wearable biometric sensors
CREDIT: LING ZHANG, PENN STATE/CHENG LAB AND HARBIN INSTITUTE OF TECHNOLOGY

Wearable sensors are advancing from watches and electrodes to bendable gadgets that give far more exact biometric measurements and comfort for users. Presently, a global team of scientists has made the evolution 1 step further by printing sensors directly on human skin without the use of heat.

“In this article, we report a simple yet universally applicable fabrication technique with the use of a new sintering aid layer to empower direct printing for on-body sensors,” said first writer Ling Zhang, a scientist in the Harbin Institute of Technology in China and in Cheng’s lab.

Driven by Huanyu “Larry” Cheng, Dorothy Quiggle Career Development Professor in the Penn State Department of Engineering Science and Mechanics, the group published their outcomes in ACS Applied Materials and Interfaces.

Cheng and his partners previously created flexible printed circuit sheets for use in wearable sensors, yet printing directly on the skin has been upset by the bonding process for the metallic segments in the sensor. Called sintering, this process commonly requires temperatures of around 572 °F(300°C) to bond the sensor’s silver nanoparticles together.

The skin surface can’t withstand such a high temperature, obviously, Cheng said. To get around this limitation, we proposed a sintering aid layer—something that would not hurt the skin and could help the material sinter together at a lower temperature.

 

By adding a nanoparticle to the mix, the silver particles sinter at a lower temperature of about 212°F (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,” Cheng said, who noted that about 104°F (40°C) could still burn skin tissue. “We changed the formula of the aid layer, changed the printing material, and found that we could sinter at room temperature.

The room temperature sintering help layer comprises polyvinyl alcohol paste – the main ingredient in peelable face masks- and calcium carbonate – which contains eggshells. The layer decreases printing surface roughness and takes into consideration an ultrathin layer of metal patterns that can bend and fold while keeping up electromechanical capacities. At the point when the sensor is printed, the scientists use an air blower, for example, a hairdryer set on cool, to remove the water that is used as a solvent in the ink.

The outcome is profound,” Cheng said. “We don’t need to rely on heat to sinter.”

The sensors are capable of precisely and continuously capturing temperature, humidity, blood oxygen levels, and heart performance signals, according to Cheng. The researchers also linked the on-body sensors into a network with wireless transmission capabilities to monitor the combination of signals as they progress.

The process is also environmentally friendly, Cheng said. The sensor remains robust in tepid water for a few days, but a hot shower will easily remove it.

“It could be recycled since removal doesn’t damage the device,” Cheng said. “And, importantly, removal doesn’t damage the skin, either. That’s especially important for people with sensitive skin, like the elderly and babies. The device can be useful without being an extra burden on the person using it or to the environment.”

Next, the scientists plan to alter the technology to target specific applications as needed, for example, a precise on-body sensor network placed to monitor the particular symptoms associated with COVID-19.

More information: Ling Zhang et al. Wearable Circuits Sintered at Room Temperature Directly on the Skin Surface for Health Monitoring, ACS Applied Materials & Interfaces (2020). DOI: 10.1021/acsami.0c11479