Chemists Develop Switchable Tattoo Ink That Appears and Vanishes With Light

Two or three seconds of bright ultraviolet light is all it takes. Under it, a colorless pigment sitting beneath the skin closes a new ring in its molecular structure and turns into a colored one. A blast of bright white light breaks the ring again, and the color switches off. That is the core of a tattoo ink developed by researchers at the University of Colorado Boulder, described in the journal Matter & Light.

The team, led by Carson Bruns, made designs with the ink, including a lantern that shows its glow only when activated and a set of multicolored scales in the style of the children's book "The Rainbow Fish." Once switched on with a UV flashlight, a design stays visible until white light turns it off.

The pigments are photochromic dyes, compounds that change color when exposed to light, the same family used for years in color-changing sunglasses. Light carries enough energy to trigger a chemical change that alters the shape of the molecule and with it the molecule's properties. Different molecular structures produce different colors.

The chemistry has long had two problems. The colored form of the dye was unstable and tended to slip back to its colorless state, especially in heat. The dyes also suffered fatigue: after enough on-off cycles they degraded and settled into a permanent tint, which is why transition-lens sunglasses weaken with age. Recent work has produced far sturdier dyes that hold their color for months and survive more than 10,000 activation and deactivation cycles.

Bruns' group showed these dyes could be made into tattoo inks by encapsulating them in poly(methyl methacrylate), or PMMA, a polymer already approved for medical use in bone cement, dentures and intraocular lenses. The family of inks is called PhotoTat.

A red version is already on sale. The Colorado biotech company Hyprskn launched it in 2024 as Magic Ink, and the ink complies with the EU's Reach chemical safety regulations. A 2-milliliter vial costs £77, roughly 100 times the price of ordinary tattoo ink, a premium the source attributes to the specialized dyes and formulation. Tattoo-community reaction was mixed: some saw a revolutionary creative tool, others an overpriced gimmick.

Now Bruns' team has added three more PhotoTat inks, blue, magenta and yellow, which can be combined to make 16 other colors. Bruns says the only person to test the ink so far is himself. "I actually have hundreds of these tattoos on my legs now and they've all gone really well," he says.

Several problems remain. Direct sunlight carries enough UV to activate the tattoos unless sunscreen protects them, and indoor light will slowly switch them off over several days. The blue ink currently degrades after only 10 to 100 activations, leaving a permanent purple mark. Most importantly, the inks need thorough toxicity and safety testing before reaching tattoo shops, which will likely take several years.

Bruns also sees uses beyond fashion. Patients who need permanent skin marks to guide radiation therapy or to identify biopsy sites could have markings that go invisible once the clinic no longer needs them. He argues that such marks otherwise "become a permanent reminder of a traumatic experience." For now, the next steps are fixing the blue ink's fatigue and completing the safety work.