Piezoelectric power harvesting

Converting incident ultrasound into electrical power inside an implant using a piezoelectric transducer, which turns oscillating acoustic pressure into an oscillating voltage that is rectified and stored to run the device. For deep millimetre-scale implants, ultrasound generally beats radio-frequency and inductive links, because the acoustic wavelength in tissue at low-megahertz frequencies is millimetric (well matched to a tiny receiver) and tissue attenuates ultrasound far less than it does the electromagnetic fields a comparably small antenna would need.

Delivered power is bounded jointly by the mote's size relative to wavelength, the acoustic impedance match between transducer and tissue, beam focusing and alignment, and the regulatory ceilings on acoustic intensity (the mechanical and thermal indices). Common piezoelectric materials include lead zirconate titanate (PZT) and, for biocompatibility, lead-free alternatives and single crystals such as PMN-PT.