Electronic tattoos: Intuitive control of mobile devices via distinctive body parts

Computer scientists at Saar University and the U.S. company Google are giving wrinkles, knuckles and birthmarks a completely new meaning. Similar to chewing gum tattoos for children, the researchers apply ultra-thin electronic tattoos to prominent body parts. Users can touch, squeeze and drag them to intuitively control mobile devices such as a music player or simply light up symbols. The advantage is that the body locations are so familiar that the individual controls can even be operated with eyes closed. In addition, they enable a completely new type of interaction and also provide operating instructions in a natural way.

The Saarbrücken computer scientists had already proven in 2015 that the human body can be used excellently as a touch-sensitive input surface for mobile devices. Together with researchers from Carnegie Mellon University in the U.S., they developed "iSkin" touch-sensitive stickers for the skin made of flexible silicone and conductive electrosensors. When applied to the forearm and connected to the smartphone, pressing the sticker made it possible to answer a call or adjust the volume of the song currently playing. However, this method was only suitable for certain parts of the body, as it required a relatively flat surface. In addition, the stickers were comparatively large.

"We want to go to body parts where no interaction was possible before. But placing electronics precisely on the skin and then in such a way that it adapts to bone structures such as the knuckles or microstructures such as wrinkles is very complicated," explains Martin Weigel, a doctoral student working with Jürgen Steimle, professor of human-computer interaction at Saarland University. But it was also clear to the researchers: it would be worth it for users. "If you have to press the first knuckle of your left hand, you intuitively know where it is. The same goes for the inside of your index finger," Weigel continues.

Together with Alex Olwal from Google, Weigel, his colleague Aditya Shekhar Nittala and Professor Jürgen Steimle tinkered with the right combination of conductive ink and printing process to print the conductive tracks and electrodes as compactly and as thinly as possible on the temporary tattoo paper. After quite a few tests, the breakthrough finally came. A conductive plastic called PEDOT:PSS was the solution. With it, the researchers were able to print the tattoo even thinner than a hair, ensuring that it would both fit over the knuckles and capture wrinkles, while also being flexible enough to withstand compression and stretching.

The researchers call the electronic tattoos SkinMarks. They are transferred to the skin with water and come off after a few days. In the lab, the scientists need only 30 to 60 minutes to print such a tattoo. "It can be done even faster. We are convinced that in the future everyone will be able to make their own e-tattoo in less than a minute on a standard printer," explains Professor Jürgen Steimle.

The researchers also used the prototypes to test new forms of input. The respective e-tattoo was connected via conductive copper adhesive tape to an Arduino mini-computer that sits close to the body. For example, they stuck an e-Tattoo to the inside of the index finger. When it is extended, the wearer could swipe another finger across the tattoo to turn a music player louder or softer. When the finger is curved, the wearer pressed on one of the three segments to stop the current song or select the next or previous one.

During their experiments, the scientists identified four classes of suitable landmarks on the body. They also exploited the accumulation of pigment-forming cells. They stuck a heart-shaped tattoo over a subject's mole. When an electrical voltage is applied, it glows blue. "Coupled to the corresponding smartphone app, it could light up when the close person is available," Steimle explains the application, adding, "You touch the heart and the call starts."