Tattoos could be radically reimagined following the development of inks that can be turned on and off by exposing them to light. This could be used medically – or to hide the butterfly on your ankle from a judgemental relative.

“You can simply turn the tattoo off whenever you don’t want to see it and it disappears from view,” says Carson Bruns at the University of Colorado. “If you never want to see it again, just never turn it on again.”

Bruns has had a tattoo of fish scales using this method for the past two years, and says he has experienced no known side effects. “I get a lot of compliments on the fish scale tattoo,” he says. “After over two years in my skin, I haven’t seen any obvious fading yet.”

Instead of traditional inks, which people have used for thousands of years to decorate their bodies, Bruns and his colleagues employ photochromic dyes, which change from a transparent state to a coloured one when exposed to certain wavelengths of light. They call the new technology, which has been tested on silicon, “PhotoTat”.

There is little mechanical difference between a traditional tattoo and the new inks, says Bruns. Both are pigment particles implanted in the dermis with a needle, he says. They also both stay there permanently and are applied with the same machines and methods.

The difference is the pigment, says Bruns. With PhotoTat, “ultraviolet light writes the colour in, bright visible light erases it, and the cycle can be repeated”, he says. “The tattoo is permanent, but its appearance is not.”

This could be useful in medical applications, such as for the tattoos used by doctors to guide radiation beams, which leave many patients permanently marked, says Bruns.

The light-sensitive tattoos can currently be made from 16 colours, which are clearly distinguishable from each other. From these colours, there are at least 300 different shades. White is unavailable because it reflects light, rather than absorbing it, so can’t be switched on and off like other colours. The researchers have also been unable to develop a true cyan.

Another issue is that the blue ink gradually develops a permanent, purplish colour as the dye slowly photo-degrades over many on/off cycles. Someone with one of these tattoos would also need to apply sunscreen to prevent the colour activating when they are exposed to sunlight.

In terms of safety, Bruns says the polymer that encapsulates the dyes is non-toxic and has a long history of use in permanent biomedical implants. “The colour-changing [dye] toxicity is unknown, but the toxicity of most tattoo pigments is equally unknown,” he says.

Still, extensive safety studies need to be carried out before the tattoos will be available to the public.

Signe Bedsted Clemmensen at the University of Southern Denmark says tattoo ink in the body is a foreign substance, which has been linked to cancer, regardless of whether it is visible or invisible. “Therefore, many of the same health considerations and concerns that apply to traditional tattoo ink are also likely to apply to invisible tattoo ink,” she says.

Earlier this year, it emerged that red tattoo ink caused one man to lose all his hair and stop sweating. Tattooing has also recently been linked to localised damage to the immune system.