gms | German Medical Science

Purpose: Letter acuity of prosthetic central vision in AMD patients with the photovoltaic subretinal prosthesis PRIMA consistently matched the 100 μm pixel size. Miniaturizing such pixels further is challenging since the local return electrodes constrain the electric field, precluding efficient stimulation with smaller flat pixels. Moreover, clinical studies demonstrated the presence of a subretinal debris layer, separating the implant from the inner nuclear layer by about 40 μm.

Methods: We developed pillar electrodes to enhance the electric field penetration and assessed the associated resolution and selectivity of the network-mediated retinal stimulation. Pillars are electroplated on top of photovoltaic arrays, with the height matching the subretinal debris layer. Due to tissue migration in subretinal space, stimulating electrodes deposited on top of the pillars will be located near the target bipolar cells. We evaluated the stimulation thresholds and resolution with photovoltaic pixels down to 20 μm in size in rats and assessed the selectivity of the bipolar cell stimulation vs. direct activation of ganglion cells.

Results: Grating acuity with 40 μm arrays matched the pixel pitch, while with 20 μm pixels, it reached the 28 μm limit of the natural resolution in rats. Computational modeling demonstrated that pillar electrodes designed to penetrate the subretinal debris layer in human patients should enable the 20 μm resolution with the stimulation strength, contrast and selectivity of the network-mediated retinal stimulation similar to that achieved with the PRIMA implants.

Conclusions: In humans, where the 20/20 vision corresponds to 5μm resolution on the retina, the 20 μm pixels with pillars may provide central vision with acuity as high as 20/80. Significantly smaller fonts can be resolved using electronic zoom. To ensure selective network-mediated activation of the retina with large patterns, electric field confinement should be optimized using frame-multiplexing.