Developing new smart hydrogel systems and new fabrication methods is critical for fundamental research and industrial applications, particularly in the intelligent human–machine field, but still faces great challenges. In this work, we present bilayer poly(N-isopropylacrylamide)/graphene oxide (polyNIPAM/GO) hydrogels with dual thermo- and near-infrared (NIR)-responsive properties using a simple, in situ polymerization-centrifugation method. By tuning the GO concentration and centrifugation speed, a transparent polyNIPAM layer and a dark-brown GO-rich layer are formed, where each layer displays distinct network structures and swelling behavior. Due to the asymmetric double-layer structure, the polyNIPAM/GO bilayer hydrogels enable one to realize fast, controllable, bidirectional bending under thermal or NIR stimulation within 1 min, and the bending orientation and degree can be reversibly, repeatedly, and precisely controlled by the temperature- or NIR-induced cooperative swelling–shrinking properties of both layers. Based on the reversible, bi-directional bending nature of polyNIPAM/GO hydrogels, we further design two proof-of-concept hydrogel actuators to highlight the advantages of this hydrogel system: one acts as a wiggler to mimic robot arms to move objects, while the other serves as an electric switch to turn on/off a light. In addition, our polymerization-centrifugation method can also be used to prepare polyNIPAM/SiO2 bilayer hydrogels, demonstrating its general applicability to other polyNIPAM–nanoparticle hydrogels with a bilayer structure. This work provides both a new fabrication method and hybrid hydrogel systems for the development of smart, programmable, and versatile hydrogel-based actuators.
Shengwei Xiao,Xiaomin He,Jie Qian,Xiaohui Wu,Guobo Huang,Huajiang Jiang,Zhicai He,Jintao Yang
The development of hydrogel–organogel bilayer films has greatly promoted the application of asymmetric structures in bionics, biointegrated electronics, and other basic industries. However, the ability to bond the hydrogel and organogel tightly remains a significant challenge. Herein, we propose a simple one-pot method for the fabrication of multiresponsive hydrogel–organogel bilayer films, consisting of a hydrophobic poly(butyl acrylate) (polyBA) layer, a salt-/thermo-responsive poly(3-(1-(4-vinylbenzyl)-1H-imidazol-3-ium-3-yl)propane-1-sulfonat) (polyVBIPS) layer, and interfacially distributed vinyl-functionalized nanoparticles. The presence of functionalized nanoparticles leads to strong adhesion between the hydrogel and organogel via copolymerization between the hydrogel and organogel precursors. Since the hydrophobic organogels and hydrophilic hydrogels exhibit independent swelling–shrinking behaviors, the resultant bilayer films undergo rapid, large-amplitude, reversible, and repeatable bidirectional bending under various external stimuli, such as temperature, salt solutions, and organic solvents. The excellent multiresponsive bending behaviors were further demonstrated in biomimetic manipulators which show excellent “capture–transport–release” complex mechanical motions. Additionally, on the basis of the salt response of polyVBIPS, a conceptual hydrogel–organogel bilayer pipe has been designed for the removal of oil/seawater pollution. Overall, this work provides a new strategy for enhancing interfacial strength in immiscible binary systems. A new hydrogel–organogel bilayer film has been developed with great potential for use in the applications of soft robots and sewage treatment systems.
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