The development of stimuli-responsive soft materials gives rise to soft robotics, which are capable of flexible deformation to adapt to diverse situations. Compared to conventional rigid robotics, soft robotics are more analogous to muscle in vivo, showing broad prospects in biomimetic field. Among all the stimuli, light stands out for its accurate, remote, contactless control over actuation on the basis of mature modulation technique including wavelength, intensity, polarization etc. Current light-driven actuators generally have poor mechanical properties and slow deformation rate and need extra stimuli to recover, all of which limit their application.
Professor Haifeng Yu’s group in the College of Engineering at Peking University proposed a brilliant bimorph strategy taking advantage of tough polymer films to hold a layer of light-responsive molecules. Photo-liquefiable azobenzene liquid crystals that liquefy under UV irradiation and recrystal upon the removal of light lead to reversible deformation. While uniaxial-rubbed low density polyethylene aligns the mesogens and enable anisotropic changes. Combined together, the bilayer actuator is capable of reversible spiralization and despiralization, which can grasp various objects mimicking pythons’ predation. J. Mater. Chem. C 2018, 6, 10815-10821)
Figure 1, 2. The composite actuator grasp different objects mimicking pythons’ predation.
In order to solve the two remained problems: one is the detachment of liquefied light-responsive layer despite the capillarity action, and the other is that the recovery process still takes about 5s, the group improved the bimorph strategy by compositing azobenzene liquid crystalline polymers (ALCPs) and Kapton films. In this situation, the ALCPs demonstrate reversible volume expansion under UV irradiation while Kapton is chosen for stronger mechanical property and firm combination. Both the deformation and recovery process take only approximately 1s here. They designed artificial leaves that open under UV irradiation and close without UV light. Besides, the light-intensity-decided bending degree can perfectly mimic the deformation process of leaves during “dawn” or “dark”. (J. Mater. Chem. C 2019, 7, 622-629)
Figure 3, 4. The imitation of circadian rhythm behavior of leaves with coupled bilayer films.
Moreover, they demonstrated that the fast, reversible deformation enabled rhythmic oscillation and accordingly designed a swimmer that exhibited continuous, everlasting and controllable motion at liquid/air interface. Through regulating irradiation intensity and site as well as the swimmers’ aspect ratio, the oscillatory bilayer film can realize similar function of dolphin’s tail fin or pectoral fin. Thus, the highly controllable swimmers have great prospects in accurate transportation for tiny objects such as micro-sensors, detectors or chemical powders as catalysts though more delicate shape design and larger driving force are desired. (Angew. Chem. Int. Ed. 2019, 58(9), 2655)
Figure 5. Illustration of composite films’ rhythmic oscillation mimicking dolphins’ behavior.
Figure 6. One complete cycle of transportation.
Jing Hu, Xiao Li and Shudeng Ma, doctoral candidates at Peking University, are the first authors of the three papers respectively. Prof. Yu is the corresponding author of all the papers and acknowledges financial support from Nation Natural Science Foundation of China.
The above-mentioned three papers are:
(1) Ma, Shudeng; Li, Xiao; Huang, Shuai; Hu, Jing Hu; Yu, Haifeng * "A Light-Activated Polymer Composite Enables On-Demand Photocontrolled Motion: Transportation at the Liquid/Air Interface", Angew. Chem. Int. Ed. 2019, 58(9), 2655-2659
(2) Li, Xiao; Ma, Shudeng; Hu, Jing; Ni, Yue; Lin, Zhiqun; Yu, Haifeng*, “Photo-activated Bimorph Composites of Kapton and Liquid-Crystalline Polymer Towards Biomimetic Circadian Rhythms of Albizia Julibrissin Leaves”, Journal of Materials Chemistry C, 2019, 7(3), 622-629. (DOI: 10.1039/C8TC05186K)
(3) Hu, Jing; Li, Xiao Li; Ni, Yue; Ma, Shudeng; Yu, Haifeng*, Programmable and biomimetic photo-actuator: a composite of photo-liquefiable azobenzene derivative and commercial plastic film, Journal of Materials Chemistry C 2018, 6,10815-10821 (DOI: 10.1039/C8TC03693D)