Using a process described as “a lint roller in reverse,” engineers from the University of California, Berkeley, have created a pressure-sensitive electronic artificial skin from semiconductor nanowires. This “e-skin,” as it’s called, could one day be used to allow robots to perform tasks that require both grip and a delicate touch, or to provide a sense of touch in patients’ prosthetic limbs.
"Humans generally know how to hold a fragile egg without breaking it," said Ali Javey, associate professor of electrical engineering and computer sciences and head of the UC Berkeley research team. "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."
Some previous attempts at artificial skin have used organic materials, as they are flexible and relatively easy to process. Their main drawback has been that they are poor semiconductors, so devices using them would require large amounts of power. The e-skin, however, is made from inorganic single crystalline semiconductors. It requires only a small amount of power, is reportedly more chemically stable than organic skin, and maintains a high degree of flexibility thanks to its wire strip construction.
The team created the e-skin by growing germanium/silicon nanowires on a cylindrical drum, which was then rolled across a sticky polyimide film (although they stated that a variety of other materials would also work). The nanowires stuck to the film in a controlled, orderly fashion, forming the platform for the complete e-skin. The researchers also experimented with another process, in which the nanowires were first grown on a flat substrate, then transferred to the film by rubbing.
The test sheet of e-skin measured 7 x 7 centimeters (2.76 inches), and was divided up into an 18 x 19 pixel matrix – each pixel contained a transistor, composed of hundreds of nanowires. The outside surface of the sheet was then coated with pressure-sensitive rubber. Using just 5-volts of power, the e-skin was shown to be able to detect pressure ranging from 0 to 15 kilopascals, which is about what people typically use when typing or holding an object. It was also able to maintain its integrity after more than 2,000 bending cycles.
"This is the first truly macroscale integration of ordered nanowire materials for a functional system – in this case, an electronic skin," said study lead author Kuniharu Takei. "It's a technique that can be potentially scaled up. The limit now to the size of the e-skin we developed is the size of the processing tools we are using."
The research is described in the current issue of the journal Nature Materials.