Highly Elastic Anti-fatigue and Anti-freezing Conductive Double Network Hydrogel for Human Body Sensors

As a functional material, conductive hydrogel has been widely used due to its stretchability and flexibility, especially in the field of flexible wearable sensors. However, preparing a hydrogel sensor with high elasticity, fatigue resistance, and low-temperature resistance is still challenging. In this work, a novel hydrogel was synthesized with acrylic acid and sodium p-styrene sulfonate in an acidic solution of chitosan under ultraviolet light. Then, the hydrogel was immersed in the sodium chloride solution to obtain a chitosan/poly(acrylic acid-sodium p-styrene sulfonate)/sodium chloride (CS/P(AA-co-SS)/NaCl) dual network (DN) hydrogel. The sodium chloride caused the molecular chains of the hydrogel to entangle. The hydrogel shows excellent mechanical properties (tensile strength is as high as 532.2 kPa, elongation at break is as high as 620%, energy to break is 1200 kJ/m3, compressive strength at 80% is 661 kPa, and compressive toughness is 83.5 kJ/m3) and high electrical conductivity (up to 4.5 S/m). At the same time, the hydrogel possesses excellent resilience and fatigue resistance due to hydrogen bonds, electrostatic interactions, and the existence of hydrophobic domains. Moreover, the hydrogel also exhibits outstanding frost resistance, excellent mechanical properties, and electrical conductivity even at −20 °C. The wearable sensor made of the CS/P(AA-co-SS)/NaCl DN hydrogel will have high sensitivity (under 100% strain, gauge factor = 2.4) and repeatability, which can accurately detect various movements of the human body.