Smart anti-biofouling composite coatings for naval applications

Smart anti-biofouling composite coatings for naval applications

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J. Telegdi, L. Trif and L. Roma´nszki Research Centre for Natural Sciences, Hungarian Academy of Sciences, Hungary

5.1

Biofouling: description, disadvantages

The term fouling refers to an unwanted process, where a surface becomes encrusted with materials from the environment. In the case of biofouling, this material consists of micro- and macroorganisms as well as their metabolites, plants, and animals on structures submerged in seawater. From the beginning of navigation, marine biofouling has been a major problem for shipping as it causes reduced speed, higher fuel consumption, and increased corrosion. It also affects offshore structures. Fouling causes huge material and economical loss in marine culture, shipping, naval vessels, and seawater pipelines. Increased frictional drag, which is the consequence of fouling on ship hulls, reduces speed by up to 10% (Townsin, 2003). Additionally, fuel consumption of a fouled vessel may increase by even 40%, resulting in increased fuel costs (around 20 billion Euros/year) and greenhouse gas emission (about 20 million tons/year). In 1981, the US Navy consumed 18 million barrels of fuel, with 3.3 million attributed to biofouling losses (Railkin, 2004; Jones, 2009). When the biofilm is 1 mm thick, this can increase the ship hull friction by 80%, which translates into a 15% loss in speed (Gordon and Mawatari, 1992). Furthermore, a 5% increase in biofouling increases fuel consumption by 17%. Foulers on ship hulls and ballast tanks can be transported worldwide and can spread non-native species. Other examples of environmental impact and financial losses, which cause significant problems related to fouling prevalent in marine fields, are demonstrated in other works (Walker et al., 2000; Railkin, 2004; Hellio and Yebra, 2009). Biofouling colonizes ships, buoys, sonar devices, pontoons, offshore structures, oil installations, platforms, underwater cables, underwater acoustic instruments, and seawater-cooling systems. Issues include increased costs, reduced speed, environmental concerns, and corrosion and safety hazards (Melo et al., 1988; Fingerman et al., 1999; Railkin, 2004; Hellio and Yebra, 2009; Copisarow, 1945; Woods Hole Oceanographic Institute, 1952; Ray, 1959). Areas with the best aeration, such as a ship’s waterline, propeller, and rudder blade experience the highest amount of fouling (Lebret et al., 2009).

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As shown, biofouling has a negative impact on surfaces of materials as it can destroy them. Biofouling algae on structures and walkways create a slippery coating, leading to increased safety risk. Extra loading due to biofouling can also damage fishing nets and sink buoys (Jones, 2009). The extra mass on the ship hull causes increased fuel consumption because of the increased roughness (Dobretsov, 2009a), the increased stress from the grown hydrodynamic drag decreases the throughput of the ship’s engine (Jones, 2009), and the marine platform suffers from the plus load and stress; hereby the metal will be deteriorated via biocorrosion (Lebret et al., 2009). In addition to the enhanced fuel consumption caused by the high frictional resistance due to the generated roughness and the subsequent speed reduction and loss of maneuverability, the number of the dry-dock operations increases, the hull should be cleaned, the paint removed and repainted, which explains the increased costs due to biofouling.

5.2

Microbiologically influenced corrosion (MIC)

Microbiologically initiated/influenced corrosion was first discussed by Garrett (1891). He gave account on deterioration of lead-covered cables by bacteria. The next report on the harmful activity of microbes was given by Gaines (1910) who related the presence of sulfur in the corrosion product with the activity of microorganisms. However, intensive research only began in the middle of the last century when (at last) the existence of the microbiologically influenced corrosion (MIC) was accepted. MIC is a special type of corrosion initialized by microorganisms by their presence and by their aggressive metabolites. Extensive study of MIC requires an understanding of chemistry, biochemistry, microbiology as well as of metallurgy. Some works summarize and explain the essentials of MIC, in particular on the activity of microorganisms (Borenstein, 1994; Kearns, 1994; Videla, 1996; Little and Lee, 2007; Javaherdashti, 2008).

5.2.1 Biofilm formation and composition What happens beneath the biofilm? Microbiologically influenced corrosion, which is a major problem in areas where biofilms can develop, refers to corrosion influenced by the presence of corrosionrelevant microorganisms adhering to surfaces in biofilms. The biofilm developed by corrosion-relevant microbes was first discussed by Zobell (1943), but became the focus of extensive research only in recent decades (Little et al., 1991a, 1991b, 1992, Lenhart et al., 2014). From the point of view of biofouling, the most dangerous microbes are the ones that initiate biofilm formation and, at the same time, excrete aggressive metabolites (e.g., sulfide ions and acids) as well as exopolymeric substances (EPSs), which are the most important component in the formation of a gelatinous biofilm. EPS acts as a glue that ensures a “safe life” for microbes, where they can securely grow,

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propagate, excrete metabolites, and are able to communicate with each other and with other microorganisms through quorum-sensing molecules. By their presence and aggressive metabolites, the microorganisms initiate pitting corrosion, which starts beneath the biofilm. In the biodeposite, concentration cells are formed that lead to localized corrosion. Later, the pits grow together, and the whole surface is deteriorated via general corrosion beneath the biofilm. The biomatrices of different origins are divergent (Costerton et al., 1987). In most cases, not only one type of biofilm is formed; the whole community of the microbes creates a mixed biofilm (Flemming et al., 1996). The composition of EPS is characteristic of the microbes (Wilderer and Charaklis, 1988; Decho, 1990). The EPS together with other organic molecules ensure the 3D arrangement of the components and formation of a gelatinous matrix where the microbes live an immobilized life (Hoppe, 1984). Microorganisms involved in the MIC are bacteria, algae, fungi, plants, and animals. They are less dangerous in planktonic form than in sessile one. The most dangerous groups of microbes that are involved in the MIC via their aggressive metabolite production are the anaerobic sulfate-reducing species (Desulfovibrio, Desulfomonas); they produce hydrogen sulfide (which causes sulfide stress cracking). They render the environment corrosive and increase the rate of material deterioration. Other groups responsible for MIC are aerobic microorganisms (e.g., Thiobacillus thiooxidans, Thiobacillus ferrooxidans, Sphaerotilus, Pseudomonas, Gallionella). They can oxidize the sulfur (and produce sulfuric acid) and metal ions (like Fe21, Fe31, and Mn41). Other authors (Railkin, 2004; Hellio and Yebra, 2009) offer other examples of the biological corrosion caused by acid-producing bacteria located in biofilm. There are many factors that influence the biofilm formation (type of microorganisms, surface roughness, dissolved chemicals, hydrodynamics, and diffusive transport) (Lewandowski, 1998; Beyenal and Lewandowski, 2002).

5.2.2 Formation and composition of biofilms The biggest part of the biofilm is water (75 90%), and the other components are as follows: 1. Microorganisms: bacteria (autotroph, heterotroph), fungi, algae, cyanobacteria, protozoa 2. Exopolymeric substances (EPS): polysaccharides, humic acids, proteins, nucleic acids, lipids (Chen et al., 1995; Nielsen et al., 1998) 3. Inorganic and organic materials

Since the biofouling starts with biofilm formation, it is important to review the progression of the biofilm formation. The steps of biofilm formation are shown in Figure 5.1. When a chemically inert substrate is immersed in seawater, an almost immediate accumulation of organic molecules starts onto the wetted surface, an initial conditional film forms; its composition depends on the ions, glycoproteins, humic and fulvic acids. The forces responsible for the adsorption are electrostatic interaction

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and van der Waals forces the same ones that are responsible for the biofilm structure. The initial attachment of cells is controlled by physical adhesion between the microorganism and the substrate. Also known as adsorption, the initial colonists attach to a surface through weak, reversible van der Waals bonds (that are slightly stronger than electrostatic repulsive forces) as well as by hydrogen bonds. Irreversible attachment is accomplished with secretion of the EPS, which forms a sponge-like matrix. This adhesive permanently binds the microorganisms to one another and collectively to the surface (Flemming et al., 1999; Stoodley et al., 2002; Railkin, 2004). The process of biofilm formation takes place at numerous length scales: First, an electric double layer (that mediates the adsorption of organic molecules, mainly proteins) is established at the surface of the solid in less than a second. In the second step, subunits of fibrinonectin adsorb to the surface and mediate the binding of microbial cells to the solid surface (this happens first reversibly then irreversibly) via the cell membrane. When bacterial cells are bound to the surface, they undergo a phenotypic change and excretion of the EPS continues. In this period, the cells grow, replicate, metabolize, produce more EPS, and form a cohesive structure known as gellike biofilm (Costerton et al., 1978). Though the main component of this collagenous film is water, it also contains different organic and inorganic ions, molecules from the

Formation of conditioning film

Microbial adhesion

Colonization

Biofilm formation

1 hour

1 day

1 week

1 month

Continuous biofilm on the solid surface 30.0

20.0

10.0

0

10.0

20.0

0 30.0 µm

Figure 5.1 Time-dependent stages of biofilm formation from cooling water illustrated by atomic force microscopic images (AFM images by J. Telegdi).

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aqueous environment, and, of course, microorganisms. There is a gradient in the film: anaerobic microbes reside mainly near the solid surface while aerobic microorganisms are mainly located nearby the solution/air interface. Biofilm microorganisms live in customized microniche in a complex microbial community that exhibits homostasis, a primitive circulatory system, and metabolic cooperativity, and each of these sessile cells reacts with its special environment in a way that it distinguishes fundamentally from the planktonic cells of the same species (Costerton et al., 1995). Microbes involved in biofilms are mainly bacteria, fungi, and spores. The sessile microorganisms induce most environmental problems, in the form of degradation (in the case of metals this means corrosion, on other materials, other types of deterioration occur). When the biodeposition starts, the microorganisms make a transition from a free-floating planktonic to a stationary sessile lifestyle, thus forming a biofilm. The biofilm continues to grow and becomes more diverse by attracting more microorganisms through chemical messengers.

5.3

Biofouling

The biofilm that forms on solids immersed in aqueous environment enables the adhesion of macroorganisms that cause the most visible fouling. With regard to organisms, there are two types of processes that drive the biofouling: micro- and macrofouling (Clare and Aldred, 2009). Microfouling consists of microbes like bacteria, unicellular algae, diatoms; macrofouling contains fouling organisms such as mussels, algae, barnacles, etc. (Figures 5.2 and 5.3). Accepting the classification of Atlar (2008), the organisms can be termed as plants (microalgae (slime) and macroalgae (weeds)), and this group can be subdivided into red-, brown-, and green algae (green algae Ulva australis, brown algae Ectocarpus) (Callow, 1986) as well as soft-bodied animals (unlimited and limited)

Figure 5.2 Truncoconical crustaceans (balanus) adhered to rock (photo by J. Telegdi).

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Figure 5.3 Various organisms and algae on reef (photo by J. Telegdi).

and hard-shelled animals (barnacles, tube worms, and mussels). The macroorganisms involved in the biofouling range in size from bacteria to seaweeds and molluscs. Some of the organisms colonize on submerged surfaces that already have a microbial film present that attracts larger macrofoulers (Railkin, 2004; Hellio and Yebra, 2009; Stoodley et al., 2002). Tube worms prefer settling on biofilms, but bryozoans and barnacles do not require a thick biofilm (Ralston and Swain, 2009). The permanent attachment allows initial growth, final growth, and dispersion (Railkin, 2004; Stoodley et al., 2002). There is another possibility, which can be classified into three major groups according to the macroorganisms responsible for the biofouling: mussels, algae, and barnacles (Petrone, 2013). The adult organisms are not dangerous but their larvae and spores are because when they are located in a favorable site, they adhere to the solid surface and later undergo metamorphosis and start adult life. There is a difference in the composition of secreted adhesives determined by whether it was excreted in their temporary or permanent life cycle. Only little information is available about the composition of the adhesive materials. One of them (Petrone, 2013) mentioned the presence of some molecular groups involved in these molecules (phosphate, monoester sulfate, carboxylate, cathecol). When a microorganism binds to a hydrophilic surface, these molecules will replace the surface-bound water molecules or other adsorbates. The most studied adhesives of foulers are the materials excreted by mussels and barnacles. Barnacle cyprid deposits trails during surface invasion, and mussel veligers secrete glycoprotein. The rate of biofouling depends on the organism and substrate (Railkin, 2004; Jones, 2009; Griebe and Flemming, 2000). Additional factors that affect the

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fouling growth are the environment (temperature, illumination, salinity, alkalinity, etc.) and the hydrodynamics (flow rate, shear stress, turbulence, acceleration, Reynolds number). As a consequence, the biofilm and biofouling negatively affect the hydrodynamics of a ship’s hull by increasing the drag and the propulsive power that leads to increased fuel consumption and extra costs. Underwater environments are ideal for biofouling as the watercourse delivers the nutrients necessary for microbial life and carries away the excreted metabolites, i.e., wastes. All these processes promote the colonization of floating organisms. Organism transportation to a surface can take place either passively or actively. Active mechanisms include electrostatic repulsion, Brownian motion, turbulent pulsations, and cell outgrowth (Railkin, 2004; Ralston and Swain, 2009), which could result in massive, thick biofouling (Railkin, 2004; Monroe, 2007; Stoodley et al., 2002). The strength of bioadhesion depends on the organism type, the substrate, and the separating fluid (Callow, 1986), which are all influenced by the electrostatic forces and surface wettability (Bhushan, 2003; Feng and Jiang, 2006; Koch and Barthlott, 2009; Sheng et al., 2010).

5.3.1 Combination of bio-, organic and inorganic fouling; influence of the wettability One also has to consider the effect of other molecules present in the aquatic environment. Inorganic fouling composed of non-living particles may also form (in addition to or independently of) a mixture of biofouling, i.e., a mixture of inorganic and organic fouling. Particles may originate from corrosion, crystallization, and from suspended particles. Salts from aqueous solutions may crystallize and deposit on surfaces. Inorganic deposits (scales) may result from minerals found in water such as magnesium and calcium salts (Walker et al., 2000; Chan and Wong, 2010). Biofouling may initiate inorganic fouling, where biocorrosion causes the formation of corrosion particles. Biofouling and inorganic fouling depend on surface factors such as wettability, microtexture, color, and contours (Fingerman et al., 1999; Railkin, 2004; Hellio and Yebra, 2009). For instance, bryozoan and mussel larvae prefer hydrophobic surfaces (Gordon and Mawatari, 1992); hydroids, bryozoans, and ascidians prefer microtextured surfaces; larvae, sponges, barnacles, ascidians, and molluscs prefer light-colored surfaces; barnacles prefer convex contours; and calcareous sponges prefer concave contours (Railkin, 2004). Surface wettability influences fouler colonization, which ranges from superhydrophobic to superhydrophilic surfaces (Fingerman et al., 1999; Railkin, 2004; Hellio and Yebra, 2009; Jung and Bhushan, 2009; Genzer and Efimenko, 2006). A hydrophobic surface exhibits low wettability and low surface energy, whereas a hydrophilic surface exhibits high wettability and high surface energy (Roma´nszki et al., 2013, 2014a, 2014b). Water droplets on a hydrophobic surface will “bead up,” while droplets on a hydrophilic surface will spread out evenly (Roma´nszki, 2015). The degree of

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wettability is determined by contact angle measurements, where surface of contact angles less than 10 are superhydrophilic and over 150 are superhydrophobic. Low-adhesive, superhydrophobic surfaces promote contaminant removal through an action simply called “self-cleaning.” In the case of a low-adhesive, superhydrophobic surface, the water droplet slides past the particles on the hydrophilic surface, while the water droplet rolls and collects particles on the superhydrophobic surface. This is the well-known lotus leaf, self-cleaning effect. Antifouling superhydrophilic surfaces attract water with an evenly distributed water layer, which can interfere and disrupt microorganism settlement (Harder and Yee, 2009). Many microorganisms prefer to colonize on hydrophilic surfaces, but some of them favor hydrophobic ones. E.g. Ulva linza prefers hydrophobic surfaces (Scardino, 2009), whereas Balanus amphitrite prefers hydrophilic surfaces. The sheating action of sliding liquid on a superhydrophilic surface can promote self-cleaning, where the liquid collects and removes fouling. Surface microtexture influences organisms such as hydroids, bryozoans, and ascidians that seek shelter against strong currents by settling in grooves, pits, cracks, and crevices (Fingerman et al., 1999; Railkin, 2004). Micro- and macroorganisms prefer to settle in areas slightly larger than themselves for maximum protection of the surface area contacted with the substrate. When the attachment extends only for several points between the microorganism and substrate, it represents lower bioadhesive strength (Scardino et al., 2008), which is also affected by the fluidity of the EPS produced by microorganisms that can fill in the rough unevenness. When the adhesive EPS covers only the surface peaks of microtexture, the force to break bonds is significantly reduced (Vladkova, 2008).

5.3.2 Mitigation of biofouling for naval application 5.3.2.1 Historical review of ship biofouling, the earliest fouling control Biofouling has been a problem for as long the oceans have been sailed. Plutarch first mentioned (Plutarch, 1909) fouling and its impact on ship speed: “When weeds, ooze, and filth stick upon its sides, the stroke of the ship is more obtuse and weak.” From the beginning, sailors applied several techniques to avert the undesired effect of the biodeposition. The use of pitch and copper plating as antifouling techniques is attributed to ancient seafarer nations such as the Carthaginians and Phoenicians (1500 300BC). Later applications of wax, tar, and asphalt as well as arsenic and sulfur materials were recorded. In the middle ages, three substances served as antifoulant: 1. white stuff, a mixture of train oil, rosin, and sulfur; 2. black stuff, a mixture of tar and pitch; and 3. brown stuff, when the “black stuff” was mixed with sulfur. At the beginning of the eighteenth century, Charles Perry suggested copper sheathing against fouling but it was not put into use until 1761. The copper, which was effective against the fouling by tube worm and weed, produced a copperoxychloride film that interacted with the fouler organisms and killed them.

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1500BC– 300BC

400 – 1860

1860 – 2003

Pitch, copper Wax, tar, plating asphalt, train oil, rosin, sulfur, arsenic

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2003 – 2011

2005 –

Organotin, copper

Self-polishing coatings with biocides

Copper sheathing and heavy metal based coatings

A

B

C

D

E

Figure 5.4 History of the antifouling techniques; A: Phoenician ship; B: http://commons. wikimedia.org/wiki/File:Victory_Portsmouth_um_1900.jpg C: Typical merchantship from the nineteenth century http://hdl.loc.gov/loc.pnp/det.4a25817; D: Container ship from the twentyfirst century; Frank Schwichtenberg GNU Free Documentation License (GFDL); E: Ta´ncsics” wafter, twentieth century; http://hu.wikipedia.org/wiki/F%C3%A1j.

Since then, this treatment has functioned well and the term “copper-bottomed” became common, meaning risk-free sailing (Figure 5.4). In the nineteenth century when iron-hulled ships came into general use, copper sheathing lost its importance because of galvanic corrosion (undesired interaction between the copper and iron). Research into solutions against biofouling started, and in 1860 the first practical “McIness” hot plastic paint was introduced in Liverpool, though this treatment had a short service life, was expensive, and turned out to be relatively ineffective. By the mid-twentieth century, the application of copper rose from the dead and copper-oxide-based paints were used on ship hulls. However, the problem that soon became apparent was the short service life due to rapid leaching of the antifouler copper, and the accumulation of copper salts in the aquatic environment. The 1960s brought a breakthrough when a so-called self-polishing paint was developed that stored the antifouling inhibitor involved in the polymer/copolymer in the main component of the coating and allowed its slow dissolution. This new paint contained a biotoxin organotin compound such as tributyltin oxide. The significant advantage of this coating was the prolonged service life; it could control biodeposition up to 4 years. However, its severe toxic impact on marine life, even at very low concentration (1 ng/L), led to its worldwide ban in 2001. This prohibition revived the application of copper in ablative or self-polishing paints, and, at the same time, the expected service life (5 years) increased. However, the problem was that it could not actively control the adhesion of diatoms and algae, and it had an unacceptable environmental impact.

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5.3.2.2 Paint and coating technologies for control of marine fouling, composition of coatings There are three approaches to control the biofouling problem: a) mechanical removal/detachment of depositions (this is not discussed in this chapter); b) killing the micro- and microorganisms that form the biofilm/biofouling; c) surface modification which turns the surface to non-sticky, non-adhesive (Vladkova, 2007). Protective coatings of special polymers have plaid a significant role in the creation of antifouling surfaces (Charney et al., 2011) that can reduce the undesired inorganic and organic depositions (Chambers et al., 2006).

5.3.2.2.1 Antifouling paints Antifouling systems can be defined as the coating, paint, and surface treatment used on a solid (e.g., ship hull) to control or prevent the attachment of unwanted organisms. The main components of these compositions are biocides, which slowly “leach out” into the seawater, killing barnacles and other marine life that have attached to the solid surface. However, studies have shown that these compounds persist in the water, threaten sea life, harm the environment, and possibly enter the food chain. Antifouling paints can be categorized as: G

G

G

G

“Hard” bottom paints, or “non-sloughing” bottom paints (Teflon and silicone coating too slippery for microbial adhesion; the active ingredient leaches out while the paint film stays mostly intact); these are hybrid paints. “Contact leaching” paints are porous films on the surface that contain biocides in the pores for slow release. “SealCoat systems” have small fibers sticking out from the coating surface that prevent adhering and growth of microbes. “Sloughing bottom paints,” or “ablative” paints, which ablate slowly and the water can always freshly leach the biocide. Both the biocides and the paint film disappear over time. These coatings need continuous motion; they can be used in multi-season but they wear away with use.

The most important biocides used in antifouling paints are summarized as follows. It’s important to remember that antifouling coatings should control not only biodeposition but, at the same time, microbiologically influenced corrosion, too (Telegdi et al., 2014). In other words, they should not cause corrosion.

5.3.2.2.2 Antifouling paints with biocides The use of organotin compounds in coatings is already banned and soon the same will happen to the copper applied in antifouling paints. Thus, we have to keep in mind that antifouling paints formulated with copper and organotin compounds should be replaced with other additives that contain biocides that can also influence the growth of barnacles, algae, and other marine organisms. This explains the intensive search for new, effective biocides or biofouling polymer composites that are as effective as those that contain tin and copper.

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5.4

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Biocides used in antifouling paints (Guardiola et al., 2012)

The function of biocide in the antifouling paint is to prevent the settling of organisms on the hull by killing them.

5.4.1 Tributyltin (TBT), tributyltin oxide This is a short summary of this very efficient but very toxic and already banned ingredient of antifouling paints to demonstrate the reason for its use. For many years (it was discovered in the 1950s in the Netherlands but the application started at the end of 1970s), TBT was used as a biocide in antifouling bottom paints. Its use prevented and slowed down the growth of organisms attached to the hull. As it leaches from the ship’s hull paint, it has a toxic effect on organisms at all points in the food chain, including mammals, that is why it is particularly dangerous for most organisms (Konstantinou, 2006). Like TBT, the tributyltin oxide also acts as fungicide and molluscide. One of the most problematic aspects of TBT is its accumulation in sediments and its long half-life of about 2 years. This means that although the use of TBT is now banned, it continues to have toxic effects on the environment, remaining in the ecosystem for up to 30 years (Champ and Seligman, 1996). TBT was mixed into paint to extend the life of antifouling coatings, and ships were able to continue operations for longer. This paint ensured fuel efficiency and delayed costly ship repairs (Abbott et al., 2000). However, the TBT used on ship hulls was found to leach out and severely damage the marine, brackish, and freshwater environment (web1). TBT has been shown to impair invertebrate (stenoglossan gastropods) development. For example, TBT was shown to disrupt the endocrine system by inhibiting Cytochrome P450 molecule, which normally converts androgen (male hormone) to estrogen (female hormone). This inhibition leads to masculinization in females, and as a consequence, population decline (Bray, 2006; Gibbs et al., 1987). Vertebrates become affected by TBT by being in water contaminated with TBT, and by consuming organisms that have already been poisoned. In the case of Oryzias latipes, slowed developmental rate and tail abnormalities were reported (Bray, 2006). Studies have shown that TBT is detrimental to the immune system, can lead to immunosuppression in mammals such as sea otters and dolphins. In humans, organotin compounds have been detected both in blood and in liver (Antizar-Ladislao, 2008). Due to its strong ecotoxicity and relatively high levels in water as well as in port sediments, its use is illegal. Other organic biocides introduced as replacements for organotin compounds were at first period mainly copper or its derivatives. Biocides, used because of their potential to destroy a wide range of organisms, can be classified into 23 product groups, each containing subgroups. In the next section, we summarize the most commonly used biocides today.

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5.4.2 Copper, copper oxide When a ship or another solid body (seashore building, landing stage, pier, etc.) immerses into (sea)water, antifouling paints with pure copper powder or cuprous oxide content are exposed to water. Copper is a highly effective antifouling agent that deters growth of marine organisms. However, it transforms into the unstable cupric hydrochloride, and the accumulated silt and slime is washed away by the movement of the water, a new, fresh copper rich surface reveals, and the antifouling process recommences (Guardiola et al., 2012). The copper salt leaches from the boat surface and enters the water as Cu1 ion, which is oxidized to Cu21 ion, a well-soluble form of copper. Copper is toxic to aquatic organisms, actively regulates the life of crustaceans and algae, and accumulates in barnacles, bivalves, and aquatic insects. Its toxicity is most likely caused by impairment of the osmoregulation and ion regulation in aquatic animals, since it could inhibition of enzymes, interference with Ca21 homeostasis, and depression of the transcription of key genes.

5.4.3 Organic additives Other organic additives that can control the microbial life and biofouling are listed below. Chlorothalonil (2,3,5,6-tetrachloroisophtalonitril) was first used as pesticide and later as biocide in marine paints and was considered a good replacement of the banned tin with antifungal activity of broad spectrum. The toxic effect of chlorothalonil on marine organisms (e.g., crustaceans, molluscs, and tunicates) was studied in detail. Dichlofluanid (N-dichlorofluoromethylthio-N’,N’-dimethyl-N-phenylsulfamide) has a relatively lower toxicity but in the case of Glyptocidaris crenularis it shows embryotoxicity. DCOIT (4,5-dichloro-2-n-octyl-4-isothiazolin-3-one) biodegrades very easily and effectively. Its metabolites are five orders of magnitude less toxic than the parent compound. It effectively influences the life of crustaceans, molluscs, echinoderms, tunicates, and teleosts. Diuron (1-(3,4-dichlorophenyl)-3,3-dimethylurea) is present in seawater in higher concentration than in sediment, and in suspended form is available for sea animals. It is very toxic for reproduction of green algae and to certain bacterial species. Irgarol (2-methylthio-4-tercbutylamino-6-cyclopropylamino-s-triazine) is slightly soluble in water; it inhibits electron transport in photosynthetic organisms. In the case of algae it inhibits the growth and increase of the cells as well as photosynthetic activity. TCMS pyridine (2,3,5,6-tetrachloro-4-methylsulfonyl pyridine) is a relatively new antifouling compound; it causes immunotoxic effects and oxidative stress. Zinc pyrithione bis(2-pyridylthio)zinc 1,1’-dioxide is one of the most important tinreplacing antifouling biocides and shows algaecide, bactericide, and fungicide activity. It is highly toxic to aquatic animals but is considered environmentally neutral because of its easy photodegradability. Capsaicin, Econa, and Medetomidine. Capsaicin is a promising compound for application as antifoulant as a metal-free biocide. Econa can replace copper. Medetomidine is able to effectively prevent the settlement of barnacle cyprid by interfering with its regulation of cement production.

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Other compounds that could replace toxic biocides are 5-chloro-2-(2,4-dichlorophenoxy)phenol) (Choi et al., 2007), phosphorylcholine (PC)-substituted methacrylate, benzoic acid, and sodium benzoate. A less toxic antifoulant, zosteric acid, a natural product present in eelgrass, has been found to prevent the attachment of some bacteria and barnacles.

5.5

Polymers with antifouling activity

Development of antifouling surfaces that can control micro- and macrobiological adhesion and deposition involves many responsibilities. There are considerations such as their lifetime, cost, processability, and tunable properties that should be evaluated during design. Bioactive and biopassive coatings can be classified by the function they fulfil. Some aspects are summarized in the work by Charnley et al. (2011). An antifouling or foul-release coating cannot be globally effective if it does not perform well in a range of environmental conditions, against a diversity of fouling organisms, together with anticorrosive activity. New non-biocidal coatings are now in high demand. Scientists have developed new polymers, coatings obtained by improving the original silicone (polydimethylsiloxane) formulation patented in 1975. The high cost of silicones, especially of fluoropolymer-modified silicones, has prevented their large-scale diffusion. In 2009, traditional antifouling coatings using copper oxide formulated in copolymer paints still represented 95% of the global market volume of antifouling paints. The sol gel nanochemistry approach to functional materials has emerged as an attractive candidate for creating low-fouling surfaces as the sol gel formulations easily bind to all types of surfaces, (e.g., steel, aluminium, and wood), they can cure at room temperature, and form thin glassy coatings different from the thick silicone elastomeric foul-releasing coatings. Good to excellent performance against biofouling, low cure temperature, enhanced and prolonged chemical and physical stability, ease of application, and the waterborne nature of sol gel coatings all support the diffusion of these paints to efficiently reduce the accumulation of fouling layers on valued surfaces immersed in seawater. There are many opportunities for chemists to develop novel sol gel derived coatings to prevent biofouling and enhance the hydrodynamic properties of boat and ship hulls (Detty et al., 2014).

5.5.1 Polymer coatings with antifouling additives 5.5.1.1 Copper-based antifouling coatings Paints with copper can be classified as a) white copper: requires 50% less copper content than the dark copper used in conventional antifouling paints; b) low-density copper: the cuprous oxide is partly replaced by environmentally friendly materials. Copper coatings developed in the 1980s and available to the public since 1991 are one of the most powerful and long lasting antifouling polymer coatings.

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Today, one of the commercially available products is Coppercoatt (web2), a combination of a specially developed, solvent-free epoxy resin and high purity (99%) copper that protects successfully tens of thousands of boats and prevents refouling. The epoxy carrier is filled with the maximum amount of copper and keeps fouling at bay in virtually all conditions.

5.5.1.2 Izothiazolinone derivative and molecules with quaternary ammonium group in polymers In the presence of three marine bacteria, the antifouling characteristics of two series of biocide-containing coatings were evaluated. One was the commercially available poly(methyl methacrylate) (PMMA) and silicone elastomer (DC) physically blended with an organic antifouling biocide Sea-Nine 211 (SN211) (4,5-dichloro-2n-octyl-3(2H)-isothiazolone), and the other was a silanol-terminated polydimethylsiloxane (PDMS-OH) reacted with an alkoxy silane-modified polyethylenimine containing bound ammonium salt groups (PEI-AmCl). The SN211 blended coatings showed that with increasing biocide concentration the biofilm formation decreased in both PMMA and DC. When compared to PMMA, DC consistently showed an equal or greater reduction in biofilm retention as the level of SN211 loading increased, although at lower loading concentrations. The evaluations of the antifouling efficacy of PEI-AmCl/PDMS-OH coatings showed that all PEI-AmCl loading concentrations significantly reduced biofilm retention by a surface contact phenomenon (Stafslien et al., 2007). The antifouling efficiency of silicone elastomer (SE) polymers and biocide 3-(trimethoxysilyl) propyloctadecyldimethyl ammonium chloride (DC5700)enhanced silicone polymers was investigated in diatom film attachment to both control and biocide-enhanced polymer surfaces. Cell viability tests indicated a toxic effect and a reduction of fouling on the DC5700 SE polymers compared to the control SE. Fouling characterized by a slime film with barnacle and hydroid settlement and on the DC5700 SE surfaces by a mixed fouling community characterized by macroalgal fouling showed the incorporation of a biocide into an SE polymer, without changing the surface energy of the polymer (Clarkson and Evans, 1995).

5.5.1.3 Silicon oil in non-toxic fouling release coatings Fouling-release capability may be improved by adding non-bonding silicone oils to the coating matrix. The efficiency has been tested by comparing the adhesion strength of hard- and soft-fouling organisms on a cured polydimethylsiloxane (PDMS) network to that of the same network containing free polydimethyldiphenylsilicone oil. The toxicity of these coatings was minimal to shrimp and fish. Less than 1.1 wt% of the incorporated oil was lost from the coating over 1 year (Truby et al., 2000). The comparison of shear adhesion strength of barnacles, oysters, and tube worms on eight silicone fouling-release coatings containing different silicone oil additives

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showed that adhesion strength differed among species and coating types. In most cases, oysters and tube worms had higher adhesion strength than barnacles. The adhesion strength of barnacle deceased on all coatings with oil additives (Kavanagh et al., 2001). Non-eroding silicone-based coatings can effectively reduce fouling of ship hulls and are an alternative to biocidal and heavy-metal-based antifoulants. The products consisting of a silicone resin matrix may contain unbound silicone oils (1 10%). If oil films build up on sediments, infiltration may inhibit pore water exchange. PDMS do not bioaccumulate in marine organisms and soluble fractions have low toxicity to aquatic and benthic organisms. The PDMS has very low water solubility and bioavailability, which do not necessarily preclude damage to marine environments (Nendza, 2007).

5.5.1.4 Phosphorylcholine-substituted methacrylate Anti-biofouling polymers containing phosphorylcholine-substituted methacrylate units, prepared by copolymerization with dodecyl methacrylate, can significantly reduce the adhesion of marine bacteria (more than 70%) (Navarro-Villoslada et al., 2001).

5.5.1.5 Crosslinked siloxane polyurethane coatings These coatings were designed, synthesized, and characterized from 72 novel hydroxyalkyl carbamate and dihydroxyalkyl carbamate-terminated PDMS oligomers and their carbamate-linked block copolymers with poly(ε-caprolactone). The surface energy analysis demonstrated the presence of PDMS on the surface with a polyurethane underlayer. Pseudo-barnacle adhesion and the attachment strength of re-attached live barnacles (Balanus amphitrite) were in good agreement. Some of the coatings performed in algal (Ulva), bacterial (Cytophaga lytica, Halomonas pacifica), and barnacle (Balanus amphitrite) laboratory screening assays pointed to their potential utilization in practice (Ekin et al., 2007).

5.5.1.6 Triclosan-incorporated silicone coatings Biocide-incorporated silicone coatings for antifouling/fouling release applications that involved the biocide Triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) were modified with alkenyl moieties and incorporated into a silicone backbone through covalent bonds. The presence of the biocide on the coating surface was expected to deter fouling organisms from attaching to the surface of the coating. Allyl glycidyl ether was used to provide crosslink functionalities. The coatings prepared from biocide-incorporated silicones with the appropriate bulk modulus significantly reduced macrofouling (Thomas et al., 2004). The preparation, characterization, and biotesting of biocide Triclosanincorporated silicone coatings for marine applications have been conducted. To a silicon backbone, derivatives of the biocide were covalently attached. The mechanical properties as well as antifouling/fouling-release performance tests showed

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significant reduction in macro-fouling with sustained fouling-release characteristics for these coatings (Choi et al., 2007).

5.5.1.7 Benzoic acid and sodium benzoate Benzoic acid and sodium benzoate, two less toxic antifoulants as compared to currently used biocides in marine biofouling protection, can be incorporated into silicone coatings and have release behaviors and antifouling capabilities. The benzoic acid, although exhibits excellent antifouling capability, forms large crystals inside the coating (regardless of the solvent used to increase its miscibility with silicone) that leach out quickly, decreasing the efficiency of the coating. Sodium benzoate, which is immiscible with silicone, can be incorporated into silicone in the form of small aggregates. Leaching of sodium benzoate from silicone is much slower than that of benzoic acid, and a direct relationship between the leaching rate and the number of aggregates was observed. The sodium benzoate incorporated coating exhibits enhanced antibacterial performance (Al-Juhni and Newby, 2006).

5.5.1.8 Zosteric acid Zosteric acid prevents the attachment of some bacteria and barnacles and is effective at reducing the early stages of biofouling, such as the attachment of bacteria that leads to a biofilm. It was found that zosteric acid with a concentration of one-tenth of its EC50 is able to reduce bacterial attachment by more than 90%. The zosteric acid incorporated into silicone coatings at 1% concentration is slowly released and such coatings reduces the bacterial attachment up to 75% (Newby et al., 2006).

5.5.1.9 Non-toxic antifouling coatings Non-toxic antifouling coatings are possible successors to toxic antifouling paints, e.g., polymers containing fluorine or silicon are partially effective for different reasons. What we expect from these elastomers is to foul slowly, clean easily, be durable in the marine environment, and for organisms bond to it only weakly. The surface energy, elastic modulus, and thickness of the elastomer may be varied over a wide range to meet different performance requirements (Brady and Aronson, 2003). Coatings designed for applications as non-toxic, non-fouling coatings could suffer from lack of adhesion to the substrate and lack of mechanical strength. Both of these disadvantages could be overcome by using a two-coat system: a basecoat, which provides adhesion to the substrate, mechanical strength, and reactivity with the topcoat, and a topcoat, which provides a non-toxic antiadhesive surface to discourage attachment of marine organisms (Adkins et al., 1996). In the following section some of the antifouling polymer coatings without biocide will be discussed.

Silicon-based polymers; organic polysiloxane composites In these coatings, diglycidyl ether bisphenol-A type epoxy resin acts as a base material either as biocide-free or biocide-loaded composite. These formulations can

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be applied on zinc-phosphated mild steel specimens in a thickness of around 200 μm. According to the different efficiency analyzing techniques these coatings show anticorrosion and antifouling activities that are ideal for marine environments (Boopalan and Sasikumar, 2011). When proteins, tissues, microbes, algae, and invertebrates adhere to solid surfaces, the minimal long-term adhesion is associated with surfaces having initial surface tensions between 20 and 30 mN/m, i.e., with low surface energies. However, all surfaces are modified rapidly with adhesion of conditioner films, (which influence the permanent adhesion of organisms), with attachment of both micro- and macrofouling to surfaces in a range of surface energies that can elicit different responses in different organisms. For most organisms, the low adhesion is associated with low surface energy. Silicone elastomers and fluoropolymers have received the most attention regarding their potential use as foul-release coatings (Callow and Fletcher, 1994). The fluoropolymer and silicone coatings use different mechanisms to release fouling but their activity can be described by the fracture mechanics between a fouling organism and a coating surface (Brady, 2001). The resistance to marine macrofouling by barnacles was investigated on room temperature vulcanized silicone surfaces with designed surface microstructure and surface chemistry. An interesting factor that was supposed to influence the biofouling was the optimal surface topography compared with the macrofouling mechanisms. PDMS surfaces were microstructured by casting the pre-polymer on microfabricated molds. The microstructured PDMS surface consisted of different microstructure sizes and geometries (pyramids or riblets with a mean roughness ranging from 5 17 μm). Films were chemically homogeneous down to the submicron level. Hydrophobicity and contact angle hysteresis increased with increased surface roughness. Field tests revealed that the microstructure containing the largest riblets (profile height 69 μm) reduced the settling of barnacles by 67%, whereas the smallest pyramids had no significant influence on settling compared to smooth PDMS surfaces (Petronis et al., 2000). A PDMS layer on a solid surface could significantly decrease the surface energy. Adsorbed particles on PDMS coating decrease the hydrophobicity (Keefe et al., 2012), and at the same time increase the surface energy (Wynne et al., 2000). The oxidation of the PDMS surface generates hydrophilic antifouling properties, but because of the low glass transition temperature, surface reconstruction happens via internal rearrangement and the hydrophilicity will be destroyed (Keefe et al., 2012). The effect of acrylic polyol composition on the properties of crosslinked siloxane polyurethane coatings was explored. The siloxane polyurethane coatings were prepared from 3-aminopropyl-terminated PDMS, the acrylic polyols, and a polyisocyanate crosslinker. These coatings showed a low-force release in the pseudo-barnacle pull-off adhesion test. Fouling-release properties of the siloxane polyurethane coatings with algae and green seaweed were also demonstrated (Pieper et al., 2007).

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Fluoro block copolymers with antifouling activity These types of block copolymers with amphiphilic side chains have the ability to influence the adhesion of marine organisms. These surface-active polymers were synthesized by grafting of fluorinated molecules with hydrophobic and hydrophilic blocks as precursor. Algal species that adhered strongly to hydrophobic and hydrophilic surfaces showed weak adhesion to the amphiphilic surfaces. These organisms secrete adhesive macromolecules with different wetting characteristics, to attach to underwater surfaces. The amphiphilic surfaces, when they are in contact with the extracellular polymeric substances, undergo an environment-dependent transformation, which could be the reason for their antifouling nature (Krishnan et al., 2006). Fluorinated surface groups were introduced into PDMS coatings by plasma treatment. Surprisingly, the PDMS surface was more hydrophilic after the introduction of fluorine. This could be explained by an increased exposure of oxygen-containing moieties toward the surface upon re-orientation of fluorinated groups toward the bulk. Experiments in the presence of strains of marine bacteria with panels of different surface energies showed a significant decrease in bacterial attachment upon fluorination of the PDMS surface (Cordeiro et al., 2009). The surface free energies govern the adhesion of micro- and macroorganisms. In the case of barnacle adhesive the coatings should have lower surface energy than 12 mJ/m2 to prevent attachment. Perfluorinated surfactants that have the lowest known surface free energies were mixed in epoxy coating matrix, in acrylate, methacrylate and siloxane (silicone) polymers and these polymers showed much lower surface free energies than Teflon type perfluorinated polymers and showed excellent antifouling characteristics (Lindner, 1992). As in the case of other types of antifouling coatings it was shown that the fouling-release materials do not rank (in terms of adhesion strength) identically for all fouling organisms, and thus development of an effective hull coating requires testing against a diversity of encrusting species (Holm et al., 2006). Multiwalled carbon nanotubes (MWCNTs), natural silicate (sepiolite) in coatings The antifouling and fouling-release properties of silicone-based coatings with either synthetic multiwalled carbon nanotubes (MWCNTs) or natural sepiolite (NS) were investigated in cases of soft-fouling (Ulva) and hard-fouling (Balanus) organisms. The bulk properties of the coatings in the presence of fillers in low concentration did not change in contrast to the surface properties, which were modified on exposure to water. The MWCNT and NS in low concentration allowed releasing the Ulva sporelings (young plants). The adhesion strength of adult barnacles significantly decreased when they grew on silicone elastomer containing a small amount of MWCNTs. In the case of filled materials of these types, the surface properties affect settlement and the fouling-release behavior (Beigbeder et al., 2008). The addition of a small (0.05%) amount of carbon nanotubes improves the fouling-release properties of the silicone matrix. The improvement depends on the amount of MWCNTs filler (max. 0.1 wt%). The method for dispersing the carbon nanotubes in the silicone matrix is also important and influences the fouling-release

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properties of the coatings. The improvement is directly related to the state of nanofiller dispersion within the crosslinked silicone coating (Beigbeder et al., 2010). Amphiphilic triblock copolymers Two series of amphiphilic triblock surface active block copolymers were prepared through chemical modification of two polystyrene-block-poly(ethylene-ran-butylene)-block-polyisoprene ABC triblock copolymer precursors. The methyl ether of polyethylene glycol (PEG550) and a semifluorinated alcohol (CF3(CF2)9(CH2)10OH) were mixed at different molar ratios to build in both hydrophobic and hydrophilic groups into the isoprene segment. In biofouling experiments, the attachment of zoospores of the green algae was higher for more hydrophobic surfaces, while surfaces with a large proportion of the PEG550 side chains inhibited their settlement. The number of cells of the diatom Navicula attached after exposure to flow decreased as the content of PEG550 to F10H10 side chains increased (Park et al., 2010).

5.6

New antifouling strategies, what nature can teach us?

5.6.1 Biomimetics: imitation of living organisms As was demonstrated in the previous sections, biofouling is generally undesired. Here, we discuss two types of fouling: biofouling when organisms colonize and inorganic fouling, which is the result of deposition of non-living particles (Bixler and Bushan, 2014). One engineering challenge solved by nature is biological fouling, commonly referred to as biofouling. The type and extent of fouling depend on the local environment, inorganic deposits, and organisms. Marine biofouling generally includes a complex combination of biofilm, macrofouling, and inorganic fouling (Melo et al., 1988; Fingerman et al., 1999; Walker et al., 2000; Railkin, 2004; Hellio and Yebra, 2009; Copisarow, 1945; Woods Hole Oceanographic Institute, 1952; Ray, 1959). Biofouling morphology is characterized by the thickness, density, structure, composition, bioadhesive strength, and weight of fouling organisms. As biofouling ranges from microorganisms to macroorganisms, a variety of measurement techniques are necessary to record their properties. Such properties are measured with weight scales as well as light, transmission electron, scanning electron, atomic force, and fluorescence microscopy techniques (Hellio and Yebra, 2009; Marrie and Costerton, 1984; Keevil et al., 1999; Flemming et al., 2000; Chan and Wong, 2010; Trinidad et al., 2010). In contrast to the previously described antifouling strategies, i.e., the application of special coatings to avoid the undesired deposition, nature offers many solutions to control fouling through various physical and chemical control mechanisms. Examples include low drag, low adhesion, wettability (water repellency and attraction), microtexture, grooming, sloughing, various miscellaneous behaviors, and chemical secretions. The control of biofouling is accomplished in a variety of ways. The antifouling methods applied in practice include surface textures, coatings, cleaning methods,

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and experimental techniques. Common controls exist, such as low-drag and low-adhesion surfaces, that reduce biofouling. In fluid flow, a low-drag surface will promote removal (washing away) of microorganisms, while low-adhesion surfaces prevent microorganism colonization through reduced adhesive strength. New antifouling methods will presumably incorporate a combination of physical and chemical controls. Inspired by the nature, researchers are adopting the “knowledge” of the world’s flora and fauna to solve real technical challenges. Attention is given to nature’s structures, materials, and surfaces in order to after thorough investigation apply them on commercial scale. Biomimicry or biological mimicking means to copy nature’s designs. Biomimetics addresses the importance of surface textures and chemistry that influence antifouling properties. In biomimicry the goal is to understand and learn lessons from nature and to apply them. Examples include submarines inspired by the low-drag shape of dolphins, “self-cleaning” windows inspired by the superhydrophobic lotus leaf, and wallclimbing robots inspired by adhesive gecko feet (Collins and Brebbia, 2004; Reis and Weiner, 2004; Bhushan, 2009a; Bhushan, 2010; Bhushan et al., 2009b; Allen, 2010; Armstrong et al., 2010; Bar-Cohen, 2011). As these examples prove, looking at nature for antifouling lessons reveals several examples of both physical and chemical control methods and a combination thereof (Bhushan, 2009a; Bar-Cohen, 2011). Physical controls include low drag, low adhesion, wettability, microtexture, grooming, sloughing, and various other behaviors, and chemical controls include products of various secretions. Fouling is both prevented and removed by these physical and chemical controls. At ambient environment, plants and insects adapt these controls, and in marine environment, plants, corals, and fish apply them. Specific antifouling examples range from the lotus leaf (Barthlott and Neinhuis, 1997; Neinhuis and Barthlott, 1997; Nosonovsky and Bhushan, 2009; Bhushan and Jung, 2011) to shark skin (Carman et al., 2006; Kesel and Liedert, 2007; Dean and Bhushan, 2010). Considering antifouling methods, many strategies exist in nature and are applied at industrial scale. Due to the complexity of fouling organisms, in nature a combination of physical and chemical controls is often configured. Industry mimics this approach through various antifouling technologies (Melo et al., 1988; Fingerman et al., 1999; Walker et al., 2000; Railkin, 2004; Hellio and Yebra, 2009; Ralston and Swain, 2009). Antifouling mechanisms commonly found in nature provide the examples we need for mimicking (Bhushan, 2009a; Bar-Cohen, 2011; Fingerman et al., 1999; Hellio and Yebra, 2009; Neinhuis and Barthlott, 1997; Ralston and Swain, 2009). Low-drag, fast-swimming shark skin is never biofouled because of its riblet microtexture, flexion of scales, and a mucous layer. The skin contains scales called dermal denticles, which are covered by specially sized and spaced riblets oriented parallel to the swimming direction. Low drag is achieved by the riblets lifting and constraining the naturally occurring fluid vortices, which reduces the transfer of momentum and total shear stress. When the vortices are pinned just above the riblet tips, the cross-stream movement and entanglement of stream-wise vortices are limited, thus reducing the transfer of momentum. The total shear stress is reduced as the vortices contact only the small riblet tips, when we compare it

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with the total surface area. Lower drag also allows the water layer next to the skin to move faster, which reduces the time for evolution of micro-organism settlement and helps to wash them away (Dean and Bhushan, 2010; Bechert et al., 1997). In addition to low drag, shark-skin microtexture deters certain microorganisms, as they prefer particular groove widths and depths for settlement. Because of these mechanisms, microorganisms have difficulty adhering to and colonizing on shark skin (Carman et al., 2006; Kesel and Liedert, 2007; Dean and Bhushan, 2010). Desirable wettability properties include low-adhesive superhydrophobic and superoleophobic self-cleaning surfaces that resist fouling as water washes away the contaminating particles (Neinhuis and Barthlott, 1997; Jung and Bhushan, 2009). Submerged fish scales are oil resistant because of their hierarchical structure, which consists of scales of 4 5 mm in diameter covered by papillae 100 300 μm long and 30 40 μm wide (Liu et al., 2009). Other examples of superhydrophobic surfaces are broccoli, the lady mantle, and cicada (Lee et al., 2004). Certain microtextured surfaces resist biofouling as organisms seek ideal surface features for settlement, and may be deterred if no suitable surface is found. The barnacle Cypris larvae are deterred by microtextured surfaces if the features are of the same size or slightly smaller than juveniles. The mussel Mytilus deters foulers with a micro-haircovered surface, and the dogfish shark egg case deters foulers with a microtextured surface (Ralston and Swain, 2009; Davenport, 1999). In addition, the pilot whale (Globicephala melas) and the common dolphin (Delphinus delphis) deter biofouling with microtextured skin. Grooming is the physical removal of biofouling from the host, which effectively controls slow- and fast-growing biofouling. Decapods and crustaceans groom other creatures with special brush structures for removing fouler from gills and appendages. Echinoderms and bryozoans use special structures called pedicellaria to groom macroepibionics, while crayfish depend on branchiobdellid annelids to feed on foulers in their gills. Sloughing is the slow shedding of the outermost layer of an organism, which effectively controls slow-growing biofouling. This method is employed by organisms such as crustaceans, stonefish Synanceia horrida, and seaweed (Ralston and Swain, 2009). Miscellaneous antifouling behaviors include any physical action that prevents or removes fouling. Such behaviors include activities such as burrowing, hiding in the dark, flexing, and mechanical cleaning. These may be used in conjunction with antifouling mechanisms such as low drag, low adhesion, and change in wettability, microtexture, grooming, sloughing, and chemical secretions. Burrowing scrapes off foulers, and hiding in the dark inhibits algae formation. Algae prevent biofouling by flexing and bending, unlike rigid foulers such as barnacles and tube worms (Walters et al., 2003). A good example of mechanical cleaning, combined with wiping via chemical secretion found in the nature is the human eye, where lachrymal gland secrets tears, puncta collect tears, and eyelid wipes the surface of the cornea. Chemical secretion methods range from preventing to removing biofouling. The red seaweed (Delisea pulchra) uses a halogenated furanone to manipulate colonizing bacteria “attraction” messages (Yen, 2010). Other chemical examples

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include snails that leave a predatory mucous trail, antimicrobial coral egg shells, mucus on shark skin (Ralston and Swain, 2009), and antifouling chemicals produced by the bacteria Roseobacter gallaeciensis (Rao et al., 2006). Several bioinspired antifouling methods could be in practice beneficial for various marine applications (Reis and Weiner, 2004; Bar-Cohen, 2011). Coatings are perhaps the most common method of biofouling control. Low-drag surfaces provide antifouling benefits, and drag reduction is demonstrated by saw-tooth, scalloped, and bullnose shark-skin inspired riblets. Laboratory experiments show that such microtextures can reduce drag by up to 10%, with an optimized relationship between the blade thickness and spacing (Walsh and Lindemann, 1984; Bechert et al., 1997). Both two-dimensional and three-dimensional riblets have been studied for their effectiveness in drag reduction, but no significant improvement was observed with three-dimensional versus two-dimensional riblets (Wilkinson et al., 1988). Styles of riblets include aligned segmented blade, offset segmented blade, offset threedimensional blade (Bechert et al., 2000), and three-dimensional shark-skin replicas (Lang et al., 2008; Jung and Bhushan, 2010). A review paper on riblet optimization suggests that a blade riblet height divided by the spacing equalling 0.5 is optimal for drag reduction, regardless of riblet length (Dean and Bhushan, 2010). Liedert and Kesel (2005) tried to verify the efficiency of biologically inspired surface microstructures as an alternative to biocide paints using shark skin as the analogue. The highest antifouling performance (about 95% reduction of settlement of barnacles) was achieved by a combination of the following parameters: surface microstructure, Rz 5 76 μm, soft silicone material (shore A 5 28), and low surface energy (25 mN /m1). In a separate study, Kesel and Liedert (2006) reported that the best antifouling performance (67% reduction of barnacle settlement) was observed on soft silicone surfaces with microtopographies between 40 μm and 2 mm. Sullivan and Regan (2011) produced synthetic shark-skin samples using a silicone elastomer (Dow Corning Sylgard 184 PDMS) and a slow swimming catshark Scyliorhinus canicula as the template. In comparison with a smooth planar surface of identical elastomer, contamination rates were reduced with smaller, densely packed denticles attracting the least fouling. Multi-level hierarchical patterning could produce surface topographies at many scales, which would provide antifoulant protection of broad spectrum. Polyester, polyamide, nylon, or polyacryl fiber flock coatings made by adhering electrostatically charged fibers perpendicular to the coating surface appear promising against barnacles. Schumacher et al. (2005) have developed a biomimetic design, Sharklet AFt microtopographical surfaces, inspired by the structure of shark scales, which reduces the settlement of algae spores by over 60% when the characteristic dimension of the surface is 4.2 μm or less. This operates on the principle that microorganisms are deterred from hydrophobic surfaces and crevices that are slightly smaller than themselves. Research suggests that appropriately-sized topographies prevent colonization by various microorganisms including Ulva spores and Balanus amphitrite cyprids (Carman et al., 2006). Protective paints on a ship’s hull can involve extracted natural products as antifouling compounds in polymer coatings. In this case, it is very important that these

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materials be compatible with the paint components. The other important factor, i.e., the delivery rate of the effective natural material to the coating surface, determines the applicability of the natural fouling inhibitor (Chambers et al., 2014). Another example is the bio-inspired polymer that consists of methoxytermainated polyethylene glycol conjugated to the adhesive L-3,4-dihydroxyphenyl-alanine, which resisted successfully protein and mammalian cell fouling. The superior performance of this polymer over a standard silicone fouling release coating shows the effectiveness of this bio-inspired polymer applied against marine antifouling (Statz et al., 2006). Grooming and sloughing methods to control biofouling are used in many applications. For instance, the US Navy sponsored the development of a ship hullcleaning tool called the Bio-inspired Underwater Grooming robot (http://www.onr. navy.mil). This robot cleans a ship hull similar to a groomer on a host organism. Surface renewal in self-polishing paints mimics the actions of sloughing prevalent in nature. Yang et al. (2014a) have recently reviewed a variety of functional “brushes” and coatings developed to combat marine biofouling and biocorrosion while minimizing environmental burdens arising from the systems. Surface-tethered brush polymer can be engineered with specific functionalities. For the inhibition of marine biofouling, the systems have specific fouling-resistant (prevention of attachment), fouling-releasing (reduced adhesion strength), and organism dissuasion (degrading or killing) actions designed to provide a nonbiocide releasing, environmentally benign surface. For prevention of attachment, hydrophilic polymers (e.g., polyethylene glycol or PEG) have been shown to resist protein absorption (Yang et al., 2013) and cell adhesion (Yang et al., 2014b). For reduced adhesion strength, fluoropolymers and silicones minimize the adhesion strength and allow easy release of the organism. To dissuade settlement, antimicrobial polymers containing cations compromise microbes by disrupting cellular membranes. In the case of exfoliating/self-polishing surfaces, the flow of water over the hull continually erodes the surface of the coating exposing a fresh layer of biocide. A thick coating can provide multi-season performance subject to an annual check and revival of the coating by abrasion/water-jetting. Such coatings are not normally suitable for drying moorings. Hoare and Thompson (1997) and (Thompson et al., 2004, 2005) have raised the issue of the final destination of plastic particles that enter the marine environment. Current commercially available biocidal self-polishing paints are considered low maintenance, effective, and repairable. Such paint types include biocidal-free association (released from resinous matrix) and ablative (bonded in soluble matrix that eventually peels off). Self-polishing copolymer paints contain antifouling toxins that erode over time, shedding the attached biofouling and continually exposing fresh toxins. ePaint.com LLC introduced an antifouling coating that relies on a combination of visible light, oxygenated water, and a catalyst in the coating to generate a steady source of hydrogen peroxide (H2O2) (Porter, 2007). After its release, the H2O2 rapidly decomposes to oxygen and water molecules. The H2O2 is effective against hardfouling agents (e.g., barnacles, mussels, and tube worms) and is complemented by zinc pyrithione to combat soft-fouling agents (e.g., algae).

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Pinori (2013) et al. at the University of Gothenburg and at the SP Technical Research Institute of Sweden in Bora˚s have proposed a new eco-friendly method to fight against the accumulation of barnacles on the hulls of boats and ships. Most marine organisms that attach to vessels (e.g., mussels and algae) can be easily scraped off but during this process the coating could be damaged. Barnacles penetrate the surface, so the new technique holds the poison inside the paint, minimizing release into the water, until the barnacles penetrate the surface and the toxin is released. The toxin used is ivermectin, produced by the Streptomyces avermitilis bacterium. An effective concentration is 0.1% and it lasts for many years. Self-polishing paints without biocides are not very effective as the rate of removal is too slow to control fouler growth. Natural antifoulant compounds and surfaces are being explored and incorporated into coatings. Cannabinoids such as anandamide had antifouling properties against Dreissena polymorpha in 48-hour exposure experiments. Anandamide prevented the byssal formation in D. polymorpha, without producing toxic effects upon the mussels. Extracts of Pseudomonas sp., particularly the bacterial strain NUDMB50-11, incorporated into paints demonstrated excellent antifouling performance against bacteria, Balanus amphitrite barnacle cyprid, and Ulva lactuca algal zoospores (Dobretsov et al., 2009b). Halogenated furanones extracted from red algae are effective antifoulants. Sodium benzoate and tannates from chestnut, mimosa, and quebracho produce a narcotic effect on nauplii of B. amphitrite when incorporated into a soluble matrix antifouling coating and reduced settlement during a four-month panel experiment in marine waters. Dobretsov et al. (2009b) reviewed antilarval compounds derived from marine bacteria and identified several compounds that inhibited the settlement and attachment of B. amphitrite including ubiquinone from Alteromonas sp.; 6-bromoindole-3carbaldehyde from Acinetobacter sp.; phenazine-carboxylic acid, hydroxyphenazine, heptylquinol one, and nonylquinol one from Pseudomonas sp.; and unknown exopolysaccharides, polymers and other substances from Halomonas marina, Vibrio campbelli, Micrococcus sp., and Rhodovulum sp. Behavior antifouling methods include freshwater rinsing for ships. Researchers found that when a fouled ship from saltwater moves into freshwater (such as in the Panama Canal) for 9 days and then back into the saltwater, about 90% of biofouling will be removed (Ralston and Swain, 2009). Chemical control for antifouling examples includes low-adhesive surfaces using zosteric acid extracted from eelgrass (Zostera marina) and polymers from sea urchin and killer whales (Callow and Callow, 2002). Antifouling coatings include bioinspired dopamine from mussels (Yen, 2010) and natural enzyme additives. Additionally, researchers have studied antifouling coatings with metabolites from microorganisms, living bacteria as well as dopamine (Ralston and Swain, 2009). So-called “living paints” contain inhibitory bacteria to reduce biofouling, as demonstrated by incorporating Pseudoalteromonas tunicata into hydrogels (de Nys and Steinberg, 2002).

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5.6.2 Quorum quenching A very special method to control the microbial life within the biofilm could be the application of compounds that can stop the cell-to-cell communication of microorganisms. One of the first detailed examples about this phenomenon was published in 2005 (Walters and Bassler, 2005). This chemical communication influences growth and metabolizing processes by detecting and responding to small molecules called autoinducers. These hormone-like molecules, such as derivatives of lactones (mainly homoserine- and gamma-butyro-lacton derivatives), are able to control the connections, synchronize the activity of larger groups of cells, a number of species present in a biofilm community. The result of the effect could be the prevention of the colonization. In this case, the effect is called quorum quenching. Some autoinducers are extremely species-specific. The application of molecules with this ability could inhibit the biofilm formation and, as a consequence, the biofouling, too. Additional information about this topic can be found in the work of Kalia (2013).

5.7

Conclusion

This chapter was dedicated to the problem of biofouling and the potential answers, resolutions to decreasing its deteriorating effect. Biofouling affects mainly ships, naval services, naval ports, underwater components, or superstructures and the maintenance of all solid surfaces surrounded by an aqueous environment for long period. Biofouling is a nuisance and leads to costly consequences as the undesired biodeposition causes serious problems (e.g., increased frictional drag, fuel costs, and greenhouse gas emission, reduced speed, etc.). The problem is approached by understanding the sequential steps of biofilm formation, by elucidating the elementary processes which lead to the microbial adhesion colony formation, and complete overlaying by microbial slime. In the course of the biofouling, the next step is the adhesion/deposition of macroorganisms (macro algae, hard-shelled foulants like barnacles, tube worms, and bivalves, etc.). The chapter provided information on the history of the fight against the biodeposition starting from ancient times and followed the development of chemicals used against the biological deterioration. There are two approaches. One focuses on the killing of the micro/macroorganism, and the other one focuses on the inhibition of microbial adhesion. In the latter case, it is not the microbial life, but its adhesion, that is influenced by special additives. Namely, when we can avoid their deposition, their undesired negative effects will not be an issue. The treatments that address this problem started with the application of wax, copper sheets, and later tin derivatives (tributyltin self-polishing polymers), proved to be the most effective. However, their toxicity was revealed, which led to the ban of these treatments. The need for increasingly environmentally friendly substitutes for tin-based products has encouraged researchers to develop replacements for the tin compounds and to develop effective non-stick, fouling release coatings,

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as well as chemicals. However, the efficacy of these treatments has not been so far able to match that of tin and its derivatives. This chapter summarized the technologies, solutions for controlling marine fouling, such as paints and coatings, which can control/reduce biofouling with high efficacy. There are antifouling paints, polymers, copper-based antifouling coatings, paints with different additives (like izothiazolinone derivatives, quaternary ammonium molecules), inorganic coatings alone or in combination with organic molecules (silicon-based polymers, organic polysiloxane composites), coatings with multiwalled carbon nanotubes, and fluoropolymers that render the surface superhydrophobic. While this discussion is not complete, the most frequently used materials have been included. The newest antifouling solution is one that mimics biology: communication between the microorganisms could be inhibited by the so-called quorum quenching method, by application of quorum-sensing molecules. For instance, the anaerobic sulfate reducer microorganisms consume sulfate ions and produce sulfide ions; on the other hand, aerobic sulfur oxidizer microbes use as energy source sulfide ions and convert them to sulfates. This is a recycling process. As both types of microorganisms are generally involved in the same biofilm, when the communication of one of the microbes is inhibited, this will influence the activity of both.

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