13 September 2010

Pressure-sensitive crystalline semiconductor used to create artificial skin

by Kate Melville

Engineers at the University of California, Berkeley, have developed a pressure-sensitive electronic material from semiconductor nanowires that will help overcome a key challenge in robotics: adapting the amount of force needed to hold and manipulate a wide range of objects. The artificial skin has been dubbed "e-skin" and the researchers say it is the first such material made out of inorganic single crystalline semiconductors.

"The idea is to have a material that functions like the human skin, which means incorporating the ability to feel and touch objects," said Ali Javey, head of the UC Berkeley research team developing the artificial skin.

"Humans generally know how to hold a fragile egg without breaking it," said Javey. "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."

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. But historically, they have been inflexible and easy to crack. In this regard, works by various groups, including ours, have recently shown that miniaturized strips or wires of inorganics can be made highly flexible - ideal for high performance, mechanically bendable electronics and sensors."

The UC Berkeley engineers utilized an innovative fabrication technique where the nanowires are "grown" on a cylindrical drum and then rolled onto a sticky substrate. The substrate used was a polyimide film, but the researchers said the technique can work with a variety of materials, including other plastics, paper or glass. As the drum rolled, the nanowires were deposited, or "printed," onto the substrate in an orderly fashion, forming the basis from which thin, flexible sheets of electronic materials could be built.

For the e-skin, the engineers printed the nanowires onto an 18-by-19 pixel square matrix measuring 7 centimeters on each side. Each pixel contained a transistor made up of hundreds of semiconductor nanowires. Nanowire transistors were then integrated with a pressure sensitive rubber on top to provide the sensing functionality. The matrix required less than 5 volts of power to operate and maintained its robustness after being subjected to more than 2,000 bending cycles. The researchers demonstrated the ability of the e-skin to detect pressure from 0 to 15 kilopascals, a range comparable to the force used for such daily activities as typing on a keyboard or holding an object.

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Source: University of California - Berkeley