Light‑Activated Tattoos Offer On‑Demand Visibility

Researchers at the University of Colorado have developed a new tattoo ink that can be turned on and off with light. The photochromic dyes change from transparent to coloured when exposed to ultraviolet light and revert to clear under visible light, allowing the same permanent tattoo to appear or disappear at will. Carson Bruns, who has worn a fish‑scale design for two years, reports no side effects and notes that the ink remains visible for months. The technology, dubbed “PhotoTat,” can be applied with standard tattoo equipment and is available in 16 base colours—over 300 shades—though blue fades to purple after repeated cycles. Users must apply sunscreen to prevent accidental activation by sunlight. While the polymer used is non‑toxic, the long‑term safety of the dyes is still under study, and similar concerns that affect traditional inks—such as potential cancer links—may apply. The team also highlighted a temporary graphene tattoo that can monitor blood pressure with high accuracy, pointing to future medical uses of light‑responsive skin markings.