Researchers at the University of Colorado Boulder have developed tattoo ink that becomes invisible and can be toggled on and off with light. The work, published in Matter & Light, uses photochromic dyes that change color when exposed to ultraviolet light and revert to colorless when exposed to white light.
Photochromic compounds change color by absorbing light energy, which alters the molecular structure. In these tattoo inks, two to three seconds of bright UV light forms a new ring in the dye molecule, switching it from colorless to colored. White light reverses the reaction. The pigments are encapsulated in poly(methyl methacrylate), a polymer used in bone cement and dentures, creating inks called PhotoTat.
Carson Bruns' team demonstrated designs ranging from a lantern that reveals light when activated to multicolored scales. A red version called Magic Ink launched in 2024 by Colorado biotech company Hyprskn at £77 per 2-milliliter vial, about 100 times the cost of regular tattoo ink. Bruns has tested the inks extensively on himself, with hundreds of tattoos on his legs. The team has since developed blue, magenta, and yellow PhotoTat inks that can combine into 16 additional colors.
Challenges remain. Direct sunlight contains enough UV radiation to activate the tattoos without sunscreen protection. Indoor light slowly deactivates them over several days. The blue ink currently degrades after only 10 to 100 activations, leaving a permanent purple mark. Most significantly, the inks require thorough toxicity and safety testing before general release to tattoo shops, a process expected to take several years.
Bruns envisions medical applications beyond fashion, such as skin markings to guide radiation therapy or identify biopsy sites. He suggests invisible tattoos could reduce psychological impact by allowing patients to hide permanent clinical markings once they are no longer medically necessary.