Electronics-Free Deep-Tissue Sensing via Passive Ultrasonic Backscatter

Monitoring physiological signals in deep tissue is often constrained by limitations in power availability, device miniaturization, and wireless signal penetration. Conventional implantable sensors rely on active electronics and electromagnetic transmission, both of which restrict scaling and suffer from severe attenuation in biological tissue. In this work, we present a fully passive ultrasonic sensing approach that encodes capacitive sensor states as antiresonance shifts in the backscattered signal of a piezoelectric transducer. We experimentally validate this encoding scheme by demonstrating that antiresonance shifts in the backscatter directly correspond to resonance changes measured at the implant site. This novel passive method overcomes the need for active circuitry, offering a scalable and long-term solution for implantable biosensing.