A soft robotic fish can quickly swim and turn with a fully integrated onboard system for power and remote control.

No abstract

An effective approach to develop a novel macroscopic anisotropic bilayer hydrogel actuator with on–off switchable fluorescent color‐changing function is reported. Through combining a collapsed thermoresponsive graphene oxide‐poly(N‐isopropylacrylamide) (GO‐PNIPAM) hydrogel layer with a pH‐responsive perylene bisimide‐functionalized hyperbranched polyethylenimine (PBI‐HPEI) hydrogel layer via macroscopic supramolecular assembly, a bilayer hydrogel is obtained that can be tailored and reswells to form a 3D hydrogel actuator. The actuator can undergo complex shape deformation caused by the PNIPAM outside layer, then the PBI‐HPEI hydrogel inside layer can be unfolded to trigger the on–off switch of the pH‐responsive fluorescence under the green light irradiation. This work will inspire the design and fabrication of novel biomimetic smart materials with synergistic functions.

Inspired by the assembly of Lego toys, hydrogel building blocks with heterogeneous responsiveness are assembled utilizing macroscopic supramolecular recognition as the adhesion force. The Lego hydrogel provides 3D transformation upon pH variation. After disassembly of the building blocks by changing the oxidation state, they can be re-assembled into a completely new shape.

For a hydrogel coating on a substrate to be stable, covalent bonds polymerize monomer units into polymer chains, crosslink the polymer chains into a polymer network, and interlink the polymer network to the substrate. In existing methods of hydrogel coating, the three processespolymerization, crosslinking, and interlinking-usually concur. This concurrency is unnecessary and hinders the widespread applications. In particular, many hydrogels are made by free-radical polymerization, involving toxic monomers, toxic initiators, and oxygen-free environment. For example, in the free-radical polymerization of a covalently crosslinked polyacrylamide hydrogel, when subject to UV light, the vinyl groups of acrylamide monomer and N,N′-methylenebisacrylamide crosslinker are activated concurrently. The former results in polyacrylamide chains and the latter results in a polyacrylamide network. When the substrate is involved, polymerization of monomers, crosslinking of poly mer chains, and interlinking between the hydrogel and substrate proceed concurrently. Since free radical polymerization is sensitive to oxygen, molding is usually required. Besides, molding is also used to control the shape and thickness of hydrogel coating. Depending on the geometry of the substrate, coating can be technically challenging for highly curved surfaces (e.g., 1D structures), or even impossible for hollow or cage structures. Furthermore, the monomers (e.g., acrylamide, acrylic acid, etc.) are usually toxic. Consequently, free-radical polymerization is unsuitable for everyday operation.For a hydrogel coating on a substrate to be stable, covalent bonds poly merize monomer units into polymer chains, crosslink the polymer chains into a polymer network, and interlink the polymer network to the substrate. The three processes-polymerization, crosslinking, and interlinking-usually concur. This concurrency hinders widespread applications of hydrogel coatings. Here a principle is described to create hydrogel paints that decouple polymerization from crosslinking and interlinking. Like a common paint, a hydrogel paint divides the labor between the paint maker and the paint user. The paint maker formulates the hydrogel paint by copolymerizing monomer units and coupling agents into polymer chains, but does not crosslink them. The paint user applies the paint on various materials (elastomer, plastic, glass, ceramic, or metal), and by various operations (brush, cast, dip, spin, or spray). During cure, the coupling agents crosslink the polymer chains into a network and interlink the polymer network to the substrate. As an example, hydrogels with thickness in the range of 2-20 µm are dip coated on medical nitinol wires. The coated wires reduce friction by eightfold, and remain stable over 50 test cycles. Also demonstrated are several proofofconcept applica tions, including stimuliresponsive structures and antifouling model boats.A hydrogel-coated substrate unites the superior properties of the substrate (e.g., strength, stiffness, and toughness) and the superior p...