The tattoo that can keep a secret: ink that appears under UV light and vanishes under white

How handy would it be if a tattoo could clock off? Researchers at the University of Colorado Boulder have created an ink, mixed with ordinary tattoo ink, whose designs colourise under ultraviolet light and fade away again under white light — a lantern that lights only when switched on, scales in the colours of The Rainbow Fish. The pigments are photochromic dyes: molecules that change colour with light. Beyond the party trick, the team imagines medical uses — permanent skin markings that guide radiotherapy or mark biopsy sites, visible only when a clinician needs them. The research is published in the journal Matter & Light.