Feature

Savvy Skins

Researchers pour new functions into coatings

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5:10pm, January 29, 2007
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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 doorknobs, 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 multilayered 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: Layered Approach). 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.

Impaled bacteria and viruses gradually build up on the coating, reducing its effectiveness, notes Klibanov. But "if you take a sponge and wash the surface with soapy water, the surfaces are rejuvenated and are as good as new," he says.

The group's initial studies show that bacteria don't develop resistance to the coatings, but the researchers haven't tested whether viruses can do so. Klibanov envisions the coatings applied throughout hospitals: on air-duct surfaces, walls, doorknobs, uniforms, and so on. He says that Boeing has expressed interest in coating surfaces on its planes that passengers touch.

Kinder coatings

Advanced coatings could also find their way into the human body. This reflects a change in opinion on how to design biomaterials, notes Joerg Lahann, a chemical engineer at the University of Michigan in Ann Arbor. Previously, "you tried to make [biomaterials] as inert as possible," he says, but some researchers are now devising surfaces with properties "more similar to what the biological system looks like."

A way to achieve this is to attach biomolecules normally found in the body, such as enzymes, to an implant. To do this, Lahann and his colleagues create coatings that bind biomolecules.

The researchers begin by attaching a chemical group to a monomer, the individual unit of a polymer chain. They choose this anchoring group depending on what sort of molecule—sugar, alcohol, or protein, for example—they want to bind to the coating. After being converted into a gas, the monomers settle on a surface and link into a polymer. These polymers can coat materials from stainless steel to plastic and can effectively cover objects with complex geometries, Lahann says.

In the final step, a researcher immerses the polymer-covered object in a solution of the chosen biomolecule, which then binds to the anchor group.

The team has recently increased the selectivity of the anchor groups, which enables the researchers to control the ratios of biomolecules that attach to the surface. They described this step toward making coatings with a modular design in the June 15, 2006 Advanced Materials.

Among the implants that Lahann and his colleagues have targeted are stents, the metal scaffolds used to prop open blocked arteries. Some coated stents release drugs to prevent cell growth in the vessel. Although drug-releasing stents have been popular since their approval by the Food and Drug Administration in 2003, they have since been linked to an increased risk of blood clots and heart attack.

For the new stent project, Lahann has teamed up with Mark E. Meyerhoff, a chemist at the University of Michigan. Meyerhoff's group developed a system that makes molecules naturally found in the blood generate nitric oxide, the same chemical that the cells lining blood vessels normally release to prevent clotting. A copper catalyst jump-starts the reaction.

To devise a clot-stopping stent coating, the researchers anchored the copper catalyst to certain areas of the coating. In their initial tests in vials of blood, Meyerhoff and Lahann found that the coating generates nitric oxide concentrations comparable to those that a layer of healthy vessel-lining cells makes.

"This is nature's solution to the clotting, and we try to mimic that," says Lahann. The team has begun testing coated stents in pigs.

Caring coatings

Bridges and ships could also use a coating upgrade. Braun says that the U.S. Navy treats the majority of its ships for corrosion every 3 to 5 years. Extending this protection would reduce the cost significantly, he says.

Dmitry G. Shchukin of the Max Planck Institute in Potsdam, Germany, and his coworkers have devised a gel coating that halts corrosion soon after it starts. Within the gel are 70-nanometer-diameter particles of silica, each covered with polymer layers containing reservoirs of benzotriazole, a chemical that inhibits corrosion.

Corrosion begins whenever a scratch to the covering exposes the underlying metal to oxygen and water. However, this changes the acidity of the damaged spot, triggering the release of benzotriazole. Within about 24 hours, the chemical inhibitor coats the damaged area and hinders the corrosion process, Shchukin says. The team described the coating in the July 1, 2006 Advanced Materials.

Braun's group, by contrast, has developed a thin-film polymer that heals scratches before corrosion has a chance to start. The coating is related to a self-healing composite material devised by Braun's colleague Scott R. White of the University of Illinois (SN: 2/17/01, p. 101: Scientists develop self-healing composites).

The coating includes two self-healing components: an encapsulated catalyst and droplets of polymer ingredients. The researchers mix this liquid pre-polymer and the capsules containing the tin-based catalyst into the coating before it cures, says Braun.

A scratch to the coating—minor or deep—cracks open some of the capsules holding the catalyst, which then flows into the scratch. Some liquid prepolymer also seeps in. The catalyst reacts with the liquid and forms a solid that repairs the damage, preempting any corrosion.

At the 2006 Materials Research Society Fall Meeting in Boston last November, Braun's group presented the results of corrosion tests on steel strips coated with films containing both the catalyst and the prepolymer or one without the other. Scratches to the single-component films became rusty after immersion in salt water for 120 hours. But the dual-component film kept its metal rust-free under the same conditions.

Future films

As coating research progresses, more items that might benefit from an extra skin become apparent. Lahann foresees prosthetic body parts with coatings that perform just as well as natural surfaces in the body. For example, integrating an artificial foot would require the prosthesis to mesh with many surfaces—bone, skin, muscle, and so on.

"I think multifunctional [coating] materials will really be critical in that respect," says Lahann.

Braun envisions a variety of complex coatings still to come: clear coverings that would enable a car windshield or a laptop screen to self-repair scratches; self-healing coatings for hip replacements and other implants; and even systems that could repeatedly heal damage to the same spot, perhaps by incorporating a network of plumbing that could replenish healing components.

"Every one of those that I mentioned I think is possible," says Braun. The trick, he says, "will just be coming up with the right combination of materials science and chemistry."

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