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Electronic retinal prostheses seek to restore sight to patients suffering from retinal degenerative disorders. Implanted
electrode arrays apply patterned electrical stimulation to surviving retinal neurons, producing visual sensations. All
current designs employ inductively coupled coils to transmit power and/or data to the implant. We present here the
design and initial testing of a photovoltaic retinal prosthesis fabricated with a pixel density of up to 177 pixels/mm2.
Photodiodes within each pixel of the subretinal array directly convert light to stimulation current, avoiding the use of
bulky coil implants, decoding electronics, and wiring, and thereby reducing surgical complexity. A goggles-mounted
camera captures the visual scene and transmits the data stream to a pocket processor. The resulting images are projected
into the eyes by video goggles using pulsed, near infrared (~900 nm) light. Prostheses with three pixel densities (15, 55,
and 177 pix/mm2) are being fabricated, and tests indicate a charge injection limit of 1.62 mC/cm2 at 25Hz. In vitro tests of the photovoltaic retinal stimulation using a 512-element microelectrode array have recorded stimulated spikes from
the ganglion cells, with latencies in the 1-100ms range, and with peak irradiance stimulation thresholds varying from 0.1
to 1 mW/mm2. With 1ms pulses at 25Hz the average irradiance is more than 100 times below the IR retinal safety limit.
Elicited retinal response disappeared upon the addition of synaptic blockers, indicating that the inner retina is stimulated
rather than the ganglion cells directly, and raising hopes that the prosthesis will preserve some of the retina's natural
signal processing.
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