Towards a mm-Sized Free-Floating Wireless Implantable Opto-Electro Stimulation Device

Next generation of brain-machine interfaces (BMI) will be able to interface with large-scale neuronal ensembles over large brain areas with a distributed architecture consisting of multiple tiny implants. This paper presents a system-on-a-chip (SoC) prototype towards a mm-sized free-floating wirelessly-powered implantable 16-ch opto-electro stimulation (FF-WIOS2) device. FF-WIOS2 is wirelessly powered and controlled through a 3-coil inductive link at 60 MHz. Forward telemetry link delivers stimulation parameters to the FF-WIOS2 via on-off keying (OOK) to configure stimulation patterns. Back telemetry link reports the FF-WIOS2 rectified voltage in a closed-loop fashion via load-shift-keying (LSK) to stabilize the implant received power. The SoC, fabricated in a 0.35-µm standard CMOS process, employs switched-capacitor-based stimulation (SCS) by charging an off-chip capacitor up to 5 V at 37% charging efficiency to provide large instantaneous current. At the onset of stimulation, the capacitor delivers charge, with peak current values of either 1.7-12 mA to a LED array for optical stimulation or 100-700 μA to a microelectrode array (MEA) for biphasic electrical stimulation. The latter mechanism is also actively charge-balanced to ensure stimulation safety.