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 colorful, customizable 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 color, 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 today (July 13) in the Proceedings of the National Academy of Sciences.
Wearable healthcare technologies are powered by electrode contacts attached to the body, explained 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. Brain signals can be processed into EEG readings, which mirror a patient's neural activity; heart activity can be processed into ECG signals, meaning the team can measure a patient's heart rate during exercise and record a detailed view of the electrical activity occurring in the heart to monitor for issues; and muscle activity can be processed into EMG signals to track muscle contractions.
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, explained 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 explained. "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 explained. "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 personalize the wearable to a person's preference."
