A simple paper sensor featuring smiley and frowny faces drawn in UV-sensitive ink has been produced by researchers in a bid to keep us safe in the sun.

Scientists say the different expressions appear in sequence as UV exposure increases, offering a low-tech way for people to gauge when it is time to cover up.

The team add that they have produced different versions of the wristband for different skin tones – something they say is important given that darker skin can tolerate greater UV exposure before damage occurs, and more exposure is needed to produce vitamin D.

Further, the sensor can respond to varying levels of the different components of UV radiation: while UVA is thought to cause skin ageing and wrinkles, it is mainly UVB that causes sunburn and leads to skin cancer.

“We are always concerned whether we are exposing ourselves too much to the sun, and we might get a skin cancer, cataract, ageing, or other skin diseases. But if we don’t expose ourself enough to the sun we take the risk of being vitamin D deficient,” said Prof Vipul Bansal, lead author of the study from RMIT University in Australia.

While Bansal noted weather reports often contain a UV index to give a guide to the strength of UV radiation, it does not tell people whether they are within safe levels of exposure at that time, and is based on metrics for fair skin.

Writing in the journal Nature Communications, Bansal and colleagues describe how they sought to tackle the issue by creating a cheap device based on phosphomolybdic acid, or PMA. This substance turns from colourless to blue in the presence of lactic acid and UV radiation.

The team then used this mixture of lactic acid and PMA as an ink to draw four invisible faces on paper, and used transparent sheets of the sort used for overhead projectors to create filters that they put on top of them. This allowed the researchers to tweak the intensity of UV radiation reaching the ink so that the faces changed colour in sequence as exposure to UV radiation rose from 25% to 50%, then 75%, and finally 100% of a “safe” dose.

The team found that the PMA-based ink turns a darker blue colour under UVB than UVA over the same period of time, meaning that the higher the proportion of UVB in the sunlight an individual is exposed to, the quicker the sensor will indicate a 100% dose of UV radiation.

By further adjusting the number of filters for each of the four faces, the team were able to adjust the system to produce different sensors to work for six different skin tones.

Bansal said that while devices were designed using UV light in the laboratory, they are currently being tested in outdoor conditions. The team hope that the sensors will be on the market in a year, costing about a 1AUD (55p) a piece.

Matthew Gass of the British Association of Dermatologists, who was not involved in the research, said said the product does “have the potential to be another tool that helps people avoid excessive sun exposure, but they are not a replacement for actual sun protection in the form of sunscreen, protective clothing, and making use of shade.”



