A new mint-sized, battery-free patch that alerts wearers to potentially harmful sunlight exposure in real time might become a powerful weapon in preventing skin cancer.

Powered by the sun while designed to measure its rays, the patch automatically transmits sun readings to a user's smartphone. It works wet or dry, is fully reusable, and weighs next to nothing.

"In the U.S., we're in a skin cancer epidemic, which is driven by excessive UV exposure," noted study author Dr. Steve (Shuai) Xu. He is a dermatology instructor at Northwestern University Feinberg School of Medicine in Chicago.

"Thus, this technology would be useful for the majority of individuals by empowering them to know how much UV they are actually getting," he said.

So, what does it look like and how does it work?

Xu said the device weighs less than a single tic tac, is half the diameter of a dime, and thinner than a credit card.

What's more, "the devices are virtually indestructible," said Xu. "We've washed them, dunked them in boiling water. They will last forever."

As to function, Xu said a solar-powered sensor embedded in the patch picks up UV, infrared and/or visible light readings, sending exposure numbers wirelessly to the wearer's smartphone app.

Caregivers could also use the patch to monitor blue light phototherapy when treating jaundice (in newborns), psoriasis and/or atopic dermatitis, Xu explained.

But the prized benefit is that "we're able to give actionable, accurate information to the user" about sun exposure in real time, he noted. In fact, his team's earlier work with a sensor prototype found that nearly two-thirds of patch users got fewer sunburns, while roughly one-third said they wore more sunscreen and looked for more shade.

"We're expecting even better results with this sensor," Xu said. "It's more accurate and sensitive than anything else out there."

Xu is also medical director for Northwestern's Center for Bio-Integrated Electronics.

In the study, two outdoor UV patch trials involving more than 10 participants per experiment were conducted in the sunny locales of Rio de Janeiro and St. Petersburg, Fla. In addition, blue light therapy patch trials were conducted in three babies undergoing neonatal care in a hospital setting.

The cosmetics company L'Oreal contributed research funding (along with the U.S. National Cancer Institute and the U.S. National Institutes of Health), and has recently launched a UVA-monitoring version of the patch for consumers.

On the downside, the trials highlighted a "fundamental limitation" of the patch: Given that not all parts of the body get the same degree of sun exposure, the small detection area of the patch means that readings may not truly represent sun exposure across the full surface of the body.

But the results indicated that the patch was easily wearable on those parts of the body that might be of "critical" sun exposure interest, including the shoulders and ears. It could even be placed on a pair of sunglasses, the researchers noted.

The findings were published Dec. 5 in the journal Science Translational Medicine.

Arielle Grabel, public relations manager for the Skin Cancer Foundation in New York City, pointed to the foundation's cautionary position statement on the device.

The foundation "can't speak to the technology and reliability of these [wearable] devices," the statement read.

The group also warned "against relying on these devices to determine when to take sun protection measures. Rather, the Skin Cancer Foundation counsels the public to consider sun protection a healthy habit to be practiced daily. This includes seeking shade during peak sun hours, covering up with clothing, hats and sunglasses, and applying sunscreen daily. When spending time outdoors, sunscreen should be reapplied every two hours or immediately after swimming or sweating," the statement concluded.

Each year, there are 5.4 million new cases of basal cell carcinoma and squamous cell carcinoma of the skin in the United States, alongside 178,000 new cases of melanoma, resulting in an estimated 9,000 deaths, the investigators noted.

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