Biomimetic antifouling coating
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A biomimetic antifouling coating is a treatment that prevents the accumulation of marine organisms on a surface. Typical antifouling coatings are not
biomimetic Biomimetics or biomimicry is the emulation of the models, systems, and elements of nature for the purpose of solving complex human problems. The terms "biomimetics" and "biomimicry" are derived from grc, βίος (''bios''), life, and μίμησ ...
but are based on synthetic
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
s that can have deleterious effects on the environment. Prime examples are
tributyltin Tributyltin (TBT) is an umbrella term for a class of organotin compounds which contain the (C4H9)3 Sn group, with a prominent example being tributyltin oxide. For 40 years TBT was used as a biocide in anti-fouling paint, commonly known as botto ...
compounds, which are components in paints to prevent
biofouling Biofouling or biological fouling is the accumulation of microorganisms, plants, algae, or small animals where it is not wanted on surfaces such as ship and submarine hulls, devices such as water inlets, pipework, grates, ponds, and rivers that ...
of ship hulls. Although highly effective at combatting the accumulation of barnacles and other problematic organisms, organotin-containing paints are damaging to many organisms and have been shown to interrupt marine food chains.1. Gittens, J. E., Smith, T. J., Suleiman, R., Akid, R., "Current and emerging environmentally-friendly systems for fouling control in the marine environment", Biotechnol. Adv. 2013, 31, 1738. Biomimetic antifouling coatings are highly lucrative because of their low environmental impact and demonstrated success. Some properties of a biomimetic antifouling coating can be predicted from the contact angles obtained from the Wenzel equation, and the calculated ERI. Natural materials such as shark skin continue to provide inspiration for scientists to improve the coatings currently on the market.


Chemical methods

Most antifouling coatings are based upon
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
s that inhibit fouling. When incorporated into marine coatings, these
biocide A biocide is defined in the European legislation as a chemical substance or microorganism intended to destroy, deter, render harmless, or exert a controlling effect on any harmful organism. The US Environmental Protection Agency (EPA) uses a slig ...
s leach into the immediate surroundings and minimize fouling. The classic synthetic antifouling agent is
tributyltin Tributyltin (TBT) is an umbrella term for a class of organotin compounds which contain the (C4H9)3 Sn group, with a prominent example being tributyltin oxide. For 40 years TBT was used as a biocide in anti-fouling paint, commonly known as botto ...
(TBT). Natural biocides typically show lower environmental impact but variable effectiveness. Natural biocides are found in a variety of sources, including
sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...
s, algae,
coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...
s,
sea urchin Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...
s, bacteria, and sea-squirts, and include toxins,
anaesthetic An anesthetic (American English) or anaesthetic (British English; see spelling differences) is a drug used to induce anesthesia ⁠— ⁠in other words, to result in a temporary loss of sensation or awareness. They may be divided into two ...
s, and growth/attachment/
metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some inse ...
-inhibiting molecules. As a group, marine microalgae alone produce over 3600 secondary
metabolites In biochemistry, a metabolite is an intermediate or end product of metabolism. The term is usually used for small molecules. Metabolites have various functions, including fuel, structure, signaling, stimulatory and inhibitory effects on enzymes, c ...
that play complex ecological roles including defense from predators, as well as antifouling protection, increasing scientific interest in the screening of marine natural products as natural biocides. Natural biocides are typically divided into two categories:
terpenes Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n > 1. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predominantly by plants, particularly conifers. Terpenes ar ...
(often containing unsaturated
ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
groups and electronegative oxygen functional groups) and nonterpenes. Various
tannins Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. The term ''tannin'' (from Anglo-Norman ''tanner'', f ...
(nonterpene), naturally synthesized by a variety of plants, are effective biocides when combined with copper and zinc salts. The tannins are able to flocculate with a variety of cations, which then exhibit antiseptic properties. 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. Bufalin is however expensive. A few natural compounds with simpler synthetic routes, such as
nicotinamide Niacinamide or Nicotinamide (NAM) is a form of vitamin B3 found in food and used as a dietary supplement and medication. As a supplement, it is used by mouth to prevent and treat pellagra (niacin deficiency). While nicotinic acid (niacin) may ...
or 2,5,6-tribromo-1-methylgramine (from ''Zoobotryon pellucidum''), have been incorporated into patented antifouling paints. A significant drawback to biomimetic chemical agents is their modest service life. Since the natural biocides must leach out of the coating to be effective, the rate of leaching is a key parameter.
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.


Shark skin mimetics

One class of biomimetic antifouling coatings is inspired by the surface of shark skin, which consists of nanoscale overlapping
placoid scales A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as we ...
that exhibit parallel ridges that effectively prevent sharks from becoming fouled even when moving at slow speeds. The antifouling qualities of the shark skin-inspired designs appear highly dependent upon the engineered roughness index (ERI). 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. Using this equation, the amount of microfouling spores per mm2 can be modeled. 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. 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%. The
contact angle The contact angle is the angle, conventionally measured through the liquid, where a liquid–vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation. A given system of solid, liq ...
s obtained for hydrophobic surfaces are directly related to surface roughnesses by the
Wenzel Wenzel is a male given name (long version Wenzeslaus) as the German and Old English form of the Czech given name Václav or Venceslav, meaning "praised with glory". Variations are Вячеслав (Ukrainian and Russian), Vencel (Hungarian), Wacła ...
equation.


See also

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Biofouling Biofouling or biological fouling is the accumulation of microorganisms, plants, algae, or small animals where it is not wanted on surfaces such as ship and submarine hulls, devices such as water inlets, pipework, grates, ponds, and rivers that ...
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Fouling Fouling is the accumulation of unwanted material on solid surfaces. The fouling materials can consist of either living organisms (biofouling) or a non-living substance (inorganic or organic). Fouling is usually distinguished from other surf ...
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Anti-fouling Biofouling or biological fouling is the accumulation of microorganisms, plants, algae, or small animals where it is not wanted on surfaces such as ship and submarine hulls, devices such as water inlets, pipework, grates, ponds, and rivers that ...
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Biomimicry Biomimetics or biomimicry is the emulation of the models, systems, and elements of nature for the purpose of solving complex human problems. The terms "biomimetics" and "biomimicry" are derived from grc, βίος (''bios''), life, and μίμησ ...
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Bionics Bionics or biologically inspired engineering is the application of biological methods and systems found in nature to the study and design of engineering systems and modern technology. The word ''bionic'', coined by Jack E. Steele in August 1 ...
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Tributyltin Tributyltin (TBT) is an umbrella term for a class of organotin compounds which contain the (C4H9)3 Sn group, with a prominent example being tributyltin oxide. For 40 years TBT was used as a biocide in anti-fouling paint, commonly known as botto ...
(TBT) * Sharklet


References

{{reflist Bionics Fouling Paints Coatings