Abstract
Recently, soft robots, which are made of soft and light organic materials, have attracted much attention because of improved safety for daily interactions with humans. Mechanically responsive materials that can move macroscopically by external stimuli, such as light and heat, have been studied extensively over the past two decades, and they are expected to be applicable to soft robots. Among them, mechanically responsive crystals are attractive in terms of a larger Young’s modulus and faster response speed compared with polymers and gels. However, it is impractical to use one piece of a single crystal as a crystal machine; it is difficult to control the size of crystals and obtain large crystals. Hybridization of crystals with polymers is one way to create actuators with more realistic movements. Herein, we report a hybrid crystal assembly in which plate-like salicylideneaniline crystals are aligned in polymer films by a “rubbing” technique, a new approach which is inexpensive, easy, and applicable to a wide range of crystals and polymers. The hybrid films bent reversibly upon alternate irradiation with ultraviolet and visible light. The hybrid films bent as fast as single crystals, even when larger than single-crystal size, showing great mechanical performance originating from the advantages of both molecular crystals (fast response time) and polymers (large size). This work enriches the development of light-driven hybrid actuators composed of molecular crystals and polymers.
Highlights
Robots are playing increasingly important roles in society
In contrast to hard-bodied robotics, soft robotics has emerged as a new research field (Palagi and Fischer, 2018; Zeng, et al, 2018; Majidi, 2019; Koshima, 2020) because soft-bodied robots, which are basically made of organic materials such as polymers and gels, tend to be soft and light and, more suitable for daily interactions with humans
We demonstrated that the plate-like crystals of N-3,5-di-tert-butylsalicylideneaniline-3nitroaniline (1) in enol form reversibly bent away from the light source under UV light exposure and returned to the original straight shape when illuminated with visible light (Koshima et al, 2011) (Figures 1A–C)
Summary
Robots are playing increasingly important roles in society. We must consider the symbiotic relationship between humans and robots, as robots may help to improve our lives in the near future. In terms of mechanical properties, crystals have the advantage of a larger Young’s modulus (Taniguchi et al, 2019) than polymers and gels as well as a faster response to external stimuli (Naumov et al, 2020); this has motivated the development of crystal soft robots composed of mechanically responsive crystals. It is, impractical to use a single crystal piece as a crystal machine, yet it is difficult to control the size of crystals and obtain crystals with sizes ranging from a few centimeters to several tens of centimeters. The hybrid films bent reversibly upon alternate irradiation with UV and visible light, and we revealed that the hybrid films bent as fast as single crystals, even at sizes larger than a single crystal (Koshima et al, 2011)
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