Drawing of artificial e-skin with nanowire circuitry  (Source: Kuniharu Takei and Ali Javey/University of California, Berkeley)
E-skin could help patients with prosthetics feel again

Artificial skin made out of electronic, pressure-sensitive material and semiconductor nanowires has been developed by engineers at the University of California, Berkeley. This sort of finding has the potential to overcome challenges in robotics like "adapting" the amount of force needed to hold and maneuver a variety of objects. 

"Humans generally know how to hold a fragile egg without breaking it," said Ali Javey, associate professor of electrical engineering and computer sciences. "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."

Kuniharu Takei, post-doctoral fellow in electrical engineering and computer sciences and lead author of the study, along with Javey and their team of researchers, are calling the artificial skin "e-skin". It is the first of its kind in that it is made out of inorganic single crystalline semiconductors

"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."

To create the skin, germanium/silicon nanowires were grown on a cylindrical drum, and then rolled onto a sticky substrate, which is a polyimide film. The nanowires are printed onto the substrate as the drum rolls, and flexible, thin sheets are formed where electrical materials can be built. 

Researchers also utilized a different method where nanowires were grown on a flat surface and then undergo a direction-rubbing process in order to transfer to the polyimide film. Nanowires are printed on an 18-by-19 pixel square matrix where each pixel holds a transistor made of hundreds of semiconductor nanowires. For sensing functionality, the transistors are integrated with pressure-sensitive rubber on top of them. 

E-skin could help researchers figure out the amount of force needed hold and maneuver a variety of objects, and could also help patients with prosthetic limbs regain their sense of touch. With e-skin requiring only 5 volts of power to function and lasting beyond 2,000 bending cycles, it's a perfect advancement for the field of robotics. 

"This is the first truly macroscale integration of ordered nanowire materials for a functional system - in this case, an electronic skin," said 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."

This study was published in Nature Materials on September 12. 

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