When mussels are mentioned, a nice plate of moules-frites might come to mind, but some bivalves are peskier than they are tasty. Dozens of mussel species living in both fresh and saltwater are a huge nuisance, known for stubbornly sticking to rocks, piers and boats and hitchhiking their way to new locales. But a new review of studies in the journal Matter suggests the thing that makes mussels so maddening—their adhesive power—could have all sorts of applications in engineering.
In fact, Susie Nelson at NPR reports that the underwater sticking power of mussels is so impressive, it’s spawned an entire field of study called mussel-inspired chemistry. Researchers reviewed recent advances inspired by the bivalves, finding that mussel stickiness could have all sorts of applications, including cleaning up oil spills, purifying water, and creating a universal red blood cell.
Why are mussels so good at clinging to rocks and boat propellers? According to a press release, that sticking power comes from thin byssus threads, filaments they use to cling to rocks and often referred to as the mussel's "beard." Those threads use an amino acid group called dihydroxyphenylalanine, or DOPA, which uses a few chemistry tricks, like hydrogen bonding, that allows it to form a super-tight bond with all sorts of substrates.
Based on those threads, chemists have developed an artificial version of the mussel adhesive called polydopamine, or PDA. And that’s the stuff that engineers are finding new uses for.
Study co-author Seth Darling, director of the Center for Molecular Engineering at Argonne National Laboratory, for instance, has experimented with PDA to clean heavy metals out of water because the compound repels charged particles and attracts water, NPR’s Nelson reports. Tweaking the material could also perform a similar clean up functions with organic pollutants, pathogens and with oil spills, something that has been tested on the small scale.
“If you put an oil-water mixture against that membrane, the oil will transmit spontaneously through the pores, and the water will stay behind,” Darling says.
According to the press release, a group of scientists in China is also using the material to develop a universal red blood cell, which could be used for anyone regardless of blood type. A coating inspired by the mussel adhesive placed on the blood cell essentially hides it from the body’s immune system, which normally attacks incompatible blood cells. The material could also have applications in solar steam generation, in which sunlight converts water to steam. That process can be used to distill and sterilize water and could also be used in power generation.
Georgia Tech chemical engineer Blair Brettmann, who was not involved in the study, tells Nelson that the potential water clean-up applications are exciting, and the materials could also lead to things, like medical adhesives that can remain sticky in a wet environment.
But there are still some major roadblocks to a mussel-inspired future, says co-author Hao-Cheng Yang, an engineer at Sun Yat-Sen University.
“Despite simplicity and effectiveness, there are still some inherent limitations,” Yang says. “Alkaline conditions are usually needed to realize the polymerization of dopamine, so it cannot be applied to materials that are unstable under alkaline conditions. Moreover, the deposition of PDA is a time-consuming process—it takes tens of hours to form a uniform coating on most material surfaces.”
It’s also possible that part of the mussels' sticking power comes from the fact that they form dense colonies—the aggregation of mollusks may combine certain amino acids to make the colony even more adhesive. In the paper, the researcher say dopamine is expensive to produce, so they suggest materials scientists look deeper into similar plant-based compounds like tannic acid.
In other words, there’s still a lot to learn about the stickiness of mussels before the mollusks come off of our plates and into a bioengineered future.