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 < 1.2% static nonlinearity of the received signal over a ±10-mV input range. The IC dissipates 37.7 μW, while the neural recording front end consumes 4 μW and achieves a noise floor of 5.3 μ V_rms in a 5-kHz bandwidth. This work improves the sub-mm recording mote depth by >2.5 ×, resulting in the highest measured depth/volume ratio by ∼ 3×. 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.