A tattoo is supposed to last a lifetime, but what if you could make it disappear before a job interview, bring it back for a night out, or change its colours without removing the ink?

Researchers at the University of Colorado Boulder have developed PhotoTat, a light-sensitive tattoo ink that makes this possible. This ink can be activated, faded and rewritten using different wavelengths of light—allowing it to switch between vivid colours and near invisibility.

In simple words “PhotoTat is like a switch. You turn it on with UV light, but you can turn it off again when you go to grandma’s house,” Carson Bruns, lead researcher and an associate professor at the University of Colorado Boulder, told 9News. 

This technology could transform body art into something you can control, while offering a way to hide medical tattoos until doctors need them.

The key to PhotoTat is a class of chemicals called photochromic dyes, which change colour when exposed to particular wavelengths of light. 

The researchers encapsulated three commercially available dyes in tiny spheres of poly(methyl methacrylate), or PMMA, a transparent plastic also known as plexiglass. Each capsule is only a few hundred nanometres across.

The dyes remain nearly colourless under ordinary visible light but develop vivid colours when exposed to ultraviolet (UV) light. The team selected dyes that respond to 365-nanometre UV light, which is readily available from LEDs. 

It takes about two to three seconds of bright light to activate or deactivate the pigments. UV cameras can reveal details that are otherwise invisible under ordinary lighting, illustrating how different wavelengths can change what we see.

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