Recent research of hydrogel actuators is still not sophisticated enough to meet the requirement of fast, reversible, complex, and robust reconfiguration. Here, we present a new kind of poly( N-isopropylacrylamide)/graphene oxide gradient hydrogel by utilizing direct current electric field to induce gradient and oriented distribution of graphene oxide into poly( N-isopropylacrylamide) hydrogel. Upon near-infrared light irradiation, the hydrogel exhibited excellent comprehensive actuation performance as a result of directional bending deformation, promising great potential in the application of soft actuators and optomechanical system.

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.

Poly(N-isopropylacrylamide)/Laponite (PNIPAM/Laponite) gradient nanocomposite hydrogel actuators are developed as temperature-controlled actuators with excellent performance using a facile electrophoresis method. The actuators exhibit a rapid (20 s response time) and reversible response, as well as large deformation (bending angle of 231°), which is due to the graded forces generated by the thermo-induced anisotropic shrinkage and extension of the gradient hydrogels. A good linear relationship is observed between the maximum bending angles and the corresponding temperatures for the actuators with fixed sizes. Moreover, the gradient hydrogel with high water content achieved larger actuation angles and shorter response time than the one with low water content, showing an interesting water-promoted effect. Meanwhile, different types of actuators are designed to suit for more specific scenarios, and may be used for various applications, such as biosensing, artificial organization, and transportation of targeted objects.

To balance the requirements of transparency, mechanical strength, stable conductivity, and biocompatibility of traditional electronic conductive hydrogels in intelligent devices is still a formidable challenge. The increase of ionic conductive gels has provided decent transparency, stretchability, and wearability in artificial skins but the dilemma still exists between stability and functionality. This article reports a facile strategy to develop a visual thermosensitive physically and chemically dual cross-linked ion-based conductive hydrogel through in situ free-radical copolymerization, achieving robust mechanical properties, an obvious response, and a multiple sensing process. As an archetypical template, the ion-based conductive hydrogel offers an evaluation and monitoring of electrocardiogram (ECG), which is comparable to commercial electrodes. Intriguingly, such kinds of conductive gels exhibit tunable upper critical solution temperature (UCST) behaviors. Taking advantage of their high temperature responsive accuracy, the visualized qualitative observation for smart response and digitized measurement and calibration for thermal stimulus can be simultaneously achieved. We therefore believe that this work will inspire the design of skinlike sensing materials, promote the preparation of biocompatible and multiple sensing abilities and the synthesis of intelligent hydrogels, and realize their applications in biosensors, wearable devices, and biomedicine.

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