A new multicolored tattoo ink called PhotoTat is designed to remain invisible under ordinary conditions and appear in bright colors when exposed to ultraviolet light. The researchers say the technology could eventually provide discreet body art and hidden medical markings, including the small tattoos used to guide radiation therapy. The work is described in a 2026 paper in Physical Review Letters, DOI: 10.1103/gbkc-6nlj.
Invisible tattoos that appear under ultraviolet light
Many people have tattoos but may sometimes want to conceal them. PhotoTat could eventually offer a way to do that: The ink is invisible to the human eye until ultraviolet light activates it, causing it to appear blue, magenta, or yellow. By combining primary-color dyes, the researchers can produce full-color images.
“There is a stigma against body art in science and in many cultures,” co-author Carson Brands said in a statement. Brands, a professor at the University of Colorado Boulder, has a green lion tattooed on one arm. “I’m proud to be against it.”
From electronic tattoos to light-activated ink
Epidermal electronics attached to the skin through temporary tattoos, often called e-tattoos, have been developed for more than a decade. These devices can connect with the skin without conventional adhesive and are generally nearly invisible. Ultrathin polymers embedded with circuit elements allow them to measure electrical signals as well as properties such as temperature and strain.
In 2024, researchers developed a polymer-based conductive ink that could be printed onto a person’s scalp to measure brain waves, even through hair. The approach could enable mobile electroencephalography monitoring and other applications outside clinical settings.
Earlier this year, scientists at Pennsylvania State University developed another conductive ink with a customizable, colorful design. Once applied directly to the skin and allowed to dry, it becomes a functional electrode for biological monitoring.
How PhotoTat works
Brands and his colleagues created PhotoTat using a color-changing dye enclosed in tiny spheres of polymethyl methacrylate, or PMMA. The material is used in dermal fillers and other medical applications. The tattoo remains hidden until ultraviolet exposure activates the dye. It fades after several hours and disappears more quickly under bright LED light.
The work remains a proof of concept. Brands tested the dye on himself by creating the black outline of a fish’s scales. Under ultraviolet light, the pattern changes to red, orange, and blue, evoking a mermaid’s scales.
The same technology could also have a medical application. Radiation therapy often uses small tattoos to help doctors align treatment accurately. PhotoTat could make those markings invisible during everyday life while allowing them to be seen when needed during treatment.
Source: arstechnica.com
