Having previously developed flexible printed circuit boards for use in wearable sensors for soft body area sensor network health monitoring, the researchers continued to search for an approach that did not rely on sophisticated fabrication techniques such as lithography or direct printing on a carrier substrate before attaching to the body. Now they have developed what they describe as a simple yet universally applicable fabrication technique with the use of a novel sintering aid layer to enable direct printing for on-body sensors.
Sintering is a process that typically requires temperatures of around 572°F (300°C) to bond the sensor's silver nanoparticles together.
"The skin surface cannot withstand such a high temperature, obviously," says Huanyu “Larry” Cheng, Dorothy Quiggle Career Development Professor in the Penn State Department of Engineering Science and Mechanics. "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, say the researchers, 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 says, who notes 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 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 use an air blower, such as a hair dryer set on cool, to remove the water that is used as a solvent