Soft ‘e-skin’ Sends Brain-Talking Nerve Impulses

Human skin’s mechanoreceptors can feel the delicate weight of a butterfly, the heat of a nearby flame or a cool drink, whether a hand is raised in a fist or a peace sign, and a loved one’s pulse with a gentle touch. Engineers seeking to construct artificial e-skin have created soft, flexible materials that replicate each of these extraordinary senses, but they have never created a single sheet with skin-like materials that can directly talk to the brain—until now.

Stanford University researchers have developed soft integrated circuits that convert pressure or temperature into electrical pulses that the brain can read. Previous efforts required rigid electronics to convert the sensed signal into electrical pulses. Researchers seek to direct the signals to peripheral nerve wireless communication chips to allow amputees to operate prosthetic limbs. New implantable or wearable medical gadgets may be used.

“We’ve been working on a monolithic e-skin for some time. The hurdle was not so much finding mechanisms to mimic the remarkable sensory abilities of human touch, but bringing them together using only skin-like materials,” said Zhenan Bao, K.K. Lee Professor in Chemical Engineering and senior author of the study appearing in the journal Science.

“Much of that challenge came down to advancing the skin-like electronic materials so that they can be incorporated into integrated circuits with sufficient complexity to generate nerve-like pulse trains and low enough operating voltage to be used safely on the human body,” said Weichen Wang, a doctoral candidate in Bao’s lab, who is a first author of the paper. Wang has been working on this prototype for 3 years.

Tech layers
Soft integrated circuits that resemble sensory receptors and run effectively at low voltage were the target. Wang’s first attempts required 30+ volts and didn’t work well. “This new e-skin runs on just 5 volts and can detect stimuli similar to real skin,” he said.

New prosthetic limbs that restore mobility and functions like gripping and offer sensory feedback (proprioception) will need artificial skin. The sensory-skin material must also stretch and return repeatedly without losing its nerve-like electrical properties.

The researchers devised a tri-layer dielectric structure that increased electrical charge carrier mobility by 30 times compared to single-layer dielectrics, permitting low-voltage circuits. Nitrile, used in surgical gloves, is one of the tri-layer layers. Most e-skin has numerous layers of skin-like materials. Organic nanostructure networks in each layer send electrical signals even when stretched. These networks can detect pressure, temperature, strain, and chemicals.

Sensory inputs each have integrated circuits. Then all sensory layers must be sandwiched into a monolithic material that does not delaminate, tear, or lose electrical function.

Each electronic layer is a few tens to hundred nanometers thick, and the finished substance of half a dozen layers is less than a micron.

“But that’s actually too thin to be handled easily, so we use a substrate to support it, which brings our e-skin to about 25-50 microns thick—about the thickness of a sheet of paper,” Bao said. “It is in a similar thickness range of the outer layer of human skin.”

Future innovations
The first technology that integrate sensing and all the desired electrical and mechanical properties of human skin in a soft, durable form could be employed in next-generation prosthetic skins and new human-machine interfaces to deliver a human-like experience of touch.

Bao, Wang, and crew are now adding wireless functionality, intricacy, and brain-body interfaces to their prototype.

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