Electronic skin has a strong future that stretches, study suggests

A material that mimics human skin in strength, stretch and sensitivity could be used to collect biological data in real time. Electronic skin, or e-skin, can play an important role in next-generation prosthetics, personalized medicine, gentle robotics and artificial intelligence.

“The ideal e-skin mimics the many natural functions of human skin, such as sensing temperature and touch, accurately and in real time,” said KAUST postdoc Yichen Cai. However, it is a challenge to create suitable flexible electronics that can perform such delicate tasks while enduring the knocks and scrapes of everyday life, and every material involved must be carefully designed.

Most e-skins are made by layering an active nanomaterial (the sensor) on a stretchable surface that adheres to human skin. However, the bond between these layers is often too weak, which reduces the durability and sensitivity of the material; alternatively, if it is too strong, flexibility is limited, making the circuit more likely to crack and break.

“The landscape of skin electronics continues to change at a spectacular rate,” said Cai. “The emergence of 2D sensors has accelerated efforts to integrate these atomically thin, mechanically strong materials into functional, durable artificial skins.”

A team led by Cai and colleague Jie Shen has now created a durable e-skin using a hydrogel reinforced with silica nanoparticles as a strong and stretchable substrate and a 2D titanium carbide MXene as a detection layer, bonded together with highly conductive nanowires.

“Hydrogels are made up of more than 70 percent water, making them highly compatible with human skin tissue,” explains Shen. By pre-stretching the hydrogel in all directions, applying a layer of nanowires, and then carefully controlling its release, the researchers created conductive paths to the sensor layer that remained intact even when the material was stretched to 28 times its original size.

Their prototype e-skin could sense objects from 20 centimeters away, respond to stimuli in less than a tenth of a second and, when used as a pressure sensor, distinguish handwritten letters. It continued to work fine after 5,000 distortions, recovering in about a quarter of a second each time. “It’s a remarkable achievement for an e-skin to maintain toughness after repeated use,” says Shen, “mimicking the elasticity and rapid recovery of human skin.”

Such e-skins can track a variety of biological information, such as changes in blood pressure, which can be detected from tremors in the arteries to movements of large limbs and joints. This data can then be shared via WiFi and stored in the cloud.

“One remaining obstacle to the widespread use of e-skins is scaling up high-resolution sensors,” added group leader Vincent Tung; “However, laser-assisted additive manufacturing offers new promise.”

“We envision a future for this technology that goes beyond biology,” added Cai. “Stretchy sensor tape could one day monitor the structural health of inanimate objects, such as furniture and airplanes.”

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