Physical Review Letters, 2026. DOI: 10.1103/gbkc-6nlj.

Invisible tattoos that show up under light

Many people sport tattoos, but there are situations where hiding them is desirable. A new line of multicolored tattoo inks could one day make that dilemma moot; they are invisible to the human eye unless one illuminates them with UV light, according to a paper published in the journal Matter & Light. “There’s a stigma, in science and in many cultures, around body art,” said co-author Carson Bruns of the University of Colorado, Boulder, who sports a green lion tattoo on one arm, among others. “I’m proud to be pushing against that.”

As previously reported, epidermal electronics attached to the skin via temporary tattoos (e-tattoos) have been around for more than a decade. So-called e-tattoos connect to skin without adhesives, are practically unnoticeable, and are typically attached via temporary tattoo, allowing 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 onto a person’s scalp to measure brain waves, even if they have hair. This could one day enable mobile EEG monitoring outside a clinical setting, among other potential applications. And earlier this year, scientists at Pennsylvania State University developed a novel conductive ink that can be painted directly onto the skin in colorful custom designs, turning into a functional electrode for biomonitoring after drying.

Bruns et al. created their inks, known as PhotoTat, from color-changing dyes injected into tiny spheres of polymethyl methacrylate (PMMA), a material used in dermal fillers among other medical applications. Mixing the primary-colored dyes can create full-color images. The tattoos are invisible until exposed to UV light, when they turn blue, magenta, or yellow. They fade after a few hours, faster if exposed to bright LED light. It’s still a proof of concept, but Bruns did test the dyes on himself, inking a black outline of fish scales that turn red, orange, or blue, like mermaid scales, under UV light. The dyes could also help cancer patients undergoing radiation therapy, who need small tattoos so doctors can guide the beams. PhotoTat would make such tattoos invisible except when the treatment is being administered.