Fascination Technology Colorful Tattoos as Sensors for Heart, Brain, and Robotic Hands

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In our section "Fascination Technology," we present impressive projects from research and development to designers every week. Today: tattoo sensors for drawing on, measuring heart signals, and controlling robotic hands.

Engineers at Penn State University have developed drawable tattoos that can detect heart attacks early, control robotic prosthetics, and read brain waves—in a colorful, customizable system that can be easily washed off and reapplied.

Conductive Ink for Drawing On

The team has filed a provisional patent application for a conductive ink that can power sensors and be painted directly onto the skin in any design. The ink can be colored in any desired color and uses materials that bond better with the skin—making the sensors not only stylish but also more sensitive, durable, and accurate than other wearable sensor designs.

The Limits of Traditional Electrodes

Wearable healthcare technologies, according to Larry Cheng, corresponding author of the study, are powered by electrode contacts attached to the body. These can interpret the health status and activity of the heart, muscles, and brain through electrical signals—as EEG, ECG, or EMG signals.
Traditional electrodes made of rigid, metal-based materials provide stability but adhere poorly during movement or sports. Experimental designs with hydrogel—which Cheng has been researching for over ten years—can stretch with the body but dry out over time, losing adhesion and elasticity.

First author Wanqing Zhang explains that not only does the detachment of electrodes lead to inaccurate measurements, but also their application: "Most commercial electrodes are prefabricated in the lab or factory and then placed on the skin, creating an air gap between the skin and the electrode that negatively affects sensor performance. To address this, we developed a conductive ink that can be painted directly onto the skin. Once dried, it serves as a functional electrode."

Like Makeup, just Functional

The team mixed various polymers and acidic additives into a water-based solution to create the ink. In its wet state, it has the consistency of glue but dries on the skin in under ten minutes—the process can be accelerated with a hairdryer."The ink itself behaves almost like makeup," says Cheng. "It is initially almost transparent, but you can color it with food dye and paint it into any desired design—like a cartoon or Superman. This allows the wearable to be completely tailored to a person's preferences."

The painted-on sensors adapt better to the skin structure through direct application, which improves measurement accuracy. The structure in detail:

• The conductive connection area is painted onto a porous silver textile that acts like a metal mesh. 
• The wet ink flows into the textile before it cures and adheres to the skin.
• The connection is then plugged into a larger electronic module worn under clothing.
• This module transmits the signals via Bluetooth to a computer.
• The porous structure allows the electrodes to stretch to over 150 percent of their original size without tearing.
• At the same time, this allows them to adhere more evenly to the skin surface.

Tested in Everyday Life and During Sports

In an experiment, the team successfully monitored the ECG values of a co-author during daily activities for over twelve hours. In another test, the electrodes remained adhesive and accurate even during a workout program. Additionally, the team managed to capture the EMG signals from the forearm of a co-author and transmit them to a robotic prosthesis—enabling the test subject to control a robotic hand without touching it.
"Although we tested the daily application for a period of 12 hours, this is not the limit of these electrodes," says Cheng. "The electrodes themselves can be washed off and easily reapplied. The basic idea is that in the future, you could have a more expensive sensor module that remains separate from the system, while the electrodes themselves are disposable. A single bottle of ink is enough to paint sufficient electrodes for several days or a week."

The electrodes themselves can be washed off and easily reapplied.

Erstautorin Wanqing Zhang

Outlook: More than just Heart and Muscles

The team plans to further develop the electrodes to also measure biomarkers such as cortisol or glucose in the future. In addition to commercial use in healthcare, the researchers are also exploring unusual applications—such as "smart plants" that could provide scientists with detailed information about chemical pollutants in the environment.

The team presented its work in a study published on July 13 in the Proceedings of the National Academy of Sciences.