Chinese researchers have developed two methods that make printed information invisible for the naked eye, and allow reusing the same paper several times. For their technique, they used a simple currently used jet printer, but replaced ink with water.
In their study published in the journal Matter, the researchers led by Qiang Zhao of the Nanjing University of Posts and Telecommunications, said the water-jet printing method is environmentally safe.
"Most fluorescent security inks on the market used to record confidential information are environmentally unfriendly and cannot be erased," noted Zhao. That what makes him and his colleagues bet on the eco-friendly manganese complex in the development of their new technique. The water-paper contact melts the material where the water settles. The message reveals itself under UV light. The printing paper coating is made of three layers of plastic, all composed of polyethylene glycol with the manganese complex added to the middle layer.
Although the water decomposes the mixture's components, the manganese complex persists for a long time due to its association with the plastic, which means the printed material remains readable.
To erase the information, simply heat the paper for 30 seconds with a hair dryer in order to remove the layers and recover the manganese. Once the content is erased, you can reuse the paper again. The research team has taken another step forward, by developing a second method that allows making printed information unreadable even when using neon light.