Scientists at Pennsylvania State University have developed a novel conductive ink that can be painted directly onto the skin in colorful custom designs, turning into a functional electrode for biomonitoring after drying.
The scientists give more light on the invention in a new paper published in the Proceedings of the National Academy of Sciences (PNAS). As previously reported, epidermal electronics attached to the skin via temporary tattoos (e-tattoos) have been around for more than a decade.
So-called e-tattoos connect to skin without adhesives, are practically unnoticeable, and are typically attached via temporary tattoo, allowing electrical measurements (and other measurements, such as temperature and strain) using ultra-thin polymers with embedded circuit elements.
However, these e-tattoos have their limitations, most notably that they don’t function well on curved and/ or hairy surfaces, as well as requiring personalized electrode placement design to cover larger areas, since biosignals are spatially distributed.
So scientists have been getting creative. For instance, in 2024, researchers developed special polymer-based conductive inks that can be printed onto a person’s scalp to measure brain waves, even if they have hair.
This could one day enable mobile EEG monitoring outside a clinical setting, among other potential applications. Penn State mechanical engineer, Larry Cheng, a coauthor of the new PNAS paper, has been working on electrode designs for biomonitoring applications for more than 10 years, including EEGs, ECGs (for heart activity), and EMGs (for muscle contractions).
Using rigid materials, like metals, makes for a stable biomonitor, but it is easily dislodged when the wearer moves too much, such as during exercise. Hydrogels have emerged in recent years as alternative materials, since they can absorb water, swell, and stretch with the body’s skin during movement.
But hydrogels degrade rather quickly and lose those benefits with prolonged use. Sweat or hair can also reduce the accuracy of recording biosignals. That’s because commercial electrodes are prefabricated and then applied to the skin, creating an air gap that weakens sensor readings.
Cheng et al. decided to develop their conductive ink to address that issue. They mixed together several different kinds of polymers and acidic additives in a water-based ethanol/ polyvinyl alcohol solution. PEDOT:PSS—aka poly (3,4-ethylenedioxythiophene): poly (styrene sulfonate)— provided electrical conductivity, along with DBSA (4-dodecylbenzenesulfonic acid), which also served as a plasticizer to give the ink flexibility.
The team mercifully shortened the abbreviated moniker of their new conductive ink to WE-PPD. “The ink itself almost behaves like face paint,” said Cheng. “It starts out almost transparent, but you can use food dye to pigment the ink into whatever colors you need to paint whatever design you have in mind—like a cartoon or Superman.
This allows us to completely personalise the wearable to a person’s preference.” Because the ink fills the contours of the skin, the resulting electrode has very high skin connectivity, and hence better signal recording.
It can also be incorporated into a porous silver texture and integrated with rigid devices. The painted sensors were tested in the lab on human subjects to monitor heart activity while running on a treadmill and lifting weights; gesture recognition to control a prosthetic robotic hand; and brain activity (EEG), monitored through hair, as a co-author went about their daily activities.
The painted electrodes were able to stretch up to 170 percent before failing, per the authors, had much higher water vapor permeability than standard medical-grade films, and caused no skin irritation over prolonged use.
“Although we tested the daily use application over a 12-hour period, this is not the limit for these electrodes. “The electrodes themselves can be washed away and easily reapplied. The big idea behind this is that in the future, you could potentially have a more expensive sensing module that remains separate from the system, but the electrodes themselves can be disposable.
A single bottle of ink could provide enough material to paint multiple electrodes over the course of several days or a week,” Cheng said. The team has filed a provisional patent for their conductive ink, but there are still a few limitations.
While the absence of imaging artifacts holds promise for MRI imaging, the authors note that there still needs to be a comprehensive safety evaluation before deploying the painted sensors in a clinical setting
