Wearable sensors are evolving from watches and electrodes to flexible devices that provide users with more accurate biometric measurements and comfort. Now, an international team of researchers has taken evolution a step further by printing sensors directly onto human skin without using heat.

Under the leadership of Dorothy Quiggle, Career Development Professor in the Department of Engineering Science and Mechanics at Pennsylvania State University, Huanyu "Larry" Cheng, the research team published their findings in ACS Applied Materials & Interfaces.

"In this paper, we report a simple but universally applicable manufacturing technique that uses a novel sintering aid layer to directly print human body sensors," said a Chinese researcher from Harbin Institute of Technology in Cheng's laboratory.

Cheng and his colleagues had previously developed flexible printed circuit boards for wearable sensors, but printing directly on the skin was hindered by the bonding process of the metal components in the sensors. Called sintering, this process typically requires temperatures of about 572 degrees Fahrenheit (300 degrees Celsius) to bond the silver nanoparticles of the sensor together.

"Obviously, the skin surface cannot withstand such high temperatures," said Cheng. "To overcome this limitation, we proposed a sintering aid layer that does not harm the skin and helps the materials sinter together at lower temperatures."

By adding nanoparticles to the mixture, the silver particles sinter at a lower temperature of about 212 F (100 C).

"It can be used to print sensors on clothes and paper, which is useful, but it is still higher than our skin temperature," said Cheng, noting that about 104 F (40 C) would still burn skin tissue. "We changed the formulation of the auxiliary layer and the printing material, and found that we could sinter at room temperature."

The room-temperature sintering aid layer consists of polyvinyl alcohol paste (the main ingredient in peel-off face masks) and calcium carbonate (composed of eggshells). This layer reduces the roughness of the printed surface and allows the formation of ultra-thin metal pattern layers that can bend and fold while maintaining electromechanical performance. When printing the sensors, the researchers used a blower, such as a hair dryer set to cool, to remove the water used as a solvent in the ink.

"The results are profound," said Cheng. "We do not need to rely on heat for sintering."

Mr. Cheng stated that these sensors can accurately and continuously capture temperature, humidity, blood oxygen levels, and cardiac performance signals. The researchers also connected the body sensors to a network with wireless transmission capabilities to monitor the combined signals during processing.

Cheng said that this process is also environmentally friendly. The sensors can last for several days in warm water, but they are easily removed by a hot shower.

"Since it can be removed without damaging the device, it can be recycled," said Cheng. "Moreover, importantly, removal does not damage the skin. This is especially important for people with sensitive skin, such as the elderly and infants. The device is very useful and does not impose an additional burden on the user or the environment."

Next, the researchers plan to adapt the technology as needed for specific applications, such as placing it in precise human body sensor networks to monitor specific symptoms related to COVID-19.

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

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