Want invisible ink? Just put water in your inkjet

But you’ll need this special paper to make it work.

Chinese researchers have developed a new paper that provides a cheap and convenient way of keeping secrets. Anything printed on it with water is invisible to the naked eye and can be revealed only under ultraviolet lighting at a particular frequency.

What’s more, the protected information can be erased by briefly heating the document with a blow dryer, and the blank document can then be reused in the same way at least 30 times.

Qiang Zhao and colleagues, from the Nanjing University of Posts and Telecommunications, created two prototypes of their new paper: one that uses plain water for everyday low-level security, and another that uses additional chemicals for even higher levels of secrecy.

They write in the journal Matter that their work “could be considered a major step forward toward rewritable and multi-level security printing”.

Using fluorescent security inks that are only visible under ultraviolet light is one of the most popular ways to make printed documents secure today. However, this approach has known weaknesses, the biggest being that fluorescent inks are easily identified and thus provide insufficient security for important military and economic information.

Further, the inks cannot be erased and are environmentally unfriendly.

Zhao and his team approached the problem from a different angle. Instead of focusing on the ink, they worked on the paper.