gms | German Medical Science

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 show that 3-dimensional electrodes penetrating through the debris layer help overcome such limitations.

Methods: An end-to-end system 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 the implant design in an open-loop sampling-based optimization. Implant performance in various geometries was evaluated with respect to voltage drop across bipolar cells (representing the stimulation strength), pattern contrast, and selectivity ratio between the bipolar and ganglion cells (BC/RGC).

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. Additionally, the BC/RGC selectivity ratio varies with pattern size, which may allow image multiplexing for improved performance.

Discussion: Using this novel 3-D electrode design, the pitch of the photovoltaic array can be decreased to 20um, while providing stimulation strength exceeding that of the flat 100um PRIMA pixels. In humans, 20um resolution on the retina corresponds to a visual acuity of 20/80 – well above the legal blindness limit.

Acknowledgment: NIH (R01-EY-035227) and DoD (W81XWH-22-1-0933) grants.