Mussels can generate their own self-healing sticky material, which allows them to attach to rocks and to repair microtears caused by breaking waves and sand abrasion, but the elastic gel attached to this one was created in the laboratory. A patent is pending on the gel, which is described online in the Proceedings of the National Academy of Sciences Early Edition. Credit: Tara Fadenrecht, Niels Holten-Andersen |
Scientists can now manufacture a synthetic version
of the self-healing sticky substance that mussels use to anchor themselves to
rocks in pounding ocean surf and surging tidal basins. A patent is pending on
the substance, whose potential applications include use as an adhesive or
coating for underwater machinery or in biomedical settings as a surgical
adhesive or bonding agent for implants.
Inspiring the invention were the hair-thin
holdfast fibers that mussels secrete to stick against rocks in lakes, rivers
and oceans. “Everything amazingly just self-assembles underwater in a
matter of minutes, which is a process that’s still not understood that
well,” said Niels Holten-Andersen, a postdoctoral scholar with chemistry
professor Ka Yee Lee at the Univ.
of Chicago.
Holten-Andersen, Lee and an international team of
colleagues are publishing the details of their invention in the Proceedings
of the National Academy of Sciences Early Edition. Holten-Andersen views
the evolution of life on Earth as “this beautiful, amazingly huge
experiment” in which natural selection has enabled organisms to evolve an
optimal use of materials over many millions of years.
“The mussels that live right on the coast
where the waves really come crashing in have had to adapt to that environment
and build their materials accordingly,” he said.
Many existing synthetic coatings involve a
compromise between strength and brittleness. Those coatings rely on permanent
covalent bonds, a common type of chemical bond that is held together by two
atoms that share two or more electrons. The bonds of the mussel-inspired
material, however, are linked via metals and exhibit both strength and
reversibility.
The sticky material that mussels have evolved has inspired an international team of scientists to design a new artificial, self-healing gel that lends itself to underwater applications. The mussels pictured here are attached to a rock on Onetangi Beach of Waiheke Island, New Zealand. Credit: Steve Koppes |
“These metal bonds are stable, yet if they
break, they automatically self-heal without adding any extra energy to the
system,” Holten-Andersen said.
A key ingredient of the material is a polymer,
which consists of long chains of molecules, synthesized by co-author Phillip
Messersmith of Northwestern
Univ. When mixed
with metal salts at low pH, the polymer appears as a green solution. But the
solution immediately transforms into a gel when mixed with sodium hydroxide to
change the pH from high acidity to high alkalinity.
“Instead of it being this green solution, it
turned into this red, self-healing sticky gel that you can play with, kind of
like Silly Putty,” he said. Holten-Andersen and his colleagues found that
the gel could repair tears within minutes.
“You can change the property of the system by
dialing in a pH,” said Ka Yee Lee, a professor in chemistry at UChicago
and co-author of the PNAS paper. The type of metal ion (an electrically charged
atom of, for example. iron, titanium or aluminum) added to the mix provides yet
another knob for tuning the material’s properties, even at the same pH.
“You can tune the stiffness, the strength of
the material, by now having two knobs. The question is, what other knobs are
out there?” Lee said.
The PNAS study reports the most recent in a series
of advances related to sticky mussel fibers that various research
collaborations have posted in recent years. A 2006 PNAS paper by Haeshin Lee,
now of the Korea Advanced Institute of Technology, Northwestern’s Phillip
Messersmith and UChicago’s Norbert Scherer demonstrated an elusive but
previously suspected fact. Using atomic-force microscopy, they established that
an unusual amino acid called “dopa” was indeed the key ingredient in
the adhesive protein mussels use to adhere to rocky surfaces.
Last year in the journal Science, scientists at Germany’s Max Planck Institute
documented still more details about mussel-fiber chemical bonds. The Max Planck
collaboration included Holten-Andersen and Herbert Waite of the Univ. of California,
Santa Barbara.
Holten- Andersen began researching the hardness and composition of mussel
coatings as a graduate student in Waite’s laboratory.
“Our aspiration is to learn some new design
principles from nature that we haven’t yet actually been using in man-made
materials that we can then apply to make man-made materials even better,”
he said.
Being able to manufacture green materials is another advantage of drawing
inspiration from nature. “A lot of our traditional materials are hard to
get rid of once we’re done with them, whereas nature’s materials are obviously
made in a way that’s environmentally friendly,” Holten-Andersen said.
