Developing untethered soft robots with programmable configurations is of great value yet a grand challenge in robotics. Recently, this challenge has been successfully attempted by a research team led by Dr Haimin Yao, Associate Professor from the Department of Mechanical Engineering of the Hong Kong Polytechnic University (PolyU), jointly with Dr Tao Chen, Professor from Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences.

The research team started with the transferring of a layer of stacked graphene assembly (SGA) on a polyethylene (PE) substrate. The as-prepared SGA/PE bilayer exhibits curling behavior upon the variation of temperature (Fig. 1a, d). More interestingly, if an SGA/PE bilayer is pre-treated with a tempering process (heating followed by cooling) in a constrained space, it will coil spontaneously into a roll when it is freed (Fig. 1a-c). Surprisingly, the PE layer now is wrapped inside the SGA layer, which is opposite to that of the un-tempered sample. Such unusual morphing behavior of SGA/PE bilayer was found attributed to the asymmetric elastoplastic property of the SGA layer as demonstrated by the molecular dynamics simulations (Fig. 2a, b).

Fig. 1 Illustration of thermal-induced morphing behaviors of SGA/PE bilayer film. (a-c) Schematics showing the process of constrained tempering on SGA/PE film. (d, e) Schematics showing the thermal-induced shape morphing of a pristine SGA/PE film and a tempered SGA/PE film respectively.

Fig. 2 (a) The stress-strain curves of an SGA layer under uniaxial tension and compression as calculated by MD simulation. (b) Schematics showing the deformation mechanisms of SGA under tension and compression. (c) Dependence of curling curvature of SGA/PE on tempering temperature and SGA layer thickness.

It is the asymmetric elastoplastic property of SGA that allows them to prepare a variety of morphing systems with programmable initial morphologies (Fig. 3). These structures can be further assembled to construct more complicated actuation systems, such as artificial mimosa (Movie 1). More importantly, the tempered SGA/PE roll can achieve rolling locomotion under infrared lighting, resulting in an untethered light-driven motor (Movie 2).

Fig. 3 Complex configurations of SGA/PE films programmed by applying patterned SGA and/or localized tempering pretreatment.

Movie 1. An artificial mimosa prepared with SGA/PE bilayer Movie 2. A light-driven motor made by SGA/PE rolling on wavy sands

“Our work not only demonstrates an alternative strategy in creating untethered soft robots and reconfigurable devices but also provides a new philosophy for fabricating 2D material-based smart materials and structures,” said Dr Yao.

This work has been recently published in Nature Communications [“Asymmetric elastoplasticity of stacked graphene assembly actualizes programmable untethered soft robotics”, https://www.nature.com/articles/s41467-020-18214-0]. Ms Yang Gao and Mr Anran Wei, two PolyU ME PhD graduates are the co-first authors and Dr Yao is one of the corresponding authors.