Lignin precipitation-driven fabrication of gradient porous hydrogel actuator with temperature response

An operative and straightforward precipitation-driven approach was reported to fabricate an anisotropic hydrogel actuator with temperature response. Through in situ deposition of lignin nanoparticles (LNP) in the process of polyacrylamide (PAM) polymerization with the presence of hydroxypropyl cellulose (HPC), an inhomogeneous hydrogel network (PAM/HPC/lignin hydrogel, PHL hydrogel) with distinct gradient porous structure was achieved that could be tailored to form a hydrogel actuator. The PHL hydrogels exhibit faster shape deformation as responding to temperature and higher mechanical properties caused by introducing the LNP and HPC chains. The deformation direction and rate of the hydrogel actuator could be influenced by the lignin content, temperature, and as well as their shape. The maximum bending angle could reach near 360° with 60 s as it was exposed to 60 °C. Due to the excellent bending behavior of the PHL hydrogel, the potential applications as grippers and valves were studied, and the results showed its sensitive response to temperature, suggesting its potential application as an intelligent actuator in the future.