June 13, 2023 | A team of researchers from the U.S. and China have developed a flat, adhesive wearable sensor for detecting signals of individual and population health through the skin, and it can be manufactured at low cost using components that are readily available pretty much anywhere. Initially used in “smart diapers” to detect wetness, the multifunctional device platform now has a superhydrophobic coating making it waterproof so it can accurately monitor health variables without moisture or humidity interfering with the sensing, according to Huanyu Cheng, associate professor of engineering science and mechanics at Penn State.
This “pencil-on-paper" sensor—so named because it uses graphite mechanically exfoliated from an ordinary pencil—can pick up multiple signals of interest, including body temperature indicative of infection and the action potential of muscles, the brain, and the heart, he says. It also has the potential to be used clinically to administer thermal therapy by sending a current through the skin.
Discussions are already underway with physicians about conducting pilot studies under “rapid, extreme conditions” to verify its diagnostic utility in individuals with different diseases, adds Cheng. To date, the sensor has been tested only on the benchtop and in healthy individuals whose temperature, concentration of gas molecules, and electrical physiological signals would logically differ from individuals in a diseased state.
As reported recently in Chemical Engineering Journal (DOI: 10.1016/j.cej.2023.142774), the wearable electronics are uniquely easy to manufacture as well as disposable, making them ideal for resource-limited settings and routine use—even at home. A batch of the sensors could be produced using any type of paper and a ruler or, alternatively, a computer printer to draw out a pattern that then gets filled in with the graphite at the core of standard pencils that is conductive like metal, Cheng explains.
Multiple layers of the overlapping graphite can be added, as needed, to get the desired flow of charge, he continues. The drawing is connected to a tiny lithium battery that powers data transmission to a smartphone via Bluetooth.
A silica coating applied to the sensor makes it virtually waterproof, says Cheng. The sensor’s design applies the principles of the traditional Chinese paper-cutting art of kirigami to create foldable, bendable circuits that increase its stretchability and ability to comfortably conform to the skin’s surface. The addition of the graphite layer from the pencil is all that’s needed to provide electrical conductance.
The sensor system can be fashioned into face masks to detect breathing patterns that might be used to detect when someone is experiencing respiratory arrest or shortness of breath, Cheng says. Human skin, the body’s largest organ, exhibits smaller changes in humidity that may likewise signal a problem. As he has shown, the pencil-on-paper wearable can detect when the skin is not processing moisture correctly even after someone has exercised or touched contaminated surfaces.
Cheng cites several possible clinical use cases for the platform, among them the detection of heart attacks and facilitation of human-machine collaboration by assessing residual motor function in amputees. While population health is the primary focus, due to the scope of vital signs the device could measure, he points also to the potential to improve the health of individuals by sensing low concentrations of gas molecules that make it through the sweat glands onto the skin surface.
The measurement of electrophysiological signals and temperature, together with heating for thermotherapy, suggest the device would also have potential applications for the diagnosis of epilepsy and Parkinson’s disease. It would also likely find utility in monitoring lung levels of nitrogen dioxide (NO2), produced by automobile exhaust, exposure to which has been found to both cause and worsen chronic obstructive pulmonary disease (COPD) and a host of other health effects, says Cheng.
He previously demonstrated the ability of the sensor system to measure nitrogen dioxide concentrations in breath as well as in the environment for air quality monitoring purposes. In the newly published paper, the sensor was shown to exhibit a large response to nitrogen dioxide gas at elevated temperature from self-heating and with minimal interference from changes in ambient temperature, relative humidity, and mechanical deformations.
As biomarkers, gas molecules are equivalent to liquid molecules typically identified in blood samples except they occur in much lower concentrations, says Cheng. Even glucose is detectable in the breath, making a needleless diabetes diagnosis theoretically possible. “We just need it to be super-accurate.”
Literally thousands of disease biomarkers might be discoverable using the pencil-on-paper sensing platform, he adds. Although only the COPD application is explored in the latest study, the research team hopes to hear from physicians about the most pressing problems to tackle first. This might include sensing signals to diagnose a disease as well as indicate how well a treatment is working to improve health outcomes.
Development of the pencil-on-paper platform began with the detection of moisture in diapers because the paper itself, even without kirigami structures, is often highly porous, Cheng says. In this case, the hand-drawn electrode sensor is treated with a sodium chloride solution. As water molecules are absorbed by the paper, the solution becomes ionized and electrons begin to flow to the graphite, setting off the sensor that detects changes in humidity in the environment and sends a signal to a smartphone.
When it came to health signals beyond humidity, the critical challenge was blocking the huge moisture effect, which is where the superhydrophobic coating layer came into play to prevent condensation on the sensor, Cheng says. Enabling the wearable platform to simultaneously detect changes in body temperature and gas molecules was a bit trickier.
Here, a pair of similar sensing units are used for detecting the biopotential, or voltages, generated by the two physiological processes, he explains. The sensors self-heat to 50 degrees Celsius to eliminate fluctuations generated by the outside environment, and the unit for monitoring temperature is coated with a thin layer of a silicone polymer that blocks out the gas molecule.
Another rationale for taking this strategy is that the applied voltage can be modulated to increase skin surface temperature, says Cheng, which expands the device’s usefulness to thermal therapy to treat joint injuries, improve blood circulation, promote relaxation, sterilize medical dressing, and facilitate wound healing. Electrical stimulation might prove to be particularly helpful for individuals with uncontrolled diabetes who frequently suffer from chronic, non-healing wounds that are vulnerable to infection, he notes.