Mussels reveal how to stick to it

U. CHICAGO (US) — Taking a cue from mussels, scientists have created a synthetic sticky substance for use as an adhesive or coating for underwater machinery, in biomedical settings as a surgical adhesive, or as a bonding agent for implants.

Inspiring the invention were the hair-thin holdfast fibers that mussels secrete to anchor themselves to rocks in pounding ocean surf and surging tidal basins.

“Everything amazingly just self-assembles underwater in a matter of minutes, which is a process that’s still not understood that well,” says Niels Holten-Andersen, a postdoctoral scholar with Ka Yee Lee, professor of chemistry at the University of Chicago.

Details of the invention are reported in Proceedings of the National Academy of Sciences Early Edition.

“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,” says Holten-Anderson.

Many existing synthetic coatings involve a compromise between strength and brittleness that 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.

“These metal bonds are stable, yet if they break, they automatically self-heal without adding any extra energy to the system,” Holten-Andersen says.

A key ingredient of the material is a polymer, which consists of long chains of molecules, synthesized by co-author Phillip Messersmith of Northwestern University.

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,” that repairs tears within minutes, Messersmith says.

“You can change the property of the system by dialing in a pH,” says Lee. 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 says.

“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,” says Holten-Andersen.

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.”

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