Phase-Separation-Optimized Porous PVDF-TrFE Piezoelectric Sensor with Liquid Metal Electrodes for Wrist Motion Recognition

Poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE)-based piezoelectric sensors have demonstrated considerable promise for wearable physiological monitoring and human–machine interaction owing to their self-powered operation and mechanical compliance. However, the piezoelectric output of such devices remains constrained by two coupled bottlenecks: the intrinsically limited piezoelectric coefficient and force-to-electricity conversion efficiency of plain PVDF-TrFE films, and the charge dissipation arising from microscopic air gaps and contact resistance at the rigid-electrode/soft-polymer interface due to mechanical mismatch. Here, we propose a synergistic strategy that couples nonsolvent-induced phase separation (NIPS) for porous structure engineering with microchannel-confined liquid-metal electrodes, simultaneously addressing performance limitations at both the film-body and electrode-interface levels. By systematically tuning the volumetric ratio of N,N-dimethylformamide (DMF) to water in the coagulation bath, PVDF-TrFE films with pore architectures were fabricated; at the optimal DMF:water ratio of 1:9 composition, the film simultaneously achieved 39.69% porosity and 93.80% β-phase content. Quantitative decoupling analysis reveals that the NIPS-derived porous architecture contributes an approximately 2.3-fold output enhancement, upon which the liquid-metal electrode, through gap-free conformal contact enabling efficient charge collection, delivers a further ∼15-fold amplification. The resulting flexible piezoelectric sensor exhibits a peak output voltage of 4.10 V under a 12 N normal load, a sensitivity of 0.790 V/N in the low-force regime (0.5–3 N), and excellent operational stability with no discernible signal degradation over 10,000 loading cycles. As a proof-of-concept for wearable human–machine interfacing, the single-channel sensor combined with an XGBoost classifier achieves 91.74% accuracy in discriminating five representative wrist motions. These findings confirm that pore engineering at the film level and interface engineering at the electrode level represent two mutually decoupled and functionally complementary optimization dimensions, whose synergistic integration offers a viable technological pathway toward high-performance flexible piezoelectric sensors with low fabrication complexity.