Giving robots skin as soft and sensitive as a cosmetics model is difficult when it must be built using tough, inflexible electronics. But Japanese researchers have created super-stretchy circuits from a new elastic conductor material that could be the solution.
The rubbery conductor was developed by Takao Someya's team at the Univ. of Tokyo. They aim to produce "e-skin"—an affordable layer of pressure and temperature sensors flexible enough to completely cover a robot without limiting its movement.
"Without human skin-like sensitivity, robots cannot be used in everyday life. Imagine the danger if a robot did not recognize when it had accidentally bumped into a young child," says Someya.
Some stretch
The team previously made flexible sensor sheets punched with holes that could be stretched by 25%. But that isn't stretchy enough, says Someya. Some parts of a skin, for example over the joints, need to be more deformable.
A solid strip of their new material can be stretched by 38% without any drop in conductivity. A net of it was still working after being stretched by 134%, albeit with a drop in conductivity.
Integrated circuits, like those needed to make pressure sensors, made from the material still function when being stretched 70%. Regular wire snaps if it is stretched by more than 1-2%.
Rubber mix
The new material is made by mixing conducting carbon nanotubes with rubber.
The team used a novel production technique that uses an ionic liquid—consisting of charged ions and not molecules like most liquids—to prevent the nanotubes clumping together.
That made it possible to add more nanotubes without fear that a high density of them would form lumps. The final material is around 20% nanotubes by weight.
Current flows through the material by hopping from nanotube to nanotube. Someya explains that the time during which a route across the material exists when it is stretched and the tubes are pulled apart is maximised by using long nanotubes.
Wearable gadgets
"In any area where you need electronics that can stretch and bend without compromising performance, for example in wearable electronics to monitor movement, this is going to be quite a breakthrough," says Gordon Wallace of the Univ. of Wollongong, Australia.
Stretchable conductors made from wavy gold wires embedded in rubber have been made before and are better conductors, a property necessary for high-speed stretchy electronics.
But the nanotube-based conductor is likely to be more physically robust, a requirement for applications like an e-skin in which speed is not critical, says Someya.
It is also cheaper to build, he adds, because the rubber and nanotube solution can be printed out in sheets.
