Miniaturized solar cell arrays for retinal prosthesis

Research output: Book/ReportPh.D. thesis

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Abstract

Blindness due to age-related macular degeneration (AMD) and retinitis pigmentosa (RP) impact millions of individuals worldwide. In both conditions, the photoreceptor cells in the retina are degenerated and these cells are the primary point of light detection, playing a crucial role in visual processing. With limited options for treatments in restoring lost vision, retinal implants present a promising solution. These implants bypass the damaged photoreceptors and directly stimulating the remaining retinal cells with electrical signals to restore some visual function. These implants are composed of arrays of pixels with each pixel featuring microelectrodes and solar cells, capture light, convert it into electrical impulses, and send these signals to the brain, mimicking the natural visual process. This innovative technology comes in three different categories: subretinal, epiretinal and suprachoroidal, based on their location in the retina. ach type offers distinct advantages, with subretinal implants being particularly effective for patients with AMD and RP, aiming to restore partial vision.
The existing literature on retinal implants illustrate the trend towards miniaturizing the pixels and incorporating 3D stimulating microelectrodes, which add the benefit of reducing the distance to target cell and improve the stimulation capability. This direction of research has demonstrated few state-of-the-art designs for subretinal implants with pixels incorporating 3D electrode architecture, with a biocompatible and conductive metal electrode. So far, pyrolytic carbon has not been explored as a material for stimulating electrodes, despite its well-known biocompatibility and excellent electrical conductivity. The capability to fabricate 3D structures via photolithography and then pyrolyze them into biocompatible pyrolytic carbon offers a promising alternative and innovative approach for potentially enhancing the performance of neural interfaces.
Therefore, this thesis explores cleanroom fabrication techniques for creating 3D pyrolytic carbon stimulating electrodes and state-of-the-art dimension pixels with p-n junction solar cells integrated with 3D pyrolytic carbon electrodes. Initially, the research focused on investigating stimulation strategies for porcine retina using 3D pyrolytic carbon stimulating electrodes in a custom 3D-printed electrophysiological setup. Direct current stimulation experiments established the stimulation threshold voltages between 500-600 mV, through statistical analysis of ganglion cell firing, demonstrating that 3D pyrolytic electrodes outperform 2D pyrolytic carbon electrodes. The 40 μm wide pixels, featuring pyrolytic carbon stimulating electrodes, successfully delivered threshold voltages nearly 500 mV under AM 1.5G solar simulation, exhibited superior performance of solar cells when using pyrolytic carbon interface electrodes compared to traditional gold electrodes.
Preliminary electrophysiology studies on rat retina validated the potential of implant with 40 μm pixels featuring gold interface electrodes by stimulating ganglion cells using near-infrared light. The incorporation of 3D pyrolytic carbon electrodes into 40 μm pixels emphasized for further fabrication optimization. Additionally, larger 200 μm pixels fabricated alongside the state-of-the-art design demonstrated successful ganglion cell firing in porcine retina using 3D pyrolytic carbon electrodes under near-infrared light, underscoring the potential of these devices for retinal stimulation applications.
A unique fabrication process was developed chips with 3D pyrolytic carbon electrode membranes, incorporating backside gold electrodes to facilitate scaling down the chip size and fit in the specific electrophysiology setup in Japan. This process was further extended to create miniaturized implants specifically designed for mice retina.
Overall, this research provided significant insights into the use of pyrolytic carbon as stimulating electrodes for retinal implants, demonstrating the feasibility of integrating these electrodes into state-of-the-art miniaturized pixel sizes. The findings from this project contribute valuable knowledge for advancing the design and fabrication of next-generation retinal implants, highlighting the potential of 3D pyrolytic carbon electrodes in enhancing device performance and scalability.
Original languageEnglish
PublisherDTU Nanolab
Number of pages206
Publication statusPublished - 2024

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