Flexible 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.”
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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.
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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.