Despite the ubiquity of tablet computers and e-readers, we simply cannot erase our addiction to paper. An estimated four billion trees are felled every year to make paper or cardboard, an energy-intensive process with a vast environmental footprint. Now chemist Yadong Yin of the University of California, Riverside, and his colleagues have developed “rewritable” paper that could help curb that impact.
The researchers coated conventional paper with nanoparticles of two chemicals: Prussian blue, the pigment that gives blueprints their characteristic color, and titanium dioxide, a substance used in sunscreens. A blast of ultraviolet light makes the titanium dioxide nanoparticles donate electrons to their Prussian blue neighbors. That jolts the pigment into shifting its color from midnight blue to milky white.
By shining that UV light through a transparent screen marked with black text, the researchers “printed” blue text on a white background. The text lasts about five days and then spontaneously fades away: “Every morning I could just push a button, and a printer would give me a fresh newspaper to read over breakfast,” Yin says.
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The paper can also be reset by heating and reused more than 80 times, a significant improvement over previous types of rewritable paper. “The key advantages are high reversibility and stability, easy handling, low cost and low toxicity,” says Sean X. Zhang, a materials scientist at Jilin University in China, who was not involved in the study but has also worked on developing rewritable paper. By comparison, technologies such as electronic ink—used for Amazon's Kindle Paperwhite—involve moving charged black-and-white particles around, which requires electronics.
Since reporting their invention in Nano Letters early this year, the scientists have MacGyvered a digital projector to replace their transparent screen. They are now working on increasing the number of times the paper can be reused. Zhang says a key hurdle will be persuading companies to develop the unconventional UV zappers needed for widespread use. Even though commercialization could be a few years away, Yin says, “We've had a lot of discussions with industry investors.”