Miniaturized ultrasonic transducers for wearable and implantable applications

While skin-integrated electronics have revolutionized healthcare by enabling continuous monitoring, extracting critical physiological information from deeper within the body remains a significant challenge for existing wearable and implantable devices. This dissertation demonstrates how decades-old ultrasound technology can be effectively integrated into wearable and implantable medical devices to access information hidden beneath the skin. We first evaluated state-of-the-art ultrasonic transducer technologies through fabrication and characterization of single element piezoelectric transducers (bulk PZT versus PVDF-TrFE-based PMUTs) to assess their feasibility for wearable applications. Building on this foundation, we developed a fully integrated wearable ultrasonic patch for continuous bladder volume monitoring, featuring efficient air-backed transducers with a −6 dB bandwidth of 37.9%. These air-backed transducers with precisely laser-micromachined matching layers enabled accurate measurements even with low driving voltages (+/- 15 V), offering a important advantage for long-term use with batteries. The device was initially validated in-vitro with a mean relative error of 14.85% across various shaped flasks mimicking diverse bladder geometries in clinical scenarios, followed by in-vivo measurements on five healthy volunteers with a mean relative error of 11.17% across various bladder shapes and volumes ranging from 100 to 800 mL. To further improve the wearability and user comfort, we introduced electronics-free ultrasonic tags, drastically reducing on-body footprint by relocating the electronics and battery off-body. This allowed for an on-body footprint as low as 0.5 cm^2, a major advantage compared to the latest wearable ultrasound devices that often exceed 25 cm^2 in total size, with transducers covering only a very small portion of that area. Furthermore, such an inductively coupled approach enabled the development of a smart ultrasonic contact lens for ophthalmologic ultrasound for the first time. Lastly, to extend the functionality of wearable ultrasonic devices beyond traditional diagnostics, we established an ultrasound-based passive communication method for medical implants, utilizing simple pulse-echo measurements. The proof-of-concept 1 cm^2 ultrasonic antenna revealed a 168 Hz/pF shift at a 100 Hz measurement rate during in-vitro underwater measurements. The devices and methodologies presented in this dissertation establish a foundational framework for next-generation wearable ultrasound in deep tissue health monitoring.