Biofouling ofartificial structures such as ships hulls is a common problem in marineenvironments. Development of protective coatings for these surfaces is vitalfor many industries as a result of economic losses from damages to shipcomponents through fouling; cost of removing fouling organisms once docked andincreased fuel consumption as a consequence of increased hydrodynamic drag.The difficultiesfaced with developing such a coating are that the coatings need to be environmentallyacceptable, successfully inhibit a wide spectrum of fouling organisms and notbe so expensive to produce that financial problems are still prominent inaffected industries.This paperpresents an insight into the process of biofouling followed by an overview ofpreviously used toxic antifouling methods and then non-toxic methods being usedto design an environmentally acceptable, broad spectrum antifouling orfoul-release coating for ship’s hulls of all sizes.
Finally, the approachesthat currently show the most promise will be used to consider the bestdirection for future research.The process ofbiofouling can be split into key growth stages. These stages are the initialamassing of adsorbed organic molecules, the settlement and growth of bacteriaresponsible for creating a biofilm and the settlement of various micro andmacrofoulers. A procedure forthe order that these marine fouling events occur is generally accepted (Fig. 1)however the order of these events is not easily determined due to some macrofouling organisms being able to exploit shallow recessesin the coatings before a biofilm has been able to develop. Formation of a conditioning film is the first stage and occurs bythe adsorption of organic molecules and ions primarily made up fromcarbohydrates, proteins and uronic acids.
Uronic acids such as D-glucuronicacid (Fig. 2) are of particular importance to other fouling bacteria as itmakes the surface more favourable for attachment.1 This is possiblydue to the alcohol groups within the structure of uronic acids enablingcondensation polymerisation to occur with monosaccharides within the diatoms. Fromhere a biofilm develops as microorganisms such as diatoms attach to theconditioning film.
These bacteria adhere tenaciously and colonise mostresistant surfaces to form a complex biofilm matrix. Adhesion is supported bycolonising microorganisms secreting extracellular polymeric substances (EPS) toanchor them to the surface as a unit. EPS is made up of uronic acid and similarstructures so that they can bind together strongly as a polysaccharide. Thisallows the microorganisms within the EPS to stay attached whilst havingprotection from environmental stress, exchange of nutrients and enzymaticexchange. This will change the surface chemistry thus stimulating furthergrowth in the form of macroorganisms including barnacles, tubeworms and mussels.1,3The adhesion techniques used by the various fouling organisms arevery diverse which makes targeting the settlement of a large collection ofspecies in a single coating extremely difficult. For example, at thecritical larval development stage of a barnacle, a temporary adhesive proteinis used whilst searching the surface until it finds a permanent place tosettle.
The barnacle then secretes a hydrophobic protein to permanently adhereto the surface. This approach is not taken by mussels however as they use ahydrophilic polyphenolic adhesive protein which crosslinks to the surface via aredox reaction in the presence of an enzymatic catalyst. This highlights how inorder to focus on fouling organisms as a collective it is fundamental to findsimilarities in organisms binding mechanisms.3 Fouling organismsare generally classified into hard foulers and soft foulers. Hard foulers areseen as organisms with hard skeletons, tubes or shells, such as barnacles,mussels and bryozoans. On the other hand, soft foulers are organisms that lackthese hard structures, like macroalgae, sponges, diatoms and ascidians.
Many scientistsand groups have tried to establish a coating that prevents the settlement ofboth hard and soft foulers however the list of currently accepted requirementsa coating requires is quite extensive (Table 1) which makes the process muchmore difficult.Surface properties such as surface roughness and wettability have beenfound to have a profound effect on the fouling ability of organisms. Forexample, Whitehead et al investigated the effect of surface properties on theattachment of fungal spores to coatings. They found that surface wettabilitydid influence the strength of spore attachment to surfaces however surfaceroughness did not. This is supported by the observation that the roughestsurface, polytetrafluoroethylene (PTFE) was least easily wetted yet had thehighest spore attachment whereas the smoothest surface, silicone was the mosteasily wetted and had the lowest spore attachment.4 This is unexpectedas previous studies have shown that rough surfaces should have a small contactangle thus be easily wetted and the opposite for smooth surfaces (Fig. 3).
Theseresults imply that the surfaces have the same surface energy therefore onlysurface roughness is different between the two surfaces. This would mean thatother factors will be influencing these results such as the stiffness of thematerials which is important for biomaterials in tissue culture. Another issue withthe findings in this research is that the spores only had momentary contactwith the surfaces which doesn’t realistically replicate real life as in thenatural environment there would be constant contact as the ship moves throughthe water therefore some fungal spores may not have had long enough to attachto the surfaces. It was also seenthat the shape of the spores has an impact on their fouling ability.4This shows that many factors impact a species ability to attach to a coatingand different strains of the same species show different characteristics so maynot be affected by a coating in the same way.
Discovery of newantifouling coatings has helped to define the criteria for an antifoulingcoating (Table 1). Coatings that are developed can be classified as eitherantifouling or foul-release. Foul-release coatings are biocide free polymermatrices, which work by providing a smooth, low-friction surface by physicalmeans onto which fouling organisms have difficulty adhering.
Antifoulingcoatings have a very similar effect on foulers however achieve this by chemicalmeans.The economiceffect of the growth of biofilms on biocidal and foul-release coatings issignificant. The increase in shear stress and drag, leads to increased fuelconsumption.
Lewthwaite et al found that a 1mm thick biofilm developed on a 23mship caused an 80% increase in friction drag and caused a 15% loss of shipspeed.6 Schultz et al found that the formation of ‘heavy slime’ onnaval ships incurs an additional fuel cost of $1.15M per vessel per year due toan increased fuel consumption of 10.3%.7 Both of these studies wereestablished through the accumulation of data previously found followed bycalculations to establish the increase in friction drag and the monetary effectthat biofouling has.
This allows the issue of biofouling to be quantified andportrays the urgency at which a new approach to tackle marine fouling inneeded.