When engineers look at mussels, they usually look in awe at how they anchor themselves to almost any surface imaginable, all underwater. The fibers they use to bond are incredibly strong and the glue works wet or dry on all kinds of materials. For the most part, engineers are looking for a substance with similar properties.
This week, however, brings an exception: engineers who want to prevent clams from sticking to everything. Zebra mussels, a species that has invaded the Great Lakes, are estimated to cost utilities hundreds of millions of dollars a year because of clogged pipes and inlets. Ships, buoys and pretty much anything else we put in the water eventually needs to be cleared out too.
The international team behind the new work has designed a material that mussels seem unable to grasp. It’s not because the clam’s glue is failing; instead, the clam itself doesn’t seem to know what it’s touching when it’s set down on the material.
Oil and water
Anyone who’s been near water has probably seen some form of shellfish sticking to rocks or piers, but most of us probably haven’t looked closely at how they do it. In the case of mussels, the animal uses an appendage (called the foot) that extends from the shell to create a web of fibers one at a time. The result is resilient, as the loss of a fiber still holds the shell firmly. And the fibers are hard to break and stick to the surface, even though the glue the mussel uses is just a mix of simple chemicals and proteins.
Several materials have been devised that promise to limit clam sticking. But every year we spend those millions of dollars removing them from ships, pipes, piers and other hardware.
The team behind the new work decided to try a class of materials I didn’t even know existed: a lube-infused material. It’s less complicated than it sounds. The lubricant in this case is silicone oil, which has a backbone of silicone oxide linked to hydrocarbons. The material poured into it is just something that has an affinity for uncharged chemicals like those hydrocarbons. The silicone oil simply works its way into surface pores and holes and stays there due to its chemical compatibility. Silicone oil repels water (it is hydrophobic), so submerging it in water has no effect on the lubricant.
The team made two types of lubricant-infused materials. One was a standard polymer (polydimethylsiloxane), the second a porous surface made of fused silica nanoparticles. While the latter can probably contain a bit more of the silica oil, the former can be made much thicker.
To test it, they simply covered some surfaces, dropped them in water and put some clams on top. For controls, they used some untreated surfaces, as well as two commercial coatings that are supposed to repel animals. In all of their tests, only a single piece of the oil-soaked polymer had a clam stick. It handily beat both commercial coatings (the oil-soaked nanoparticles were as good as a commercial offering). In the event that mussels stuck, it took less force to pull them off and none of the mussels’ adherent proteins remained afterwards.
The team then took the material for a field test in the Atlantic Ocean. Compared to the two commercial products, the oil-soaked polymer ended up with either a quarter the number of mussels on it or a thirtieth.
What is this stuff?
Why is it so effective? To find out, the authors dropped some mussels on the surface and filmed what happened. And it seemed that the oil-soaked material confused the mussels. As in normal cases, the mussel put out its foot to examine the surface it was on. But when it came into contact with the oil-soaked material, it didn’t try to stick glue to it. Instead, the researchers observed one of three behaviors. In some cases, he simply retracted his foot into the shell. In other cases, it gave up and attached some glue to its own shell. In one case, the mussel simply sprayed a little glue into the water.
“From the ‘mussel perspective,'” the authors suggest, “[oil-infused] surfaces present a 400 nm thick smear layer after an initial capillary adhesion, where no solid surface would be observed.
Of course, the clam’s perspective involves some real biology. Here, unfortunately, the work comes across as a bunch of engineers trying out biology. They show that the mussel leg contains some receptors encoded by a family of genes involved in sensing stress. The team suggests this is likely responsible for the mussel’s ability to sense the properties of the surface it’s on. However, depending on the species, they also feel heat, cold, pain and other phenomena. So this is not a particularly convincing conclusion.
Either way, the work makes it clear that this material could reduce the problem of clams everywhere. And it’s practical to boot. The authors say it can be made into a sprayable paint.
Science2017. DOI: 10.1126/science.aai8977 (About DOIs).