Tian, X.; Spyrou, A.; Iordanidis, T. N.; Stemme, G.; Roxhed, N.

Implantable microdevices capable of autonomous operation over extended lifetimes are promising enablers for minimally invasive diagnostics and therapy. Microelectromechanical systems (MEMS)-based piezoelectric ultrasonic energy harvesters (PUEH) have emerged as a compelling approach for powering implantable microdevices, where both miniaturization and efficient wireless energy transfer are essential. Here, we present a highly miniaturized (5 x 5 x 5 mm3) ultrasonic energy-harvesting platform enabling sustained in vivo operation of implantable microdevices. The platform integrates a MEMS-PUEH, a high-efficiency power management system, an energy storage element, and representative functional electronics. We first investigate the effect of backside cavity boundary conditions on MEMS-PUEH performance and show that a sealed air-filled chamber significantly outperforms an open water-filled cavity, yielding a 46% increase in root-mean-square output voltage and a 117% increase in average output power across a 2 kOhm resistive load under identical incident acoustic intensity at the respective optimal operating frequencies. We then demonstrate system-level integration and characterization. In a tissue-mimicking phantom, under an incident acoustic intensity of approximately 257 mW/cm2, the device charges an 11.5 mF supercapacitor, a 5 uAh solid-state microbattery, and a 100 uF capacitor to their nominal voltages in less than 5 min, 3 min, and 20 s, respectively. Finally, in vivo validation demonstrates fully autonomous operation of representative functional electronics following ultrasonic charging of the onboard energy storage element. These results establish a highly miniaturized and fully integrated ultrasonic energy-harvesting platform that advances MEMS-based power solutions for implantable biomedical microdevices.