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A miniature wearable sensor has been designed to let you know if you’ve spent too much or too little time in the sun.

Excess exposure to the sun’s ultraviolet (UV) rays can cause sunburn, premature skin ageing and skin cancer, while underexposure can lead to vitamin D deficiency and worsen skin conditions like eczema and psoriasis.

Several devices exist for monitoring sun exposure, but they are usually bulky and expensive. Most are also non-waterproof, making them inappropriate for use during sweaty or water-based activities.

To address these issues, at Northwestern University in the US and his colleagues developed a small, low-cost, waterproof UV sensor that can be stuck directly to the skin or attached to jewellery or clothing.

The circular sensor is 8 millimetres wide and 1 millimetre thick and weighs 50 milligrams. It converts UV light to an electric current, the magnitude of which indicates the level of sun exposure. An inbuilt radio antenna transmits this information wirelessly to the user’s smartphone.

Sunburn risk

The individual’s sunburn risk is then calculated based on their UV dose and other information they provide about their skin type and how much sunscreen and clothing they are wearing. “This allows users to receive a very specific recommendation,” says Xu.

The sensor was tested by 9 volunteers in Brazil and 13 in Florida. They stuck it to their fingernails, hands or arms while they did recreational activities on a rooftop, went swimming, showered with soap, or went for long walks.

The sensor measured UV dose as accurately as a commercial device, and its bendy, plastic casing made it comfortable to wear and resistant to heat, soap and water.

Xu says he hopes the technology will help to curb the current skin cancer epidemic in the US and Australia by letting people know when they should apply sunscreen or go indoors. At the same time, it can be used by people with eczema and psoriasis to make sure they get adequate sun, he says.

The sensor and smartphone app have been available to buy for $60 since last month after being commercialised by cosmetics company L’Oréal.