How handy would it be if you could make your tattoos invisible for a job interview in the morning, and reveal them again for a party in the evening? This sounds like science fiction, but it may soon be reality.
Researchers at the University of Colorado Boulder have developed invisible tattoo ink than can be revealed with a UV torch and stays visible until switched off with white light. They have published their research in the journal Matter & Light.
They created different motifs with this ink ranging from a lantern that reveals its light when activated, to multicoloured scales like those in the much-loved children’s book The Rainbow Fish.
The pigments used in these inks are so-called photochromic dyes (from the Greek “photos”, which means “light” and “chroma”, which means “colour”). In other words, they are compounds that change colour when exposed to light. These dyes have been used for years in colour-changing sunglasses.
Light carries energy, and photochromic compounds are designed in such a way that this energy is enough to cause a chemical change that alters the shape and therefore the properties of the molecule.
In the case of the “invisible” tattoo ink, two or three seconds of bright ultraviolet (UV) light is enough to form a new ring in the dye molecule, which switches a colourless pigment to a coloured one. A variety of colours are available depending on the exact structure of the molecule.
A blast of bright white light reverses the reaction, breaking the ring and turning the colour off again. For a long time, the challenge with this chemistry was that the coloured form of the dye was relatively unstable and quickly reverted to the more stable, colourless form – especially at higher temperatures.
Another issue was fatigue: after a certain number of on-off cycles, the dyes degraded and took on a permanent coloured hue. Have you ever bought transition lens sunglasses and noticed that they became less effective over time? This is due to the same deterioration of photochromic compounds in the lenses. But thanks to recent research efforts, remarkably stable dyes have been designed that reliably hold their colour for months and can undergo over 10,000 activation and deactivation cycles without degrading.
Carson Bruns’ team at the University of Colorado Boulder showed that these pigments could be formulated into tattoo inks by encapsulating them in poly(methyl methacrylate) (PMMA), a polymer approved for medical use in bone cement, dentures and intraocular lenses. This family of inks is known as PhotoTat.
Matter & Light, Bruns et al., CC BY-SA
Time to market
So, when will you be able to get one of these switchable tattoos? A red pigment was already launched in 2024 under the name Magic Ink by Colorado-based biotech company Hyprskn. A 2 millilitre (ml) vial will set you back £77, which is about 100 times more expensive than regular tattoo ink due to the specialised dyes and formulation process required. The ink is fully compliant with the EU’s Reach (Registration, Evaluation, Authorisation and Restriction of Chemicals) safety regulations.
Now, Bruns’ research team has developed three more PhotoTat inks – blue, magenta and yellow – that can combine into 16 other colours. “So far,” Bruns says, “the only person who’s tested this ink is me. I actually have hundreds of these tattoos on my legs now and they’ve all gone really well.”
Some challenges remain though: direct sunlight contains enough UV radiation to activate the tattoos unless they are protected by sunscreen. Indoor light will slowly switch them off over a period of several days.
Miriam O’Duill
In addition, the blue ink currently degrades after only 10–100 activations, leaving a permanent, purple-coloured mark. Most importantly, the inks need to undergo thorough toxicity and safety testing before they can be rolled out to tattoo shops for general use, which will likely take several years. Don’t get your hopes up just yet about your new “business during the day, party at night” look.
When Hyprskn launched Magic Ink, the reception in the tattoo community was mixed. While some tattoo enthusiasts viewed it as a revolutionary creative tool, others thought it was an overpriced gimmick. Bruns does not only see his photochromic tattoo inks as fashion accessories, however.
He also envisages medical applications, for example for patients who need permanent skin markings to guide radiation therapy or identify sites for biopsies. Bruns argues that these markings “become a permanent reminder of a traumatic experience” which can be mitigated by making the tattoo invisible once it is no longer needed in a clinical setting.
