Programmable Volume Phase Transition of Hydrogels Achieved by Large Thermal Hysteresis for Static-Motion Bilayer Actuators

In order to achieve a hydrogel capable of programmable volume change, poly(N-isopropylacrylamide)-graft-methylcellulose hydrogel (PNIPAm-g-MC) was prepared through the grafting of PNIPAm onto a MC backbone and simultaneous cross-linking of the chains. PNIPAm-g-MC exhibited large thermal hysteresis in its volume change, which results from the stable hydrophobic junctions between the MC strands formed during heating. By combining photothermal magnetite nanoparticles as a heat transducer with the prepared hydrogel, programmable volume phase transition between the shrunken and swollen state could be triggered by visible light irradiation and excessive cooling, respectively. Based on this programmable feature, a bilayer actuator capable of static bending was fabricated. The developed programmable hydrogels are expected to provide a platform for the next generation of origami, microvalves, and drug delivery systems.