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.�
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