Bacterial cellulose-liquid metal/polyacrylic acid hydrogels for freezing-tolerant strain sensing

Developing conductive hydrogels with high stretchability, robust conductivity, anti-freezing performance, and reliable strain sensing across wide temperature ranges remains challenging. Herein, poly(acrylic acid)/gallium-bacterial cellulose/NaCl (PAA/Ga-BC/NaCl) conductive hydrogel sensors are fabricated via a facile two-step strategy. Critically, Ga liquid-metal nanoparticles (LMNPs), stabilized by BC nanofibers via a Pickering mechanism, directly initiate polymerization of acrylic acid. Subsequent NaCl post-soaking introduces Na⁺/Cl^- as mobile charge carriers. Optimized at 30 min soaking, the hydrogel achieves tensile strength of 112.0 kPa, elongation of 1527%, toughness of 0.89 MJ m^-3, and conductivity of 1.8 S m^-1. NaCl converts free water into tightly bound hydration clusters, depressing the freezing point and preserving 1444% elongation after conditioning at -20 °C, with retained flexibility after conditioning at -70 °C. As a wearable sensor, the hydrogel exhibits strain-dependent gauge factors of 0.67 (0-100% strain), 1.20 (100-250%), and 1.56 (250-400%), a stable piezoresistive response over the 2-400% strain range, and 300-cycle durability at 100% strain, enabling multi-scale human-motion monitoring. The dynamic ionic network additionally confers rapid electrical self-recovery, with the circuit reopening within 22.5 ms of disconnection and re-closing within 13.5 ms of re-contact. This ionic engineering strategy establishes a green and broadly applicable paradigm for environment-tolerant multifunctional hydrogel electronics.