Next time you see an adult with a stick-on tattoo, don’t laugh – that person might have a metabolic problem, or they could be a high-performing athlete who’s getting their training schedule fine-tuned. No, really. A team lead by Dr. Joseph Wang at the University of California, San Diego, has created a thin, flexible metabolic sensor that is applied to the skin ... and it takes the form of a smiley-face tattoo.

For some time now, medical researchers and athletic trainers have been using devices known as ion-selective electrodes (ISEs). By detecting changes in the pH levels of sweat on the skin, these can be used to assess a person’s metabolic state, or to tell when an athlete is tired or dehydrated. These devices can be bulky, however, plus it’s hard to keep them adhered to overly-sweaty skin.

The new sensor, called a tattoo-based solid-contact ion-selective electrode, performs the same sort of function as an ISE, but it’s sleek and stays put. It was made using a standard screen printer, which laid down consecutive layers of silver, carbon and inks, followed by a process known as electropolymerization of aniline. The smiley face’s eyes are actually the sensor’s two electrodes, while its ears serve as conductive contacts for a separate measurement device.

It’s even applied like a regular stick-on tattoo – it’s placed onto the skin, then a paper towel soaked in warm water is held against it, allowing its backing paper to come loose and slide off. In lab tests, the sensor remained on the skin of exercising test subjects even when they were sweating heavily.

In its present form, the tattoo is able to detect metabolic stress due to exertion. By using different sensing materials, however, it is claimed that the sensor could also be used to measure concentrations of substances such as sodium, potassium or magnesium in the sweat.

A paper on the research was recently published in the journal Analyst.