Mussels have long been the scourge of owners of boats and static maritime equipment, like pipes and heat exchangers. They induce drag and damage by clinging tenaciously to the boat’s hull; they clog pipes and limit a heat exchanger’s efficiency. Coatings have been developed to prevent such biofouling, but some are toxic, and there is much room for improvement. Now, by performing molecular-scale studies of adhesion proteins deposited by mussels, nanoscale measurements of the adhesion forces of mussels and various surfaces, and macroscale observations of the behavior of mussels during the attachment process, researchers have shown that a nontoxic, lubricant-infused polymer-based coating has much promise in solving the biofouling problem and preventing mussel adhesion.

“The multiple length scales of this research is key,” says Onye Ahanotu, a senior research scientist at the Wyss Institute for Biologically Inspired Engineering at Harvard University. “We looked into whether adhesive proteins are left on the surface after adhesive plaque detachment and rinsing, as well as what the organism is doing on these surfaces and which ones they would rather stick to. The mussels actively avoided certain coatings and sought out others.”

The extraordinary ability of mussels to attach to virtually any surface comes from adhesive proteins secreted in threads from a gland at the end of the mussel’s foot, and therefore called mussel foot proteins (Mfps). The foot is normally housed within the two shells of the mussel, but it can be extended through the opening between the shells to attach to surfaces. The proteins produce a complex adhesive that remains intact in water, and is capable of secure attachment to any surface.

In this collaboration between Harvard University and Nanyang Technological University in Singapore, the researchers studied the adhesion of the Asian green mussel P. viridis—an extremely aggressive species that has invaded multiple geographical locations through fouling of boat hulls. They assayed the adhesion of the mussels to surfaces protected by various coatings: (a) a polymer coating based on polydimethylsiloxane (PDMS) infused with silicone oil (i-PDMS); (b) a silica nanoparticulate coating deposited by a layer-by-layer (LBL) technique and infused with silicone oil (i-LBL); and (c) two commercially available foul-release coatings. A plain glass substrate and non-infused versions of PDMS and LBL were used as controls. The results showed no adhesion plaques on i-PDMS, a small number of plaques on i-LBL and the two commercial coatings, and a large amount on the three controls.

The researchers used a micro-tensile testing machine to determine the macroscopic adhesion strength of the mussel plaques, and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry as a nanoscale biochemical test for the presence of P. viridis foot proteins (Pvfps) on the various surfaces. In both analyses, i-PDMS was shown to have the best properties, including the lowest mean adhesion strength and no Pvfps on its surface.

To determine whether the secreted protein threads were simply not sticking to the i-PDMS surface, or whether the mussel chose not to secrete the proteins at all, the researchers observed mussel behavior in the laboratory on i-PDMS, i-LBL, and non-infused PDMS and LBL samples. Predictably, mussels quickly probed the surface and deposited adhesive proteins on the non-infused materials. In the case of i-PDMS, the mussels showed three unusual behaviors: (a) after surface exploration, the mussel foot did not deposit threads, instead attaching threads to its own shell or another nearby substrate; (2) it secreted a viscous gel that did not solidify and readily dispersed in seawater; and (3) the mussel swiftly retracted its foot after 1 s of surface exploration without secreting a thread.

In an effort to understand the mechano-stimulus that mussels may be detecting with the sensing organ in their foot, the researchers conducted depth-sensing nano-contact mechanics measurements, which measure the contact forces experienced by the mussels during probing.

“When they probe the surface with their foot, mussels expect to feel the compressive force of a hard surface; the nano-contact mechanics measurements on the i-PDMS coating reveal that the mussel feels a tensile force from the lubricant, which deceives its mechano-sensing organ,” says Joanna Aizenberg, professor of Chemistry and Chemical biology at Harvard and the corresponding author of the article on this work published in a recent issue of Science. “On the biochemical level, adhesive proteins that mussels use for attachment displace water from the surface to attach the adhesive proteins to the naked solid; however, the i-PDMS lubricant layer does not allow the proteins to displace water and reach the solid.”

Studies of i-PDMS in the field over 16 weeks in Scituate Harbor, Massachusetts, showed that this lubricant-infused polymer system successfully prevents mussel adhesion in real marine conditions.

“Fouling organisms carry on two levels of negotiation with target surfaces” says Herbert Waite, a Distinguished Professor at the Marine Sciences Institute, University of California–Santa Barbara, who was not involved in the current study. “There's the superficial level in which the mussel is asking "do I want to kiss this surface?" and there's the interfacial level in which the question becomes "how do I best kiss this surface?” Waite, who has studied mussel adhesion for almost 40 years, says that most researchers have investigated these two mechanisms one at a time, which can affect results. When mussels were presented only with “repellent” surfaces, for example, their fussiness regarding attachment decreased. He concludes, “The primary innovation of this study was that their experimental design effectively tackled both operative levels.”

Read the abstract in Science.