Physical Review Letters, 2026. DOI: 10.1103/gbkc-6nlj.
Invisible tattoos that become visible under light
Photo credit: Matter & Light, Bruns et al./CC BY-SA
Many people wear tattoos, but there are situations in which it is desirable to hide them. A new line of multi-colored tattoo inks could one day solve this dilemma; According to an article published in the journal Matter & Light, they are invisible to the human eye unless you illuminate them with UV light. “Body art is stigmatized in science and in many cultures,” said co-author Carson Bruns of the University of Colorado at Boulder, who, among other things, has a green lion tattoo on one arm. “I’m proud to take action against this.”
As previously reported, epidermal electronics applied to the skin via temporary tattoos (e-tattoos) have been around for more than a decade. So-called e-tattoos are attached to the skin without adhesives, are practically invisible and are typically attached by temporary tattoos. They enable electrical measurements (and other measurements such as temperature and strain) using ultra-thin polymers with embedded circuit elements. In 2024, researchers developed special polymer-based conductive inks that can be printed on a person’s scalp to measure brain waves, even if they have hair. This could one day enable, among other things, mobile EEG monitoring outside of a clinical setting. And earlier this year, scientists at Pennsylvania State University developed a novel conductive ink that can be applied directly to the skin in colorful, custom designs and, once dry, transforms into a functional electrode for biomonitoring.
Bruns et al. have developed their inks, known as PhotoTat, from color-changing dyes injected into tiny beads made of polymethyl methacrylate (PMMA), a material used in dermal fillers for medical applications, among other things. By mixing the primary dyes, full-color images can be created. The tattoos are invisible until exposed to UV light and then turn blue, magenta or yellow. They fade after a few hours, faster in bright LED light. It’s still a proof of concept, but Bruns tested the dyes on himself, coloring in a black outline of fish scales that turn red, orange or blue under UV light, like mermaid scales. The dyes could also help cancer patients undergoing radiation therapy who need small tattoos so doctors can direct the beams. PhotoTat would make such tattoos invisible except during treatment.
