Researchers from RMIT University in Australia have developed an ink that changes colour when exposed to different types of ultraviolet (UV) radiation. They used the ink to create low-cost wearable sensors for monitoring UV exposure throughout the day. Such sensors could help people manage vitamin absorption while avoiding sun damage (Nature Communications 9 3743).
Humans need a modest level of sun exposure to maintain healthy levels of vitamin D. Excessive exposure, however, can cause health problems. UVA (315–400 nm) radiation penetrates deep into the skin and results in skin aging and wrinkling. UVB (280–315 nm), meanwhile, is particularly effective at damaging DNA. Too much UVB exposure can cause sunburn, which increases the likelihood of developing skin cancer and cataracts.
Healthy exposure levels, however, depend upon an individual’s skin colour classification, which can range from very fair (type I) to darkest brown (type VI). The minimal erythemal dose (MED) — the threshold UV dose that produces sunburn — is five times higher for type VI skin than for type I, while darker skin types require longer time in the sun to absorb healthy amounts of vitamin D. In addition, some diseases and medications can increase skin photosensitivity or reduce the ability to absorb vitamins through diet.
Monitoring sun exposure thresholds is thus a highly individual process. To address this, the researchers created colour-changing sensors in six variations to reflect the range in human skin tone. “We are excited that our UV sensor technology allows the production of personalized sensors that can be matched to the specific needs of a particular individual,” says co-senior author Vipul Bansal.
The sensors are made from a photoelectrochromic phosphomolybdic acid–lactic acid (PMA–LA) mixture that turns from transparent to blue upon UV exposure. The researchers used PMA–LA-based ink to draw smileys on four filter paper discs attached to a flexible band. Each smiley is covered with an increasing number of low-cost transparency film filters (TFFs) that increasingly reduce UV transmittance.
In each sensor, the first two smileys are happy faces that appear at 25% and 50% of the UVB MED. The third one is a flat smiley representing 75% MED, which provides a pre-warning of UV exposure threshold. Finally, a frowning face appears, warning that the user has approached the maximum safe threshold and must avoid further exposure.
By changing the number of TFFs covering each smiley in the sensor strip, the researchers could personalize the UV sensors to specific skin types. They created six sensors for the six skin colour classifications.
Bansal says that this development could help to provide people with an accurate and simple measure of their personal exposure levels throughout the day. Importantly, the fabrication of these skin-specific UV dosimeters only requires readily available, low-cost components such as filter paper, a fountain pen and transparency sheets.
“We can print our ink on any paper-like surface to produce cheap wearable sensors in the form of wrist-bands, head bands or stickers for example,” he explains. “The low cost and child-friendly design of these UV sensors will facilitate their use as educational materials to increase awareness around sun safety.”
