What is PhotoTat?
Scientists at the University of Colorado in Boulder have created a novel tattoo pigment called PhotoTat. Unlike conventional inks that sit permanently visible on the skin, PhotoTat remains virtually invisible until it is illuminated with ultraviolet (UV) light. The formulation consists of microscopic beads of polymethyl‑methacrylate (PMMA), a biocompatible polymer already used in bone cement, intra‑ocular lenses and dermal fillers. Each bead, hundreds of times thinner than a human hair, carries a dye that can be triggered to change colour when exposed to UV radiation, producing a tattoo that can be turned on and off at will.
Why invisible tattoos matter for cancer survivors
Patients who undergo radiotherapy often receive tiny permanent dots on their skin to guide the radiation beam for each treatment session. After therapy ends, those marks linger as daily reminders of a painful episode, and a recent survey showed that roughly 70 % of breast‑cancer survivors view them negatively. PhotoTat offers a way to hide those markers: the dots stay hidden under normal lighting and only become visible when a clinician shines a UV lamp over the area, allowing precise localisation without the psychological burden of a permanent scar.
From permanent reminders to temporary signals
The technology also opens the door to other medical applications, such as indicating the exact spot where a skin biopsy was taken or providing a discreet guide for future procedures. Because the colour only appears under UV, the ink can be used in hospital rooms, outpatient clinics, or even at home without compromising privacy. In addition, the same principle could be adapted for temporary makeup that patients can switch on for special occasions and erase afterward.
How the ink works
When UV photons strike the PMMA beads, the embedded dye molecules undergo a rapid structural rearrangement. This change alters the way the molecules absorb and reflect visible light, causing the tattoo to flash into a vivid hue—typically blue, magenta or yellow—within seconds. The effect is comparable to photochromic sunglasses, but with a crucial difference: the colour in PhotoTat does not fade automatically when the UV source disappears. Instead, it requires exposure to bright white light to revert to its invisible state, which can take minutes to hours depending on the intensity of the illumination.
The science behind the colour shift
Because the beads are engineered to scatter light similarly to surrounding skin, the tattoo is essentially invisible under ambient conditions. The three primary dye colours can be mixed in varying proportions to generate a limited palette of secondary shades, enabling the creation of simple graphics or even full‑colour designs. Researchers demonstrated the concept on porcine skin, successfully reproducing radiation marks and a mock‑biopsy location that only appeared under a handheld UV lamp.
Beyond medical use: artistic possibilities
Artists and tattoo enthusiasts are already intrigued by the switchable nature of PhotoTat. The ink can be applied with a standard tattoo machine, allowing traditional techniques to be used while offering a new dimension of interactivity. By selectively erasing specific colour components with targeted laser light, creators can produce dynamic images that change when the viewer shines a UV pointer across the skin. Although the colour range is narrower than that of conventional inks—true cyan and white are still missing—the novelty of a tattoo that can be written, erased, and rewritten at will is compelling.
Real‑world test on a tattooed leg
Lead researcher Carson Bruns, an avid tattoo‑collector himself, used PhotoTat to ink a pattern of black fish‑scale motifs on his own leg in early 2024. Over the following eighteen months the design remained stable, and he could repeatedly reveal smiley faces, hearts and musical notes simply by flashing a UV torch. The ink persisted without fading, proving that the technology can survive the rigours of daily life on light‑coloured skin.
