Spatiotemporally-regulated multienzymatic polymerization endows hydrogel continuous gradient and spontaneous actuation

There are several natural materials which have evolved functional gradients, ingeniously attaining maximal efficacy from limited components. Herein, we utilized the spatiotemporal distribution of initiator acetylacetone to regulate the multienzyme polymerization and fabricate a chitosan-polymer hydrogel. The temporal priority order of acetylacetone was higher than phenol-modified chitosan by density functional theory calculation. The acetylacetone within the gelatin could gradually diffuse spatially into the chitosan hydrogel to fabricate the composite hydrogel with gradient network structure. The gradient hydrogel possessed a transferring topography from the two-dimensional pattern. A continuously decreased modulus along with acetylacetone diffusion was confirmed by atomic force microscope-based force mapping experiment. The water-retaining ability of various regions was confirmed by low-field nuclear magnetic resonance (NMR) and thermogravimetric analysis (TG) analysis, which led to the spontaneous actuation of gradient hydrogel with maximum 1821°/h curling speed and 227° curling angle. Consequently, the promising gradient hydrogels could be applied as intelligent actuators and flexible robots.