A Sub-mm³ Ultrasonic Free-Floating Implant for Multi-Mote Neural Recording

A 0.8-mm3-wireless, ultrasonically powered, free-floating neural recording implant is presented. The device is comprised only of a 0.25-mm2 recording integrated circuit (IC) and a single piezoceramic resonator that are used for both power harvesting and data transmission. Uplink data transmission is performed by the analog amplitude modulation of the ultrasound echo. Using a 1.78-MHz main carrier, >35 kb/s/mote equivalent uplink data rate is achieved. A technique to linearize the echo amplitude modulation is introduced, resulting in $\mu \text{W}$ , while the neural recording front end consumes $4~\mu \text{W}$ and achieves a noise floor of 5.3 $\mu V_{\text {rms}}$ in a 5-kHz bandwidth. This work improves the sub-mm recording mote depth by >2.5 $\times $ , resulting in the highest measured depth/volume ratio by $\sim 3\times $ . Orthogonal subcarrier modulation enables simultaneous operation of multiple implants, using a single-element ultrasound external transducer. Dual-mote simultaneous power-up and data transmission are demonstrated at a rate of 7 kS/s at the depth of 50 mm.