Electronic tattoo monitors body functions

Flexible electronics with sensors, solar cells and radio module adheres securely and imperceptibly to the skin

Urbana (USA)/Singapore - Without any adhesive, a versatile and flexible sensor module adheres to the skin and permanently measures important bodily functions. This adhesive tattoo, which is much thinner than a sheet of paper and only the size of a postage stamp, was developed by an international team of researchers. In addition to flexible electrical conductors, the scientists integrated sensors for heartbeat and brain waves, light-emitting diodes, a radio module and even tiny solar cells to generate electricity. In addition, the module can be supplied with energy via electromagnetic waves - similar to radio tags for expensive products. The researchers present their prototype, which sticks securely to the skin for up to 24 hours, in the journal Science.

"This technology can connect you to the cyber world and the physical world through a very natural way," says Todd Coleman of the University of Illinois at Urbana-Champaign. Together with colleagues from Singapore and China, he deposited flexible conductive traces in a serpentine pattern on an ultra-thin transparent substrate made of polyester. In addition, the researchers printed the numerous electronic components onto this carrier. Encapsulated in a waterproof manner, the adhesive tattoo could be placed on the skin of test subjects with the help of a water-soluble film made of polyvinyl alcohol. For up to 24 hours, weak adhesion forces (van der Waals forces) ensured a secure hold. And despite the many movements and tensions of the skin, all components remained intact.

Supplied with power from the integrated solar cell and an induction coil, heartbeat and brain waves could be reliably measured depending on the position of the electronic adhesive tattoo. Since this data can be sent directly to a computer by radio, the technology is suitable for convenient continuous monitoring of high-risk patients. Since they do not feel the adhesive tattoo at all and do not have to consider its stability, they do not suffer any restrictions on their activities. "The best way to record neural signals in a natural environment is to use modules like this that are invisible to the user," Coleman says.

But the same sensor that detects heart rate can also be used to electronically sense rhythmic movements of the skin. An adhesive tattoo that the researchers had placed on the neck near the larynx as a test demonstrated this ability impressively. Using the movements alone, the module was able to recognize several spoken words. With a hit rate of over 90 percent, one test subject even controlled the actions in a video game via the motion data sent over the air. With even more sensitive sensors, it's even conceivable that patients with vocal cord problems could audibly participate in conversations again using a small computer and speakers.

"This work is in the very early stages," emphasizes John Rogers, who leads the Urbana-Champaign research group. More sensitive sensors could greatly expand the medical applications of electronic skin. For power supply, Rogers is also thinking about integrating tiny batteries and piezoelectric mini power plants that generate electricity from mechanical movements alone. The development of smart electronic clothing could also drive this technology. It also cannot be ruled out that, in addition to light-emitting diodes, small, flexible monitors, loudspeakers and mobile phone modules could also be integrated into these skin electronics in the future. This would make it conceivable to have a wafer-thin smartphone that users could simply stick on their palm or forearm.