Effects of Soft Encapsulation on the Receive Performance of PMUTs for Implantable Devices

Recent studies present ultrasound (US) as a promising candidate for powering implantable devices, requiring in-tegrated and encapsulated receivers to ensure longevity. Conventional hermetic packaging can hinder acoustic transmission, making polymer-based approaches desirable. This study evaluates how polymers commonly used for implants (i.e., thermoplastic polyurethane, parylene-C, medical-grade silicones, and polyimide) affect the receive performance of piezoelectric micromachined ultrasound transducers (PMUTs). Simulations and measurements between 1 and 7 MHz show transmission coefficients above 94 % for material thicknesses in the nm and μm ranges. A theoretical analysis of the mechanical properties guides material selection for later PMUT encapsulation, focusing on polyurethane, parylene-C, and two medical-grade silicones (MED-1000, MED2-4213). In a complete system comprising encapsulated PMUTs, mechanical and acoustic properties, along with interface mismatch between the encapsulation and the PMUTs, influence the receive performance of the devices. Finite element modeling (FEM) and measurements evaluate the impedance and receive sensitivity of encapsulated PMUTs. The results show that residual stress or higher stiffness in some polymers, reduces the receive sensitivity, an effect not evident from only analysing the acoustic transmission through coatings. However, this study demonstrates that upon careful consideration of the acoustic and mechanical properties as well as thickness selection, polymers commonly used for implantable devices can effectively be used for PMUT encapsulation.