A material that can go from stiff to plastic to change its shape and back to stiff has been developed by American researchers.
A mixture of a metallic alloy and soft porous rubber foam, the hybrid material could find many applications in soft robotics but possibly also pave the way for futuristic aircraft that could change or adjust the shape of its wings in mid-flight.
The researchers used the so called Field’s metal - an alloy consisting of bismuth, indium, and tin – that melts at 62 °C and mixed it with porous silicone foam. This low melting point of Field’s metal is key for the shape-changing properties.
"It's sort of like us - we have a skeleton, plus soft muscles and skin," said Rob Shepherd, who led the research group based at Cornell University, the USA. "Unfortunately, that skeleton limits our ability to change shape - unlike an octopus, which does not have a skeleton. 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 elastomer foam, with pores about 2mm in diameter, is dipped into the molten metal and placed into a vacuum chamber so that the air in the foam's pores is removed and replaced by the alloy. The size of the pores enables the researchers to change the properties of the material between stiffer and more flexible. In tests, researchers were able to repeatedly switch between the stiff and flexible mode by re-heating and re-cooling the material.
"Sometimes you want a robot, or any machine, to be stiff. But when you make them stiff, they can't morph their shape very well,” said Shepherd. “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."
The research will appear on the cover of the upcoming issue of the journal Advanced Materials.