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

Abstract

A 0.8-mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> -wireless, ultrasonically powered, free-floating neural recording implant is presented. The device is comprised only of a 0.25-mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{W}$ </tex-math></inline-formula> , while the neural recording front end consumes <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4~\mu \text{W}$ </tex-math></inline-formula> and achieves a noise floor of 5.3 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu V_{\text {rms}}$ </tex-math></inline-formula> in a 5-kHz bandwidth. This work improves the sub-mm recording mote depth by >2.5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> , resulting in the highest measured depth/volume ratio by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 3\times $ </tex-math></inline-formula> . 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.