A Sub-mm³ Wireless Neural Stimulator IC for Visual Cortical Prosthesis With Optical Power Harvesting and 7.5-kb/s Data Telemetry

This article proposes StiMote, an untethered, free-floating and individually addressable stimulator mote designed for visual cortex stimulation, aimed at vision restoration. The system is optically powered by a custom photovoltaic (PV) layer. In addition, the photodiode (PD) layer captures the light modulation and forwards it to the optical receiver (ORX) including a tranimpedance amplifier. Translated instructions can assign a unique slot, up to 1024 available, to each mote within the time-division multiple access (TDMA) framework. In this work, we propose an automatic charge balance (CB) technique that monitors the injected charge to balance in bi-phasic switched-capacitor stimulation (SCS). The chip was confirmed fully functional when operated completely wirelessly using harvested light. Measurement results revealed a power consumption of 4.48 $\mu$ W with a 7.5-kb/s optical downlink data rate, corresponding to continuous updates at 2.5 Hz of 1024 motes to their individual 3-b stimulation intensity levels. The dc–dc converter, responsible for providing high voltage for stimulation, demonstrated 4.3-V output voltage when unloaded, with a maximum efficiency of 67.4%. The proposed CB circuit exhibited linear controllability of stimulation charge up to 16 nC, with a charge imbalance of less than 0.2 nC. Furthermore, in vitro testing confirmed the absence of chemical reactions at electrodes, and in vivo experiments conducted on live rats verified the effectiveness of the stimulation through StiMote.