Beijing: An international team of researchers has taken the evolution of wearable electronics further by printing sensors directly on the human skin without the use of heat.
"In this article, 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," said study author Ling Zhang from Harbin Institute of Technology in China.
The research team previously developed flexible printed circuit boards for use in wearable sensors, but printing directly on the skin has been hindered by the bonding process for the metallic components in the sensor.
Called sintering, this process typically requires temperatures of around 572 degrees Fahrenheit (300 degrees Celsius) to bond the sensor's silver nanoparticles together.
"The skin surface cannot 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," Cheng added.
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," Cheng added. 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.
When the sensor is printed, the researchers used an air blower, such as a hair dryer set on cool, to remove the water that is used as a solvent in the ink.
"The outcome is profound. We don't need to rely on heat to sinter," Cheng said. 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. The sensor remains robust in tepid water for a few days, but a hot shower will easily remove it," the authors wrote. This study was published recently in the journal 'ACS Applied Materials & Interfaces'.