Invisible tattoos that appear when illuminated

A team of engineers in Colorado, USA, has designed a new line of multicolored tattoo inks that switch on and off with different kinds of light. Described in the Cell Press journal Matter & Light, the technology could one day help people hide their tattoos when they have an important business meeting. They could even doodle on their own skin using a simple laser pointer.

“Tattooing is a very ancient technology that remains mostly untapped as a platform for today's technologies,” commented lead author Carson Bruns of the University of Colorado Boulder.  

The new inks, known as PhotoTat, are made from dyes and a plexiglass-like material common in many medical technologies on the market today, including dermal fillers. The research is a passion project for Bruns, who has many of his own tattoos, including a green lion on his arm. “I love body art because of how personal it is,” he said. 

To make the inks, Bruns and his colleagues start with a material called polymethyl methacrylate, or PMMA, which forms incredibly small spheres that look like they are made of glass. The team then inserts a range of color-changing dyes into the spheres. 

“By mixing these primary colors, we can generate a full color image,” said Bruns.

Normally, these dyes are completely invisible to the human eye. But when illuminated by ultraviolet light, they change color in a matter of seconds—turning a vibrant blue, magenta, or yellow. The colors fade over several hours, but they can also be switched off faster by exposing them to a bright LED light. 

Photonic ‘switch’

“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,” said Bruns.

PhotoTat is not yet available in tattoo parlors. But Bruns and his colleagues hope to continue improving their designs in the coming years, eventually making the inks available for fans of body art across the country. 

Beyond cosmetics, Bruns also foresees practical applications for the technology. For example, many people undergoing radiation therapy for cancer have to get small tattoos on their body to help doctors guide the radiation beams. Those markers can be scarring, both mentally and physically. 

“They’re not beautiful, and they become a permanent reminder of a traumatic experience,” said Bruns. “With PhotoTat, we could turn an involuntary medical tattoo into something that's only visible to the doctor when you go in the clinic, and then it can be invisible the rest of the time.” 

Bruns has not yet determined whether the inks are safe for people. But he has tried the inks on himself. He has a black outline of fish scales on his leg, which he’s filled in with PhotoTat inks.  With a little light, Bruns can turn those scales red, orange, blue, and more—almost like “magical mermaid scales,” he said. 

“There is a stigma, in science and in many cultures, around body art,” Bruns said. “I'm proud to be pushing against that.” 

This research was supported by funding from HYPRSKN Inc., the U.S. National Science Foundation, and the College of Engineering and Applied Science at the University of Colorado Boulder.