Engineers Print Wearable Sensors Directly On Skin Without Heat

Wearable sensors are evolving from watches & electrodes to bendable devices that provide much more precise biometric measurements, comfort and luxury for users. Now, International team of researchers has taken the evolution one step further by printing sensors directly on human skin without the utilization of warmth or heat.

Led by Huanyu “Larry” Cheng, Dorothy Quiggle Career Development Professor in Penn State Department of engineering & Mechanics, the team published their leads to ACS Applied Materials & Interfaces.

“In this article, we report an easy yet universally applicable fabrication technique with the utilization of a completely unique sintering aid layer to enable direct printing for on-body sensors,” said first author Ling Zhang, a researcher in Harbin Institute of Technology in China & Cheng’s laboratory.

Cheng and his colleagues previously developed flexible PCB (printed circuit board) to be used in wearable sensors, but printing directly on skin has been hindered by the bonding process for the metallic components in 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 temp., obviously,” Cheng said. “To get around this limitation, we proposed a sintering aid layer—something that might not hurt the skin and will help the material sinter together at a lower temp.”

By adding a nanoparticle to the combination , the silver particles sinter at a lower temp. of about 212 F (100 C).

“That are often wont to print sensors on clothing and paper, which is beneficial , but it’s still higher than we could stand at skin temperature,” Cheng said, who noted that about 104 F—40 C—could still burn skin tissue. “We changed the formula of aid-layer, changed the printing material and located that we could sinter at 20-25 degree temperature .”

The room temperature sintering aid layer consists of polyvinyl alcohol paste—the main ingredient in peelable face masks & calcium carbonate—which comprises eggshells. The layer reduces printing surface roughness and allows for an ultrathin layer of metal patterns which will bend and fold while maintaining electromechanical capabilities. When the sensor is printed, the researchers use an air blower, like a hand blower assail cool, to get rid of the water that’s used as a solvent in ink.

“The outcome is profound,” Cheng said. “We do not 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, consistent with Cheng. The researchers also linked the on-body sensors into a network with wireless transmission capabilities to moniter mixture of signals as they progress.

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

“It might 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, just like the elderly and babies. The device are often useful without being an additional burden to the person using it or to the environment.”

Next, the researchers decide to alter the technology to focus on specific applications as required , like a precise on-body sensor network placed to moniter the actual symptoms related to COVID-19.