Imagine it. A colorful tiger, shark, or rainbow tattoo that can read your brain waves, power robotic prosthetics, and even detect heart attacks sooner. Engineers at Penn State University have filed a provisional patent for these colorful, customizable wearable devices that can be painted directly onto a patient’s skin. In addition to using all the colors of the rainbow, the sensors use materials that adhere better to the skin, making them more sensitive, durable and accurate. The elegant sensors are described in a study published today in the journal Proceedings of the National Academy of Sciences (PNAS).

Handheld shark sensor

Improving wearables

All wearable health technologies work through electrode contacts attached to the body. These electrodes can then interpret the health and activity of a patient’s heart, muscles or brain by recording the different electrical signals produced by the body. The brain signals are processed into EEG readings, which reflect the activity of the patient’s neurons. Heart activity is processed into ECG signals, allowing doctors to measure a patient’s heart rate when they exercise and record a detailed view of the heart’s electrical activity to monitor for problems. Muscle activity can be processed into EMG signals, which can track muscle contractions.

Electrode designs generally use rigid metal-based materials. While these materials offer stability, they often have difficulty staying attached to the body when the patient moves. Many experimental designs use a soft, gelatinous material called a hydrogel, which can absorb and swell with water. This helps them stretch and adapt better to body movement, but can become dehydrated over time. Then the electrodes lose adhesion and elasticity.

According to PhD student and study co-author Wanqing Zhang, electrode detachment is not the only factor contributing to less accurate sensor readings. The actual act of applying the sensors can affect their operation. Many commercially available sensors also struggle to accurately record what is happening inside a patient’s body, especially when applied to hairy or sweaty skin.

“Most commercial electrodes are prefabricated in a laboratory or factory and then layered on the skin, which means there is an air gap between the skin and the electrode, which negatively affects the detection performance,” Zhang explained in a statement. “To solve this problem, we have developed conductive ink that can be painted directly on the skin. After drying, it acts as a functional electrode.”

More than fun colors

To create these new sensors, the team mixed several different types of polymers (or plastics) with acidic additives in a water-based solution to create the colorful ink. The new ink has the consistency of glue when wet, but can dry on the skin in less than 10 minutes. The drying process can be accelerated even with the help of a regular hair dryer.

“The ink itself almost behaves like face paint,” explained study co-author Larry Cheng. “It starts out almost clear, but you can use food coloring 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 customize the wearable to each person’s preferences.”

In addition to being fully customizable based on what the patient can imagine, the team says that the sensors driven by its electrodes are extremely responsive. Zhang explained that painting the material directly on the skin helps it conform more closely to the texture of the skin and, in turn, improves measurements.

To improve stability between the electrodes and the sensors they report to, a connective region of the electrodes is painted on a porous silver fabric (something like chicken wire) placed on the skin. The wet ink then flows onto the fabric, before hardening and adhering to the surface of the skin. That connective part then clips into a port on the larger electrical module. The electrical component is taped to the user’s skin under clothing and wirelessly transmits the electrical signals collected by the ink to a computer via Bluetooth.

According to the study, the porous structure of the textile allows the electrodes to stretch to more than 150 percent of their original size without breaking. At the same time, it helps them adhere more evenly to the skin’s texture and register electrical signals more effectively.

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

tattoo

In one experiment, painted electrodes were able to effectively track ECG readings during a test subject’s daily activities over the course of 12 hours. During another test, a different subject tracked their readings while exercising, demonstrating that the electrodes maintain their stickiness and accuracy even during physical activity. The team also tracked EMG signals from a subject’s forearm and fed them to a robotic prosthesis, allowing the individual to control the robotic hand without touching it.

“Although we tested daily use application over a 12-hour period, this is not the limit for these electrodes,” Cheng said. “The electrodes themselves can be easily washed and reapplied. The big idea behind this is that in the future, you could 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 further develop their electrodes so that one day they can be used to detect more advanced biomarkers, such as cortisol or glucose. The fun colors and design possibilities could also be especially useful for pediatricians, since going to the doctor isn’t exactly fun for most kids.

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