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Xin Lei, Tiffany Wanshing Huang, Thomas Anthony Flores, Henri Lorach, Ludwig Galambos, Theodore Kamins, James Harris, Keith Mathieson, Daniel V Palanker; Photovoltaic Subretinal Prosthesis with Pixel Sizes Down to 40 um. Invest. Ophthalmol. Vis. Sci. 2017;58(8):4269.
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© ARVO (1962-2015); The Authors (2016-present)
Following the demonstration that spatial resolution of prosthetic vision in rats with retinal degeneration matches the 70 um pixel pitch of the subretinal photovoltaic arrays, we developed new arrays with optimized pixel configuration and reduced sizes: 70, 55 and 40 um. We evaluate their performance in-vitro and retinal stimulation thresholds in-vivo.
To increase the active area of the photodiodes, we developed ultrathin isolation trenches separating the diodes inside the pixels and the adjacent pixels, and shared the return electrodes between the adjacent pixels. Hexagonal pixels of 70, 55 and 40 um in size had active electrodes of 18, 14, and 10 um in diameter, and circumferential return electrodes shared with adjacent pixels were 11.4, 9, and 6 um in width. The 1 mm diameter implant included 157, 250 and 502 pixels of 70, 55, and 40 um in width, respectively. Arrays were implanted into subretinal space in rats with retinal degeneration (RCS), and stimulation thresholds were measured using Visually Evoked Potential (VEP) recording.
Light-to-current conversion efficacy of the photodiodes was 0.38 A/W. Under 1 mW/mm2 irradiance, pixels of 70, 55 and 40 um in size are generating 0.5, 0.3, 0.16 uA of current. In physiological medium with 500 Ohm*cm resistivity, simulating the retinal impedance, pixels operated in the current-limited regime at pulse repetition rates below 10 Hz and irradiances up to 10 mW/mm2, and reached the voltage-limited regime at 20 - 40 Hz frequencies. With pulse durations in the range of 1 - 10 ms, stimulation thresholds in rats were approximately 0.5, 1 and 4 mW/mm2 for arrays with 70, 55 and 40 um pixels, respectively.
Photovoltaic arrays with pixel sizes as small as 40 um can elicit retinal responses under safe illumination conditions by near-infrared light (880 nm). The higher threshold with smaller pixels is consistent with reduced current and tighter confinement of electric field between the active and return electrodes as the pixel size decreases. Implantation of larger or multiple modules allows scaling the number of pixels to thousands, thereby providing foundation for restoration sight with high visual acuity.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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