Bilayer Flexible Films with Controllable Porous/Dense Structure via Single-Step Fabrication for Advanced Piezoelectric Wearables

Polyvinylidene fluoride (PVDF) and its copolymer poly(vinylidene fluoride-<i>co</i>-trifluoroethylene) (PVTF) have attracted significant attention in energy harvesters and piezoelectric sensing devices due to their inherent piezoelectric properties and exceptional flexibility. However, their limited piezoelectric performance restricts practical applications. Inspired by microstructural design principles and based on the traditional nonsolvent-induced phase separation (NIPS) method, we developed a low-pressure-assisted in situ nonsolvent-induced phase separation (LPA-NIPS) technique and successfully fabricated high-performance 3D porous PVTF-based flexible piezoelectric films with a bilayer sponge-like structure. By controlling the content of the nonsolvent phase (H₂O), the thickness of the porous layer and the pore size within the film can be precisely tuned over a wide range. This bilayer sponge-like PVTF film exhibits excellent strain behavior and the ability to withstand high electric fields. Under a polarization voltage of 2500 V, the porous film, owing to its superior compressibility, higher piezoelectric coefficient (<i>d</i>₃₃ ∼ 15.96 pC/N), and lower dielectric constant, exhibits a 41% increase in <i>V</i>_op, reaching 4.58 V, compared to the dense film. Even under a maximum polarization voltage lower than that required by the dense film, the film with a bilayer sponge-like structure outperforms its dense counterpart, achieving a <i>d</i>₃₃ of 20.3 pC/N, a piezoelectric output of 6.17 V, a power density of 1.28 μW/cm², and a force sensitivity of 158 mV/N. Furthermore, the piezoelectric sensing prototype maintains stable power generation performance over 1000 cycles without structural collapse. By harvesting energy from subtle vibrations, the film exhibits highly discriminative capability for various human motions in daily activities, which reflects its potential for active monitoring and healthcare applications. This straightforward method offers significant promise for the large-scale production of piezoelectric composite films with remarkable flexibility, sensitivity, and suitability for wearable sensing devices.