Emerging Tech

E-Skin for Robots Could Lead to Touchy-Feely Prosthetics for Humans

Robots can be made made strong, robots can be made tireless, but a big problem with robots so far is that they can’t be made to have a sense of touch as humans do.

The same issue challenges designers of prosthetic limbs. Imitating the motor movements of joints and muscles is one thing, but imitating human skin with all its myriad ways of detecting sensation is quite another.

Now, a team at the University of California, Berkeley, has developed a pressure-sensitive electronic material from semiconductor nanowires that they’re calling “e-skin.” It is the first such material made out of inorganic single crystalline semiconductors, according to the researchers.

An optical image of a fully fabricated e-skin device with nanowire active matrix circuitry. Each dark square represents a single pixel. Credit: Ali Javey and Kuniharu Takei, UC Berkeley

Inorganic Advantage

Previous attempts to develop artificial skin have utilized organic materials. Those materials tends to be flexible and easier to work with than inorganic ones. However, organic materials do not conduct electricity well; thus, any device that used that type of artificial skin had to also use relatively high voltages to ensure proper communication of the signals.

“Inorganic materials, such as crystalline silicon, on the other hand, have excellent electrical properties and can operate on low power. They also are more chemically stable,” explained team leader Ali Javey, associate professor of electrical engineering and computer sciences at Berkeley.

Using miniaturized wires made of inorganic materials can allow researchers to create a surface that is both very conductive and also very flexible. This is the kind of surface required for such applications as prosthetic limbs — like hands — that require great sensitivity.

The Berkeley researchers were unavailable for further comment by this story’s deadline.

Completing the Feedback Loop

Almost all of the research work done so far in prosthetics has focused on motor movements, Steven Hsiao, professor of neuroscience at Johns Hopkins University, told TechNewsWorld.

Scientists now are turning their attention to the other part of the feedback loop.

“How to feed those signals back to the subjects so they feel the limb,” said Hsiao, “has been a problem for a long time.”

The technology innovations required to complete the feedback loop involve excellent sensor arrays — which the Berkeley project has developed — along with ways to get the signals created by those arrays back to the nervous system of the person using the prosthetic.

Thus, the e-skin research represents “the very beginning” of the journey toward prosthetic limbs that can both accomplish tasks and also be modulated by a user who is receiving information back from the device, explained Hsiao.

Lint Roller in Reverse

To create the e-skin, the researchers started by growing the germanium/silicon nanowires on a cylindrical drum, which was then rolled onto a sticky substrate. They described it as a “lint roller in reverse.”

The substrate used was a polyimide film, but the technique could be used with a variety of materials, including other plastics, paper or glass, the team said. The nanowires were deposited by the drum onto the substrate in an orderly fashion, and then the substrate was processed into thin, flexible sheets.

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 e-skin developed by the team can detect pressure from 1 to 15 kilopascals. That’s about the amount of pressure used to complete daily activities like typing on a keyboard or holding an object, the researchers said.

Such sensitivity is necessary to create a human prosthetic hand that can distinguish between the force necessary to, say, hold an egg without cracking it and the force necessary to lift a heavy pot without dropping it.

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