Scientists Made a Tattoo You Can Erase and Redraw With a Flashlight

A tattoo that can be switched on, switched off, and even redrawn using nothing but light just passed a two-year test inside real human skin. Researchers at the University of Colorado Boulder tattooed the ink into a person’s leg and watched it for more than two years; at the end, it still switched colors on command.

Called PhotoTat, the ink works something like a dimmer switch for skin art. Shine ultraviolet light on it, and color appears within seconds. Shine ordinary bright white light on it, and it fades back to nearly invisible. The pigment never leaves the skin, so in that sense it’s as permanent as a regular tattoo, but the color it shows at any given moment depends entirely on who points a light at it, and when.

Lead researcher Carson J. Bruns tattooed the experimental ink into his own leg and let his team track it over time. Their results, published in the journal Matter & Light, point to something bigger than a lab curiosity: under real human skin, the ink kept switching on and off, kept being erasable, and showed no obvious decline in function after more than two years.

How the Color-Changing Tattoo Actually Works

Behind the effect is a class of molecules called photochromes: compounds that change color when hit with light and then hold that new color until a different type of light switches them back. These molecules aren’t new. For two decades, scientists have used them in rewritable paints, nail polish, and even 3D-printed objects that change color on command. What nobody had figured out was how to put them safely and permanently inside living skin.

To solve that, the researchers packed the color-changing molecules into plastic particles so small they’re invisible to the naked eye, roughly the size of specks of dust used in ordinary tattoo ink. The plastic shell, made from the same material used in some medical implants like bone cement and artificial lenses, is designed to seal the molecules away from the surrounding tissue. The particles come in three base colors: blue, magenta, and yellow. Mixed together, they produce a decent range of hues, though the researchers admit the overall range is narrower than what a commercial printer can achieve.

Putting the ink into skin required no special tools. The team used standard tattoo machines, the same needles and equipment any tattoo artist works with, to apply the ink to practice skins made of silicone, to pig skin samples, and eventually to the lead researcher’s own leg. Before testing on a real person, they confirmed the ink showed up clearly across a range of skin tones, from very light to darker, using silicone models built to mimic that range.

Once tattooed, the color could be switched on with a handheld UV flashlight in just a few seconds, and switched off with an ordinary bright light almost as fast. Indoors, under normal room lighting, a tattooed image stayed visible for hours or even days without any extra effort, meaning the ink doesn’t flicker or drift on its own. It holds whatever state it is left in until a light changes it again.

What the Two-Year Skin Test Showed

This in-skin trial was small by design: it involved exactly one person, the lead researcher, who tattooed himself with dozens of patches of the ink between January 2024 and January 2025 and tracked how they performed over time. Because this was one individual rather than a group of patients, the authors describe the results as an early proof of concept rather than a final answer. Larger studies are reportedly already underway.

What that one-person test did show was encouraging. Blue, magenta, and yellow all kept switching on and off after more than a year in living skin, with some patches still working at the two-year mark. The researcher drew a smiley face on one tattoo using a UV flashlight fitted with a makeshift narrow tip, erased it with white light, then drew a heart using a printed stencil, erased that, and drew a musical note with a different stencil, all on the exact same patch of skin. Each cycle of drawing and erasing took only a few seconds.

Researchers also tested whether different colors of light could erase just one shade out of a mixed-color tattoo while leaving the others untouched. Using a portable projector that shoots red, green, and blue laser light, they found they could nudge a dark, three-color mixture toward distinct hues depending on which color laser they used, confirming that the underlying trick works even through real, living skin.

Researchers also stumbled onto a practical side use: sunscreen testing. Because the ink reacts to UV light, applying SPF 30 sunscreen over an activated tattoo made it fade completely within minutes in sunlight, while an unprotected patch stayed visibly colored. That points to a possible side role for the ink, flagging whether sunscreen is blocking UV rays, an effect shown so far only in skin models and one that builds on earlier tattoo designs made specifically for UV monitoring.

Where the Technology Falls Short

This isn’t a finished product. The researchers flagged two big problems. First, the blue pigment showed signs of wearing out after repeated on-off cycling, developing a faint purple tint that wouldn’t fully disappear even after long exposure to white light. That kind of residue undercuts the whole pitch of a tattoo that can go fully invisible, since the appeal depends on a clean off state. Second, the current three-color system can’t produce true cyan or white, which limits the color range the tattoo can display compared to a printed photo.

A single-subject trial also can’t answer some questions. The researchers note that how skin tone affects the tattoo’s look and performance over time still needs study across a wider group of people, and that writing a full, multi-color image directly into living skin, rather than just showing the color-shifting mechanism works, remains a task for later research. A full round of safety testing, including checks for toxicity and purity, still needs to happen for each color before the ink could see broader use, according to the paper.

Why a Color-Changing Tattoo Could Matter

A traditional tattoo’s appeal and its biggest drawback are the same thing: permanence. People spend years regretting tattoos, and current removal methods are reportedly painful, expensive, and often don’t fully work. A rewritable tattoo sidesteps that problem by making the ink itself the variable, not the skin underneath it.

Most of the compelling uses described in the paper have little to do with fashion. Radiation therapy patients are sometimes marked with permanent tattoos to help line up treatment beams, marks that patients often come to resent once treatment ends, according to the research. A rewritable version could appear only when a clinician needs it and disappear the rest of the time. The same logic applies to permanent makeup, which currently locks a person into one look for good. What this study shows, carefully and in just one person over two years, is that turning skin into something closer to a programmable screen is no longer just a thought experiment. It worked, at least once, and that’s enough to take seriously.

Disclaimer: This research is a single-subject proof of concept. The in-skin results come from one person, the study’s lead author, who co-founded and holds financial interests in the companies commercializing the technology. The findings are not generalizable across different people or skin tones, and the ink has not completed the toxicity, sterility, and purity testing required before any medical or commercial use.

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