Photovoltaic stimulation of mouse and pig retina with pyrolytic carbon microelectrodes integrated on high-density silicon solar cell arrays

Photovoltaic retinal prothesis have the potential to restore vision for patients suffering from retinal degenerative diseases. These implants consist of high-density arrays of miniaturized solar cells and for neurostimulation the integration of microelectrodes on each pixel remains a key challenge. In this work, we introduce carbon as novel electrode material in retinal prosthesis. We fabricate Si-based photovoltaic retinal implants with single p-n junctions in 40 μm and 200 μm pixels and successfully integrate pyrolytic carbon electrodes. Single pixels provide an open circuit voltage of up to 0.49 V under near-infrared (NIR) illumination, demonstrating the functionality of the miniaturized solar cells despite the high thermal budget of the pyrolysis process. In electrophysiological ex vivo experiments with mouse retinal tissue, the intra- and extracellular spike activity significantly increased upon NIR-induced photovoltaic stimulation with the retinal implant compared to spontaneous activity, while spike amplitudes were comparable to natural response to white light. Furthermore, spike amplitudes of evoked potentials were slightly higher for carbon electrodes compared to Au counterparts. Photovoltaic retinal implants with 3D pillar electrodes were explored for ex vivo stimulation of porcine retinal tissue, confirming the ability of carbon to evoke action potentials in retinal neurons.