"This is a collaborative project that started with theoretical work by Anna Balazs at Pittsburgh," explained team member Todd Emrick. "Using computer simulations, she predicted that if nanoparticles were held in a certain type of microcapsule, they could probe a surface and release the nanoparticles into certain specific regions. This concept applies particularly well to damaged surfaces, where the defective regions typically possess characteristics that are very different to the undamaged part of the sample – in terms of their topography, wetting properties, roughness and chemical functionality."

Such a "repair-and-go" approach is inspired by naturally occurring biological mechanisms in the body that exploit structures such as leukocytes, which can probe, identify and heal wounded or diseased tissue. And in medicine, cancer drugs are routinely encapsulated to ensure that they preferentially permeate into "leaky" cancer tissue rather than healthy surrounding tissue, says Emrick.

Rolling capsules
Using a polymer surfactant that stabilizes oil droplets in water, the researchers encapsulated cadmium selenide nanoparticles in such a way that the particles could be released when desired. This was possible because the walls of the capsules are very thin – about the same size as the diameter of the nanoparticles themselves. They then found that the capsules roll or glide over damaged substrates and selectively deposit their nanoparticle contents into the damaged or cracked regions thanks to hydrophobic–hydrophobic interactions between a nanoparticle and the cracked surface. The nanoparticles can easily be tracked too because cadmium selenide is fluorescent.

"Our research could have numerous practical applications," Emrick told nanotechweb.org. "For one, it could help massively lower the amount of material required when repairing a damaged object or sample, thus avoiding the need to coat an entire surface when only a very small fraction of it is damaged."

The technique could also be exploited as a precise method for detecting damaged substrates, by depositing sensor material into the regions of concern, he adds.

"Looking forward, this rapid and efficient coating mechanism might come in useful for repairing a wide range of objects – from aeroplane wings to microelectronic components and biological implants," said Emrick. "Having realized the concept experimentally, we now plan to demonstrate how the mechanical properties of coated objects can be recovered by adjusting the composition of the nanoparticles being delivered."

The work was reported in Nature Nanotechnology.