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Design of a High Density Optoelectronic Retinal Neural Interface


Degenerative retinal diseases such as Age-Related Macular Degeneration (AMD) are among the leading causes of irreversible blindness today for which there are no effective treatments to recover lost vision. Retinal prostheses have been developed to replace the lost photo-sensing with implanted optoelectronic pixels that transduce light into electrical current to stimulate patterns of retinal activity. A fundamental goal of retinal prosthesis design is the realization of a densely packed stimulating array that can restore high visual acuity. However, the efficacy of optoelectronic subretinal stimulation for high visual acuity retinal prosthesis is not as well understood, particularly because passive photodiodes used in clinical implants today lack sufficient photoresponsivity (< 1 A/W) to produce sufficient photocurrent to stimulate retinal neurons at size scales suitable for high density retinal interfaces. Here, we evaluated an optoelectronic approach to retinal prosthesis that offers a unique solution to the problem of minimizing the photosensor, current source, and stimulating electrode size for high density retinal interfaces. First, we characterized the feasibility of retinal stimulation with a fully implantable nanowire based subretinal prosthesis on the basis of electrically evoked potentials measured in the visual cortex of an in vivo rabbit model. We then established criteria for stimulation at a single pixel level using an ex vivo model of photoreceptor cell degeneration. We determined stimulation thresholds and dynamic range of current required to evoke spiking responses in retinal ganglion cells within the charge injection limits of 10-30µm iridium oxide electrodes. We showed that the minimum size for effective stimulation approaches 20µm diameter. Next, to meet these established current requirements we developed a novel optoelectronic pixel architecture consisting of a vertically integrated photo junction-field-effect transistor (Photo-JFET) and neural stimulating electrode. We demonstrated that optically addressed Photo-JFET pixels can utilize phototransistive gain (>100 A/W) to produce a broad range of neural stimulation current. At 13µm pixel size, a single Photo-JFET pixel can effectively stimulate retinal neurons ex vivo. The compact nature of the Photo-JFET pixel can enable high resolution retinal prostheses with the smallest reported optoelectronic pixel size to help restore high visual acuity in patients with degenerative retinal disease.

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