Sandwich-structured piezoelectric ultrasound harvester for wireless power charging of implantable biomedical electronics

Implantable biomedical electronics (IBMEs) require reliable power sources for long-term operation and minimize frequent of battery-replacement surgeries. Piezoelectric ultrasound harvesters (PUSHs) have emerged as a promising solution for ultrasound-based wireless power transfer (US-WPT). However, their output power is constrained by regulatory limits on ultrasound transmission intensity and the small size of implantable harvesters. In this study, we propose a sandwich-structured ultrasound harvester (SW-PUSH) to maximize energy harvesting efficiency under these constraints. The SW-PUSH consists of matching layered front PUSH and separation layered rear PUSH, where the rear PUSH captures the ultrasound energy that passes through the front PUSH, thereby improving overall energy conversion efficiency. The optimized structure of SW-PUSH was designed through simulation and subsequently fabricated. The outputs of both PUSHs are electrically combined, achieving a power density of 497.47 mW/cm² and a total power of 732.27 mW in water, sufficient to fully charge a 140 mAh battery in 1.7 h. In tests using 30 mm thick porcine tissue, the SW-PUSH generated 312.34 mW and charged a 60 mAh battery in 1.4 h. These results demonstrate that SW-PUSH offers a high-performance, efficient solution for powering IBMEs, overcoming conventional limitations and enabling extended functionality in next-generation IBMEs.