Researchers at the University of California at Riverside have developed a treatment for ordinary paper that makes it light-sensitive, allowing it to be marked using an ultraviolet light pen and reused up to 80 times.
Printing pages with the removable ink could help to reduce a source of pollution, as paper production accounts for almost 1% of worldwide carbon emissions, even though recycled paper is increasingly in use.
The treatment consists of nanoparticles, which are one million times smaller than the thickness of a strand of human hair, treated with Prussian blue dye or a structurally similar compound, and titanium oxide, which is often used in white paint.
Ultraviolet light is then applied to the mixture after it is applied to paper to form white letters on blue pages or blue letters on a white background.
The paper goes back to its original state after five days, or after around 10 minutes when heated to 120 °C (250 °F).