Invisible ink is rewritable

Most electronic information systems have more sophisticated security than paper. It is not impossible, however, to make paper secure and rewritable, to boot.

Researchers from the University of Tokyo, Tokyo University of Science, Tokushima Bunri University and Japan Science and Technology Agency have fabricated a rewritable security paper whose contents are only visible under ultraviolet light.

The paper can be erased and reused by heating it above 50 degrees Celsius. The paper can be used to handle confidential information, and is environmentally friendly because it is reusable, according to the researchers.

The paper is coated with a clear plastic whose molecules contain copper atoms. The plastic appears clear under regular light, and ordinarily appears pink under ultraviolet illumination. When information is written into the plastic by heating portions of it with a thermal printhead like those used in fax machines, the connections between the branched molecules that make up the plastic change, making those portions of the material appear blue under ultraviolet illumination.

Images printed this way last as long as a year, according to the researchers.

The secure paper and plastic ink system could be used practically within five years, according to the researchers. The work appeared in the June 19, 2005 issue of Nature Materials (Rewritable Phosphorescent Paper by the Control of Competing Kinetic and Thermodynamic Self-Assembling Events).