Development of hierarchically constructed robust multifunctional hydrogels for sensitive strain sensors by using guar gum and polydopamine to encapsulate liquid metal droplets

Conductive hydrogels have aroused tremendous interest as strain sensing materials because of their good biocompatibility, high stretchability, and structural similarity to natural soft tissues. However, fabricating a multifunctional conductive hydrogel that simultaneously exhibits high toughness, high sensing performance and a low detection limit remains a challenge. Herein, we developed a new conductive hydrogel with a hierarchical cluster network structure by using soft Ga droplets encapsulated by guar gum (GG), in situ polymerized polydopamine (PDA) as conductive materials and in situ synthesized/Ga3+-crosslinked poly acrylic acid (PAA) as a ductile hydrogel matrix. GG and PDA around Ga droplets constituted physically crosslinked dense rigid networks. Because of its completely physically crosslinked hierarchical cluster network structure with Ga droplets, the hydrogel, which was named GG-PDA-Ga-PAA, exhibited high toughness (1.8 MJ/m3), good moldability, adhesiveness, antibacterial activity and excellent self-healing properties. Its conductivity recovered more than 90 % within 83 ms, while its tensile strength (213.2 kPa) and elongation at break (1364 %) recovered 92.8 % and 100 %, respectively, within 6 h. When applied as a strain sensing material, the GG-PDA-Ga-PAA hydrogel displayed high sensitivity (GF=10.83), short response/recovery time (167/167 ms), ultralow detection limit (0.2 %) and good fatigue resistance and cyclic stability (1000 cycles). Due to its excellent comprehensive properties, the hydrogel can be applied as a competent wearable stain sensor for monitoring various human activities from pulses in the radial artery to human joint bending. This work provides a new approach for producing multifunctional conductive hydrogels for strain sensors.