Metallic Wearable Sensors Printed Directly On The Skin Require No Heat
A team of international researchers, including engineers from Penn State University, has developed a process that allows printing metallic wearable sensors directly on the skin without requiring heat. The new process has helped wearable sensors evolve from simple electrodes to bendable devices able to provide biometric measurements and comfort for the user. The first author for the study, Ling Zhang from Harbin Institute of Technology in China, says the team created a simple and universally applicable fabrication technique using a novel sintering aid layer, allowing direct printing for on-body sensors.The team had previously developed flexible printed circuit boards that can be used in wearable sensors. However, printing directly on the skin was impossible using the previous technology because of the bonding process required for the metallic components in the sensor, known as sintering. That process typically requires temperatures of around 572 degrees Fahrenheit to bond the silver nanoparticles in the sensor together.
Obviously, the skin can't withstand such high temperatures. Researchers added a nanoparticle to the mix that allows silver particles to sinter at a lower temperature of about 212 degrees Fahrenheit. The method was low enough to enable printing sensors on cloth and paper but was still too high for the skin.
After changing the aid layer's formula and changing the printing material, researchers were able to sinter at room temperature. The room temperature aid layer consists of polyvinyl alcohol paste, the main ingredient in peelable facemasks, and calcium carbonate found in eggshells.
The new layer reduces surface roughness and allows an ultrathin layer of metal patterns to bend and fold while maintaining its electromechanical capability. After printing, the researchers used a hairdryer on cool to remove the water used as a solvent in the ink. Resulting sensors can continuously capture temperature, humidity, blood oxygen levels, and heart performance signals. Sensors can also be linked in a network with wireless transmission capability. The printed sensors are also easily removable in a warm shower.