Different types of self-cleaning materials that incorporate nanoparticles have been developed in the past. Stain-repellant fabrics and paints that are currently on the market typically have a nanoparticle or nanofiber coating that causes drops of liquid to roll off instead of getting absorbed into the material. The liquid drops take small particles of dirt and grime with them.
More materials are in the research stage. These include microstructured, Teflon-like materials that bounce oil off their surface. (See "No More Thumbprints.") Purdue's Youngblood has made a material that changes its structure depending on whether it's in contact with oil or water, causing water to spread out into a thin film and oil to bead up so that it runs off or is easily wiped off with water. (See "Self-Cleaning, Fog-Free Windshields.")
All of these materials are based on making the surface oil or water repellant, says Youngblood. This is a concept that is completely different from that of the new titanium dioxide coating. "We're controlling wettability and surface interaction," he says. Titanium dioxide coatings, on the other hand, degrade organic matter. "It has nothing to do with surface wettability whatsoever. Here, you're not removing what's on the surface: you're burning it off."
Each of these techniques to make self-cleaning materials has its own limitations. Superhydrophobic materials, which repel water, are typically good at removing dirt particles but "don't deal with oils well," Youngblood says. Materials that repel oil, such as the one that he has developed, might not work with certain types of oil. The titanium-dioxide-coated materials, on the other hand, will not work unless they are exposed to sunlight for hours.
The sunlight requirement has not stopped the technology from getting commercial interest. Several wool manufacturers have suggested that they'd like to evaluate the technology, Daoud says. He expects self-cleaning wools to be available in the market within two years, once sufficient laboratory and industrial trials have been completed.