An Ultrasonic Wireless Power Transmission System with High Conversion Efficiency Based on 30% ScAlN Piezoelectric MEMS Transducers

This study proposed an ultrasonic wireless power transmission system for implantable medical devices (IMDs) using high receiving sensitivity piezoelectric micromachined ultrasonic transducers (PMUTs) with a 30% scandium-doped aluminum nitride (ScAlN) thin film. PMUT arrays with various element numbers were designed, fabricated and characterized with electrical impedance, acoustic receiving sensitivity, directivity, and power conversion efficiency for various electrical loads. The experimental results showed that increasing element number in PMUT arrays improved receiving sensitivity while making the PMUT array more directional and more sensitive to alignment with the ultrasonic transmission probe. An optimized 25×25 PMUT array with 128μm pitch maintains high receiving sensitivity (5V/MPa at ~500kHz) and reasonably good directivity (18°), making it less sensitive to the lateral alignment. Additionally, the study investigated the relationship between ultrasonic power conversion efficiency and load impedance matching. The optimized system was found to be capable of providing 130μW power for a 20kΩ load with a power conversion efficiency of 6.2%, enabling it to charge a 220µF capacitor to 3V within 550 seconds.