Barnacle Bioadhesive Bonding Formation, Strength and Control

Abstract: The current technology for keeping ship hulls free of marine biofouling requires the use of toxic compounds such as copper, organic biocides and tributyl tin (TBT). The compounds that are released have been shown to have negative effects on non-target organisms, and a worldwide ban of TBT is currently being introduced. An environmentally attractive alternative would be to reduce the strength of the bioadhesive bonds formed by the marine organisms. Thus, fouling organisms may be removed by grazing fish and crabs, by their own weight or by forces that are present when ships move through the water. There has been considerable interest in such systems, usually termed non-stick surfaces or fouling release coatings, for biofouling control and coatings made from poly(dimethylsiloxane) (PDMS) have shown good performance. Nevertheless, the bioadhesion still needs to be reduced for the concept to satisfy the requirements for successful biofouling control. Accomplishing that objective requires a fundamental understanding of the adhesive mechanisms and the mechanisms that control release from surfaces.

The work presented in this interdisciplinary thesis has focused on the formation, the strength and the possible control of bioadhesive bonds with an emphasis on the Balanus improvisus barnacle, which is the main macrofouler found on ship hulls in Swedish waters. The major components of the bioadhesive usually termed cement were found to be three polypeptides with molecular masses of 42, 57 and 94 kD, respectively. The granular sub-micron morphology of the cement indicated a complex between the polypeptides. Incorporation of CaCO₃ in the form of calcite was detected. The incorporation of CaCO₃ seems to increase the cohesive strength of the adhesive plaque together with the adhesive strength. Moreover, the substrate properties were found to affect the morphology and chemistry of the adhesive plaque, and it is important to consider this barnacle response in future coating design.

The barnacle bioadhesion strength was found to be affected not only by surface chemistry of the coating but also on the modulus of the coating and with decreased modulus release was facilitated. Furthermore, the surface chemistry of a condensation cured PDMS network was tailored by the incorporation of a fluorinated cross-linker. The reduced surface energy of the fluorinated coating had no significant effect on barnacle bioadhesion bond strength compared to a non-fluorinated control. However, the morphological and chemical stability of the coating surface was greatly improved, which resulted in significantly lower barnacle bioadhesion bond strength measured on the fluorinated coating compared to the non-fluorinated control after prolonged exposure to seawater.

The results presented in this thesis demonstrate the importance to understand and characterize the biointerface between the adhesive plaque produced by the barnacle and the synthetic polymer. With this knowledge we now have a new platform for developing innovative fouling-release coatings.