Like all plastics the new material contains long chains of polymers, except that the chains are linked by small molecular bridges that break and change the plastic's shape when it is scratched. This shape-change produces the bruise, which disappears when energy from the light enables the bridges to reform and the plastic to heal. Urban, who presented the research at a meeting of the American Chemical Society in San Diego, California in March 2012, says the new material is not expensive. "It could be used anywhere really, the sky is the limit." It sounds like the stuff of a Harry Potter film, in which the wizard has his cracked spectacles fixed with a flash of light from a magic wand. But researchers are reporting this week that they can indeed fix cracks in materials with a flash of light - laser light. Engineers from the US Army Research Lab in Maryland, Case Western Reserve University in Cleveland and the University of Fribourg in Switzerland have initially performed their trick using a special class of rubber called a metallo-supramolecular polymer.
A Dash of Zinc
Unlike regular polymers like the polystyrene that vending machines cups are made from, or the clear polycarbonate that CDs are pressed with, the molecules in the new rubbery polymers are not linked by strong covalent bonds – which involve atoms sharing electrons. Instead, the spaghetti-like base units of the materials terminate in 'ligand' groups that like to link to free zinc based groups through ionic bonding. When ultraviolet light hits these zinc groups, they absorb energy and convert it into heat. It's that heat that can be used for self-repair.
Christopher Weder at Case Western and colleagues took a 0.4 millimetre-thick sheet of the polymer and gouged fissures between 0.2 and 0.3 millimetres deep in it. They then fired a 1 watt UV laser at each crack for 30 seconds – and found the fissure healed completely. The team are far from alone in the field of self-repairing materials. In 2008, Ludwik Leibler, a chemist at the Industrial Physics and Chemistry Higher Educational Institution (ESPCI) in Paris, harnessed another (weaker) type of bonding – hydrogen bonding – to make a self-repairing rubber that heals itself when two broken sides are simply compressed together.
And auto maker Nissan in Japan has used similar techniques to make self-repairing scratch-proof paint for cars and cellphones. The Case Western team now want to try healing a wide range of different polymers using lasers – but they have to be careful about the mixture. "Materials with higher metal content healed less well," they report in Nature today. Who knows? One day they might even make polycarbonate glasses like Harry Potter's.