Transparent, photothermal and stretchable alginate-based hydrogels for remote actuation and human motion sensing.

Multifunctional hydrogels show potential applications in actuators and wearable sensors. However, it is still a challenge to develop a photothermal responsive conductive hydrogel with high transparency, mechanical properties, broad sensing range, and low-temperature resistance. In this work, a transparent, photothermal responsive, and highly stretchable alginate-based hydrogels was feasibly constructed by adding two-dimensional non-layered molybdenum dioxide nanosheets (2D-MoO) to sodium alginate/polyacrylamide mixture and then soaking into the calcium chloride solution. The introduction of 2D-MoO renders the hydrogels excellent photothermal properties and controllable photomechanical deformation under near-infrared irradiation, while maintaining high transparency (~60 %).The calcium ions give the hydrogel excellent mechanics, conductivity, and freezing tolerance concurrently. The transparent hydrogel-based sensor shows wide sensing range (0-1800 %) and cycling stability in detecting deformations and real-time human motions even in harsh environments. Therefore, this work provides a new route for generating transparent multifunctional hydrogels towards the applications of remote actuation and strain sensing.