Engineers print wearable sensors directly onto the skin with no heat

Wearable sensors are evolving from watches and electrodes to bendable devices that provide users with far more precise biometric measurements and convenience. Now an international team of researchers has taken the development one step further by printing sensors directly onto human skin without the use of heat.

Led by Huanyu “Larry” Cheng, Dorothy Quiggle’s career development professor at Penn State Department of Engineering and Mechanics, the team published their findings in ACS Applied Materials & Interfaces.

“In this article we report on a simple but universally applicable manufacturing technique using a novel sintering aid layer to enable sensors to be printed directly on the body,” said first author Ling Zhang, a researcher at the Harbin Institute of Technology in China and Cheng’s Laboratory.

Cheng and his colleagues previously designed flexible circuit boards for wearable sensors, but printing directly on the skin has been hindered by the bonding process for the metal components in the sensor. This process, known as sintering, typically requires temperatures of around 300 degrees Celsius to bond the sensor’s silver nanoparticles together.

“The surface of the skin obviously cannot withstand such a high temperature,” said Cheng. “To get around this limitation, we suggested a sintering aid layer – something that doesn’t damage the skin and allows the material to sinter together at a lower temperature.”

By adding a nanoparticle to the mixture, the silver particles sinter at a lower temperature of about 100 ° C.

“That can be used to print sensors on clothing and paper, which is useful, but it’s still higher than we can stand at skin temperature,” said Cheng, noting that around 40 ° C (104 ° F) could still burn skin tissue. “We changed the formula of the auxiliary layer, changed the printing material and found that we can sinter at room temperature.”

The sintering aid layer at room temperature consists of polyvinyl alcohol paste – the main component of peelable face masks – and calcium carbonate – which comprises eggshells. The layer reduces the roughness of the printing surface and allows for an ultra-thin layer of metal patterns that can bend and fold while maintaining electromechanical capabilities. When the sensor is printed, the researchers use an air blower, e.g. B. a cool hair dryer to remove the water that is used as a solvent in the ink.

“The result is profound,” said Cheng. “We don’t have to rely on heat when sintering.”

According to Cheng, the sensors can precisely and continuously record temperature, humidity, blood oxygen levels and cardiac output signals. The researchers also connected the sensors on the body into a network with wireless transmission functions to monitor the combination of signals as they progressed.

The process is also environmentally friendly, Cheng said. The sensor remains robust in lukewarm water for a few days, but can easily be removed with a hot shower.

“It could be recycled because removing it won’t damage the device,” Cheng said. “And most importantly, removing it doesn’t damage the skin. This is especially important for people with sensitive skin, such as the elderly and babies. The device can be useful without adding any additional burden to the person using it or the environment. “”

Next, the researchers plan to modify the technology so that it targets specific applications when needed, such as a precise network of sensors on the body that will be used to monitor the specific symptoms associated with COVID-19.

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Other contributors are Hongjun Ji, Senpei Xie, Yaoyin Li, Ziheng Ye, Tiesong Lin, Xiangli Liu, Xuesong Leng, Mingyu Li, Pengdong Feng, Jiaheng Zhang and Xing Ma, all of whom are affiliated with the Harbin Institute of Technology. Houbing Huang and Xiaoming Shi, both from the Beijing Institute of Technology; and Ning Yi of the Penn State Department of Materials Science and Technology.

This work was supported by Penn State, the National Science Foundation, the Petroleum Research Fund of the American Chemical Society, the Shenzhen Science and Technology Program, the Shenzhen Bureau of Industry and Information Technology, and the National Science Foundation of China.