Polymeric hydrogel actuators refer to intelligent stimuli‐responsive hydrogels that could reversibly deform upon the trigger of various external stimuli. They have thus aroused tremendous attention and shown promising applications in many fields including soft robots, artificial muscles, valves, and so on. After a brief introduction of the driving forces that contribute to the movement of living creatures, an overview of the design principles and development history of hydrogel actuators is provided, then the diverse anisotropic structures of hydrogel actuators are summarized, presenting the promising applications of hydrogel actuators, and highlighting the development of multifunctional hydrogel actuators. Finally, the existing challenges and future perspectives of this exciting field are discussed.

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.

Surface melting temperature is well-known significantly lower than the bulk melting point. But we find that the interior melting temperature in ultrathin nanowires is lower than that of the surface melting . The thermal stability of helical multi-walled cylindrical gold nanowires is studied using molecular dynamics simulations. The melting temperature of gold nanowires is lower than the bulk value, but higher than that of gold nanoclusters. An interesting interior melting is revealed in the gold nanowires and the thermodynamics is closely related to interior structures. The melting starts from the interior atoms, while the surface melting occurs at relatively higher temperature. We propose that the surface melting represents the overall melting in ultrathin metallic nanowires. 61.46.+w, 68.65.-k, 82.60.qr The melting behavior of nanoparticles and nanorods have been demonstrated dramatically different from the bulk both experimentally and theoretically 1-13 . The melting process of a crystalline starts from the surface layer and propagates into the interior. Thus, the surface melting temperature is significantly lower than the bulk melting point. Similarly, people may ask whether the surface melting temperature is lower than the overall melting temperature in clusters and nanowires. Berry considers that "dynamic coexistence" or surface melting happens in the melting process of small clusters before the total melting 3 . For crystalline nanowires, Tosatti found that surface melting temperature for Pb wires was also lower than the total melting temperature 4 . Experimentally, surface melting is involved in the melting process of nanoparticles and nanorods 5-7 . Especially, Schmidt found a broad peak for heat capacity of Na + 139 clusters, maybe implying the coexistence of solid-like and liquidlike phases before the total melting 8 . Two major effects are responsible for these different melting behavior in nanoparticles and nanorods. One is the large surface-tovolume ratio in these nanostructures. The other is quan- * tum confinement effect in finite size systems. Surface atoms have fewer nearest neighbors and weaker binding, which may induce an earlier surface melting behavior. On the other hand, a close relationship between the melting and the structure features is found in the cluster 14,15 .Recent studies demonstrated that ultrathin metallic nanowires have quite different structural and properties from those of bulk, clusters and crystalline nanowires [16][17][18][19][20][21][22][23] .The helical multi-walled cylindrical structures have been widely found in 1-3nm size range of metallic nanowires both experimentally and theoretically [16][17][18][19]22,23 . This kind of novel structure will bring some bizarre melting features different from the above-mentioned systems. To our knowledge, less efforts are focused on their thermodynamics so far, although this kind of ultrathin metallic nanowires have attracted great interests 20,21 .Furthermore, the ultrathin nanowire has some characteristics similar to the cluste...