Maximizing the fidelity of a photovoltaic subretinal prosthesis for human patients.

Objective: PRIMA subretinal implants provide prosthetic vision in patients blinded by age-related macular degeneration, with acuity closely matching the sampling limit of the pixel pitch: a single 100 µm pixel per line of a letter corresponds to 20/420 acuity. Decreasing the pixel size in the same flat geometry is difficult due to the constrained electric field, especially considering a 40 µm thick debris layer separating the implant from the target neurons. Here we optimize the electrode design to help overcome such limitations. Approach: An end-to-end modeling pipeline combines the subretinal photovoltaic implant simulator (RPSim) based on the Xyce circuit simulator with an interface to COMSOL Multiphysics for electric field modelling. It was used to generate and characterize implants in an open-loop sampling-based optimization. Implant performance was evaluated with respect to voltage drop across bipolar cells (representing the stimulation strength), pattern contrast, and neural selectivity. Main Results: The highest selectivity in stimulation of bipolar cells was achieved with arrays having active electrodes on pillars and return electrodes connected in a mesh surrounding the photovoltaic pixels in the array. Such a design with pixel size down to 20 µm provides stimulation strength exceeding, and contrast similar to that of flat 100 µm PRIMA pixels. Significance: Using a novel 3-D electrode design, the pitch of the photovoltaic array can be decreased to 20 µm, while providing performance that exceeds the flat 100 µm PRIMA pixels. In humans, 20 µm resolution on the retina corresponds to a visual acuity of 20/80 - a five times improvement compared to the current clinical device.