Scientists have long studied how titanium dioxide, a chemical commonly found in paint, sunscreens and cosmetics, destroys microbes and the stubborn layer of slime they create when they colonize a surface. When titanium dioxide molecules are exposed to sunlight, it kicks off a chemical reaction that creates free radicals, which penetrate the cell walls of bacteria and fungi and damage their DNA.
To test how that reaction works outside of lab, researchers at the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB in Germany coated the armrests of some plastic lawn chairs with titanium dioxide and left other chairs untouched. They then sprayed a mixture of bacteria, moss, algae and fungi onto both sets of chairs and left the chairs outdoors. After two years, the untreated armrests gathered a layer of grime that was difficult to clean, while the titanium dioxide-treated armrests appeared clean and white, according to Fraunhofer.
In another Fraunhofer lab, engineers are working on titanium dioxide paints for outdoor walls and on titanium dioxide coatings for glass that could go into touch-screen devices.
"If you apply a thin coating of titanium dioxide to a glass surface such as a smartphone screen, the skin oils and fingerprints gradually disappear from the display by themselves," Michael Vergöhl, who leads sun-activated surfaces research, said in a statement.
The glass coating needs an hour of sunlight to work -- an improvement over the three days' worth of sun needed by previous self-cleaning screens -- but the process is still fairly inconvenient, as most people don't like to leave their electronics in direct sunlight. The next step is to develop antibacterial surfaces that are able to be get activated by indoor lights, according to Fraunhofer.