Freestanding Hierarchical-Porous BCZT Thin Films Enable Efficient Ultrasonic Energy Harvesting in Soft Tissue

Excellent electromechanical properties of piezoceramic thin films facilitate ultrasonic energy harvesting in bioelectronic systems. However, most piezoceramic thin films are fragile and cannot maintain mechanical integrity under deformation, hindering their application in soft tissue coupling. Here, we report a freestanding hierarchical-porous barium calcium zirconate titanate (BCZT) thin film that combines crack resistance with efficient ultrasonic energy harvesting. The hierarchical-porous structure, fabricated using polyvinylpyrrolidone (PVP) as a porogen, disperses tensile-side stress concentration and suppresses crack initiation, while reducing the acoustic-impedance mismatch with soft tissue and suppressing interfacial ultrasound reflection. The film exhibits reduced permittivity and a high piezoelectric voltage coefficient of g₃₃ = 97 × 10^-3 V·m/N, resulting in a maximum output power that is approximately 1.5 times that of the dense BCZT thin film under identical ultrasound excitation, evaluated at the respective optimal loads. Owing to its excellent piezoelectric performance, the film also enables reliable detection of weak vibrations, such as radial artery pulse-wave signals. This work demonstrates a hierarchical-porous structure for simultaneously improving acoustic coupling, mechanical reliability, and electromechanical transduction in freestanding piezoceramic thin films, highlighting their potential for wearable sensing and implantable ultrasound-powered bioelectronics.