Stretch-oriented chitosan/PVA-based anisotropic double network hydrogel curing by annealing and rehydration for wearable strain sensors

Abstract

Conductive hydrogel is considered as ideal candidates for flexible electronic devices. However, the limited toughness, flexibility and swelling behavior of conductive hydrogel hinder their application in various environments. In this work, an anisotropic and swelling resistant hydrogel is developed by stretching and reorienting polymer chains by combining with annealing and rehydration process, which is inspired by hierarchical structure of natural tissues. As a result, this hydrogel exhibits significant variability in mechanical properties along and perpendicular to the orientation direction. The toughness along the orientation direction is 1018.5 ± 35.6 kJ·m⁻³ and the tearing energy is 0.56 ± 0.03 kJ·m⁻². The toughness perpendicular to the orientation direction is 483.4 ± 18.6 kJ·m⁻³ and the tearing energy is 2.24 ± 0.07 kJ·m⁻². The hydrophobic interactions of 2-hydroxyethyl methacrylate, in conjunction with the dense polymer network contribute to swelling resistant of the hydrogel. The anisotropic hydrogel is further integrated into flexible sensors, which could be capable of monitoring human motion in amphibious environments, providing distinctly different feedback signals in the directions parallel and perpendicular to the orientation. This work provides a promising strategy to prepare diverse anisotropic materials, offering new solutions for sensor technology.