Researchers at Cornell University have developed a hybrid material that combines stiff metal with soft rubber foam, and can shift between the properties of both.
To make it, an alloy of bismuth, indium, and tin (called Field's metal) is mixed with a porous silicone foam. The foam is dipped in the molten alloy and then placed into a vacuum so that the air in its pores is removed and replaced by the metal.
The alloy has a low melting point of just 62C, and when heated above this level it'll melt - making the whole substance soft and pliable. When it cools, it regains its rigidity in whatever shape it was bent to. Once re-heated, it again softens and regains its original shape.
'A skeleton when you need it'
"It's sort of like us – we have a skeleton, plus soft muscles and skin," said Rob Shepherd, who lead the research team that developed the substance. "That's what this idea is about, to have a skeleton when you need it, melt it away when you don't, and then reform it."
The material was specifically designed to be biocompatible, containing no toxic substances like lead that are often found in other alloys.
"Sometimes you want a robot, or any machine, to be stiff," said Shepherd, whose group is also working on electroluminescent skin. "But when you make them stiff, they can't morph their shape very well. And to give a soft robot both capabilities, to be able to morph their structure but also to be stiff and bear load, that's what this material does."
As well as soft robots, the discovery could also pave the way for all kinds of other shape-shifting devices, like an aircraft that could alter wing shape mid-flight to adjust for air conditions. You can see it shift from one shape to another in the video below:
The details of the substance were published in the journal Advanced Materials.