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Photovoltaic Retinal Prosthesis
Daniel Palanker
Department of Ophthalmology and Hansen Experimental Physics Laboratory
Stanford University, Stanford, CA
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 sensation. 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 pixel density as high as 256 pix/mm2. Photodiodes within each pixel of the subretinal array directly
convert light to stimulation current, avoiding the use of bulky power supplies, 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 onto retina by
video goggles using pulsed, near infrared (~900 nm) light. Prostheses with three pixel densities (16, 64,
and 256 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 spikes elicited from the ganglion cells, with latencies in the 2-100ms 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 stimulation is of bipolar cells, rather than the
ganglion cells directly, and raising hopes that the prosthesis will preserve some of the retina’s natural
signal processing.