Anisotropic bilayer hydrogels with synergistic photochromism behaviors for light-controlled actuators

Smart hydrogels have attracted widespread attention and have displayed comprehensive applications in many fields. The development of a novel type of bilayer hydrogel with synergistic function of anisotropic deformation and photochromic property is extremely challenging. Many previous researches have focused on multiple response hydrogels. However, it hardly achieves the synergy of deformation and color-changing through a method with the advantages of simplicity, stability, practicality, and precise control. Herein, a near-infrared light (NIR) photothermal thermoresponsive graphene oxide/poly(N-isopropylacrylamide-beta-cyclodextrin) (PNIPAM-beta-CD/GO) hydrogel layer and a reversible photochromic ammonium molybdate tetrahydrate/poly(N-isopropylacrylamide-co-beta-cyclodextrin) (PNIPAM-beta-CD/Mo-7) hydrogel layer were combined to obtain a bilayer hydrogel through the host-guest interactions between beta-CD and the isopropyl group of NIPAM, exhibiting integrate anisotropic shape deformation and reversible photochromic behaviors. The designed actuators can undergo complex deformation owing to volume shrinkage caused by the photothermal effect under NIR of the PNIPAM-beta-CD/GO layer, while the PNIPAM-beta-CD/Mo-7 hydrogel layer shows the characteristics of quickly color changing within 5 s when irradiated by a UV light source. The multiple responsiveness of hydrogels on temperature, NIR irradiation, and UV irradiation displays valuable advantages in multifunctional smart materials, such as intelligent soft robots.