StimDust: A 2.2 mm$^3$, implantable wireless precision neural stimulator with ultrasonic power and communication.

Neural stimulation is a powerful technique for modulating physiological functions and for writing information into the nervous system as part of brain-machine interfaces. Current clinically approved neural stimulators require batteries and are many cubic centimeters in size: typically much larger than their intended targets. We present a complete wireless neural stimulation system consisting of a 1.7 mm$^3$ wireless, batteryless, leadless implantable stimulator (the "mote"), an ultrasonic wireless link for power and bi-directional communication, and a hand-held external transceiver. The mote consists of a piezoceramic transducer, an energy storage capacitor, and a stimulator integrated circuit (IC). The IC harvests ultrasonic power with high efficiency, decodes stimulation parameter downlink data, and generates current-controlled stimulation pulses. Stimulation parameters are time-encoded on the fly through the wireless link rather than being programmed and stored on the mote, enabling complex stimulation protocols with high-temporal resolution and closed-loop capability while reducing power consumption and on-chip memory requirements. Uplink data indicates whether the mote is currently stimulating; it is encoded by the mote via backscatter modulation and is demodulated at the external transceiver. We show that the system operates at an acoustic power one fifth the FDA limit for diagnostic ultrasound and is robust to expected real-world acoustic link misalignment. We investigate the performance of the system with motes acutely implanted with a cuff on the sciatic nerve of anesthetized rats and show highly repeatable stimulation across a wide range of physiological responses.