A Sub-mm Ultrasonic Free-floating Implant for Multi-mote Neural Recording.

A 0.8 mm$^3$ wireless, ultrasonically powered, free-floating neural recording implant is presented. The device is comprised only of a 0.25 mm$^2$ recording IC and a single piezoceramic resonator that is used for both power harvesting and data transmission. Uplink data transmission is performed by amplitude modulation of the ultrasound echo. A technique to linearize the echo amplitude is introduced, resulting in 2.5x, resulting in the highest measured depth/volume ratio by ~3x. Orthogonal subcarrier modulation enables simultaneous operation of multiple implants, using a single-element ultrasound external transducer. Dual-mote simultaneous power up and data transmission is demonstrated at a rate of 4.7 kS/s at the depth of 45 mm.