High-Strength, Tunable Thermoresponsive, Biocompatible, and Highly Sensitive Sensing Properties of Freeze-Thaw-Regulated Poly(acrylic acid)/Hydroxypropyl Cellulose Hydrogels.

A physically crosslinked multifunctional hydrogel based on poly(acrylic acid)/hydroxypropyl cellulose/aluminum chloride (PAA/HPC/AlCl3) was developed for flexible sensing applications. The network, constructed solely through hydrogen bonds and Al3+ coordination bonds, exhibits stretchability, adhesiveness, and reversible thermochromism. Freeze-thaw cycling serves as a green physical strategy to regulate performance: after treatment, tensile strength increased from 0.11 to 3.60 MPa, hysteresis strain decreased from 35 to 9.5%, and dissipated energy rose from 7.839 to 53.699 MJ·m-3. Low-field NMR reveals that freeze-thaw promotes water redistribution and chain aggregation, strengthening non-covalent interactions without introducing chemical crosslinkers. By adjusting Al3+ content and freeze-thaw cycles, the upper critical solution temperature (UCST) can be synergistically tuned between 15 and 54 °C, enabling reversible thermochromism. Before freeze-thaw, the hydrogel exhibits excellent adhesion, high strain sensitivity (GF = 5.4, 3-500%), high temperature sensitivity (TCR = -0.019/°C, detection limit 0.1 °C), and electromechanical self-healing, allowing accurate detection from human motion to Morse code. Post freeze-thaw, hydrogels combine high strength with stable thermoresponsive properties. In vitro hemolysis and CCK-8 assays confirmed the hydrogel's hemocompatibility and cytocompatibility. A preliminary in vivo wound contact test on rats revealed only mild, fully reversible irritation with no persistent damage. This work achieves differentiated performance regulation in a single system through a simple and environmentally friendly processing method, offering a sustainable design approach for flexible sensors.