A gel has been programmed to change shape on its own, without any external triggers.

Credit: X. Hu et al./Nature Commun.

Most shape-shifting materials require a shift in conditions — for example, temperature or humidity — to flip between two forms. But Andrey Dobrynin at the University of Akron in Ohio, Sergei Sheiko at the University of North Carolina at Chapel Hill and their team created a polymer hydrogel with two types of crosslink: permanent covalent bonds that allow the material to recover its initial shape after deformation, and hydrogen bonds that temporarily hold it in a different configuration. By varying factors including the speed at which the temporary deformation occurs and the length of time it is held in place, the researchers could control how rapidly the material regained its shape, without the need for a trigger. Using this approach, the team created an artificial flower with individually programmed petals that unfolded in sequence (pictured).

Such a material could have applications in devices such as medical implants, the authors say.

Nature Commun. 7, 12919 (2016)