Biomimetic antifouling coating

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Antifouling is the ability of specifically designed coatings to remove or prevent biofouling by any number of organisms on wetted surfaces.[1] Since biofouling can occur almost anywhere water is present, biofouling poses risks to a wide variety of objects such as medial devices and membranes, as well as to entire industries, such as paper manufacturing, food processing, underwater construction, and desalination plants.[2] Specifically, the buildup of biofouling on marine vessels poses a significant problem. In some instances, the hull structure and propulsion systems can be damaged.[3] Over time, the accumulation of biofoulers on hulls can increase both the hydrodynamic volume of a vessel and the frictional effects leading to increased drag of up to 60%[4] The drag increase has been seen to decrease speeds by up to 10%, which can require up to a 40% increase in fuel to compensate.[4] With fuel typically comprising up to half of marine transport costs, biofouling methods are estimated to save the shipping industry around $60 billion per year.[4] Increased fuel use due to biofouling contributes to adverse environmental effects and is predicted to increase emissions of carbon dioxide and sulfur dioxide between 38 and 72% by 2020.[5]

A variety of antifouling methods have historically been implemented to combat biofouling. Recently, antifouling methods inspired by living organisms have become the subjects of intense research by scientists looking for more environmentally friendly and effective ways of reducing biofouling. This type of design imitation is known as biomimicry.

Historical and current approaches[edit]

Poly(dimethyl siloxane) repeat unit

Throughout history, a variety of methods have been used to combat biofouling. Descriptions of pitch or metal sheathing were common for centuries before eventually moving into toxic but effective tributyltin (TBT) or tributyltin oxide (TBTO) in paints in the 1950s.[6] Copper has also been used as both an antifouling coating and as a biocide with less bioaccumulation or adverse environmental effects than TBT, but remaining less effective.[7] The 2008 worldwide ban on the use of TBT-containing paints by the International Maritime Organization has excited research into environmentally friendly and effective antifouling techniques.[8] Many of these focus on the development of biomimetic methods to replace the current standard of polydimethylsiloxane, or PDMS. PDMS consists of a nonpolar backbone made of repeating units of silicon and oxygen atoms. The nonpolar nature of PDMS causes the coating to have a high interfacial energy with the surrounding water of 52 mJ m−2.[9] Furthermore, the nonpolarity of PDMS allows for biomolecules to readily adsorb to its surface to lower interfacial energy. However, PDMS also has a low modulus of elasticity and surface energy (19.8 mJ m−2) that allows for the release of fouling organisms at speeds of greater than 20 knots. The dependence of effectiveness on vessel speed limits the use of PDMS on slow-moving ships or those that spend significant time in port.[2]

General idea of fouling on varied surfaces: (A) untreated surface, (B) biocide loaded coating, and (C) PDMS coating

Fouling methods and organisms[edit]

The variety among biofouling organisms is highly diverse and extends far beyond contamination by barnacles and seaweeds. According to some estimates, over 1700 species can contribute to biofouling.[6]

Biofouling does not occur in one mass sedimentation, but is often broken into a series of steps. First, within hours of the hull being exposed to the marine environment, Van der Waals forces attract small organic molecules present in the seawater, such as polysaccharides, proteins, and proteoglycans, to the hull.[10] Within 24 hours the adhesion of these organic molecules conditions the surface of the hull for the attachment of larger species, such as bacteria and algae. As more bacteria and diatoms attach to the surface, a microbial biofilm coating the hull is produced.[11] The microorganisms present in the biofilm serve as food for larger microorganisms such as microalgae and protozoans, which attach to the hull over the microbial biofilm and create a relatively rough surface. The process of biofilm development takes around one week. After the hull is coated with a rough biofilm, it is then primed for the attachment of larger macrofoulers, such as barnacles and seaweed. Over a period of a few weeks, the larvae of macrofoulers and macroalgae begin to develop, creating a sufficiently fouled hull.[3]

Biomimetic methods[edit]

Biomimetic design describes the process of using living organisms as the inspiration for new functional materials. For instance, many large and slow-moving marine animals, such as whales, are able to effectively prevent biofouling on their skin. For marine antifouling purposes, biomimetic designs typically fall into two categories: physical and chemical.

Physical methods[edit]

Biomimetic physical methods aim to create a mechanical surface that deters bioaccumulation.

Mussel adhesive proteins[edit]

One of the more common methods of antifouling comes from growing polymer chains from a surface, often by poly(ethylene glycol) or PEG.[12] However, challenges exist in creating a functionalized surface to which PEG chains may be grown, especially in aqueous environments. Researchers have been able to study the methods by which the common blue mussel Mytilus edulis' is able to adhere to solid surfaces in marine environments using mussel adhesive proteins, or MAPs. MAPs are typically comprised a number of proteins, of which the most common repeating sequence is Ala-Lys-Pro-Ser-Tyr-trans-2,3-cis-3,4-dihydroxyproline (DHP) -Hyp-Thr-3,4-dihydroxyphenylalanine (DOPA) -Lys.[13] The inclusion of the hydroxylated DHP and DOPA amino acids are thought to contribute to the adhesive nature of the MAPs. Recent studies have looked into using a short chain of DOPA residues as an adhesive end-group for antifouling PEG polymers which show promise in adsorbing onto certain metal surfaces. The increasing the number of DOPA residues to three greatly improves the total amount of adsorbed DOPA-PEG polymers and exhibits antifouling properties exceeding most other 'grafting-to' polymeric functionalization methods.[12]

The antifouling characteristics of PEG are well documented, but the service life of PEG antifouling coatings is debated due to the hydrolysis of PEG chains in air, as well as by the low concentrations of transition metal ions present in seawater.[2] Using DOPA residues as attachment points, new polymers similar in structure to the polypeptide backbone of proteins are being investigated, such as peptidomimetic polymer (PMP1). PMP1 uses a repeat unit of N-substituted glycine instead of ethylene glycol to impart antifouling properties. The N-substituted glycine is structurally similar to ethylene glycol and is hydrophilic, so easily dissolves in water. In controlled studies, PMP1-coated titanium surfaces were seen to be resistant to biofouling over a period of 180 days, even with continued addition and exposure to microfouling organisms.[12][14]

Barnacle cement[edit]

Similar to MAPs, barnacle cement is composed primarily of adhesive proteins.[15] The composition of barnacle cement is known to be both non-toxic and stable over a long period of time[16] and also shows a significant attachment strength as an adhesive.[17] The sequence and composition of these proteins is highly dependent on the surface of attachment, allowing for the barnacles to securely attach to a number of surfaces. Barnacle cement proteins contain a significant number of hydroxyl groups which can be used as starting points to grow antifouling polymer brushes from using atom-transfer radical-polymerization (ATRP). Using harvested barnacle cement applied to stainless steel and functionalized, poly(hydroxyethyl methacrylate) brushes have been grown and coupled with chitosan to create a surface that reduces the settlement of E. coli.[15]

Shark skin[edit]

Shark skin, alongside that of many other large marine animals such as dolphins and whales, exhibits a high degree of nanoscale surface roughness that imparts a low wetability which limits biofouling. The surface of shark skin consists of nanoscale overlapping plates that exhibit parallel ridges that effectively prevent sharks from becoming fouled even when moving at slow speeds. A commercial shark skin analogue coating has been developed at the University of Florida under the name Sharklet AF.[18] The antifouling qualities of the shark skin-inspired designs appear highly dependent upon the engineered roughness index (ERI).[19]

 ERI = { r \times n \over 1 - \phi}

Where r is the Wenzel roughness ratio, n is the number of distinct surface features in the design of the surface, and φ is the area fraction of the tops of the distinct surface features. A completely smooth surface would have an ERI = 0.[18]

Using this equation, the amount of microfouling spores per mm2 can be modeled according to the equation:

 \text{ln(settlement)}  { \text{spores} \over \text{mm}^2 } = -7.47 \times 10^{-2} \times ERI + 6.28

Similar to actual shark skin, the patterned nature of Sharklet AF shows microstructural differences in three dimensions with a corresponding ERI of 9.5. This three-dimensional patterned difference imparts a 77% reduction in microfouling settlement.[20] Other artificial nonpatterned nanoscale rough surfaces such as 2-μm-diameter circular pillars (ERI = 5.0) or 2-μm-wide ridges (ERI = 6.1) reduce fouling settlement by 36% and 31%, respectively, while a more patterned surface composed of 2-μm-diameter circular pillars and 10-μm equilateral triangles (ERI = 8.7) reduces spore settlement by 58%.[20] The contact angles obtained for hydrophobic surfaces are directly related to surface roughnesses by the Wenzel equation.[21]

 \cos \theta_w = r \times \cos \theta

θW refers to the observed contact angle, r to the Wenzel roughness (actual area of surface over the projected surface area), and θ refers to the real contact angle in the Wenzel equation. As seen in the Wenzel equation, for surfaces of a hydrophobic nature (θ=90°-180°), an increased roughness leads to exaggeration of the hydrophobic properties. In the case of shark skin and similar biological surfaces (lotus leaf and gecko setae),[21][22] the nano- and microstructured hydrophobic surfaces have heightened contact angles as compared to hydrophobic surfaces like polypropylene and polyethylene.

Polymer Contact Angle (θ) (°) Receding contact angle (θR) (°) Advancing contact angle (θA) (°)
Poly(dimethylsiloxane)(PDMS)[23] 113 118 72
High Density Polyethylene (HDPE)[24] 107 112 77
Polypropylene (PP)[24] 116 123 78
Sharklet AF (+3SK2 x 2)[23] 135 160 112
Lotus Leaf[21] 142.4 N/A N/A
Gecko setae[22] 160.6 159.3 160.7

Chemical methods[edit]

Most chemical methods are based upon the synthesis of naturally produced molecules that prohibit fouling, known as biocides. When incorporated into marine coatings, biocides leech out to the immediate surroundings and discourage fouling. Compared to TBT, natural biocides typically show lower environmental impact and varying effectiveness.

The chemical structure of bufalin (3,4-dihydroxybufa-20,22 dienolide)

Natural biocides are found in a variety of sources, including (sponges, algae, corals, sea urchins, bacteria, and sea-squirts),[3] and include toxins, anaesthetics, and growth/attachment/metamorphosis-inhibiting molecules.[7] As a group, marine microalgae alone produce over 3600 secondary metabolites that play complex ecological roles including defense from predators, as well as antifouling protection,[25] increasing scientific interest in the screening of marine natural products as natural biocides. Natural biocides are typically divided into two categories: terpenes (often containing unsaturated ligand groups and electronegative oxygen functional groups) and nonterpenes.

Various tannins (nonterpene), naturally synthesized by a variety of plants, have been shown to be effective biocides when coupled with additional metal biocide agents such as copper and zinc.[8] The tannins are able to flocculate with a variety of cations, which then react with biomolecules as antiseptics. The most effective natural biocide is 3,4-dihydroxybufa-20,22 dienolide, or bufalin (a steroid of toad poison from Bufo vulgaris), which is over 100 times more effective than TBT at preventing biofouling.[7] Extensive difficulty currently exists in the laboratory synthesis of many natural products, limiting their availability and commercial application. A few natural compounds with simpler synthetic routes, such as nicotinamide or 2,5,6-tribromo-1-methylgramine (from Zoobotryon pellucidum), have been recently been incorporated into patented antifouling paints. In particular, 2,5,6-tribromo-1-methylgramine shows three to siz times the antifouling activity of TBTO against barnacle attachment while remaining one-tenth as lethal to the barnacles.[7]

One of the most significant drawbacks to biomimetic chemical methods comes from their service life. Since the natural biocides must leech out of the coating to be effective, the rate of leeching must be controlled in such a way that maximizes effectiveness and service life. The total amount of biocide released into the environment must be ideally kept to a minimum as to not disrupt the ecosystem. Assuming biocide release remains relatively constant after the first two weeks of exposure to water, the total amount of biocide released can be calculated by taking into account a number of variables according to the following formula:[26]

 \text{Total biocide release} = {  L_\text{a} \times a \times W_\text{a} \times 100 \over\ SVR \times SPG \times DFT}

Where La is the fraction of the biocide actually released (typically around 0.7), a is the weight fraction of the active ingredient in the biocide, DFT is the dry film thickness, Wa is the concentration of the natural biocide in the wet paint, SPG is the specific gravity of the wet paint, and SVR is the percentage of dry paint to wet paint by volume.

References[edit]

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