17November2018

E-Skin: Nanotechnology’s Artificial Skin Breakthrough

Artificial skin, dubbed “e-skin” by UC Berkeley researchers, is the first such material made out of inorganic single crystalline semiconductors.

It’s a pressure-sensitive electronic material made from semiconductor nanowires and this sort of touch-sensitive artificial skin would help overcome a key challenge in robotics: adapting the amount of force needed to hold and manipulate a wide range of objects.

Biotech wizards have engineered electronic skin that can sense touch, in a major step towards next-generation robotics and prosthetic limbs.

The lab-tested material responds to almost the same pressures as human skin and with the same speed, they reported Nature Materials.

Important hurdles remain but the exploit is an advance towards replacing today’s clumsy robots and artificial arms with smarter, touch-sensitive upgrades, they believe.

Robots won’t break wine glasses

“Humans generally know how to hold a fragile egg without breaking it,” said Ali Javey from the University of California at Berkeley, who led one of the research teams.

“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 grip the stock pot without dropping it.”

An artist's illustration of an artificial e-skin with nanowire active matrix circuitry covering a hand. It holds a fragile egg illustrating the functionality of the e-skin device for prosthetic and robotic applications. Credit: Ali Javey and Kuniharu Takei‘E-skin’ made of sticky film

The ‘e-skin’ made by Javey’s team comprises a matrix of nanowires made of germanium and silicon rolled onto a sticky polyimide film.

The team then laid nano-scale transistors on top, followed by a flexible, pressure-sensitive rubber.

The prototype, measuring 49 square centimetres, can detect pressure ranging from 0 to 15 kilopascals, comparable to the force used for such daily activities as typing on a keyboard or holding an object.

Super-fast, like human skin

A different approach was taken by a team led by Zhenan Bao from Stanford University in California who has gained a reputation as one of the top women chemists in the United States.

Their approach was to use a rubber film that changes thickness due to pressure, and employs capacitors, integrated into the material, to measure the difference. It cannot be stretched, though.

“Our response time is comparable with human skin, it’s very, very fast, within milliseconds, or thousandths of a second,” Bao said.

“That means in real terms that we can feel the pressure instantaneously.”

Touch the biggest obstacle

The achievements are “important milestones” in artificial intelligence, commented John Boland, a nano-scientist at Trinity College Dublin, Ireland, who hailed in particular the use of low-cost processing components.

In the search to substitute the human senses with electronics, good substitutes now exist for sight and sound, but lag for smell and taste.

Touch, though, is widely acknowledged to be the biggest obstacle. Even routine daily actions, such as brushing one’s teeth, turning the pages of a newspaper or dressing a small child would easily defeat today’s robots.

Improving the new sensors

Bao added important caveats about the challenges ahead. One is about improving the new sensors. They respond to constant pressure, whereas in human skin more complex sensations are possible.

This is because the pressure-sensing cells in the skin can send different frequencies of signal – for instance, when we feel something painful or sharp, the frequency increases, alerting us to the threat.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

In addition, Bao warned, “connecting the artificial skin with the human nerve system will be a very challenging task”.

Handheld device of the future

“Ultimately, in the very distant future, we would like to make a skin which performs really like human skin and to be able to connect it to nerve cells on the arm and thus restore sensation.

“Initially, the prototype that we envision would be more like a handheld device, or maybe a device that connects to other parts of the body that have skin sensation.

“The device would generate a pulse that would stimulate other parts of the skin, giving the kind of signal ‘my (artificial) hand is touching something’, for instance.”

Sending robots into space

In the future, artificial skin could be studded with sensors that respond to chemicals, biological agents, temperature, humidity, radioactivity or pollutants.

“This would be especially useful in applications where we want to send robots into environments, including space, where it could be dangerous for humans to go,” said Bao.

“They could collect information and send it back.”

A longer term goal would be to use the e-skin to restore the sense of touch to patients with prosthetic limbs, which would require significant advances in the integration of electronic sensors with the human nervous system.

The experiment

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

“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, associate professor of electrical engineering and computer sciences and head of the UC Berkeley research team developing the artificial skin. “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.”

A longer term goal would be to use the e-skin to restore the sense of touch to patients with prosthetic limbs, which would require significant advances in the integration of electronic sensors with the human nervous system.

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 that works somewhat like a lint roller in reverse. Instead of picking up fibers, nanowire “hairs” are deposited.

The researchers started by growing the germanium/silicon nanowires on a cylindrical drum, which was 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.

In another complementary approach utilized by the researchers, the nanowires were first grown on a flat source substrate, and then transferred to the polyimide film by a direction-rubbing process.

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. In a nod to their home institution, the researchers successfully mapped out the letter C in Cal.

“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, post-doctoral fellow in electrical engineering and computer sciences. “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 National Science Foundation and the Defense Advanced Research Projects Agency helped support this research.

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