Ultrasonic Implant Localization for Wireless Power Transfer: Active Uplink and Harmonic Backscatter

Efficient ultrasonic power transfer to implantable devices requires precise transmitter beamforming to the receiver and can quickly degrade with small changes in implant location. Ultrasound localization can be used to find and track implants in the body to maintain an efficient link. We present a framework to calculate localization accuracy showing that sub-mm accuracy is obtainable using only three receive channels. A harmonic backscatter approach, which passively provides contrast in the frequency domain without active load modulation is compared to active uplink from the implant. The localization accuracy using both active uplink and harmonic backscatter from the implant power receiver is characterized using a linear array probe. The measured location standard deviation is nearly two orders of magnitude smaller than the half-power beamwidth of the array focal spot. Finally, beamforming using the measured location information increases the available power by over 20 × compared to an unfocused beam.