A helical configuration confers a great advantage in artificial muscle due to great movement potential. However, most helical fibers are exposed to a high temperature to produce the coiled helical structure. Hence, thermoset polymercomposed hydrogels are difficult to fabricate as helical fibers due to their thermal degeneration. Here, we describe a self-helical hydrogel fiber that is produced without thermal exposure as a glucose-responsive artificial muscle. The sheath−core fiber was spontaneously transformed into the helical structure during the swelling state by balancing the forces between the untwisting force of the twisted nylon fiber core and the recovery force of the hydrogel sheath. To induce controllable actuation, we also applied a reversible interaction between phenylboronic acid and glucose to the selfhelical hydrogel. Consequently, the maximum tensile stroke was 2.3%, and the performance was six times greater than that of the nonhelical fiber. The fiber also exhibited tensile stroke with load and a maximum work density of 130 kJ/m 3 . Furthermore, we showed a reversible tensile stroke in response to the change in glucose level. Therefore, these results indicate that the self-helical hydrogel fiber has a high potential for use in artificial muscles, glucose sensors, and drug delivery systems.

Spider silks represent stretchable and contractible fibers with high toughness. Those tough fibers with stretchability and contractibility are attractive as energy absorption materials, and they are needed for wearable applications, artificial muscles, and soft robotics. Although carbon-based materials and poly(vinyl alcohol) (PVA) composite fibers exhibit high toughness, they are still limited in low extensibility and an inability to operate in the wet-state condition. Herein, we report stretchable and contractible fiber with toughness that is inspired by the structure of spider silk. The bioinspired tough fiber provides 495 J/g of gravimetric toughness, which exceeds 165 J/g of spider silk. Besides, the tough fiber was reversibly stretched to-80% strain without damage. This toughness and stretchability are realized by hybridization of aligned graphene oxide/multiwalled carbon nanotubes in a polyurethane matrix as elastic amorphous regions and β-sheet segments of spider silk. Interestingly, the bioinspired tough fiber contracted up to 60% in response to water and humidity similar to supercontraction of the spider silk. It exhibited 610 kJ/ m3 of contractile energy density, which is higher than previously reported moisture driven actuators.

Controllable and miniaturised mechanical actuation is one of the main challenges facing various emerging technologies, such as soft robotics, drug delivery systems, and microfluidics. Here we introduce a simple method for constructing actuating devices with programmable complex motions. Thermally responsive hydrogels based on poly( N -isopropylacrylamide) (PNIPAM) and its functionalized derivatives (f-PNIPAM) were used to control the lower critical solution temperature (LCST) or the temperature at which the gel volume changes. Techniques for ultra-violet crosslinking the monomer solutions were developed to generate gel sheets with controllable crosslink density gradients that allowed bending actuation to specified curvatures by heating through the LCST. Simple molding processes were then used to construct multi-transform devices with complex shape changes, including a bioinspired artificial flower that shows blossoming and reverse blossoming with a change in temperature.

Underwater robots and vehicles have received great attention due to their potential applications in remote sensing and search and rescue. A challenge for micro aquatic robots is the lack of small motors needed for three-dimensional locomotion in water. Here, we show a simple diving and surfacing device fabricated from thermo-sensitive poly(N-isopropylacrylamide) or a poly(N-isopropylacrylamide)-containing hydrogel. The poly(N-isopropylacrylamide)-containing device exhibited fast and reversible diving/surfacing cycles in response to changing temperature. Modulation of the interaction between poly(N-isopropylacrylamide) chains and water molecules at temperatures above or below the lower critical solution temperature regulates the gel density through the swelling and de-swelling. The gel surfaced in water when heated and sank when cooled. We further showed reversible diving/surfacing cycles of the device when exposed to electrical and ultrasonic stimuli. Finally, a small electrically heated gel was incorporated into a miniature submarine and used to control the diving depth. These results suggest that the poly(N-isopropylacrylamide)-containing device has good potential for underwater remote-controlled micro aquatic robots.

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