A team of scientists have figured out a way to make paper that can be printed with ultraviolet light, erased by heating and then rewritten more than 80 times, as reported in NanoLetters, a journal that covers fundamental research in the theory and practice of nanoscience and nanotechnology.
"That means you don't have to spend a lot of money on the paper and the inks," said Yadong Yin, chemistry professor at the University of California, Riverside. "And it saves trees, too. Before you had to cut 88 trees to make the same amount of paper that now takes one."
Yin and other colleagues from Shandong University in China as well as Lawrence Berkeley National Laboratory think their rewritable paper could be used to make newspapers, notepads and price tags, which can be rewritten when the amount changes.
To make the inkless paper, the researchers found a low-cost, environmentally friendly way to coat conventional paper with a thin layer of nanoparticles that respond to light. For this experiment, they used two types of nanoparticles: those made of Prussian blue, a common, nontoxic blue pigment, and titanium dioxide, a nontoxic chemical used to, among other things, print the white M&Ms label on the candies.
Yin and his colleagues mixed these two kinds of nanoparticles in a solution and then applied it to a conventional piece of paper. The paper turned a deep blue.
To print text, the scientists used a mask with the letters etched out and placed it on top of the coated paper. Next, they exposed the paper and mask to ultraviolet light. When the photons in the light hit the titanium dioxide nanoparticles through the cut-out holes in the mask, the nanoparticles released electrons. Those electrons got picked up by the Prussian blue nanoparticles, which then turned colourless.
In short, some spaces stayed blue and other spaces became colourless, producing the printed text. The inkless letters remained visible for at least five days before they faded away. The team found that the letters could be erased faster by exposing the page to heat for 10 minutes. The whole process can be repeated more than 80 times.
Although the scientists used a mask overlay for this experiment to show that the UV exposure worked in principle, the team has already developed a prototype printer.
Yin said that because their version of a printer doesn't require the moving components found in laser or ink-jet printers already on the market, it would be much simpler to manufacturer and maintain. The next step for the team is to develop a printer that's able to print on a standard-size sheet of paper and then take the system to full colour.