Bioinspired multifunctional hydrogel actuator with rapid temperature/near-infrared light dual-stimuli responsiveness and strain-sensing capability.

In this work, a bilayer hydrogel with dual responsiveness to temperature and near-infrared (NIR) light, together with strain-sensing capability, was prepared via stepwise in situ free-radical polymerization. Upon heating or NIR irradiation, the poly(N-isopropylacrylamide-co-acrylamide) (PNAM)/Ti₃C₂Tₓ MXene active layer contracts, while the poly(acrylamide) (PAAm)/poly(vinyl alcohol) (PVA) passive layer provides flexible mechanical support and undergoes relatively limited deformation, resulting in directional bending of the bilayer hydrogel. Meanwhile, MXene serves as both the photothermal component for NIR actuation and the conductive component for real-time strain sensing. The hydrogel consists of a PNAM/MXene active layer and a PAAm/PVA passive layer, wherein MXene nanosheets provide excellent photothermal conversion capability and electrical conductivity, while the PAAm/PVA network offers favorable flexibility and mechanical support. Owing to its bilayer structure, the hydrogel exhibits favorable flexibility and stretchability, with a tensile strength of 12.5 kPa and an elongation at break of 397%. Under thermoresponsive actuation, it achieves a maximum bending rate of 12° s-1. Under NIR irradiation, the surface temperature of the optimized hydrogel reaches 68.7 °C within 60 s. The hydrogel can also be fabricated into various shapes to realize biomimetic behaviors. In addition, it exhibits high sensitivity (GF = 3.28) and excellent cyclic stability. Therefore, the PNAM/MXene//PAAm/PVA hydrogel developed in this study represents a multifunctional soft material with potential applications in intelligent soft actuators, wearable sensors, and human-machine interfaces.