Lignin reinforced hydrogels with fast self-recovery, multi-functionalities via calcium ion bridging for flexible smart sensing applications.

Hydrogels have found applications in many different fields. However, poor mechanical properties, such as low elasticity and lack of rapid recovery under large deformation, can severely limit their applications. In this study, we developed lignin reinforced hydrogels made of calcium ion containing ternary polymers (lignosulfonate (LS), alginate (Alg), and polyacrylic acid (PAA)). The resultant hydrogel has excellent elasticity, rapid self-recovery, and multi-functionalities. The covalent PAA network acts as the elastic scaffold of hydrogel, while calcium bridging networks of LS, Alg, and PAA, as well as the strong hydrogen bonding network in the system, function as sacrifice bonds to dissipate energy and transfer stress. The PAA/LS/Alg/Ca hydrogels exhibit rapid and durable elastic recovery ability under large deformation with the highest compressive stress of 835 kPa (95% strain), highest tensile fracture stress of 357 kPa, and highest tensile strain of 1144%. In addition, these tough hydrogels show UV resistance, self-healing, antifreeze, and excellent electro-conductivity. When assembled into a strain sensor, stable and reliable electrical responses with 375 ms response time are demonstrated. The PAA/LS/Alg/Ca hydrogel strain sensors can monitor human movements with responsive and accurate physiological signals. These results support the conclusion that the PAA/LS/Alg/Ca hydrogel strain sensors have great application potential in flexible wearable electronics and smart devices.