Mapping of Physiological Signals From Wireless Battery‐Free Implants With Plane‐Wave Ultrasound Imaging

ABSTRACT Wireless implants based on electromagnetic or ultrasonic communication provide exciting opportunities in the acquisition of physiological data in various clinical applications. Nevertheless, numerous challenges should be addressed before their use in the clinic, notably unspecific, multisite addressing. In classical ultrasonic communication, the ultrasound beam is directed to the implant through geometrical or electronic focusing. Variation in physiological signal modulates the acoustic reflectivity of the piezo‐element of the implant through a functionalized transistor. Because this approach assumes a known position of implant, it is incompatible with unspecific multisite addressing. Here, we explore plane‐wave ultrasound imaging to power a network of wireless, battery‐free implants with an electromagnetic pulse through inductive coupling using an imaging probe and record the modulated signal. Beamforming allows spatial reconstruction of acoustic energy emitted by a 350‐µm piezo‐element, whose modulation was observed in deionized water and in ex vivo samples without its known position. As a proof‐of‐concept, synthetic biosignals (5, 20, and 100 Hz sine waves) were reconstructed through pixelwise processing, and their dominant frequency was mapped on top of a B‐mode ultrasound image with an SNR of 6.25 dB. This approach enables functional imaging using implants addressed in parallel, which broadens the clinical applicability of wireless implants in the monitoring of physiological activity and could create opportunities in closed‐loop stimulation therapies.