Researchers apply wearable sensors to skin without heat

Researchers have discovered a way to apply wearable sensors to human skin without heat, and the solution involves calcium carbonate found in eggshells. (Penn State)


Researchers are printing sensors directly on human skin without the use of heat, a process that could eventually be applied to monitoring symptoms of COVID-19.

For some time, engineers have been developing flexible printed circuit boards for use as wearable sensors, but the bonding process for metallic components (called sintering) is typically 572 degrees F. —far too hot for human skin. 

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“The skin surface cannot withstand such a high temperature, obviously,” said Huanyu “Larry” Cheng, an engineering professor at Penn State, in an article posted by the university.

 “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.”

Researchers added a nanoparticle to the silver particle mixture, lowering the sintering temperature to 212 degrees F. Still too hot.

To get the sintering down to room temperature—tolerable by human skin—the researchers added to the aid layer a polyvinyl alcohol paste, which is an ingredient used in peelable face masks, along with calcium carbonate found in eggshells.  The layer reduces printing surface roughness and allows for ultrathin metal patterns that can bend and fold and still hold electromechanical capabilities.

Penn State said when the sensor is printed, the researchers use an air blower like a hair dryer set on cool to remove water used as a solvent in ink.  “The outcome is profound,” Cheng said. “We don’t need to rely on heat to sinter.”

Sensors applied directly to the skin can capture temperature, humidity, blood oxygen levels and heart signals.  From there, on-body sensors can be linked wirelessly to a network and servers for further monitoring and evaluation.  The sensor remains robust for a few days, but a hot shower will remove it.  Cheng said that removing the device doesn’t damage it, however.

Cheng worked with an international team on the process which included Ling Zhang, a researcher at the Harbin Institute of Technology in China and in Cheng’s lab. 

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