Self-polarized cellulose nanofiber-reinforced PVDF-based piezoelectric composites via direct-ink-writing 3D printing for pressure sensing and energy harvesting

With their unique physicochemical properties, PVDF-based piezoelectric materials demonstrate significant potential in various fields. Nevertheless, their benign piezoelectric performance is usually obtained through polarization post-treatment after material formation. In this study, a self-polarized direct-ink-writing (DIW) 3D printing strategy is proposed for one-step fabrication of high-performance PVDF/cellulose nanofiber (CNF) piezoelectric composites with low energy consumption. Under the synergistic effects of shear/stretching from DIW and hydrogen bonding between PVDF and CNF, α-phase PVDF undergoes gauche-trans conformational transformation into β phase. And then β-phase PVDF is dragged by high aspect ratio CNF into bead-like small crystals to form a multilayered oriented structure, resulting in abundant oriented dipole moments. PVDF/CNF composites with different weight ratios are printed using this strategy, and the results show that the composite film with 5 wt% CNF content exhibits the best piezoelectric performance. Without additional polarization treatment, it achieves a sensitivity of 103 mV/N, 2.2 times higher than conventional cast films. The composite also demonstrates good linear response and durability, meeting the requirements for human motion monitoring and mechanical energy harvesting. This work has opened up new avenues for the efficient and low-energy fabrication of flexible wearable and energy-harvesting devices.