We've already seen things such as bracelets that monitor the wearer's exposure to the sun's ultraviolet rays. Now, L'Oréal has teamed up with Illinois' Northwestern University to create a tiny thumbnail-mounted gadget that does the same job. According to the university, it is the world's smallest wearable device – of any kind.

Known as UV Sense, the solar-powered waterproof device is less than 2 mm thick, 9 mm in diameter, and is designed to be worn for up to two weeks at a time (additional adhesives can be used to reapply it after that). It contains no battery or moving parts, and is claimed to incorporate the world's most accurate UV dosimeter.

To get a reading from it, the user just runs an iOS/Android app on their smartphone, then swipes the phone over the NFC (Near Field Communication)-enabled device. The app will tell them whether they've exceeded safe limits for UVA and UVB exposure, either for that day or over time – the device can store up to three months of user data.

"We think it provides the most convenient, most accurate way for people to measure sun exposure in a quantitative manner," said Northwestern's John A. Rogers, who led the research. "The broader goal is to provide a technology platform that can save lives and reduce skin cancers by allowing individuals, on a personalized level, to modulate their exposure to the sun."

UV Sense was unveiled this week at CES in Las Vegas. It will be available on a limited basis in the US this summer, followed by a global roll-out in 2019.

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