If you’ve ever been hooked up to a wearable machine at a doctor’s office, then you’re familiar with the electrodes that are attached to your body to monitor its electrical signals. The problem with these prefabricated metal-based or hydrogel electrodes is that they don’t always stay in place during movement, for long periods or on sweaty or hairy skin.
Penn State University engineers aim to change this with paint-on tattoos that use conductive ink to power sensors for wearable devices such as EEGs, ECGs and EMGs that track brain, heart and muscle activity, respectively.
As reported in a paper published Monday in the Proceedings of the National Academy of Sciences, the engineering team filed a provisional patent for this ink, a water-based solution mixed with polymers and acidic additives that starts out transparent with a glue-like consistency. It can be pigmented with food dye to create different colors for a cute fox or shark that opens its mouth when you open your hand, and it dries on the skin in under 10 minutes. It can later be reapplied or washed off.
Thanks to its customizable, fun nature, these paintable electrodes could be especially beneficial for children who may be more likely to wear a medical device if it’s powered by a temporary tattoo of their favorite character or animal.
How the ink powers wearable devices
To connect the ink to sensors, there’s a porous silver textile with connective electrodes. Before your painted-on tattoo dries, the textile is placed on the design so that it will stick to the skin. Then, the textile is connected to a port on a wearable monitoring device. The latter is taped to the skin beneath clothing
Electrical signals collected by the ink are sent through the textile to the monitoring device, which then transmits the data to a computer
“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,” said Larry Cheng, the paper’s corresponding author and a James L. Henderson Jr. memorial professor of engineering science and mechanics at Penn State, in a press release.
The hope is that these temporary electrode tattoos can help spot heart attacks early, read brain waves or power robotic prosthetics
Plus, since it’s painted directly onto the skin, the ink is more durable and accurate than sensors attached to the skin, as there may be an air gap between them and the skin. As for the silver textile, because it’s porous, its connective electrodes can expand to over 150% their original size. This means sweat can pass through the textile without negatively impacting adhesion, accuracy or comfort.
What the team found during testing
When experimenting with the ink, the Penn State team found that the painted electrodes could monitor ECG signals for up to 12 hours. They also stayed on during exercise. When applied to a team member’s forearm, the electrodes successfully tracked muscle signals using an EMG device, enabling remote control of a robotic hand.
Since the electrodes can be washed off and reapplied, 12 hours isn’t the limit for their use.
Eventually, the painted electrodes could even power sensors that track cortisol or glucose. The team is also looking toward commercial use for doctors, such as pediatricians. Or, to create “smart plants” that provide information on chemical exposure in their environment and its impact on plant health.
Just like it’s become normal to see people wearing health tracking smartwatches and smart rings, perhaps one day no one will bat an eye when they see someone leaving a doctor’s office with a temporary tattoo.
