Artificial skin could give robots a lighter touch

Posted by Emma Woollacott

Two US teams have developed artificial skins that are sensitive enough to detect the touch of a butterfly.       

 Engineers at the University of California, Berkeley, have developed what they call e-skin, a pressure-sensitive electronic material created from semiconductor nanowires.

"The idea is to have a material that functions like the human skin, which means incorporating the ability to feel and touch objects," said research leader Ali Javey.

"Humans generally know how to hold a fragile egg without breaking it. If we ever wanted a robot that could unload the dishes, for instance, we'd want to make sure it doesn't break the wine glasses in the process. But we'd also want the robot to be able to grip a stock pot without dropping it."
               
The material could also one day restore the sense of touch to patients with prosthetic limbs.

Previous attempts to develop an artificial skin relied upon organic materials because they are flexible and easier to process.

"The problem is that organic materials are poor semiconductors, which means electronic devices made out of them would often require high voltages to operate the circuitry," said Javey. "Inorganic materials, such as crystalline silicon, on the other hand, have excellent electrical properties and can operate on low power. They are also more chemically stable"

Unfortunately, though, they've been inflexible and brittle.

But the UC Berkeley team used a new fabrication technique, growing the germanium/silicon nanowires on a cylindrical drum, which was then rolled onto a sticky substrate. As the drum rolled, the nanowires were neatly deposited, or 'printed', onto the substrate.

For the e-skin, the engineers printed the nanowires onto an 18-by-19 pixel square matrix measuring seven centimeters on each side. Each pixel contained a transistor made up of hundreds of semiconductor nanowires. The transistors were then integrated with a pressure-sensitive rubber layer to provide the sensing functionality. The matrix required less than five volts of power to operate and maintained its robustness even after being bent 2,000 times.

They found the e-skin could detect pressure from 0 to 15 kilopascals - comparable to the force used for typing or holding an object.

Meanwhile, researchers at Stanford have used a different technique, using a sheet of the elastic polymer polydimethylsiloxane (PDMS) with holes cut out. When the PDMS is squashed, its capacitance is altered, rendering it sensitive to pressure.

The team says it's so sensitive that it's able to detect a butterfly landing on it.