Non-toxic, lubricant-infused coatings deter mussels and prevent their attachment by disrupting their mechanosensory and adhesive systems.
It all began with a bet at a conference in Italy in 2013. Nicolas Vogel, Ph.D., then a postdoctoral fellow in Joanna Aizenberg’s lab at the Wyss Institute for Biologically Inspired Engineering at Harvard University and Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS), gave a talk about the group’s Slippery Liquid-Infused Porous Surfaces (SLIPS) coatings, which promised to prevent nearly anything from adhering to structures to which they were applied. In the audience was Ali Miserez, Ph.D., an Associate Professor of Materials Science and Engineering at Nanyang Technological University (NTU) specializing in biological materials who approached Vogel after the presentation and said confidently, “I bet mussels will stick to your coatings, because I still have yet to see a surface that they won’t attach to.”
Vogel accepted the challenge and sent some SLIPS samples to Miserez upon returning to Cambridge, initiating a collaboration whose results are reported in this week’s issue of Science. The study demonstrated that a certain form of SLIPS is indeed essentially mussel-proof, and shed light on how they thwart mussels’ expert attachment mechanisms. “I badly lost the bet,” says Miserez, who is a corresponding author of the paper along with Vogel (now a Professor at Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany) and Aizenberg. “I think I owe Nicolas a nice dinner.”
Mussels are one of the worst perpetrators of biofouling, or the unwanted accumulation of organisms on underwater structures like pipes, boats, industrial equipment, and docks. Not only do biofouling organisms like mussels threaten to slice open an unlucky swimmer’s foot, they have significant economic and environmental costs: the US Navy alone spends ~$1 billion per year on antifouling efforts, and many species are invasive pests that hitch rides to new environments on ships’ hulls.
The vast majority of the weapons deployed against mussels and other clingers-on are paints and coatings that contain toxic chemicals, usually copper-based, that deter or kill organisms when they come into close proximity. These materials raise concerns because they poison species indiscriminately, accumulate in waterways, likely have ecological impacts, need to be replaced regularly, and are often not as effective as desired. Non-toxic “low surface energy” coatings based on silicone or siloxane polymers (compounds similar to those used in the medical industry for catheters) have been introduced as non-toxic alternatives, but while these materials do allow for easier removal of biofouling species, they are less effective at preventing organisms from attaching in the first place, and are susceptible to damage and decay.
The Wyss’ SLIPS technology, inspired by the slick lip of a carnivorous pitcher plant that sends insects sliding down to their doom, takes advantage of the fact that it is very difficult for an organism to attach to a liquid surface. SLIPS consists of a solid surface infused with a liquid lubricant overlayer that is retained in place so that anything that comes into contact with the liquid layer will simply slide right off. SLIPS have previously been shown to be effective against bacteria and algae, but mussels represent a particularly intimidating foe. Their muscular feet produce adhesive filaments called byssal threads whose tips, called adhesive plaques, contain special adhesive proteins that remove water molecules from the target surface to enable the plaques to bind to it. “Mussels have mastered the skill of sticking in an underwater environment, despite water being the biggest enemy of adhesion,” says Miserez. This system allows them to bind to surfaces extremely well: large accumulations of mussels can weigh as much as 1,700 pounds per square foot.
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