Although a wide range of environmentally friendly surface coatings can reduce biofouling on marine structures, there is still not a fundamental understanding of barnacle adhesion upon reattachment. This study assessed the effect of hydrophobicity on adhesion in the barnacle Amphibalanus amphitrite, an abundant and widespread biofouler. Self-assembled monolayers were made on glass slides from alkyl silanes with methylated and fluorinated terminal groups to produce hydrophobic surfaces. Coated and uncoated glass slides underwent a 2-week barnacle reattachment assay. Barnacles were removed using a force gauge and critical shear stress was calculated for each substrate. Following reattachment assays, a Coomassie Blue G250 protein stain was used to quantify the amount of glue remaining on substrates by measuring pixel density with ImageJ software on glue scans.

Critical shear stress was found to be significantly higher for both hydrophobic surfaces as compared to the hydrophilic uncoated glass, and correspondingly the density of residual glue was higher on hydrophobic surfaces. Given that hydrophobic substrates can exclude water from the surface, they may provide a protected environment for glue release that is favorable for adhesive bond formation with the substrate as well as inter and intramolecular bonding within the glue layer. Critical shear stress showed a strong positive correlation with residual glue density, suggesting barnacle release occurs primarily via cohesive failure. Scanning electron microscope micrographs depicted a diverse mixture of features in the glue remnants depending on the coating and its location under the base plate. These features, which included a sponge-like matrix, globular structures, viscous fingering and nanoscale fibers contribute to adhesion strength. The design of marine coatings must continue to consider the nanoscale topography as an essential attribute to reducing biofouling as well as the ability of a coating to exclude water from the surface.

Barnacles are sessile marine arthropods that live and reproduce on nearly any available surface in the ocean. They adhere via a thin adhesive layer developed through a multistep secretory process synchronized with growth via molting. Unlike other arthropods, the combination of expansion, molting and protein secretion within the narrow adhesion interface leads to a nanofibrillar protein layer manipulated by shear stresses, protected by calcite, and containing a cocktail of chemically active molecules and proteins. Here we use in vivo imaging, mechanics, and spectroscopy of barnacle growth and development, coupled with mass spectrometry and proteomics to reveal much about the biophysics and biochemistry of barnacle adhesion. We will discuss the role of interfacial processes, self-assembly, amino acid composition, and chemical manipulation in the construction and function of the adhesive.

Fungal and bacterial fouling on military aircraft is a problem that can lead to polyurethane top coat deterioration and pose health hazards to personnel; the phasing out of hexavalent chromium in coatings is expected to worsen fouling problems. Thus, better understanding of the relevant microbiological interactions with polyurethanes is required to identify new ways to inhibit fouling and associated affects. We have screened over 400 aircraft isolates for polyurethane degradation, with Cryptococcus strains among the most aggressive polyurethane degraders. These strains were further characterized for their capability to metabolize and grow on expected hydrolysis products from polyester components of the polymers, showing that fungal growth occurs to varying degrees on the metabolites. Gas chromatography also showed that microbes metabolize polymers and hydrolysis products to CO₂. Polymer metabolization to CO₂ results in bulk polymer loss and optical profilometry confirmed that fungal cells steadily “eat” trenches into solid polyurethane films over time. Initial polyurethane film degradation processes at the micro and nano scales were analyzed by confocal Raman and AFM-IR (combined AFM and infrared spectroscopy). These results showed varying, non-uniform degradation events among cells, indicating that variations in single cell physiology play roles in early stage degradation. The spectroscopic results are consistent with lipase activity as the primary driver of degradation.

[1] M. Alles, A. Rosenhahn, Biofouling. 2015, 31, 469–480.

[2] MP. Schultz, et al., Biofouling. 2000, 15, 243-251.

[3] K. Nolte, J. Schwarze, A. Rosenhahn Biofouling. 2017, in press

Due to the complexity of adhesion mechanisms of marine life to surfaces, it is becoming apparent that combating biofouling will require coatings having complex surfaces. Specifically, coatings having mixed hydrophobic and hydrophilic surface domains are being shown to be able to mitigate the adhesion of a broader variety of marine organisms than can the silicone elastomer fouling-release coatings. Since it is also desirable to have a coating that can adhere well to a variety of substrates and stand up to occasional cleaning, a tough coating system is needed.

The incorporation of a low surface energy polymer such as a siloxane into a robust coating system such as a polyurethane results in stratification of the low surface energy component to the surface. By chemically binding hydrophilic groups to the siloxane, both functional groups stratify leading to a polyurethane coating having amphiphilic character on the surface. By varying the molecular parameters of the hydrophobic and hydrophilic components, the surface composition can be tuned to achieve a range of fouling-release properties when characterized using a broad variety of marine organisms. Different architectures of the reactive amphiphilic component have been explored including block and graft copolymers as well as dual-functionalized prepolymers.

Membranes coated with antifouling immobilized liquid layers have been recently shown to permit filtration while reducing surface fouling. In this work, we test the ability of liquid gated membranes created with expanded polytetrafluoroethylene and perfluorinated liquids to reduce the buildup of internal pore fouling using whey protein, an extremely challenging biological foulant. We find no differences in the decrease of flux or rate of fouling between coated and uncoated membranes in a dead-end filtration setup. However, upon stopping flow for 15-30 minutes, up to 70% of the original flux can be recovered with no additional energy input. This cycle can be repeated multiple times, with approximately 5% decrease in flux recovery each time. We use standard fouling equations and light microscopy to demonstrate that this zero-energy recovery may occur within the pores of the membrane due to the refilling of the pore with lubricating liquid, pushing the proteins off the pore walls. This work could have important applications in filtration processes with high fouling rates, reducing costs associated with standard chemical or physical cleaning methods.