six arms of different skin tones, each wearing UV wristband

The Granada–RMIT research team has created a wearable UV sensor that is capable of differentiating between types of UV radiation and can be customized according to skin tone. [Image: University of Granada and RMIT University of Melbourne]

Imagine a Fitbit—except that, instead of tracking steps, heart rate or sleep pattern, the accessory monitors skin cancer risk.

Researchers at the University of Granada, Spain, and RMIT University in Melbourne, Australia, have developed just such a personalized monitoring system for solar ultraviolet (UV) exposure—one you can wear on your wrist (Nat. Comm., doi: 10.1038/s41467-018-06273-3). The low-cost, paper-based chromogenic UV sensor employs invisible ink, allowing for naked-eye UV monitoring based on an individual’s skin tone.

The Sun: Friend or foe?

Skin cancer is the world’s most common type of cancer, and it is most often caused by excessive exposure to solar UV radiation. Yet sun exposure is also necessary for maintaining healthy body functionality, including the production of vitamin D. Finding the right balance of sun exposure can be challenging; add in the fact that every person requires a different amount of UV exposure depending on skin tone, and the challenge compounds.

Moreover, not all UV radiation is created equal—some types are more damaging than others. Yet, according to the Grenada–RMIT research team, the UV sensors now on the market measure only overall radiation, and do not distinguish between the three types of UV radiation: UVA (315–400 nm), UVB (280–315 nm), and UVC (100–280 nm). Each type has a different impact on human health. UVA radiation causes skin wrinkling; UVB radiation is associated with sunburn and increases the likelihood of skin cancer; and shortwave UVC radiation, with the highest energy, can cause the most damage.

“UVB and UVC radiation is retained by the ozone layer,” explains team-lead José Manuel Domínguez Vera, University of Granada, in a recent press release. “This sensor is especially important in the current context, given that the hole in the ozone layer is exposing us to such dangerous radiation.” This is particularly true in regions like Australia, where the thinned ozone layer means that an estimated two out of three people will be diagnosed with skin cancer by age 70.

Current UV sensors are expensive and not viable for consumer-based monitoring day to day. Moreover, tools such as the UV index, which is reported alongside daily weather reports, are very limited when it comes to ethnically diverse populations. Different skin types have different levels of UV tolerance—individuals with darker skin require more time spent in the sun to produce healthy amounts of vitamin D, and those with lighter skin are more susceptible to UV damage.

It’s all in the ink

Tackling these drawbacks meant developing a spectrally selective, affordable, disposable sensor for personal use in real time. So the research team designed a special invisible ink that offers a color signal for UV exposure. The ink contains UV-sensitive phosphomolybdic acid (PMA) and lactic acid (LA), which causes chemical reduction in the PMA when it’s exposed to UV radiation, producing a blue color. This multi-redox active photoelectrochromic solution generates a unique response to specific UV wavelengths, allowing for spectrally selective differentiation of UVA, B and C by the naked eye.

Scatterings image

The “smiley faces” on the wristband UV sensors correspond to differing levels of UV exposure, ranging from 25 percent to 100 percent. [Image: University of Granada and RMIT University of Melbourne]

The team tricked out a standard fountain pen to produce the invisible ink, and hand-drew happy and sad emoticon faces on a flexible paper band. Depending on the type and intensity of the UV exposure, the ink on the paper turns blue as an individual reaches 25, 50, 75 and 100 percent of their recommended daily UV dose. By coating the emoticons with an increasing number of transparency film filters (TFF), the UV transmittance is increasingly reduced on the emoticons from left to right.

Layering the low-cost, readily available TFFs over the emoticon faces is also how the team achieved skin-tone customizability. Increasing the number of TFF layers controls the UV dose reaching the sensor, so the color changes faster or slower depending on the user’s skin tone. Using this technique, the researchers tailored sensors for six different skin phototypes, ranging from very fair to dark brown.

They calculated that the minimal erythemal dose (MED), or the threshold dose of UVB that produces sunburn, ranges from 200 J m-2 for very fair skin (type I) to 1000 J m-2 for dark brown skin (type VI). In their experiments, the researchers showed that the PMA–LA ink maintains spectral selectivity and accuracy at very low UV doses (20 J m-2) and both within and beyond the prescribed UVB MED for different skin types.

More applications

According to the team, the sensor generates unambiguous real-time colorimetric warning indications of different UV scales with increasing UV radiation exposure. Further, the researchers determined that the sensors are highly durable under various extreme environmental conditions, and that the ink remains stable for at least eight weeks. The team emphasizes that production of these sensors is low-cost and should be easily scaled up, and can thus empower users with different skin phototypes to control their maximum sun-safe limits.

Beyond personal health, the team also foresees potential agricultural and industrial applications for its sensor. Crops require an optimum UV-exposure balance as well, and the sensor technology could also be applied by various industries looking to evaluate the effect of prolonged UV exposure on engineered products.