In high-performance scenarios, your skin sweats. Sticky electrode pads rub loose, disrupting their biometric accuracy. And the metal-to-skin interface of your typical smartwatch, while impressive, can only do so much—often failing on key data points, like calorie expenditures. So, how is anyone supposed to be the very best (like no one ever was) under these conditions?
An international team of scientists—including researchers at Penn State, MIT, and China’s Suzhou Institute of Biomedical Engineering and Technology—has fortuitously developed an answer: a safe and reliable polymer electrode that can be painted right onto the skin. As reported in their new paper, published Monday in the Proceedings of the National Academy of Sciences (PNAS), this paintable electrode’s “biocompatible” compounds proved excellent at tracking muscle activity, heart rate, and brain waves, while also passing multiple safety tests. In electrocardiogram (EKG) tests with a subject running on a treadmill, in fact, the electrodes managed 95.1% consistency “before and after mild sweating,” they wrote.
The researchers believe their design could be tweaked to form more permanent electrode “tattoos,” cyborg human-machine interfaces, and plant biometrics to help meet agricultural goals. But they were particularly enthusiastic about their paintable electrodes’ ability to make life easier for children in need of long-term medical monitoring.
“For example, electrodes can be designed with cartoon patterns, which may reduce anxiety and improve acceptance among pediatric users by making EP [electrophysiology] monitoring less intimidating,” they suggested in their study.
“Such personalization not only improves physiological comfort but also enhances social acceptance,” they noted. (I can think of a few electric-type Pokémon that would work especially well for this myself.)
A fresh paint job
Penn State engineering professor Huanyu Cheng and his collaborators incorporated several complex compounds to make this hyper-flexible, uniquely adhesive, electrode paint a reality. The material’s electrical conductivity was accomplished via the use of a material called poly (3,4-ethylenedioxythiophene): poly(styrene sulfonate), or PEDOT:PSS. Another additive, 4-dodecylbenzenesulfonic acid or DBSA, did double duty as both an electrically conductive additive and a plasticizer, helping to make the finished product more flexible to human body movements. And, finally, a mixture of water, ethanol, and a soft plastic polymer known as polyvinyl alcohol (PVA) completed the recipe, which the team graciously shortened to WE-PPD (for “water-ethanol-PVA/PEDOT:PSS/DBSA”).
“The long-term use of on-skin electrodes hinges on high permeability of water vapor for increased body comfort and enhanced adhesion even in the presence of sweat, while also minimizing skin irritation,” the team noted in its study.
Cheng and his colleagues tested the breathability of their paintable electrodes at two temperatures, 71.6 degrees Fahrenheit (22 degrees Celsius) and 98.6 degrees F (37 degrees C), to see how easily water vapor passed through them. The results, compared to the traditional medical-grade film Tegaderm, were impressive: WE-PPD managed to let nearly five times the amount of water vapor through at that milder temperature and over ten times at that steamy 98.6 degrees.
These electrodes also withstood “stretching of up to ~170% before failure,” the team reported, when paired with a porous silver textile to connect them to biomonitoring devices.
Skin in the game
While the researchers documented “no adverse effects on the skin” after subjects wore WE-PPD for a full 24 hours, they recognize that this and their modest battery of toxicity tests are only the beginning of the safety backstops needed before these electrodes could see real use.
One particular challenge, they noted, will be vetting these paintable electrodes for multimodal magnetic resonance imaging (MRI), where the tech has passed some preliminary MRI compatibility but could cause unintended side effects.
“Future studies should systematically investigate radiofrequency (RF)-induced heating, specific absorption rate, and electromagnetic interactions under different MRI operating conditions and pulse sequences,” they noted. (The last thing you want, in other words, is a super-adhesive wearable that’s suddenly very hot and won’t come off.)
So, the technology is promising, but it may be a while longer before you get to flaunt paintable electrodes that look like Pikachu or Raichu.
