Construction of organic–inorganic hybrid heterostructure towards solvent responsive hydrogel with high stiffness

Solvent responsive hydrogel is a kind of intelligent soft material, which can be used in soft robots. Currently, most of the solvent responsive hydrogels are based on pure organic materials which has limited stiffness for actuations. Herein, a novel organic–inorganic composite hydrogel is designed and prepared. Calcium phosphate oligomers nanoclusters are incorporated in polymer solution containing polyvinyl alcohol (PVA) and sodium alginate to form organic–inorganic hybrid copolymer suspensions. The solvent responsive hydrogel is simply prepared using co-evaporation method by optimizing the components in the colloid system. The inorganic nanoparticles work as the scaffold in the porous PVA network and the evaporation caused nonuniformity distribution further induces the formation of heterostructure, which has different shrinkage ratios along the thickness direction. The prepared hydrogel demonstrates excellent shape memory property by changing the environmental solvents between water and ethanol and its repeatability is also verified. The stiffness of hydrogel is enhanced and it has large deformation after incorporation of calcium phosphate nanoparticles. The bending angle of hydrogel can be well controlled by different water to ethanol ratios, allowing for underwater actuation. The functionality of an artificial gripper based on responsive hydrogel with high stiffness is demonstrated to transfer objects in ethanol. The design of organic–inorganic composite hydrogel with high stiffness may provide new insights for preparation of intelligent soft materials for underwater applications.