Some things are printed for long-term use or storage, but plenty of pages are read once before being thrown out. Now, engineers at the University of Cambridge have found a way to wipe these printed pages clean and reuse them. The team, led by David Leal-Ayala, created an 'unprinter' that sends lasers in short pulses towards the paper. The lasers don't affect the white cellulose fibers, but the darker toner absorbs them, generating heat that vaporizes the ink from standard printers in several thin layers. The team tried a number of different colors and pulse lengths to determine the optimal laser type, eventually settling on green light with a relatively long pulse rate of four nanoseconds. With this laser, they were able to print and erase a page three times without noticeable damage.

Since a small amount of heat gets transferred to the paper when the unprinter is used, the pages will eventually take on a yellow tint, but each sheet could be wiped clean several times before being damaged enough to recycle. Even better, the tool will work with ordinary toner and paper, although some varieties apparently work better than others. "Our ambition was to develop a method that would remove conventional toner from conventional paper in order to allow re-use of the paper," says project supervisor Julian Allwood. This sets it apart from products that require special toner or other preparations. The research was published last month in Proceedings of the Royal Society A.