Structures made from materials that can alter their shape in response to changes in their local environment could find use in soft robotics, amongst other applications. Flat, easily fabricated films that can fold up to form 3D structures may be particularly useful from a synthetic standpoint, and such materials can take inspiration from the elegant simplicity of traditional origami.

Now, a team from the University of Massachusetts Amherst and Western New England University led by Ryan Hayward has developed a material that can be pre-programmed to fold and unfold itself into complex origami shapes on cooling and heating. The material consists of a crosslinked poly(N-isoproylacrylamide) (PNIPAM)-based hydrogel copolymer that can swell and contract in response to external stimuli. The hydrogel is sandwiched between, and covalently bonded to, two thin layers of rigid poly(para-methylstyrene) (PpMS). When the material is immersed in a buffer solution, the hydrogel swells but is constrained by the top and bottom layers. Creating gaps in the two PpMS layers using photolithography results in folding of the material at those lines — and whether the gap is in the top or bottom film determines the direction of the fold. The radius of curvature of the fold is determined by the width of the gap, with folding angles between 0 and 180° possible.

Credit: © 2014 WILEY

Several traditional origami designs were patterned onto the trilayer material and shown to fold into recognizable origami objects, such as a bird, with sizes of around 1 mm. As the PNIPAM becomes hydrophobic above a certain temperature, the hydrogel contracts on heating — this phenomenon means that the origami can be reversibly unfolded and refolded. One limitation of the material is that all folds occur at once, rather than being able to fold along the patterned lines in a specific order. Nonetheless, this approach could be used to form structures or small-scale soft robots with function as well as beauty.