Ultrasonic backscatter telemetry

The communication principle behind ultrasonic motes: instead of generating its own transmission, the implant modulates how strongly it reflects an externally supplied ultrasound beam. Varying the electrical load across the piezoelectric element changes its acoustic reflection coefficient, so the recorded neural voltage amplitude-modulates the backscattered echo that the external transducer receives and demodulates.

Because the implant expends almost no energy transmitting, backscatter telemetry is far more power-frugal than an active radio, and is what makes battery-free millimetre motes feasible. Its costs are a weak, noise-prone return signal, limited data rate, and the difficulty of separating many motes' echoes (multiplexing) and rejecting tissue clutter and multipath.