A Bioinspired Swimming and Walking Hydrogel Driven by Light‐Controlled Local Density

摘要

A hydrogel exhibits a real-time depth-controllable swimming motion via light-mediated modulation of local density to mimic the volume changes found in the bladders of fish. Moreover, other motions, e.g., rolling, somersaulting, and bipedal-like walking, can also be realized by designing or combining gel shapes, and the location of light. An actuator is a kind of device that can transform a certain energy source into motion, which has been applied in a lot of fields.1 Many types of motions have been realized by researchers, e.g., crawling,2 bending/folding,[1, 3] rolling,4 jumping,5 walking,[3, 6] diving–floating,7 microjeting in aqueous solutions,8 moving on water surface,9 etc. Among the motions, diving–floating, microjeting in aqueous solutions, and moving on water surface can be included in “swimming” motions.7-10 Gao et al. first fabricated a cooperating device by combining a pH-responsive surface with hydrogen peroxide-responsive platinum on a nickel foam cube, which displays a diving–floating motion in H2O2 solution.[7] Afterwards, researchers further fabricated thermosensitive diving–floating devices for matter transportation between three phases,[7] or used diving–floating devices to convert chemical energy into electricity.[7] However, a real swimming motion in nature should be depth-controllable, and the reported “swimming” motions cannot attain this. Here, we show a hydrogel which exhibit a real-time depth-controllable swimming motion based on a new concept of light-controlled local density inspired by fish swimming. The swimming depth of the gel can be instantly controlled by near infrared (NIR) laser; moreover, other motions, e.g., rolling, somersaulting, and “bipedal-like” walking, can also be realized by designing or combining gel shapes, different gel compositions, and the location of NIR laser. Our results demonstrate that the velocity during swimming and other motions can also be modulated by NIR laser. We anticipate that the new concept of light-controlled local density of materials will be a starting point for realizing complicated swimming motions and other various motions of actuators, which will be applied in mini-robots in future. Furthermore, the actuators and mini-robots based on this concept could be composites of polymers, ceramics, or even metals, and the environment for motions could be other transparent liquids, not limited to water. As fishes swim, they modulate their buoyancy to control depth by changing the volume of swimming bladders, i.e., varying the local density of their bodies. Inspired by swimming bladders of fish, the concept of designing the swimming gel is illustrated in Scheme 1. The material is made from three ingredients: monomer A (ρA < ρwater), monomer B (ρB > ρwater), and NIR absorbent C (small amount). After copolymerization, the density of the obtained material is above ρwater by modulating the ratio of A and B, and the material contains a crystal phase of A. If temperature is higher than the melting point (Tm) of A, crystals melt, decreasing ρ. As soon as the decreasing ρ is below ρwater, the material will float to water surface. Because the NIR laser absorbent can absorb laser energy, increasing temperature, the material will exhibit not only a thermosensitive floating/diving motion but also a light-controlled swimming, rolling, somersaulting, and walking motions by modulating NIR laser location and time. Here, stearyl acrylate (SA) is chosen as mono­mer A, whose density is 0.8 g cm−3;11 methacrylic acid (MA) is monomer B, whose density is 1.0153 g cm−3 (20 °C);11 reduced graphene oxide (RGO) is used as NIR absorbent, whose concentration is fixed on 0.6 wt%. The density of the copolymer can be controlled above ρwater at room temperature by modulating compositions. The obtained copolymer of SA and MA contains a crystal phase resulting from the crystallization of the long side chains of SA. The long alkane side chains are arranged in a dense highly ordered microscopic structu