Paint-on electrodes enable smarter wearable sensors

Engineers at Penn State are blending art and science to create cute, paint-on tattoos that could help spot heart attacks early, power robotic prosthetics and read brain waves — all within a colourful, customisable system that can be easily washed away or reapplied.

The team has designed and filed a provisional patent for a conductive ink that can power sensors and be painted directly onto a person’s skin in whatever design they can imagine. The ink, which can be pigmented with any desired colour, uses materials that better bond with the skin, meaning they’re not just stylish, but more sensitive, durable and accurate than other wearable sensor designs. The team detailed their work in a paper published in the Proceedings of the National Academy of Sciences.

Wearable healthcare technologies are powered by electrode contacts attached to the body, said Larry Cheng, James L. Henderson Jr. Memorial Professor of Engineering Science and Mechanics at Penn State and corresponding author on the paper. The electrodes can interpret the health and activity of a patient’s heart, muscles and brain by recording different electrical signals produced by the body.

Traditional electrode designs use rigid, metal-based materials that offer stability, but struggle to stay attached to the body during movement or exercise. Many experimental designs, like the ones Cheng has been researching for over a decade, use a soft, jelly-like material known as hydrogel, which can absorb and swell with water to stretch and better match the body’s movement. However, this material can dehydrate over time, causing the electrodes to lose adhesion and stretchiness with prolonged use.

Wanqing Zhang, an engineering science and mechanics doctoral candidate and first author on the paper, said that it’s not just the electrodes peeling off that contributes to less accurate sensor readings, but the actual act of applying them. Many commercial sensors have trouble accurately recording what is happening inside a patient’s body, especially when applied to hairy or sweaty skin.

“Most commercial electrodes are prefabricated in a lab or factory and then layered on the skin, meaning there is an air gap between the skin and the electrode, which negatively impacts sensing performance,” Zhang said. “To address this, we’ve developed conductive ink that can be painted directly to the skin. After drying, it acts as a functional electrode.”

Don’t wear a boring patch — grab a brush and paint your own

The team mixed several different types of polymers, or plastics, and acidic additives into a water-based solution to create the ink. It has the consistency of glue when wet but can dry onto the skin in less than 10 minutes, Zhang said. Drying can be accelerated with the help of a hair dryer.

“The ink itself almost behaves like face paint,” Cheng said. “It starts out almost transparent, but you can use food dye to pigment the ink into whatever colours you need to paint whatever design you have in mind — like a cartoon or Superman. This allows us to personalise the wearable to a person’s preference.”

Image caption: The team has painted sensors in all sorts of different designs and colours, including animals like the fox pictured above, among other cute characters. Image credit: Wanqing Zhang

In addition to being customisable, the team reported that sensors powered by their electrodes are extremely responsive. Zhang explained that painting the material directly onto the skin allows it to better conform to the skin’s texture, in turn, improving measurements.

To enhance the stability between the electrodes and the sensors they inform, a connective region of the electrodes is painted onto a porous, silver textile — almost like a metal fabric — placed on the skin. The wet ink flows into the textile before hardening and sticking to the skin’s surface. The connective portion is then clipped into a port on the larger electric module, which is taped on the wearer’s skin underneath their clothes. This larger module transmits the electrical signals collected by the ink wirelessly to a computer using Bluetooth.

The textile’s porous structure allows the electrodes to stretch to over 150% their original size without breaking, while more uniformly adhering to the skin’s texture and effectively recording electrical signals.

“Over multiple days with other materials, sweat and moisture will accumulate on the electrode interface, potentially causing irritation or disconnection from the skin,” Cheng said. “By using a porous structure, we can allow moisture or hair to better pass through the material, making the electrodes more conductive, adhesive and comfortable.”

In one experiment, the team showed that the painted electrodes could effectively track the ECG readings of a co-author during their daily activities over the course of 12 hours, while in another test, a separate co-author tracked their readings throughout an exercise routine, proving that the electrodes maintain adhesion and accuracy even during physical activity. In another test, the team tracked EMG signals of a co-author’s forearm and fed them to a robotic prosthetic, enabling the individual to control the robotic hand without touching it.

“Although we tested the daily use application over a 12-hour period, this is not the limit for these electrodes,” Cheng said. “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.”

The team plans to continue developing their electrodes to potentially enable more advanced health sensing of biomarkers like cortisol or glucose. Cheng said that in addition to investigating a path towards commercial use for healthcare providers like paediatricians, the team is investigating unique applications, notably in the field of plant science, to create ‘smart plants’ that can offer scientists detailed information on chemical exposure in an environment, and the impacts that exposure might have on a plant’s health.

Top image caption: Using a conductive, face-paint-like ink, researchers can now paint electrodes to monitor a wearer’s heart, muscle or brain activity in style. Image credit: Wanqing Zhang.