University of Colorado ink lets light hide a tattoo while its pigment stays in the skin

The dyes change their molecular structure when exposed to specific wavelengths of light; the pigment remains in the skin while the tattoo's appearance changes.

"ultraviolet light writes the colour in, bright visible light erases it, and the cycle can be repeated"

Many people undergoing radiation therapy for cancer have to get small tattoos to help doctors guide the radiation beams.

"They're not beautiful, and they become a permanent reminder of a traumatic experience,"

"With PhotoTat, we could turn an involuntary medical tattoo into something that's only visible to the doctor when you go into the clinic, and then it can be invisible the rest of the time."

PhotoTat could potentially allow similar markings to be switched on during surgery and hidden afterward.

Bruns has tested each iteration of PhotoTat's inks on his own skin.

Bruns has had a fish-scale tattoo made with the method for the past two years and says he has experienced no known side effects.

"After over two years in my skin, I haven't seen any obvious fading yet."

The technology has been tested on silicon.

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,"

Bruns hasn't yet determined whether the inks are safe for people.

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,"

Bruns and his team found that all three PhotoTat channels remained switchable in the skin for between 1.4 and 2.2 years.

Earlier this year 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.

Extensive safety studies need to be carried out before the tattoos will be available to the public.

Bruns has tried the inks on himself: a black outline of fish scales on his leg filled in with PhotoTat inks.

"The tattoo is permanent, but its appearance is not."

Photochromic dyes change from a transparent state to a coloured one when exposed to certain wavelengths of light.

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

"If you never want to see it again, just never turn it on again."

Someone with one of these tattoos would need to apply sunscreen to prevent the colour activating when exposed to sunlight.

The light-sensitive tattoos can currently be made from 16 clearly distinguishable colours, with at least 300 shades; white is unavailable and the researchers have been unable to develop a true cyan.

The blue ink gradually develops a permanent, purplish colour as the dye slowly photo-degrades over many on/off cycles.

The human skin testing described across the three reports is Bruns's own.

PhotoTat is not yet available at tattoo parlours; Bruns and colleagues hope to keep improving the designs in coming years and eventually make the inks available to body-art fans.