IDEA WATCH  JULY 2017

Rewritable Paper

Print. Erase. Repeat.

By Prachi Patel

E-readers and tablets seem to be all the rage, but most people still find good-old-fashioned paper books hard to beat. A Pew Research Center survey found that 65 percent of Americans had read a print book in 2016, as opposed to 28 percent who had read an e-book. However, our paper habit takes a toll on the environment.

A rewritable paper made by researchers at the University of California, Riverside, could help. The researchers “print” the paper using ultraviolet light instead of ink, and the text stays legible for five days before fading away. “You wouldn’t want to use it for writing contracts,” says chemist Yadong Yin, who led the work. “But it would be ideal for things like newspapers and posters.” The paper can be reused 80 times and could be recycled after that, he says.

Conventional paper costs pennies but leaves a huge footprint. About 4 billion trees are cut every year to make paper or cardboard, using a process that consumes immense amounts of electricity and water. The paper and pulp industry is the world’s fifth-largest energy consumer. It uses the most water per ton of product and is a major polluter. Even paper recycling pollutes

Yin and his colleagues set out to crack the code of low-cost, low-toxicity reprintable paper. They coated conventional paper with nanoparticles of two commonly used materials: the compound titanium dioxide, which is used in sunscreens and makeup, and the paint pigment Prussian blue.

The paper starts out entirely blue. Placed under ultraviolet light, the titanium dioxide particles get excited and produce electrons that they donate to the neighboring Prussian blue particles. This makes the blue pigment lose its color and turn white. Over time, oxygen from the air absorbs the extra electrons, changing the pigment back to blue. Heating the paper to 120 degrees Celsius speeds up the process and erases the paper in ten minutes.

To “print” on the paper as a proof of concept, the researchers used a rudimentary process involving masking. They inkjet-printed text with black ink onto a transparent sheet, which acted like a mask when they shined UV light through it. Parts of the rewriteable paper under the black ink remained blue, while the surrounding areas turned white.

For commercial printing without a mask, a UV laser could be used to scan the paper surface on areas that need to remain white, Yin says. This would work similarly to the way today’s laser printers do.

The researchers report that they can print patterns at incredibly high resolution—as small as 10 micrometers, or ten times smaller than our eyes can see. And as for cost, their coated paper should be comparable to regular paper. Both titanium dioxide and Prussian blue can be produced by the ton and are inexpensive. In fact, titanium dioxide is already used to make bright white paper, Yin says.

The team is now thinking about how to make a commercially viable printing technique and multi-colored paper.

Prachi Patel, originally from Nagpur, India, is now based in Pittsburgh. She writes about energy, biotechnology, nanotechnology, and computing. She is a contributing editor at IEEE Spectrum and her work can also be found in Scientific American and Technology Review.

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