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Smart hydrogel materials have demonstrated significant advantages in application fields, such as drug delivery, tissue engineering, and smart sensing. However, developing a stimuli-responsive hydrogel that is simple to prepare, programmable, and capable of rapid response remains a challenge. Herein, we employed a polyvinyl alcohol (PVA) polymer film to hydrolyze and diffuse in a hydrogel monomer precursor solution, thereby constructing a diffusion layer and preparing an anisotropic hydrogel with a multiphase gradient structure. The diffusion-generated gradient structure creates a significant structural difference between the diffused and nondiffused layers similar to that of a bilayer structure, which leads to a significant difference in internal stress between the two layers, thus providing the hydrogel with a fast response capability (bending deformation of 527.3° in 30 s) while avoiding the interfacial confinement caused by the bilayer structure. In addition, the abundant hydroxyl groups of PVA, forming hydrogen bonding interactions with the amide groups of Poly(N-isopropylacrylamide) (PNIPAM), significantly enhance the mechanical properties of the gradient hydrogel. This enhancement is evidenced by an increase in tensile stress from 32 to 176 kPa and an increase in Young's modulus from 2.23 to 44.19 kPa. It is important to emphasize that by designing the shape of the PVA film, precise multidimensional programmable deformation can be achieved. This simple yet versatile diffusion-driven strategy offers a practical approach to the design of responsive hydrogels, potentially paving the way for future advancements in intelligent actuators, artificial muscles, and intelligent human-machine technologies.
Wang et al. (Wed,) studied this question.