Wearable sensors are evolving from watches and electrodes to bendable devices that provide more precise biometric measurements and comfort for users. Now an international team of researchers, led by Huanyu “Larry” Cheng, the Dorothy Quiggle Career Development Professor in the Penn State Department of Engineering Science and Mechanics, is pushing this further by printing sensors directly on human skin without using heat.
“We report a simple yet universally applicable fabrication technique with the use of a novel sintering aid layer to enable direct printing for on-body sensors,” says first author Ling Zhang, a researcher at the Harbin Institute of Technology in China and in Cheng’s laboratory.
Cheng and his colleagues previously developed flexible printed circuit boards for use in wearable sensors; however, printing directly on skin was hindered by the bonding process – sintering – which typically requires temperatures around 572°F to bond the sensor’s silver nanoparticles together.
To not scorch patients’ skin with high temperatures, Cheng and the team “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 about 212°F.
“That can be used to print sensors on clothing and paper… but it’s still higher than we can stand at skin temperature,” Cheng says. “So, we changed the formula of the aid layer, changed the printing material, and found we could sinter at room temperature.”
The room temperature sintering aid layer consists of polyvinyl alcohol paste – the main ingredient in peelable face masks – and calcium carbonate, which comprises eggshells. The layer reduces printing surface roughness and allows for an ultrathin layer of metal patterns that can bend and fold while maintaining electromechanical capabilities. While printing sensors, researchers use a cool air blower to remove the water used as a solvent in the ink.
According to Cheng, the sensors can precisely and continuously capture temperature, humidity, blood oxygen levels, and heart performance signals. The researchers also linked the on-body sensors into a network with wireless transmission capabilities to monitor the signals as they progress.
The sensor remains robust in tepid water for a few days, while a hot shower easily removes it, so “It could be recycled, since removal doesn’t damage the device or the skin,” Cheng says.
The researchers plan to alter the technology to target specific applications as needed, such as a precise on-body sensor network placed to monitor symptoms associated with COVID-19.