bi-phasic pulses of current to stimulate the nearby inner retinal neurons. Bright pulsed illumination is provided by image projection from video goggles and avoids photophobic effects by using near-infrared (NIR, 880-915nm) light. Retinal network mediated responses of the ganglion cells (RGCs) are modulated by pulse width (1-20ms) and irradiance (0.5-10 mW/mm2). Stimulation threshold of 0.3 mW/mm2 with 10 ms pulses is more than two orders of magnitude below the ocular safety limit. Similarly to normal vision, retinal response to prosthetic stimulation exhibits flicker fusion at high frequencies, adaptation to static images and non-linear spatial summation. Spatial resolution was assessed in-vitro and in-vivo using stroboscopic illumination (20-40Hz) of alternating gratings with variable stripe width. RGCs responded to grating stripes down to 67mm using photovoltaic stimulation in degenerate rat retina, and 28mm with visible light in normal retina. In-vivo, visual acuity in normally-sighted controls was 29mm/stripe, vs. 64mm/stripe in rats with subretinal photovoltaic arrays, matching the spacing of the adjacent pixel rows in the array, and corresponding to 20/250 acuity in human eye. Extrapolating from the observed stimulation thresholds, pixel size can be further reduced by a factor of two, thereby supporting even higher spatial resolution. Ease of implantation and tiling of these wireless arrays to cover a large visual field, combined with their high resolution opens the door to highly functional restoration of sight.