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Creatures, such as Venus flytrap and hummingbirds, capable of rapid predation through snap‐through transition, provide paradigms for the design of soft actuators and robots with fast actions. However, these artificial "snappers" usually need contact stimulations to trigger the flipping. Reported here is a constrained anisotropic poly(N‐isopropylacrylamide) hydrogel showing fast snapping upon light stimulation. This hydrogel is prepared by flow‐induced orientation of nanosheets (NSs) within a rectangular tube. The precursor containing gold nanoparticles is immediately exposed to UV light for photopolymerization to fix the ordered structure of NSs. Two ends of the slender gel are clamped to form a buckle with bistability nature, which snaps to the other side upon laser irradiation. Systematic experiments are conducted to investigate the influences of power intensity and irradiation angle of the laser, as well as thickness and buckle height of the gel, on the snapping behaviors. The fast snapping is further used to kick a plastic bead and control the switch state. Furthermore, synergetic or oscillated snapping of the gel with two buckles of opposite directions is realized by inclined irradiation of a laser or horizontal irradiation with two lasers, respectively. Such light‐steered snapping of hydrogels should merit designing soft robots, energy harvests, etc. [ABSTRACT FROM AUTHOR]

The development of broad-spectrum anti-bacterial tough hydrogels without antibiotics remains a challenge in biomedical applications. In this study, we have synthesized a novel tough anti-bacterial complex hydrogel based on Cu2+ coordination. A swollen and weak poly(acrylamide-co-4-vinylbenzyl-(trihydroxymethyl-phosphonium)chloride) (P(AAm-co-VBzTHPC)) hydrogel was prepared by the radical copolymerization of AAm and VBzTHPC monomer solutions, followed by immersion in CuSO4 solution to coordinate with Cu2+ to form a strong and tough hydrogel. Fourier transform infrared (FTIR) spectra and X-ray photoelectron spectra (XPS) were used to characterize the coordination structure between phosphorus and oxygen atoms in the VBzTHPC monomer and copper ions. The water content and mechanical properties of the obtained hydrogel varied with gel composition. The prepared toughened hydrogel exhibited excellent anti-bacterial performance because of the introduction of copper ion coordination and the slow release of copper ions, with bacterial viability of 5.1% when the mole fraction of VBzTHPC was 10 mol%. Cell viability when cocultured with the toughened hydrogel was above 85% using the Cell Counting Kit-8 (CCK-8) method, indicating the good biocompatibility of the hydrogel. Compared with the control group experiment in vivo, this tough hydrogel can also promote wound healing, making it a promising candidate for wound dressing. [ABSTRACT FROM AUTHOR]

Forming carboxyl‐Fe3+ coordination bonds as physical crosslinks is an effective strategy to develop tough hydrogels. Considering the inhibition of ferric ions on free‐radical polymerization, these coordination bonds cannot be formed during the reaction, and a soaking process of preformed hydrogels is usually required for mechanical enhancement, resulting in uncontrollable gradient structure, long preparation time, and unnecessary waste of metallic ions. A facile strategy is reported here to prepare tough metallosupramolecular hydrogels by polymerization and in situ formation of coordination bonds with phosphates as the temporal ligands of Fe3+ ions. The phosphate ligands in the precursor solution form coordination complexes with the Fe3+ ions, which avoids the inhibition and ensures the polymerization. After swelling the resultant hydrogel in water, the ligands are substituted by carboxyl groups of the gel matrix due to the variation of local pH. The equilibrated hydrogel with carboxyl‐Fe3+ coordination bonds as the physical crosslinks possesses excellent mechanical properties that can be tuned over a wide range by adjusting the polymer compositions and the concentrations of phosphate ligands and Fe3+ ions. This strategy should be applicable to other systems to enable synthesis of functional hydrogels with Fe3+ ions as the additive toward specific applications in engineering and biomedical fields. [ABSTRACT FROM AUTHOR]

We demonstrate the self-supporting 3D printing of complex hydrogel structures based on simultaneous crosslinking reactions while printing. The printing strategy is based on the Schiff base reaction and metal coordination with a two-step crosslinking process. The printing ink was first prepared by dispersing oxidized sodium alginate (OSA) and adipic dihydrazide (ADH) in poly(acrylamide-co-acrylic acid) (P(AAm-co-AAc)) polymer solutions, and was mixed and printed into 3D structures with an extrusion-based coaxial printing platform. Because of the rapid chemical crosslinking reaction between the aldehyde group in OSA and the hydrazide group in ADH, the printed structures can be solidified quickly, and are further crosslinked by forming carboxyl-Fe3+ coordination complexes to enhance their mechanical properties. The dynamic time-sweep rheological properties of the gel composed of different proportions of OSA and ADH were systematically investigated for the characteristic gelation time, and compression tests were carried out to measure the mechanical properties of the gel composed of OSA and ADH. Combining the gelation time and mechanical properties of the gel, the weight ratio of OSA and ADH was selected as 1 : 0.44 for an optimized setting in the subsequent printing. To evaluate the printability of inks, the material formula and printing parameters were systematically varied. The ink exhibited a wide printing range, self-supporting properties, and good printability. Tensile tests of the printed single fiber crosslinked by Fe3+ show that its strength and toughness are tunable. Complex 3D structures such as pyramids, cylinders, and noses were constructed to demonstrate the printability of the ink. This printing method provides a facile approach for tough hydrogel fabrication without changing the rheological properties of the ink or sacrificing the ultimate mechanical properties of the printed materials. [ABSTRACT FROM AUTHOR]