By layering tiny nanospheres onto stretchy fabric, scientists have created a material that changes color when stretched. Called "polymer opal" by the team, the material mimics the multicolored brilliance of an opal gemstone.

Using ink made from synthetic photonic crystals and a printer that can modulate voltage, the team has figured out how to print shimmering, color-changing patterns onto a stretch of flexible opal. The new printing method was described on May 22 in Advanced Engineering Materials.

"You don’t need different printing inks," said Jeremy Baumberg, director of the Nanophotonics Centre at the University of Cambridge. "You can use one pot, and then change the color produced when you print it."

Made from tiny spheres that are 200 nanometers across, the photonic crystal "ink" mimics color-producing structures found in nature.

Butterfly wings, feathers, and some gemstones derive their shimmering shades from ordered nanostructures that bend and reflect light, rather than from pigments. The shape and arrangement of these tiny nanocrystals determines which colors are produced. Unlike pigments, which look the same from any angle and can bleed and fade, structurally derived colors stay vibrant indefinitely, can appear metallic, and enigmatically change with viewing angle.

The flickering flames and shifting sheens seen in opals are produced by stacks of tiny silica spheres within the gemstone. Mimicking that arrangement in the lab, using spheres with a hard polystyrene core and softer outer shell, produced what the team calls polymer opals. Making the spheres is inexpensive and relatively simple, Baumberg says.

Layering those spheres on an elastic fabric produces a flexible opal, or material that changes color when twisted, stretched, or bent. As the space between the spheres shrinks and expands, the matrix diffracts and reflects light of different wavelengths, producing a changing rainbow of colors. The size of the spheres affects the opals' starting color, with larger spheres appearing red initially, and smaller spheres starting at blue.

A previous version of the material, shown below, shifted from an orange-yellow, to green, and finally to blue when stretched.

Now, the team can print patterns into the material, using a printhead that incorporates an electric field. As the spheres are laid down, varying the voltage affects the spacing between them, which in turn affects the color of that region. Producing patterns is as simple as altering the voltage in localized areas; the nanoparticles are then fixed into place with UV light.

Flexible opals are resilient – you can stitch over them, chop them up, emboss them, weld them, even punch a hole in them.

As for applications? We'd like to order up a stretchy, color-changing outfit for our next trip to the disco. Other uses could include incorporation into safety belts or other devices that could benefit from a visual indicator of tension or overstretching, as well as fraud-resistent inclusions in banknotes.

Videos: Cambridge University/YouTube; dres2video2share/YouTube