New circuits can get printed directly on human skin to help monitor vital signs, a new study finds.
Wearable electronics are growing increasingly more comfortable and more powerful. A next step for such devices might include electronics printed directly onto the skin to better monitor and interface with the human body.
Scientists wanted a way to sinter—that is, use heat to fuse—metal nanoparticles to fabricate circuits directly on skin, fabric or paper. However, sintering usually requires heat levels far too high for human skin. Other techniques for fusing metal nanoparticles into circuits, such as lasers, microwaves, chemicals or high pressure, are similarly dangerous for skin.
In the new study, researchers developed a way to sinter nanoparticles of silver at room temperature. The key behind this advance is a so-called a sintering aid layer, consisting of a biodegradable polymer paste and additives such as titanium dioxide or calcium carbonate.
Positive electrical charges in the sintering aid layer neutralized the negative electrical charges the silver nanoparticles could accumulate from other compounds in their ink. This meant it took less energy for the silver nanoparticles printed on top of the sintering aid layer to come together, says study senior author Huanyu Cheng, a mechanical engineer at Pennsylvania State University.
The sintering aid layer also created a smooth base for circuits printed on top of it. This in turn improved the performance of these circuits in the face of bending, folding, twisting and wrinkling.
In experiments, the scientists placed the silver nanoparticle circuit designs and the sintering aid layer onto a wooden stamp, which they pressed onto the back of a human hand. They next used a hair dryer set to cool to evaporate the solvent in the ink. A hot shower could easily remove these circuits without damaging the underlying skin.
After the circuits sintered, they could help the researchers measure body temperature, skin moisture, blood oxygen, heart rate, respiration rate, blood pressure and bodily electrical signals such as electrocardiogram (ECG or EKG) readings. The data from these sensors were comparable to or better than those measured using conventional commercial sensors that were simply stuck onto the skin, Cheng says.
The scientists also used this new technique to fabricate flexible circuitry on a paper card, to which they added a commercial off-the-shelf chip to enable wireless connectivity. They attached this flexible paper-based circuit board to the inside of a shirt sleeve and showed it could gather and transmit data from sensors printed on the skin.
"With the use of a novel sintering aid layer, our method allows metal nanoparticles to be sintered at low or even room temperatures, as compared to several hundreds of degrees Celsius in alternative approaches," Cheng says. "With enhanced signal quality and improved performance over their commercial counterparts, these skin-printed sensors with other expanded modules provide a repertoire of wearable electronics for health monitoring."
The scientists are now interested in applying these sensors for diagnostic and treatment applications "for cardiopulmonary diseases, including COVID-19, pneumonia, and fibrotic lung diseases," Cheng says. "This sensing technology can also be used to track and monitor marine mammals."
The scientists detailed their findings online Sept. 11 in the journal ACS Applied Materials & Interfaces.
This article appears in the December 2020 print issue as “You Are the Circuit Board.”
Charles Q. Choi is a science reporter who contributes regularly to IEEE Spectrum. He has written for Scientific American, The New York Times, Wired, and Science, among others.