Electro-Poling and Zwitterionic Bacterial Cellulose Nanocrystals Enable Conformation-Stabilized β-Phase PVDF for High-Sensitivity Piezoelectric Sensing

Polyvinylidene fluoride (PVDF) is a promising flexible piezoelectric polymer for wearable sensing, but its performance is limited by the relatively low content and orientation of the electroactive β-phase. Herein, zwitterionic bacterial-cellulose-derived cellulose nanocrystals (CNC-K) were introduced as a multifunctional additive and combined with electrical poling to develop a conformation-stabilized PVDF/CNC-K composite. CNC-K exhibited high crystallinity (72.23%) and a reduced fibril diameter of 5.85 ± 0.82 nm after hydrolysis, providing rigid and chemically active interfacial domains. The presence of hydroxyl (–OH) and carbonyl (C=O) functionalities enabled hydrogen bonding and dipole–dipole interactions with PVDF, promoting molecular organization and electroactive β-phase formation. Compared with pristine PVDF, PVDF-FP, and unpoled PVDF-K, the electro-poled PVDF-K Electric exhibited the highest crystallinity (70.15%) and a 2.04-fold higher β/α phase ratio (3.97), confirming enhanced electroactive phase formation. Piezoresponse force microscopy (PFM) further demonstrated enhanced electromechanical coupling, with a butterfly amplitude of 89.85 pm/V and phase hysteresis of 60.67°. The optimized sensor exhibited a near-linear response (R² = 0.9717) over a low pressure range of 0.157–3.14 kPa, with a high sensitivity of 38.24 mV/kPa, response/recovery times of 6.9/6.1 ms, and stable operation over 60 cycles with performance retained after 6 months. Distinct electrical responses were also obtained for finger bending, tapping, and dragging motions. The combination of high sensitivity, low-pressure operation, linear response, and flexibility demonstrates the potential of CNC-K as a single multifunctional additive for PVDF-based pressure sensors, particularly for wearable electronics, electronic skin, soft robotics, and human–machine interfaces.