Borrowing an idea from nature could lead to technology capable of producing full-colour prints in a fraction of a second, according to South Korean engineers.
Many insects and birds owe their bright colours to the interaction of light with finely-patterned surface textures, rather than relying on pigments. The iridescent colours of a peacock's tail are largely a result of the interaction of light with just one biological material – melanin rods.
Engineers have long experimentedMovie Camera with replicating these so-called structural colours in synthetic materials, and now Sunghoon Kwon's team at Seoul National University in South Korea has managed it.
Their M-Ink can be used to produce any colour in the visible spectrum and could lead to a new method of cheap and fast full-colour printing, Kwon says.
Just add nanoparticles
M-Ink contains three ingredients: magnetic nanoparticles 100 to 200 nanometres across, a solvation liquid, and a resin.
The nanoparticles disperse throughout the resin, giving the ink a brown appearance. But when an external magnetic field is applied, the nanoparticles immediately snap to the magnetic field lines, forming chain-like structures.
The regularly-spaced nanoparticle chains interfere with incoming light, so that the light reflected from the surface is of a particular colour. Adjusting the magnetic field strength shifts the spacing of the field lines and changes the colour, says Kwon.
"If you want to control the angle of the magnetic field [to create curves in the image, for instance] you can combine multiple electromagnets," he says.
Meanwhile the solvation liquid creates repulsive forces between the magnetic nanoparticles, ensuring they do not simply clump together in the ink.
Once the desired colour is produced, the nanoparticles can be fixed in place by exposing the ink to UV light, which cures the resin. The researchers' system uses maskless lithography to shine UV light over just the areas of the image that are of the desired colour. By then changing the magnetic fields and the UV light pattern, it's possible to build up a full-colour image.
"We first set the magnet to tune colour to red and then shine UV for 0.1 seconds [to fix the red areas of the image] and then change to blue, for 0.1 seconds again, then green," says Kwon. "You can pattern A4-size [letter-size] full-colour prints within a second."
However, Kwon cautions that the prototype system takes a few seconds to print because it can shift the magnetic field strength only relatively slowly (see video, above).
Not like a DVD
He says that other potential applications include counterfeit inks: "You can build papers displaying unique features on application of an external magnetic field."
The team is also working on reversible colour fixing, which could have applications in colour-changing gadgets.
Zhong Lin Wang at the Georgia Institute of Technology says the approach is an interesting one, but thinks an existing method involving laser beam patterning of a plastic surface – the method responsible for the iridescent pattern on the surface of a DVD – might be simpler.
Kwon says the two technologies are fundamentally different: "Ours can change the colour of the whole plastic substrate, not just the carving on the surface," he says. The UV fixation method could turn out cheaper than using a laser beam, he adds.
Journal reference: Nature Photonics, DOI: 10.1038/nphoton.2009.141