Photovoltaic Pixels for Neural Stimulation: Circuit Models and Performance

TLDR

Photovoltaic conversion of pulsed light into pulsed electric current enables optically‑activated neural stimulation with miniature wireless implants, and in retinal prostheses near‑infrared light projected onto subretinal arrays of photovoltaic pixels is converted into current patterns to stimulate inner retinal neurons. The authors describe a model of photovoltaic neural stimulation devices and evaluate the performance of their circuits, including the electrode‑electrolyte interface. They model electrode characteristics in saline as voltage‑dependent capacitances and Faradaic resistances, use the circuit model to optimize pixel design for maximum charge injection under various lighting conditions and thresholds, and introduce a shunt resistor to accelerate electrode discharge between light pulses for high‑frequency stimulation. The mathematical model predicts the dynamics of current generated by pulsed light, and measured voltages in saline with a pipette electrode above the pixel closely matched the model predictions.

Abstract

Photovoltaic conversion of pulsed light into pulsed electric current enables optically-activated neural stimulation with miniature wireless implants. In photovoltaic retinal prostheses, patterns of near-infrared light projected from video goggles onto subretinal arrays of photovoltaic pixels are converted into patterns of current to stimulate the inner retinal neurons. We describe a model of these devices and evaluate the performance of photovoltaic circuits, including the electrode-electrolyte interface. Characteristics of the electrodes measured in saline with various voltages, pulse durations, and polarities were modeled as voltage-dependent capacitances and Faradaic resistances. The resulting mathematical model of the circuit yielded dynamics of the electric current generated by the photovoltaic pixels illuminated by pulsed light. Voltages measured in saline with a pipette electrode above the pixel closely matched results of the model. Using the circuit model, our pixel design was optimized for maximum charge injection under various lighting conditions and for different stimulation thresholds. To speed discharge of the electrodes between the pulses of light, a shunt resistor was introduced and optimized for high frequency stimulation.