Just in time for tropical holiday vacations, a team of scientists at Northwestern have released a device that can tell you just how much sun is too much. It’s a game-changer if you manage not to lose it on a sandy beach. Holding onto it could be tricky, since the scientists claim it’s the smallest device of its kind in existence.

The device, called “My Skin Track UV,” was officially unveiled at Apple stores on November 13th, but a paper released Wednesday in Science Translational Medicine is the first to provide details on the design and testing of the device. The thumbnail-sized wearable has been lauded because it can track three different types of potentially harmful light and use an app to alert the user if they’re getting toasted. Useful as this is against harmful sun exposure, John Rogers, Ph.D., a biomedical engineering professor at Northwestern who helped design the device, tells Inverse that its real potential lies in its minimalist design, which could reshape the way engineers approach wearables going forward.

“It’s the smallest wearable we’re aware of.” Rogers says. “It’s always on, you never have to turn it on and off because it’s always responsive to UV light. If you look at most wearable devices, I think non-use is driven by the inconvenience of managing the battery.”

wearable technology
The device doesn't have a battery, and can stick to a fingernail

During the design process, Rogers and his team worked with cosmetics powerhouse L’Oréal to make the M&M-sized device as non-invasive as possible. It doesn’t have a battery, never runs out of power, but can still communicate with a phone to help users act upon the information it collects. By integrating with Apple’s Health Kit, it can send reminder notifications to apply sunscreen or take a break from the sun. The team achieved its hyper-minimalist design, Rogers says, by focusing on two crucial aspects.

First, Rogers swapped a traditional wearable battery for a device called a capacitor. The big difference between a battery and a capacitor is that a battery stores energy in a chemical form, whereas a capacitor stores energy in electrical field. In this case, the same UV light that the device measures creates the electrical field in the capacitor. By eliminating a battery, says Rogers, he was able to to explore different design concepts.

“If you look at most wearable devices the size and the weight and the overall feel are often dominated by the battery. So what we did here is create a device that’s powered by ultraviolet light that we’re measuring,” Rogers says.

Perhaps most importantly, however, it still has the tech to connect to a mobile app, which is what allows users to interact with the data the sensor collects. Most wearables transmit their information to phones through a tiny antennae embedded within the device. Rogers, meanwhile, re-designed the usual antennae into two smaller coils with a small space in between them, the he says is a break from traditional single-antennae designs.

“It allows us to shrink that antennae size down to exceptionally small dimensions without sacrificing the ability to communicate with the phone, Rogers says. “Previous devices have used more conventional antennae design that prevents this overall size reduction,” Rogers says.

Taken together, the team’s breakthroughs in capacitor technology and antennae size open the door to unprecedentedly smaller devices than those typically housed around chargeable batteries or antennae. They’re not just going to be used for UV exposure tech, either; Rogers believes he’s already found a way to adapt this design to other types of wearables. For instance, he’s working on one that can measure metabolic activity — the way the burns calories throughout the day.

“We think it will result in a broader trend across the wearable developer community. Certainly for us, it represents a starting point,” he says. “UV is just the beginning.”

Photos via L'Oreal , Northwestern University , Northwestern