Human skin’s mechanoreceptors are known to have the remarkable ability to perceive the subtle weight of a butterfly, sense the warmth emanating from a nearby flame or a refreshing drink, distinguish between a raised fist and a peace sign, and discern the pulse of a loved one through a gentle touch.

While advancements in the field of medical technology have been able to create materials that are flexible and soft to simulate each of these amazing senses, they haven’t succeeded in making even one skin-like sheet that can communicate with the brain. There’s good news coming from Stanford University’s Bao Research Group.

In a major breakthrough, researchers at Stanford University have developed e-skin, also known as, electronic skin, which is soft and flexible, with the ability to simulate touch and operate effectively at low voltage.

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In earlier attempts, stiff electronics were needed to turn the perceived signal into electrical pulses that the brain could understand.

The Mechanics Behind E-Skin: How Does It Work?

The team at Stanford University has developed soft integrated circuits that transform sensed temperatures or pressures into electrical signals the same as nerve impulses to communicate with the brain. Earlier attempts needed rigid electronics to translate the sensed signal into electrical pulses that the brain can read.

The researchers envision a future in which those signals could be used to control prosthetic limbs for amputees by implanting wireless communication chips in the peripheral nerve. Modern implantable devices are another example of potential applications.

Embarking on the Path of Monolithic E-Skin:

Zhenan Bao, K.K. Lee Professor in Chemical Engineering and senior author of the study, says,

“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.”

Weichen Wang, a doctoral candidate in Bao’s lab, also the first author of the paper, says,

“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.”

The Goal of Monolithic E-Skin

The objective was to create a soft integrated circuit that could effectively operate at low voltage and emulate the functioning of sensory receptors. Unfortunately, Wang’s initial attempts required more than 30 volts and were unable to produce enough circuit functionality. Wang stated that the new e-skin can detect stimuli similarly to actual skin and uses just 5 volts of power. Modern prosthetic limbs that not only restore mobility and functions, including grasping but also offer sensory feedback (proprioception) that lets the user operate the device precisely, will depend heavily on artificial skin.

Modern prosthetic limbs will rely significantly on artificial skin to provide sensory feedback (proprioception) that enables the user to manipulate the device precisely while also restoring movement and functions, including gripping.

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The Layer Technology

To enable the operation of the circuits at low voltage, a tri-layer dielectric structure was developed that helped boost the movement of electrical charge carriers by 30 times when compared to a single-layer dielectric.

It would interest you to know that nitrile, the rubber used in surgical gloves, is one of the layers of the tri-layer.

• Most e-skin is constructed from numerous layers of materials that resemble skin. Networks of organic nanostructures that can transfer electrical signals even when stretched are integrated into each layer. Most e-skin is constructed from numerous layers of materials that resemble skin.
• There is a separate integrated circuit for each sensory input.
• To ensure that a single monolithic material does not tear, delaminate, or lose electrical function, all the different sensory layers must be sandwiched together.

The total product of about six to eight layers is less than a micron thick, with each electronic layer being only a few dozen to hundred nanometers thick.

“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.”

The Next-Level Advancement

The technology is the first to integrate sensing with all the desired electrical and mechanical characteristics of human skin in a soft, durable form that might be employed in cutting-edge human-machine interfaces and future prosthetic skins to deliver a sense of touch similar to that of a human.

With the completion of the prototype, and to elevate the complexity and scalability of the technology, Bao, Wang, and their team are now working on ways to connect to the brain and other parts of the body’s peripherals as well as adding wireless functionality.

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