Humans need vitamin D for a long list of benefits including healthy bones and even preventing some forms of cancer. Sun exposure remains the best source of vitamin D. Unfortunately, according to the American Cancer Society, too much exposure to the sun can contribute to skin cancer: about 5.4 million basal and squamous cell skin cancers are diagnosed each year.

The bottom line: we get great benefit from some sun exposure but too much can be very bad for us. How do we know how much is enough? Two University of Southern California’s Viterbi School of Engineering researchers attacked the problem of measuring ultraviolet (UV) light exposure. The challenges included developing a wearable device small enough to not be intrusive, flexible so it would conform to skin contour, free of microcircuitry, and able to operate battery-free.

The USC engineers’ work resulted in a clear, flexible, waterproof UV detection patch made of a patented material using FDA-approved non-toxic polymers. The small sensor patches measure 0.5″ by 0.5″ and less than a half millimeter thick. If the user applies sunscreen the patch compensates for the reduced exposure. The UV sensitive material starts clear, but changes color with ultraviolet light exposure. First it becomes a pale yellow and then gradually darkens to a dark burnt orange color.  When the sensor is orange, the user knows she or he has gotten the recommended World Health Organization (WHO) vitamin D dose and should minimize sun exposure. According to Andrea Armani, one of the engineers in the study, 10 minutes generally suffices for the WHO recommended vitamin D dosage and exposure longer than roughly an hour leads to skin damage, both times dependent on individual skin type and chemistry.

The researchers designed the skin patch to work within varying geographic and environmental factors. The current prototype version needs more work including optimizing the sensor layer in the patch for different skin types.