Savvy skins: researchers pour new functions into coatings

Science News,  Feb 3, 2007  by Aimee Cunningham

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Among the innovations highlighted in the Dec. 10, 2006 New York Times magazine's "Year of Ideas" was a coating manufactured by Nissan. Called Scratch Guard Coat, this substitute for clear-coat car finishes--currently available on a sports utility vehicle in Japan--repairs surface scratches. Though deep scratches are beyond the resin coating's capabilities, it fills in slight scuffs in a day to a week.

Scratch Guard Coat notwithstanding, most coatings found on products today do their jobs in a much simpler way. They typically provide a passive barrier between the environment and some object prone to degradation, be it a car, a bridge, or a cheap metallic dish rack. By taking action to repair a product, the Nissan coating is a step toward a more dynamic coating world.

But in the realm of new coating possibilities, Nissan's entry only scratches the surface. Materials scientists and chemists are already developing coverings that pack in more functions and complexity. "This is certainly a major growth area," says Paul V. Braun, a materials scientist at the University of Illinois at Urbana-Champaign. "It's been picking up in the last few years."

The newest coatings incorporate multiple functions, offer chemical reactivity, or act in response to stimuli in the environment. Once out of the laboratory, they could provide germ-busting door-knobs, artery-opening stents with powerful anticlotting properties, or polymer skins that self-heal before corrosion can mar the covered product.

"People have developed ideas of how to apply new advances in material science to coatings," Braun says.

KILLER COATINGS Making surfaces germfree often takes no more than a good scrub with soap and water. But the doorknobs and walls of hospitals, for example, are continuously prone to contamination by some of the nastiest microbes around. In these cases, a coating that kills bacteria or viruses might reduce the spread of infections, particularly those from the antibiotic-resistant bugs that plague hospitals.

At the Massachusetts Institute of Technology (MIT), materials scientist Michael F. Rubner and chemical engineer Robert E. Cohen have combined their laboratories' efforts to create a multi-layered antibacterial coating that kills microbes in two ways: on contact and by chemical release. The covering, which they say can be applied to fabrics and hard surfaces, is an example of "how nanotechnology is working its way into the coating world," says Rubner.

The researchers build the coating layer by layer (SN: 8/9/03, p. 91). The first 40 layers alternate between positively charged and negatively charged polymers. Twenty additional layers provide a surface of silica nanoparticles.

Attached to the nanoparticles are molecules called quaternary ammonium compounds. Previous studies had shown that these molecules provide antimicrobial activity by disrupting bacterial membranes on contact.

When exposed to water, the coating also releases silver, a long-recognized antibacterial agent, from the polymer-only layers. Silver ions initially bind to the polymers' chemical groups. If left as such, the ions would leach out quickly on contact with moisture, and "you'd lose them within a day," says Rubner.

His group therefore performs another chemical step that gathers the silver ions into nanoparticles. When exposed to moisture, the nanoparticles break down slowly, releasing the bacteria-killing silver ions. This extends the release period to weeks or possibly months, Rubner says.

In tests against Staphylococcus epidermidis, the silver was more deadly than the ammonium compounds, Rubner notes. But when the silver was depleted, ammonium compounds held fast to the silica nanoparticles and continued to kill bacteria on contact. The researchers report on the dual-action covering in the Nov. 21, 2006 Langmuir.

Another coating, developed by MIT chemist Alexander M. Klibanov and his colleagues, kills both the influenza virus and deadly bacteria on contact.

To create the coating, Klibanov's group chemically modified a commercially available polymer to make its chains highly water-repellent and positively charged. Dissolving the polymer in an organic solvent produced a paint that can be brushed or sprayed onto a surface, or an object can be dipped into the paint. As the paint dries, the solvent evaporates, leaving behind polymer chains that repel each other because of their charges. In that arrangement, fragments of some of the chains stick out from the surface.

The researchers had previously demonstrated that a paint made of the polymer chains kills bacteria by punching holes in their cell membranes. They reasoned that the system might also damage viruses enveloped by membranes. In their new work, Klibanov and his colleagues painted the spiky coating on a glass slide and tested it against influenza A virus. The coating killed at least 99.99 percent of the virus that it contacted, the researchers report in the Nov. 21, 2006 Proceedings of the National Academy of Sciences.