Hierarchical Lead-Free Piezoelectric Composites for Ultrasound Wireless Energy Harvesting

The rapid advancement of bioelectronics has created growing requirements for flexible, biosafe, and self-powered devices. Piezoelectric energy harvesters (PUEHs), which convert mechanical energy from ultrasound into electricity, offer promising solutions for wireless power systems. However, existing piezoelectric bioelectronics typically face limitations due to their reliance on toxic lead-based materials, rigid structures, and bulky form factors. To address these challenges, we developed an innovative lead-free piezoelectric composite featuring a multilayered gradient architecture with "dense-porous-dense" structure. This was achieved through phase boundary engineering and tape-casting technology. Our approach involved: (1) embedding micron-sized polymer spheres into engineered (K, Na)NbO₃-based ceramics, and (2) precisely tuning the secondary phase content and layer thickness ratios using tape-casting combined with burned-out polymer sphere technology. The resulting material exhibits outstanding piezoelectric performance, with a g₃₃ of 73.04 × 10^-3 V·m·N^1- and g₃₃×d₃₃ of 17311.6 × 10^-15 m²·N^1- that exceed most lead-free alternatives and even rival some lead-based materials. We then fabricated a flexible PUEH by connecting diced piezoelectric composite elements with serpentine circuits, which maintained stable operation on curved body surfaces under ultrasound excitation, delivering an average charging power of 717.41 nW. Through systematic evaluation using ex vivo porcine tissue models of varying thicknesses, the device's reliable transdermal energy transmission capability was confirmed. This breakthrough provides a practical approach for wirelessly powering next-generation implantable bioelectronic devices.