Sticky Mussels Inspire Phil Messersmith Yet Again
Thursday, December 13, 2007
Mussels are delicious when cooked in a white wine broth, but they also
have two other well-known qualities before they're put in a pot: they
stick to virtually all inorganic and organic surfaces, and they stick
with amazing tenacity.
Northwestern University biomedical engineer Phillip B. Messersmith
already has developed a material that mimics the strength of the bonds;
now he has produced a versatile coating method that mimics the mussels'
ability to attach to a wide variety of objects.
Messersmith and his research team, in a study published in the Oct. 19
issue of the journal Science, report that a broad variety of materials
can be coated and functionalized through the application of a surface
layer of polydopamine.
Potential applications of the simple and inexpensive method include
flexible electronics, such as bendable and flexible displays,
biosensors, medical devices, marine anti-fouling coatings, and water
processing and treatment, such as removing heavy metals from
contaminated water.
Key to the coating method is the small molecule dopamine, commonly
known as a neurotransmitter. Dopamine, it turns out, is a good mimic of
the essential components of mussel adhesive proteins, and the
researchers use it as a building block for polymer coatings. (Dopamine
itself is not found in mussels.) So, like a mussel, Messersmith's
coating sticks to anything.
"This is an astonishingly simple and versatile approach to functional
surface modification of materials," said Messersmith, professor of
biomedical engineering at Northwestern's McCormick School of
Engineering and Applied Science, who led the research. "We dissolve
dopamine, which we buy at low cost, in a beaker of water exposed to
air. We adjust the water's pH to marine pH, about 8.5, put in an object
and several hours later it's coated with a thin film of polydopamine.
That's it."
Solid objects of any size and shape can be immersed in the solution.
(The dopamine solution is very dilute -- only two milligrams of
dopamine per one milliliter of water.) At marine pH, there are chemical
changes in the dopamine molecule that result in polymerization of the
molecules together to form a polymer, polydopamine, which coats the
object. The polymer is fairly similar to what is found in the mussel
adhesive protein.
And to make things more interesting, the polydopamine coating, in turn,
provides a very chemically reactive surface onto which the researchers
can deposit a second coating. And because the surface is so reactive in
so many different ways, a wide variety of second coatings can be
applied.
"We take advantage of that reactivity to apply the second layer," said
Messersmith. "As a simple example, I could put an iPod in the dopamine
solution, and a thin polydopamine coating would form. Then I could take
it out and put it in a metal salt solution and form a coating of copper
or silver."
This second coating, depending on what it is, promises to take
researchers and industry in multiple directions as far as applications
go. In addition to cladding objects with metal coatings, this includes
inhibiting biofouling of materials (such as for medical devices),
engineering surfaces to support biospecific interactions with cells
(such as for culture and expansion of stem cells) and applying
self-assembled monolayers to nonmetal surfaces (such as for biosensors).
Messersmith and his colleagues tested the two-step process on 25
different substrate materials (but not an iPod) with a wide range of
characteristics representing all major classes of materials, from
hydrophobic to hydrophilic, from inorganic to organic, as well as the
traditionally difficult material Teflon, all with positive results.
They then demonstrated deposition of metal and organic coatings and
self-assembled monolayers onto the polydopamine coating.
"Existing methods for modifying material surfaces are fairly restricted
to specific materials -- what works well on glass would not work well
on gold," said Messersmith. "Our method is a much more general strategy
for a variety of surfaces. We haven't found a material to which we
can't apply polydopamine."
The research was supported by the National Institutes of Health (grants DE 14193 and HL 74151).
In addition to Messersmith, other authors of the paper, titled
"Mussel-Inspired Surface Chemistry for Multifunctional Coatings," are
Haeshin Lee (lead author) and Shara M. Dellatore, both graduate
students, and William M. Miller, professor of chemical and biological
engineering, all from Northwestern.
-- Megan Fellman
