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William R Freeman, Sharif Y El Emam, Lingyun Cheng, Gabriel Silva, Massoud Khraiche; Evaluation of a High Density Photovoltaic Prosthesis in Rabbits. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1795.
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Retinal prostheses are intended to treat retinal degenerative diseases such as age related macular degeneration (AMD) and retinitis pigmentosa (RP). Current prostheses either rely on micro-photodiode light detectors or cameras to capture the image with downstream processing and power before stimulating retinal neurons. In contrast, we are developing a novel approach using high quantum efficiency and responsivity photovoltaic silicon nanowires (Khraiche, 2011}). In this work we evaluated the feasibility of subretinal implantation and neural retinal electrical stimulation with these devices in rabbits by measuring evoked responses in visual cortex.
Silicon implants 3mm x 500 x 500 microns, were fabricated using nanolithography. Implants were placed in the subretinal space of pigmented rabbits using a 20 gage vitrectomy by implanting thru an incision in a retinal bleb. The retina was re-attached using perfluoro-n-octane; eyes remained phakic. 3-7 days after implantation, electrophysiology experiments (visual or electrically evoked potentials, VEP or EEP) were carried out as were ophthalmic examinations looking for any adverse reaction to the implant. A light pipe was placed thru a sclerotomy and was directed over the normal retina or implanted device at 680 or 400 nm wavelengths (100 Hz) and at high and low light intensity.
There was no short-term toxicity or adverse effects after implantation using slit lamp and fundus examination. Mean VEP amplitudes on normal retina using a 3mm diameter spot size was compared to the EEP recorded over the implant. Using a 680 nm wavelength there was no measurable response over normal rabbit retina (as the eye is not responsive in the red wavelengths). Using 400 nm light mean VEP amplitude was 52.2 microvolts. Paired statistics showed mean amplitude over the implant of 62.50 SD 8.35uV in the high intensity group, 48.75 SD 4.7 in the low intensity group and 17.5 SD 19.7 over normal retina. Paired analysis of high intensity vs normal retina amplitudes at 680 nm wavelength was significant p< 0.006 as was low versus high intensity light retina p<0.01
Photosensitive nanowire technologies can potentially be developed into a novel retinal prostheses to replace photoreceptors in the subretinal space. We have shown here it can produce EEP’s in an animal model. The current implant is unpowered and is stimulated in our experiments with relatively high light intensities.
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