High resolution photovoltaic retinal prosthesis [electronic resource]
Keywords: Applied Physics

Age-related blindness has become a critical issue as life expectancies continue to rise. Age-related Macular Degeneration (AMD) is the leading cause of blindness in the developed world, with an incidence of 1:500 in patients age 55-64, and 1:8 in patients over 85. Retinitis Pigmentosa (RP) is the leading cause of inherited blindness, occurring in about 1 in every 4000 births. This disease afflicts patients starting in their early 20s, leaving them blind for the most productive period of their lives. Both diseases are characterized by the degeneration of the "image capturing" photoreceptor layer of the retina, while neurons in the "image processing" inner retinal layers are relatively well preserved. AMD progression can be delayed, but not prevented, while there is currently no effective treatment for RP. Visual prostheses seek to restore visual sensation to patients suffering from these diseases by electrically stimulating surviving retinal nerve cells via chronically implanted electrode arrays, in the visual analog of the successful cochlear implant. Several existing technologies have been evaluated in laboratory settings and in patients, but like the cochlear implant, all are tethered to implanted wire coil systems which deliver power to the neural stimulators. To date the most successful prostheses have enabled blind patients to read large fonts. However, the perceptual resolution of current systems is quite low - less than 10 pixels/mm², geometrically corresponding to visual acuity below 20/1200. The presented work describes the design and initial testing of a high resolution, photovoltaic retinal prosthesis in which power and data are directly delivered to photodiodes within each pixel using pulsed, near-infrared light. This direct optical data delivery maintains the natural link between eye movements and visual stimulus, while the lack of a separate power delivery system greatly simplifies the implantation of an array of such pixels, thereby decreasing the risk of surgical complications. All pixels operate autonomously, obviating the need for a wiring array and allowing separate arrays to be independently placed in different areas of the subretinal space. A goggles-mounted camera captures images of the visual scene, which are processed by a pocket computer before being projected onto the pixel array by a near-to-eye projection system. This projection system is similar to commercially available video goggles, but approximately 1000 times brighter, requiring the use of novel laser projection and de-speckling techniques. The charge injection characteristics of several dozen different circuit designs and electrode geometries were measured, before were selecting two for fabrication with 16, 64, and 256 pixels/mm². Initial tests have been performed with both single and three-diode pixels. Previous work on blind rats implanted with subretinal photodiode arrays has recorded neural activity in response to an infrared flash. However, these recordings were made from electrodes in the superior colliculus region of the brain, and therefore yield little insight into the retinal stimulation dynamics. To probe this space, we have measured photovoltaic stimulation responses from ex vivo rat retinas sandwiched between 512-microelectrode recording array and a photovoltaic stimulating array. Stimulated ganglion cell spikes were observed with latencies in the 1-100ms range, and with peak irradiance stimulation thresholds varying from 0.1 to 1.0 mW/mm². The elicited response disappeared upon the addition of synaptic blockers, indicating that stimulation is mediated by the inner retina rather than the ganglion cells directly, and raising hopes that a subretinal photovoltaic prosthesis will preserve some of the retina's natural signal processing.