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Thomas Flores, Tiffany Wanshing Huang, Mohajeet Bhuckory, Henri Lorach, Zhijie Chen, Roopa Dalal, Xin Lei, Ludwig Galambos, Theodore Kamins, Keith Mathieson, Daniel V Palanker; Honeycomb-shaped subretinal prosthesis enables cellular-scale pixels. Invest. Ophthalmol. Vis. Sci. 2019;60(9):4971.
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High-resolution retinal prostheses require small, densely packed pixels for localized neural stimulation, but limited penetration depth of the electric field formed by a planar electrode array constrains such miniaturization. We propose a novel honeycomb design of an electrode array with vertically separated active and return electrodes designed to leverage migration of retinal cells into pixel cavities. This configuration aligns the electric field vertically to match the orientation of bipolar cells in the retina to significantly reduce the stimulation threshold and decouple scaling limitations from pixel size.
1 mm wide honeycomb-shaped arrays were fabricated in silicon, with 25 µm tall walls separating inactive pixels of 40, 30 and 20 µm in width. Devices were implanted beneath the degenerate rat retina (RCS, p180-300, n=5) for 6 weeks, and then observed using OCT, immunohistochemistry, and histology. Stimulation thresholds were assessed using electric field simulations and a model of network-mediated retinal stimulation validated with previously recorded experimental thresholds.
More than half of the inner retinal neurons migrate into the honeycomb cavities of all widths. The layered structure of the retina is preserved, and glial cell response is similar to that with planar implants. Simulations of active devices show that the stimulation threshold current density with honeycombs does not significantly change with pixel size, unlike the quadratic increase with the reduced pixel size of flat arrays. Calculated thresholds with 20 µm honeycomb pixels are 34 times lower than that of planar pixels of the same size. Compartmentalization of the inner retinal neurons into cavities should allow preservation of 100% spatial contrast for all pixel sizes within the SIROF charge injection limit (<3 mC/cm2), while allowing up to 100% of tissue volume activation.
The honeycomb-shaped subretinal prosthesis decouples the penetration depth of electric field in tissue (set by the height of the walls) from the pixel width, and thereby overcomes the limitations of decreasing pixel size. This approach may enable scaling the pixel size to cellular dimensions to provide visual acuity better than 20/100. Coupled with the structural simplicity of the photovoltaic retinal prosthesis, this approach may enable highly functional restoration of central vision in patients with advanced age-related macular degeneration.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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