How convenient it would be if we could convince a torn piece of material to return to its undamaged state. Université de Sherbrooke chemist Yue Zhao has been able to purchase just such convenience with some vanishingly small amounts of gold.
The substance in question is gold nanoparticles or nanorods, which are inserted into a thin polymer specimen. When laser light passes through the structure, these agents heat up and initiate a polymer melting-crystallization process that replaces bonds broken by physical damage to the matrix around them. “This photothermal effect is not new,” Zhao explains. “But what is new is demonstrating it in healing crystalline polymers. In general it is challenging to heal a mechanically strong material.”
Zhao and his PhD student Hongji Zhang published their findings in the American Chemical Society journal ACS Applied Materials and Interfaces. They confirmed that gold nanoparticles not only enabled a polymer sample to repair a cut but also provided the material with light-controllable, shape-memory ability, so that it returned to an initial, folded state after being flattened out. “The most surprising part was the small amount of gold nanoparticles we needed,” says Zhao. “It took something like 0.003 weight/percent to generate the heat for the healing.”
Zhao adds that such a small amount also helps retain the transparency of the polymer, which is necessary so that the laser light can pass through it and activate the nanoparticles. Of course, “small” is a relative term in this context. He also points out that a cubic millimetre of this polymer will still contain upwards of a billion nanoparticles of 10 nanometres in diameter, which turn out to be more than enough to generate the necessary heat for repairs. Moreover, the use of a laser light means that it is possible to heal a polymer from a long distance so long as the laser beam can reach the damage.
Zhao concedes that the potential applications of this physical behaviour are still being discussed. But as the ability to promote self-healing becomes a practical possibility in everything from hard crystals to hydrogels, he suspects the imaginations of inventors will take over. “It’s a very general approach,” Zhao says. “It’s getting easier and easier to get gold nanoparticles, and you just need tiny amounts, so it’s not that costly.”