Yang Yang,E. Liu,Wenli An,Yan Hu,Xuehuan Xia,Shimei Xu
Mimicking the anisotropic structure and environmental adaptation of organisms in nature remains a key objective in the field of hydrogels. However, it has been very challenging due to complex fabrication and confined application only in water. Here, we demonstrate a new strategy of spontaneous fabrication of an anisotropic hydrogel based on our finding in the tropic movement of gelatin toward the Teflon template. The obtained hydrogel exhibits fast response and recovery under temperature stimuli both in aqueous and non-aqueous environments, making use of the approximate transition temperature and opposite phase transition behavior of gelatin and poly(N-isopropylacrylamide) (PNIPAm). Its recovery performance in water is more than 50 times faster than that of the PNIPAm hydrogel. Furthermore, the PNIPAm/gelatin hydrogel can achieve 3D complex deformations, stealth deformation, erasable and reprogrammed surface patterning, and multistage encryption by simply modulating the location and shape of gelatin to achieve an anisotropic structure. The work provides a simple and versatile way to obtain an anisotropic hydrogel with a definite and predictable structure, which is demonstrated across a range of different monomers. It improves the responsive performance and broadens the hydrogel application to the non-aqueous environment. Additionally, this tropic movement of gelatin can be extended for the design of new types of anisotropic materials and thus endows the materials with diverse functionality.
Fast recovery in a nonaqueous environment is a big challenge for hydrogel actuators. In this work, a temperature-responsive hydrogel actuator with outstandingly rapid recovery in air was reported. The hydrogel with bionic large-ranged gradient structure was fabricated by copolymerization of hydrophilic monomer hydroxyethyl acrylate (HEA) and N-isopropylacrylamide in the dispersion of Laponite utilizing a facile electrophoretic method. The deformation degree and time can be regulated by varying the concentration of HEA to change the lower critical solution temperature (LCST) and swelling of the hydrogel. A dynamic equilibrium between the water into and out of the hydrogel was observed, and the hydrogel showed no shrink above LCST. The synthesized hydrogels showed fast response in hot water and rapid recovery in air. Such nonshrink characteristics and excellent reversibility made it possible for these hydrogels to be used as temperature-controlled microfluidic switches. This work provided an approach to design fast recovery hydrogel actuators by the incorporation of hydrophilic monomers and extend the application of the hydrogel actuators into fields such as soft robots, micromanipulation, microfluidics and artificial muscles in various environments.
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