Electronic skin feels the heat, hears the sound

New materials show promise for covering prosthetic limbs, other medical devices

ELECTRONIC TOUCH  A new kind of electronic skin, designed to mimic the skin of human fingertips, can sense pressure, texture, temperature and even sound. 

Park et al/Ulsan National Institute of Science and Technology

A new electronic skin can feel the grain of sand paper, the heat and beat of a person’s pulse — and listen to Richard Feynman’s voice, too.

Rubbery plastic-and-graphene film mimicking the structure of human skin can detect texture, temperature, pressure and sound, Hyunhyub Ko and colleagues report October 30 in Science Advances.

It’s the first time anyone has demonstrated an e-skin that can sense so many different kinds of stimuli, says Stanford University materials scientist Alex Chortos. “That’s the innovative and impressive part of this work.”

Chortos and colleagues recently developed another pressure-detecting e-skin that sends signals directly to mouse brain cells. The cells got the message, too — they dialed activity up or down depending on how hard researchers pushed on the skin, Chortos’ team reported in the Oct. 16 Science. That work offers a blueprint for scientists to actually “bridge electronics with biology,” says Wenlong Cheng, a chemical engineer at Monash University in Australia.

SKIN DEEP The ultra-sensitive skin of people’s fingertips relies on a ridged surface and interlocking layers to detect different kinds of stimuli. Designing electronic skin with similar structures can boost sensitivity. Park et al/Ulsan National Institute of Science and Technology
Both Chortos’ skin-to-cell-communication system and Ko’s super-sensing e-skin bring lifelike artificial skins even closer to practical use. “In the future, we could combine these techniques for real, operational electronic skin,” says Ko, a materials scientist at the Ulsan National Institute of Science and Technology in South Korea.

One day, such an e-skin could cover prosthetic limbs and plug directly into people’s nerve cells, he says, letting people know if they were touching something hot or rough or sharp — just like real skin does. The artificial skin could also form the basis for soft, wearable medical devices.

Over the last several years, scientists have invented an assortment of electronic-skin elements, from different soft materials to new kinds of sensors. Some sensors can recognize more than one type of stimuli, Cheng says, but only under just the right conditions. E-skins are still “far from having the capabilities that human skin has,” he says — but the new work brings the technology closer.

Ko and colleagues designed their e-skin to detect many kinds of signals by mimicking the ultrasensitive skin of human fingertips. The researchers placed a soft ridged film over bumpy plastic-and-graphene sheets about the thickness of a few layers of Saran Wrap. Touching the e-skin pressed electrodes on the bumpy sheets together, causing current to flow through the device, which was hooked up to a machine that measures electrical signals. The amount of current depended on how much the bumps squished together, giving the researchers a sensitive way to gauge pressure.

Heating the e-skin also generated a current, showing that it could sense temperature, too. A strip of the e-skin placed on a person’s wrist let the researchers simultaneously measure skin temperature and blood pressure.

The e-skin’s ridges help it detect texture. When researchers skimmed the skin over glass or sandpaper, the ridges vibrated in different patterns detectable by the skin’s sensors. Sound waves also made the e-skin vibrate, so that it could “hear” noise from a speaker playing one of Richard Feynman’s famous physics lectures (“There’s Plenty of Room at the Bottom”). The e-skin converted his words into electrical signals, and sent them to a machine that let researchers judge how well the e-skin sensed sounds. 

It worked even better than an iPhone’s microphone, Ko says. Cheng thinks the e-skin could serve in soft, wearable hearing aids. Unlike conventional aids, he says, soft devices are comfortable because they mold to human skin.

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