'marine biofouling', the undesired growth of marine organisms such as microorganisms, barnacles and seaweeds on submerged surfaces, is a global problem for maritime industries, with both economic and environmental penalties. the primary strategy for combating marine fouling is to use biocide-containing paints, but environmental concerns and legislation are driving science and technology towards non-biocidal solutions based solely on physico-chemical and materials properties of coatings. advances in nanotechnology and polymer science, and the development of novel surface designs 'bioinspired' by nature, are expected to have a significant impact on the development of a new generation of environmentally friendly marine coatings. Marine biofouling, the colonization of submerged surfaces by unwanted marine organisms ( Fig. 1), has detrimental effects on shipping and leisure vessels, heat exchangers, oceanographic sensors and aquaculture systems. For example, it has been shown that the increased roughness presented by a heavily fouled ship hull can result in powering penalties of up to 86% at cruising speed; even relatively light fouling by diatom 'slimes' can generate a 10-16% penalty 1 . Without effective antifouling (AF) measures, in order to maintain speed, fuel consumption (and therefore greenhouse gas emissions 2 ) increase significantly. A recent analysis of the economic impact of biofouling for the Arleigh Burke DDG-51 destroyers, which comprise 30% of the ships in the US Navy fleet, estimates the overall cost associated with hull fouling at $56 million per annum 3 . This figure is based on the present AF coating system, cleaning and fouling level (typically heavy slime) of the Navy. If the analysis is extended to the entire US Navy fleet, the approximate cost of hull fouling is between $180 and 260 million per annum.Marine biofouling is ubiquitous and has been a practical problem ever since man sailed the oceans; controlling it, without simultaneously creating unacceptable environmental impacts on non-target species is a considerable challenge. Recent years have seen a resurgence of interest in the fundamental science behind the processes involved in biofouling, and in the design of novel coatings and other non-coating technologies. The main driver for this is legislation that has outlawed some highly effective AF paints, notably the use of tributyltin oxide, and posed a stricter evaluation and regulatory regime on the use of alternative biocides. 'Green' alternatives to biocide-based technologies are therefore urgently sought by the marine coatings industry, and there is considerable interest in developing biocide-free coatings that rely on surface physico-chemical and bulk materials properties to either deter organisms from attaching in the first place ('prevention is better than cure') or reduce the adhesion strength of those that do attach, so that they are easily removed by the shear forces generated by ship movement or mild mechanical cleaning devices.

Cross-linked hyperbranched fluoropolymer (HBFP) and poly(ethylene glycol) (PEG) amphiphilic networks with PEG weight percentages of 14% (HBFP-PEG14), 29% (HBFP-PEG29), 45% (HBFP-PEG45), and 55% (HBFP-PEG55) were prepared on 3-aminopropyl)triethoxysilane (3-APS) functionalized microscope glass slides for marine antifouling and fouling-release applications. The surface-free energies (gamma(s)), polar (gamma(s)(p) and gamma(s)(AB)), and dispersion (gamma(s)(d) and gamma(s)(LW)) components were evaluated using advancing contact angles by two-liquid geometric-mean and three-liquid Lifshitz-van der Waals acid-base approaches. The HBFP coating exhibited a low surface energy of 22 mJ/m(2), while the gamma(s) and gamma(s)(p) of the cross-linked HBFP-PEG coatings increased proportionally with the PEG weight percentages in the networks. The adsorption of bovine serum albumin (BSA), lectin from Codium fragile (CFL), lipopolysaccharides from Escherichia coli (LPSE) and Salmonella minnesota (LPSS) upon glass, APS-glass, HBFP, PEG, and the cross-linked HBFP-PEG network coatings were investigated by fluorescence microscopy. The marine antifouling and fouling-release properties of the cross-linked HBFP-PEG coatings were evaluated by settlement and release assays involving zoospores of green fouling alga Ulva (syn. Enteromorpha; Hayden, H. S.; Blomster, J.; Maggs, C. A.; Silva, P. C.; Stanhope, M. J.; Waaland, J. R. Eur. J. Phycol. 2003, 38, 277). The growth and release of Ulva sporelings were also investigated upon the HBFP-PEG45 coating in comparison to a poly(dimethylsiloxane) elastomer (PDMSE) standard material. Of the heterogeneous cross-linked network coatings, the maximum resistances to protein, lipopolysaccharide, and Ulva zoospore adhesion, as well as the best zoospore and sporeling release properties, were recorded for the HBFP-PEG45 coating. This material also exhibited better performance than did a standard PDMSE coating, suggesting its unique applicability in fouling-resistance applications.

The effect of feature size, geometry, and roughness on the settlement of zoospores of the ship fouling alga Ulva was evaluated using engineered microtopographies in polydimethylsiloxane elastomer. The topographies studied were designed at a feature spacing of 2 microm and all significantly reduced spore settlement compared to a smooth surface. An indirect correlation between spore settlement and a newly described engineered roughness index (ERI) was identified. ERI is a dimensionless ratio based on Wenzel's roughness factor, depressed surface fraction, and the degree of freedom of spore movement. Uniform surfaces of either 2 mum diameter circular pillars (ERI=5.0) or 2 microm wide ridges (ERI=6.1) reduced settlement by 36% and 31%, respectively. A novel multi-feature topography consisting of 2 mum diameter circular pillars and 10 microm equilateral triangles (ERI=8.7) reduced spore settlement by 58%. The largest reduction in spore settlement, 77%, was obtained with the Sharklet AF topography (ERI=9.5).

Surfaces of novel block copolymers with amphiphilic side chains were studied for their ability to influence the adhesion of marine organisms. The surface-active polymer, obtained by grafting fluorinated molecules with hydrophobic and hydrophilic blocks to a block copolymer precursor, showed interesting bioadhesion properties. Two different algal species, one of which adhered strongly to hydrophobic surfaces, and the other, to hydrophilic surfaces, showed notably weak adhesion to the amphiphilic surfaces. Both organisms are known to secrete adhesive macromolecules, with apparently different wetting characteristics, to attach to underwater surfaces. The ability of the amphiphilic surface to undergo an environment-dependent transformation in surface chemistry when in contact with the extracellular polymeric substances is a possible reason for its antifouling nature. Near-edge X-ray absorption fine structure spectroscopy (NEXAFS) was used, in a new approach based on angle-resolved X-ray photoelectron spectroscopy (XPS), to determine the variation in chemical composition within the top few nanometers of the surface and also to study the surface segregation of the amphiphilic block. A mathematical model to extract depth-profile information from the normalized NEXAFS partial electron yield is developed.

Quantitative methods and associated kinetic analyses have been used for the$rst time to study detailed aspects of the settkment and adhesion of various types of Enteromorpha popagule. Time course expm'ments showed that quadri-and biJlagellate zoospores and zygotes adhered rapidly, but a proportion within any one population appeared to be incompetent at adhm'ng to the substratum. Kinetic (Scatchard) analysis of adhesion expm'ments pe$ormed at a range of zoospore concentrations revealed density-dependent effects not previously reported, with positive cooperativity at low spore densities and negative cooperativity at high spore densities. High-resolution video microscq@ was used fw thejrst time to reveal details Ofthe various stages in the settlement and adhesion of zoospores and zygotes. Novel obseruations were made of a n initial, temporary phase of attachment via the apical papilla, followed by a permanent phase of commitment, charactm'ed by discharge of adhesive-containing cytoplasmic vesicles, as the cell contracted against the suqace, and adsm-ption of p agella. ?'he phase of commitment was followed by exploitation of the surface through amoeboid-like movements at the interj4ace. Gregarious settlement behavior was frequently o b serued leading to the formation of rafts of cells. The possible mechanisms and signifcance of density-dependent spore adhesion are discussed.

Bioadhesion and surface wettability are influenced by microscale topography. In the present study, engineered pillars, ridges and biomimetic topography inspired by the skin of fast moving sharks (Sharklet AF) were replicated in polydimethylsiloxane elastomer. Sessile drop contact angle changes on the surfaces correlated well (R2 = 0.89) with Wenzel and Cassie and Baxter's relationships for wettability. Two separate biological responses, i.e. settlement of Ulva linza zoospores and alignment of porcine cardiovascular endothelial cells, were inversely proportional to the width (between 5 and 20 microm) of the engineered channels. Zoospore settlement was reduced by approximately 85% on the finer (ca 2 microm) and more complex Sharklet AF topographies. The response of both cell types suggests their responses are governed by the same underlying thermodynamic principles as wettability.

To understand the role of surface wettability in adhesion of cells, the attachment of two different marine algae was studied on hydrophobic and hydrophilic polymer surfaces. Adhesion of cells of the diatom Navicula and sporelings (young plants) of the green macroalga Ulva to an underwater surface is mainly by interactions between the surface and the adhesive exopolymers, which the cells secrete upon settlement and during subsequent colonization and growth. Two types of block copolymers, one with poly(ethylene glycol) side-chains and the other with liquid crystalline, fluorinated side-chains, were used to prepare the hydrophilic and hydrophobic surfaces, respectively. The formation of a liquid crystalline smectic phase in the latter inhibited molecular reorganization at the surface, which is generally an issue when a highly hydrophobic surface is in contact with water. The adhesion strength was assessed by the fraction of settled cells (Navicula) or biomass (Ulva) that detached from the surface in a water flow channel with a wall shear stress of 53 Pa. The two species exhibited opposite adhesion behavior on the same sets of surfaces. While Navicula cells released more easily from hydrophilic surfaces, Ulva sporelings showed higher removal from hydrophobic surfaces. This highlights the importance of differences in cell-surface interactions in determining the strength of adhesion of cells to substrates.

Coatings based on polysulfobetaine polymers are being developed as environmentally benign, fouling-resistant marine coatings. Poly(sulfobetaine methacrylate) (polySBMA) brushes were grafted onto glass surfaces using surface-initiated atom transfer radical polymerization (ATRP). The settlement, growth, and attachment strength of marine algae were investigated on polySBMA-coated surfaces. Results showed that few spores of the green marine alga, Ulva, settled (attached) on the polySMBA surfaces, and the adhesion strength of both spores and sporelings (young plants) was low. Diatoms were also mostly unable to adhere to the polySMBA surfaces. Assays demonstrated that SBMA polymers in solution were not toxic. The data are discussed in terms of the interfacial properties presented by the polySMBA surfaces. Zwitterionic polymers and coatings exhibit great advantages for their effectiveness to resist marine fouling while being environmentally benign and are promising as ultralow fouling marine coatings.