Layered Approach
A simple technique for making thin coatings is poised to shift from curiosity to commodity
Wrap an apple in Yasa-sheet and it will stay fresh for weeks. So says Semei Shiratori of Keio University in Yokahama, Japan, who makes this high-tech plastic for preserving fruits and vegetables. To be sure, it’s a humble product. But it may be a harbinger of an enormous new class of materials and products created
in a startlingly simple process: Thin liquid layers applied one at a time create solid, multilayered coatings that mix and match a wide variety of technologically valuable properties.
As for Yasa-sheet, it’s made of alternating layers of chitosan, which is a major sugar-based ingredient of crab shells, and an enzyme-containing liquid extracted from bamboo. The protective wrapper works, according to Shiratori, by suppressing the food’s emission of ethylene gas–a naturally produced ripening agent that eventually makes fruits and vegetables rot.
Now, a decade after the layering technique first made a splash among materials researchers, it’s serving as the basis for a slew of potential products, including anticorrosion coatings, fuel cells, and biomedical implants. Known as layer-by-layer assembly, the technique is no longer a mere laboratory curiosity.
“This field is reaching the point where we’re really starting to harvest some very interesting new technologies based on these materials,” says Michael Rubner of the Massachusetts Institute of Technology, where he has championed layer-by-layer assembly techniques for nearly a decade. “This is going to be a very exciting time in the next few years and beyond.”
Double dipping
Although its roots reach back decades, the layer-by-layer technique started making waves in 1991. Gero Decher, now at the Louis Pasteur University and the Charles Sadron Institute in Strasbourg, France, reported making multilayered films using positively and negatively charged polymers, called polyelectrolytes.
These materials readily layer upon each other, and electrostatic forces hold them in place.
Decher demonstrated that he could make thin, high-quality films with a technique that’s easy to use, says chemist Nicholas Kotov of Oklahoma State University in Stillwater. After that, Kotov says, research in layer-by-layer assembly experienced “exponential growth.”
The technique’s simplicity is a big draw. In its rawest embodiment, researchers simply dip a naturally negatively charged substrate, such as a glass slide, in a solution containing a high–molecular weight, positively charged substance.
Then they rinse it, let it dry and repeat the steps using a solution containing a negatively charged substance. Each cycle of dipping and drying yields a layer just nanometers or less thick–qualifying the technique and its products to be included in the red-hot research arena of nanotechnology.
Because the layering process is so repetitive, robots often perform it. The possibility of such automation provides an advantage for commercialization.
“The method is very simple, even primitive,” says Yuri Lvov, a researcher Louisiana Tech University in Ruston who has studied layer-by-layer assembly since the early 1990s. Even so, he notes, “it allows us to work with organized layers of polymers, proteins, viruses, and nanoparticles.”
In fact, says Rubner, “any water-soluble or dispersible element that has either charges on it or hydrogen-bonding capability can be built into these films. People have just put everything but the kitchen sink in there.”
The layer-by-layer strategy has additional virtues. It usually uses good old water, rather than hazardous and toxic liquids, as the solvent. The initial surface that’s coated can be ceramic, plastic, metal, or of many other materials.
And the resulting films are uniform, which opens up many options for imparting the films with electronic, magnetic, structural, and chemical properties.
Says Rubner: “It’s really a remarkably simple processing technique that gives you nanoscale control over properties.”
Thin-tech
There’s no limit to the properties that layer-by-layer assembly can convey to materials. The strength of mother-of-pearl is one property that Kotov is emulating. Using alternating layers of negatively charged clay particles and positively charged polymers, he produced a structure on glass slides that resembles mother-of-pearl in strength and its nanoscale structure (SN: 6/21/03, p. 397: Available to subscribers at Material mimics mother-of-pearl in form and substance).
Kotov has had enough success with this and materials that incorporate such celebrity substances as carbon nanotubes that a small Stillwater company, Strala Materials, is now trying to commercialize his materials for body armor, aviation equipment, and artificial bone.
Corrosion resistance is another property amenable to the layering strategy. Joseph Schlenoff of Florida State University in Tallahassee is making anticorrosion coatings with alternating layers of polyelectrolytes known as PDDA and PSS, which are commonly found in shampoo or used to treat wastewater (SN: 4/13/02, p. 228: Steely Glaze: Layered electrolytes control corrosion). He hopes that such coatings could prove valuable for protecting water pipes and other metal surfaces that interact with water.
He’s also trying to develop membranes for sorting molecules in the pharmaceutical or chemical industries. For example, Schlenoff aims to separate drug molecules that have the same chemical structure but are mirror images of each other–a property known as chirality. The layers in his experimental films are themselves made from chiral amino acids or polymer molecules. In the June 4 Journal of the American Chemical Society, Schlenoff reports that the films permit certain molecules to diffuse through faster than their mirror-image siblings do.
As the president of a small Tallahassee company called nanoStrata, Schlenoff pursues another angle that could help the layer-by-layer process gain a foothold in industry. The company, which he founded in 2000, sells a robotic layering system developed at Florida State. Researchers around the world have purchased the system, Schlenoff says.
According to Thomas Mallouk of Pennsylvania State University in State College, the best use of layer-by-layer technology is for assembling very thin films that require only a few precisely controlled layers. These coatings might be used for high-tech products such as light-emitting diodes, solar cells, and chemical sensors.
Currently, Mallouk is attempting to make ultrathin ion-conducting layers for fuel cells that would run at higher temperatures than today’s ion-conducting materials can handle. The lower temperatures require expensive platinum catalysts. The new films include 10 or so alternating layers of positively charged zirconium- and aluminum-containing clusters and negatively charged materials called perovskites.
Mallouk hopes that this strategy will enable fuel cell makers to use less-expensive catalysts.
Mallouk is also exploring the basic physics of ferroelectric materials made from thin layers of perovskites that are just nanometers thick. Thicker ferroelectric materials are now used in sensors and actuators, but researchers would like to shrink these materials to miniaturize such devices and improve their performance.
Lara Halaoui of the American University of Beirut in Lebanon is also using layer-by-layer assembly to tackle energy issues. She aims to make a new type of solar cell by layering arrays of nanoscale semiconductor particles, called quantum dots, with polyelectrolytes. Scientists predict that quantum dots could convert sunlight into electricity more efficiently than the bulk semiconductor materials used in conventional photovoltaic cells do.
Going a step further in another project, Halaoui has created films with layers of polyelectrolytes and layers hosting quantum dots and platinum nanoparticles that can catalyze the production of hydrogen gas from water. The goal, she says, is to use the energy produced by sunlight to convert water into clean-burning hydrogen fuel.
Prescription potential
Other coatings made with layer-by-layer assembly have biological and biomedical possibilities. Kotov, for one, is aiming to use multilayer structures as scaffolds to grow tissues. The layer-by-layer technique enables him to design surface characteristics that are either inviting or repulsive to cells. The nanoscale roughness of a surface is a particularly important factor in creating a surface amenable to cell adhesion and growth, he notes.
Surfaces made through layer-by-layer assembly can also be chemically tuned to prevent bacterial buildup, blood clotting, or other conditions that commonly plague biomedical devices such as artery-opening stents, says Rubner. Polymer films that absorb a lot of water, for example, aren’t good environments for cell growth, so Rubner’s lab has been using two oppositely charged polymers, commonly called PAA and PAH, to create layered films that swell with different amounts of water depending on the acidity of their environments. In some cases, he and other researchers are making patterned coatings on which some regions attract certain proteins or cells while other parts resist their adsorption, says Rubner.
Meanwhile, some scientists are designing multilayered polymer films that might find use in delivering medicines to specific regions of the body. These materials become porous or break apart when they encounter a particular level of acidity or a certain concentration of metal ions (SN: 3/8/03, p. 150: Available to subscribers at Making Polymers That Self-Destruct: Layers break apart in controlled way).
Rather than starting with flat surfaces, Frank Caruso of the University of Melbourne in Australia begins with tiny spheres of latex, gold, and other materials. After using layer-by-layer assembly to give them a permeable coating, he places them in a solution that dissolves the interior spheres but not the coating. The resulting hollow microspheres could be loaded with drug molecules to be released, for example, by heating the spheres with a laser, says Caruso.
He’s also developing coated spheres for use as bioimaging agents in the study of cells or tissue samples. Spheres coated with lanthanum-phosphate particles are luminescent in colors corresponding to the rare earth ions with which the lanthanum-phosphate is sprinkled, or doped.
Depending on the coating applied, tiny spheres can have many more uses, says Caruso. For example, spheres coated with layers that contain enzymes or metal particles might catalyze specific chemical reactions. Or those with coatings containing antibodies might find use in immunoassays that test for the presence of disease-causing agents.
Caruso cofounded the Berlin-based company Capsulution to commercialize these technologies.
On the shelf?
“We’re at a pivotal point,” says Rubner. “If we don’t start showing some utility from these things–if our industrial partners don’t start to actually make some money–then all of the hoopla about the great promise of these materials will dissipate away.”
With the emergence of small companies such as Strala Materials, nanoStrata, and Capsulution, momentum toward the industrialization of layer-by-layer processes and products is growing. At least one big company is adding to that force.
Although the company remains cagey on details, CibaVision, headquartered in Duluth, Ga., has filed several patents on layer-by-layer assembly of materials for use in manufacturing contact lenses. At an American Chemical Society meeting in Orlando in April 2002, a CibaVision researcher also described the use of layer-by-layer assembly for making the surfaces of contact lenses water-friendly.
Shiratori, a former researcher in Rubner’s lab, has already used layer-by-layer assembly to create useful household items in his lab. He also manufactures and markets these items–albeit with an alternative production method–through his company, Shiratori NanoTechnology in Kawasaki, Japan. Besides Yasa-sheet for fruits and vegetables, he’s made items such as shoe deodorizers.
With all of these developments, layer-by-layer assembly may soon be moving out of the labs to refrigerator and store shelves everywhere.
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