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Richard Smith, Georges A Goetz, Xin Lei, Ted Kamins, Jim Harris, Keith Mathieson, Daniel V Palanker, Alexander Sher; Optimization of the Photovoltaic Subretinal Prosthesis. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1796.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the optimal polarity and number of diodes per pixel in a subretinal photovoltaic prosthesis. We examine the thresholds and dynamic range of retinal stimulation by photovoltaic arrays with pixel sizes of 70 µm and 140 µm, containing 1, 2 or 3 diodes per pixel, with anodal- or cathodal-first pulse polarity. Devices with more diodes can produce more current at the expense of increased illumination. Smaller pixels allow higher resolution, at the trade-off of higher stimulation thresholds.
The response of retinal ganglion cells (RGCs) to photovoltaic stimulation was examined in-vitro with a multielectrode array, using rat retinas with degenerate photoreceptors from the Royal College of Surgeons strain, and the wild-type. The retina was placed with the RGC layer in contact with the recording electrodes, and an array of photovoltaic pixels was placed on top of it. Photodiodes were illuminated with pulsed NIR (880nm) light through the transparent multielectrode array and the network-mediated responses of RGCs were recorded. We varied the IR pulse width and intensity to measure the thresholds for stimulation, defined as the intensity of light required to modulate the response of each cell by, on average, 0.5 spikes per pulse. Each preparation yielded 10-40 responsive neurons.
Stimulation thresholds decreased for longer pulse widths, up to but not beyond 4ms. At this pulse width, the 140µm pixel, 3 diode, anodal devices had 3 times lower thresholds than similar cathodal devices in wild-type retina (0.2 mW/mm2 vs. 0.6 mW/mm2). Among the 70µm pixel anodal devices, the arrays with 2 diodes per pixel had the lowest thresholds: at 0.7 mW/mm2. RCS degenerate retinas had thresholds 20-50% higher than their wild-type counterparts. The maximum number of elicited spikes varies significantly between neurons: a typical value is 1-3 spikes per pulse, but some cells produced over 10 spikes per pulse. More OFF cells were stimulated than ON cells, but the thresholds for stimulation were similar between these two types.
The preferable configuration of the photovoltaic subretinal arrays with 70 µm pixels has anodal polarity with 2 diodes per pixel. Stimulation thresholds for such pixels are more than two orders of magnitude below the ocular safety limit, allowing for a wide dynamic range of stimulation.
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