OREANDA-NEWS. Researchers have created a chemical process that allows them to print color images on specially coated paper, erase those images, and then print new images on the same paper.

The technique makes use of structural colors, which have different properties than the ink dyes used for standard printing.

“This could be really useful when you want to reconfigure, recolor, and reshape messages on signs or clothing…”

The standard dyes absorb all the colors of the spectrum except for the color that is visible to the eye, such as red or blue, and the colors fade over time. Structural colors are determined by the selective reflections of certain colors at certain angles. They’re made from one-dimensional stacks of layered polymers, called block copolymers.

“Copolymers are soft, stretchable, and deformable,” says Ned Thomas, a professor of engineering, materials science, and nanoengineering, chemical and biomolecular engineering, and chemistry at Rice University. “You can swell or shrink them and change their shape and dimensions, which will affect which color they reflect.”

Thomas says one of his former doctoral students, Cheolmin Park, who is now a professor at Yonsei University, wanted to collaborate on developing printable and rewritable copolymer structural colors.

The researchers found that they could use a single, colorless, water-based ink based on ammonium persulfate (APS) to control how the copolymers cross-link in various locations, which impacts their subsequent thickness and hence the structural colors that are reflected. APS stops the swelling of the copolymers, and the thin layer reflects blue. Ethanol was used to thicken the copolymers, which reflected red.

By applying varying amounts of ethanol and APS to paper that is coated with copolymers, the researchers were able to control the swelling and shrinking of the molecules and generate the colors and patterns needed to create a picture. Large amounts of APS stopped all swelling, which resulted in black images because there was no reflection.

The researchers also discovered that applying hydrogen bromide to the paper removed or erased the APS, so the reflections were neutralized, which “reset” the system so that the paper could be used again. They printed and erased images more than 50 times on the paper, with resolution similar to that of a commercial office inkjet printer.

Thomas says refinements will be needed before this technique is commercially viable. Because ethanol evaporates, the reflective patterns disappear, so the researchers are looking for a substance that is less volatile and will maintain the colors indefinitely. They also need to find an alternative to hydrogen bromide, which is toxic and not environmentally friendly.

Thomas thinks the technique has the potential to be cost-effective because it will require only one ink—the APS—and a modified inkjet printer that uses paper coated with copolymers, which should cost “pennies per sheet,” he says.

“This could be really useful when you want to reconfigure, recolor, and reshape messages on signs or clothing,” Thomas says.

The researchers will outline their technique in a paper that appears in the journal Advanced Materials. In addition to Thomas and Park, additional coauthors of the paper are from Yonsei University and Korea University.