Ultrasound (US) is a promising modality for wirelessly powering implantable devices, requiring encapsulated receivers to ensure long-term stability. Traditional hermetic packaging often limits acoustic transmission, making polymer-based encapsulation a more suitable alternative. This study investigates how implant-grade polymers, thermoplastic polyurethane (TPU), parylene-C, and medical-grade silicones (MED-1000 and MED2-4213), affect the receive performance of piezoelectric micromachined ultrasonic transducers (PMUTs). Simulations and measurements between 1 and 7 MHz show that all tested materials exhibit transmission coefficients above 94% at nanometer- and micrometer-scale thicknesses, confirming their acoustic transparency. The results show that although coated PMUTs are acoustically well matched with the surrounding water medium, the added mechanical load of the coating can hinder membrane motion and reduce the energy transferred to the PMUTs. Modeling and experimental data demonstrate that stiffer coatings, such as parylene-C, lead to a reduced sensitivity when similar thicknesses are used. Likewise, residual stress in materials like MED-1000 can also degrade the performance. These effects are not evident from acoustic transmission measurements alone, underscoring the need to assess both acoustic and mechanical properties when selecting encapsulation materials. In general, softer materials offer excellent acoustic performance for PMUT encapsulation, while stiffer materials must be applied in thinner layers to avoid impairing PMUT function.

Aluminum Nitride (AlN) Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) are gaining interest for biomedical implant power due to biocompatibility and lowtemperature processing. However, due to the low piezoelectric coefficient of AlN PMUTs, storage capacitors are often used to accumulate ultrasonic power transferred over an extended time. The accumulated energy is then used to power a DC load, which leads to a long start-up time, and insufficient duty cycle for some applications. We present an ultrasonically powered system for biomedical implants capable of delivering mW-range instantaneous power to DC loads, without pre-storing it. The system features a 25 mm2 AlN PMUT, an inductive matching network, and an application-specific power management integrated circuit(ASIC). For an acoustic intensity of 360 mW/cm2 at the surface of the PMUT, an open-circuit voltage of 1.11 V and an aperture efficiency of 30.5 % are measured. Furthermore, by connecting a series-matching inductor to the PMUT, the highest-reported power delivered to the load (PDL) of 6.4 mW is measured over an optimal load of 7.6 Ω. Finally, together with the ASIC and at the intensity of 108 mW/cm2, our system delivers 1.04 mW DC power to a 3.3 kΩ load, which is over two orders of magnitude higher than the previously reported average DC power for AlN PMUTs.