A Heterogeneous Cascaded Resonant MEMS Energy Harvester with In Situ Integrated PVDF Films for Sub‐10 Hz Ultra‐Low‐Frequency Vibration Energy Harvesting

MEMS energy harvesting under low-frequency vibrations faces significant challenges due to the mismatch between the low-frequency nature of environmental vibrations and the inherently high resonant frequencies of micro-scale inertial elements. Flexible multi-degree-of-freedom (MDOF) energy harvester offers a promising solution by achieving low resonant frequencies and multimodal resonance. However, integrating suitable piezoelectric materials into such complex microstructures remains a critical challenge. Herein, numerical modeling demonstrates the advantages of a PVDF-PDMS composite flexible suspended beam in achieving either a lower resonant frequency or generating higher normal stress in the piezoelectric layer. Furthermore, it is proposed a design strategy that reconciles rigidity and flexibility within a single micro resonant system for low-frequency energy harvesting. The designed heterogeneous cascaded structure integrates commercial PVDF thin films with a PDMS-silicon resonant structure, achieving a high effective proof mass and low stiffness. It is developed an in situ compatible microfabrication method to enable seamless integration of commercial PVDF thin films. The resulting resonant system achieves multimodal resonance with closely spaced frequency peaks, thereby enhancing its responsiveness to ultra-low-frequency vibrations. The fabricated micro energy harvester (MEH) demonstrates a normalized power density (NPD) of 13.33 µW/cm³/g² at 2 Hz and a figure of merit (FoM) of 6.67. Surpassing the sub-10 Hz frequency threshold of conventional MEMS resonant harvesters, this work preserves the ultra-low-frequency resonance demonstrated in the previous work while attaining a remarkable increase by an order of magnitude in NPD. This work provides a scalable and practical approach to the development of flexible energy harvesting systems, addressing key challenges in low-frequency vibration energy harvesting.