The legendary sticking power of mussels in a marine environment is giving coatings researchers some valuable lessons in adhesion.
Few things in nature can beat the sticking power of a mussel anchored to a rock in the crashing ocean surf. So, scientists are taking notice and have now developed their own version of the sticky byssal threads that mussels generate to keep hanging in there.
Not only do the hair-thin fibers hold fast, with both hardness and elasticity; they are even self-healing, report researchers at the University of Chicago.
Self-Healing Polymer Networks
“Inspired by the pH jump experienced by proteins during maturation of a mussel byssus secretion, we have developed a simple method to control catechol-Fe3+ interpolymer cross-linking via pH,” the team reports in “pH-induced metal-ligand cross-links inspired by mussel yield self-healing polymer networks with near-covalent elastic moduli,” published in the Jan. 24 Proceedings of the National Academy of Sciences Early Edition.
“Growing evidence supports a critical role of metal-ligand coordination in many attributes of biological materials, including adhesion, self-assembly, toughness, and hardness without mineralization,” the researchers note.
A patent is pending on how to make the substance. Potential applications include adhesives or coatings for underwater applications.
The research is being funded by the National Science Foundation, National Institutes of Health, and National Aeronautics and Space Administration.
Self-Assembly and Silly Putty
With mussels, “everything amazingly just self-assembles underwater in a matter of minutes, which is a process that’s still not understood that well,” researcher Niels Holten-Andersen told the website sify.com.
The site notes that many current coatings formulations must 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,” it says. On the other hand, the bonds of the mussel-inspired material “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 told sify.com
A key ingredient of the material is a polymer that, when mixed with metal salts at low pH, appears as a green solution. But when mixed with sodium hydroxide to change the pH from high acidity to high alkalinity, “it turned into this red, self-healing sticky gel that you can play with, kind of like Silly Putty,” he said. The gel can repair tears within minutes, he said.
Mussels on the Move
Mussels have inspired a tremendous amount of coatings research recently. Some of that work will be discussed in a meta-study of “mussel-inspired” coatings and adhesives scheduled to be published in the Annual Review of Materials Research Volume 41 in June.
As those researchers note in their abstract: “[M]uch effort has been directed to characterizing and mimicking essential features of the adhesive chemistry practiced by mussels. Studies of these organisms have uncovered important adaptive strategies that help to circumvent the high dielectric and solvation properties of water that typically frustrate adhesion.”
Synthetic polymers functionalized with the organic compounds known as catechols offer the potential for a wide range of adhesives, sealants and coatings that may prove beneficial not only in protective coatings but also in biomedical applications, experts say.