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J. Loudin, K. Mathieson, D. Boinagrov, A. Sher, D. Palanker; Retinal Stimulation With a Photovoltaic Subretinal Prosthesis. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4321.
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© ARVO (1962-2015); The Authors (2016-present)
Electronic retinal prostheses seek to restore sight to blind patients with degenerative retinal diseases by delivering patterned electrical stimulation to surviving retinal neurons. A photovoltaic retinal prosthesis directly converts pulsed infrared light into current in each pixel, offering the possibility of high resolution stimulation without the need for a separate power and/or data transmission system. This study examines retinal response to subretinal photovoltaic stimulation by recording retinal ganglion cell (RGC) action potentials with a microelectrode array (MEA) using both healthy and degenerated retinas.
The MEA consists of 512 electrodes on a glass substrate, each 5 µm in diameter and spaced 60 µm apart for an overall area of 1.7 mm2. An isolated retina was placed on the transparent MEA, RGC side down. The retinal prosthesis was then placed on top of the retina, facing the photoreceptor/bipolar cells. This preparation was mounted in the focal plane of an inverted microscope. Both visible and pulsed infrared images were projected onto the retina, so that optical and electrical stimulation could be directly compared. Experiments were performed on both degenerated RCS rat retina and normally sighted rat and rabbit controls.
Both single spike and train responses were observed, with latencies in the 5-25 ms range and a typical value of 10 ms. In sighted rat retina, RGC spikes were reliably induced with 905 nm light pulses of 2 ms in duration at 0.06 mW/mm2 peak irradiance. In rabbit retina the minimum stimulation threshold observed was for 4 ms pulses with 0.9 mW/mm2 peak irradiance, but pulses as short as 0.1 ms could elicit spiking at 1.4 mW/mm2. With a 10 Hz stimulation rate, the average irradiance was 1.4 µW/mm2, which is below the ocular safety limit by approximately a factor of 1000.
Photovoltaic stimulation is possible with even single-diode pixels, though multi-diode circuits provide a greater dynamic range. The light irradiance required for stimulation with subretinal photovoltaic arrays is well within ocular safety limits. The results indicate that these wireless photovoltaic implants used in conjunction with a pulsed infrared video goggle system may provide useful vision to blind patients with degenerative retinal diseases.
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