An Energy Focusing Flexible and Lightweight Acoustic Metamaterial for Enhanced Ultrasound Power Transfer.

Ultrasound power transfer enables the wireless charging of implantable medical devices by utilizing an energy harvester to convert incoming ultrasound into electrical energy. Most harvesters rely on Lead Zirconate Titanate (PZT) crystals for energy conversion, but since PZTs are rigid and dense ceramics, scaling them up to harvest more energy produces heavier devices that are impractical for medical applications. This study presents a metamaterial-enhanced ultrasound energy harvester (Meta-UEH) that integrates a small PZT crystal with a locally resonant metamaterial, which concentrates ultrasound energy onto the PZT and thus improves its energy harvesting performance. The metamaterial is flexible, lightweight, and entirely passive, making it well-suited for medical applications, while increasing the power of harvested electricity by more than double. Experimental and simulation results confirm that the Meta-UEH achieves enhanced efficiency even under reverberation or deformation, benefiting from standing waves that form between transducers and the metamaterial, with harvested electrical power reaching up to 350% of that obtained without the metamaterial and standing wave. The underlying mechanisms behind the improvement are analyzed, revealing that the improvement is not specific to PZTs but can also be applied to other ultrasound-electricity conversion methods, such as piezoelectric and triboelectric nanogenerators.