Glue That Sticks

Glue That Sticks to Nearly EverythingFor an easy-to-make adhesive inspired by mussels, possible applications abound. By Kevin BullisFlexible displays, water-purification filters, and materials that convert heat directly into electricity could be easier to make thanks to a new polymer that allows researchers to coat almost any object, even one made of Teflon, with microscopic patterns of metals and organic materials.
Researchers at Northwestern University designed the polymer to mimic a protein-based glue that mussels use to attach themselves to rocks, wood, plastic, and steel--indeed, just about any material they encounter. The researchers, led by Phillip Messersmith, a professor of biomedical engineering and materials science and engineering at Northwestern, identified an easy-to-make compound similar to active elements in this mussel glue. They found that under the right conditions, the compound forms an extremely thin polymer film on the surface of just about any material that it's applied to. This film can in turn chemically bind to a wide variety of materials that have useful functions. Many other methods for "functionalizing" materials have been developed, but according to Marcus Textor, a materials professor at the Federal Institute of Technology, in Switzerland, this one stands out because it's easy and extremely versatile. "What I find fascinating is that this is a relatively simple system," Textor says. "Often, one has to find a particular solution for a particular substrate. But this is a universal adhesive that works on many different surfaces."
The new adhesive will allow nearly any object to be easily and inexpensively coated with a veneer of metal or some other functional material, including materials that keep objects free of bacteria or encourage the growth of specific types of cells. The coatings would be thin enough that they wouldn't change the shape of the underlying object; a surgical instrument, Messersmith says, could be given an antibacterial coating without compromising its performance. One application that the Northwestern researchers have been exploring is water filters that use tiny pellets coated with the adhesive. As water runs through a cylinder full of the pellets, the adhesive pulls toxic metals out of the water by binding to them.
The researchers have also demonstrated that the adhesive can be carved into intricate patterns through conventional microlithography. If a solution containing metal salts washes over such a pattern, metal will stick only to the adhesive. This could be a way to print electronic circuits onto just about any object. Deposited on a flexible substrate, such circuits could be useful for flexible displays. The ability to create microscopic patterns of organic materials could also be useful to biologists. The Northwestern researchers have demonstrated that it's possible to create coatings that bind to a specific type of acid important for blood-vessel growth and stem-cell differentiation. The ability to deposit precise patterns of this and other organic materials could make it easier to build microfluidic devices that help explain biological mechanisms.
To develop the new adhesive, the researchers studied the chemical components of a protein in mussel glue, identifying important functional chemical groups. In earlier work, they'd made a glue based on one of these groups. (See "Nanoglue Sticks Underwater.") But the resulting glue worked only with inorganic materials and was difficult to make. The new adhesive contains two chemical groups found in mussel glue, rather than just one. The combination allows the adhesive to bind to both organic and inorganic materials. What's more, the new adhesive is readily available. The researchers noted that the two chemical groups, amines and catechols, are found in dopamine, a compound best known as a neurotransmitter. At the right pH level, dopamine self-assembles into polymer chains to produce thin films of the adhesive. It's also sold commercially, and it's inexpensive.
The adhesive, which is described in the current issue of Science, is already attracting the interest of other researchers. For example, Nicholas Kotov, a professor of chemical engineering at the University of Michigan, intends to use it to make thermoelectric materials--materials that convert heat directly into electricity. Such materials must conduct electricity well but heat badly. Kotov says that it may be possible to use the adhesive to bind together electrically conductive materials such as carbon nanotubes. The adhesive itself could serve as a thermally insulating layer, he says.
Another researcher, Herbert Waite, a professor of molecular, cellular, and developmental biology at the University of California, Santa Barbara, calls Messersmith's work very interesting. But he notes some limitations that could be exceeded through further study of the mussel that served as the adhesive's inspiration. Messersmith's adhesive can be applied only under conditions in which concentrations of the dopamine and pH levels are strictly maintained. Ideally, Waite says, it would be nice to have a glue that, like the mussel's, can be applied to any substrate, even in water, without external control of environmental parameters.

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