As a kid, you might have worn a temporary tattoo—Superman perhaps, or Donald Duck. (You might even have donned a design in your adult years: Such things are fashionable these days.) Well, now the same years-old technology could be useful for something else: to track your vital signs or monitor your athletic performance.
Researchers at the University of California, San Diego, and University of Toronto have come up with a lightweight, transferable tattoo that easily sticks to the skin, and barely troubles the wearer. Inside are a set of ion-selective electrodes that register the pH or salt levels of the skin, as well as traces of potassium, sodium, and magnesium.
Doctors already use the devices to non-invasively test for metabolic diseases, such as Addison’s, or to monitor an athlete’s fitness level. But the devices are bulky and not easy to wear. Vinci Hung, a researcher at the University of Toronto, describes the typical model as about the size of a Sharpie pen.
By contrast, the new tattoo is tiny, very sticky, and can "endure repetitive mechanical deformation," according to the paper announcing the research. It can even withstand heavy sweating.
The tattoo needs more testing before it’s ready for the hospital or medical applications. But Hung says it is at a stage where it can be used by sports teams. "Depending on what the coaches want to measure, you see the pH content or how much salt is in the sweat," she says.
"When athletes are doing endurance exercises, their bodies will produce a lot of lactic acid. If the athlete is someone who has always been training and is very healthy, he will get rid of lactic acid relatively quickly. With someone who hasn’t been exercising as much, their pH will change significantly, compared to the fitter person."
The tattoo is a smiley-face, with ears that serve as connection points for measurement devices, and eyes that contain the electrodes. But the tattoo can be more or less any design (an earlier iteration was in the shape of a flower).
Moreover, the sensors are very cheap and fast to produce: The team produced an initial batch of 20 without any fuss at all. "During the printing cycle you can produce many at a time, depending on how they are set up," Hung says.
The sensor itself is made up of layers of silver, carbon fiber, and insulator inks, with an "electropolymerization of aniline" on the sensing surface. You apply the tattoo the normal way: with a piece of paper soaked in water to take away the base paper.
It certainly beats visiting the doctor for something more obtrusive. Hopefully, other characters––perhaps team mascots?—will be available soon.