Biomimetic Strategies

Biomimetic

Strategiesin Antifouling Coatings by Maria Salta

Maria Salta

The lotus leaf (Qingdao, China), with its hydrophobic nature and self-cleaning ability, has been the focus for several bio-inspired applications.

Introducing Biofouling: The Challenge harbours due to shipping can affect local marine Marine biofouling is the colonization of aquatic biodiversity and have detrimental effects on the organisms on manmade structures such as ship ecosystem as a whole. hulls and platforms, and has been of major concern since the inception of naval fleets. Despite the high variability of environmental The aim in modern ship coatings is primarily conditions and the wide diversity of organisms directed towards minimizing the hull roughness involved, the formation of biofouling is caused by biofouling, as a higher roughness observed to follow a reoccurring sequence: after leads to increased drag and subsequently to the initial step during which the submerged excessive fuel consumption. Reducing marine surface undergoes biochemical conditioning biofouling is therefore analogous to reducing from the surrounding water, pioneer bacteria fuel emissions and the high costs associated will eventually attach to the surface (in a with fuel production. Biofouling has an impact process implicating physical, chemical and in a range of marine operations such as sensors, biological parameters), and progressively form aquaculture, energy systems and oil platforms. a multiple species biofilm, often commonly referred to as “slime.” The complexity increases Because of the need to replace antifouling paint if microalgae (especially phytoplankton such systems at specific intervals (approximately as diatoms) and fungi are involved. This every five years), biofouling also impacts on initial part of the fouling process is known as the through-life costs. This necessitates the “microfouling.” The subsequent attachment removal of fouling and previously applied and growth of spores of macroalgae and/or coatings before subsequent repainting, incurring larvae of various micro-invertebrates constitutes manpower, material and drydocking costs. It “macrofouling.” Although the mechanisms is true to say that naval vessels do not dock involved are not yet fully understood, solely to remove fouling build-up, but the microfoulers, or at least some of the bacteria failure to remove attachments in a timely and diatoms involved, have been reported to fashion seriously impacts on performance, i.e., regulate macrofouler settlement, especially via reduces speed and manoeuvrability, increases chemical cues, and often serve as a food source. the risk of noise signatures, and increases fuel costs. Importantly, the environmental issue of Since the late 1960s-early 1970s, effective alien species translocation between the world’s protection against marine fouling of naval

An example of alien species (invasive species) where the Japanese oysters have outcompeted the local species in Den Helder, Netherlands. vessels has been achieved through the use of interest in, and led to the examination of, marine organotin systems, with a durability of around natural systems as a possible route for novel five years. Their use, however, has become a antifouling technologies. The term biomimetics major issue since the breakdown products of as the word signifies (from the Greek βίος=bio= tributyltin (TBT) in seawater were identified life and μίμησις=mimesis=to imitate) is used to as inducing serious biological damage to describe technologies that are in this way marine invertebrates. This impact on the inspired by nature. Biomimetics have often environment resulted in the application of TBT resulted in models for numerous engineering- paint systems being completely banned by the related applications in order to overcome International Maritime Organization in 2008. scientific challenges and in the same context, As a consequence, considerable international bio-inspired antifouling technologies make a effort has been devoted towards developing strong candidate to resolve the problem of bio- environmentally friendly technologies capable fouling in an environmentally friendly manner. of providing the required degree of protection from marine fouling along with adequate Biomimetics: Chemical Defences coating adhesion and endurance. Nearly 20,000 natural products have so far been described that originate from marine The Solution: Biomimetic Approaches organisms. Sessile marine organisms (e.g., In the marine environment, a wide variety of sponges, soft corals and seaweeds) as well species appear to remain fouling-free over as microorganisms are known to elaborate extended periods of time and illustrate chemical defence mechanisms against antifouling abilities by several means, e.g., the predation and epibiont growth. The excreted use of chemical and physical defences, but also metabolites might repel or inhibit fouling symbiotic relationships between host (e.g., organisms and can act enzymatically by algae) and epibionts (other organisms that live dissolving the adhesives of the fouling on them) that prevent fouling. The impediment organism and/or act as biocide. Since the of biofouling in a natural way, as observed in early 1980s, a great number of marine natural marine organisms, has triggered the scientific extracts have been assayed against organisms

30 The Journal of Ocean Technology, Vol. 9, No. 4, 2014 Copyright Journal of Ocean Technology 2014 implied in the biofouling process and several reviews dealing with their potential use as novel antifouling biocides have been realized. The most successful story is that of the red algal species Delisea pulchra, which produces secondary metabolites, halogenated furanones, exhibiting antifouling properties. Interestingly, it was observed that in D. pulchra, furanones are encapsulated within specialized glands located at the surface of the alga. These secondary metabolites were found to be released Maria Salta from the algal surface at such concentrations (ranging from 10 to several 100 In-situ microscopy showing natural biofilm (“slime”) in real-time ng.cm–2, depending on the furanone) that the colours formed on a plastic surface after a month. These biofilms can be as thick as a few hundred micrometres and can directly settlement of microfouling in both field and affect a ship’s hydrodynamics by increasing its drag as it moves laboratory assays was largely reduced. into the water.

An important factor that contributes towards involved in inhibition have frequently been the exploitation of marine algae as natural difficult to identify, posing a problem in resources against biofouling is that these creating structural analogues for incorporation organisms are cultivable, relatively easy to into antifouling coatings. harvest and that the extract yield is relatively high. Hence, algal extracts represent cost Biomimetics: Physical Defences effective alternative antifouling compounds The other major environmentally friendly and are an attractive source for a commercially antifouling mechanism possessed by several viable product. Algae are often sedentary and marine organisms is via physical/mechanical thus act as biomimetic solutions for static ship means, mainly through ingenious topographic operational profiles. Some algae can tolerate arrangements on their surfaces. This is the exposure out of the marine environment (e.g., case for a great spectrum of marine organisms during low tides) and a ship’s hull would be ranging from mammals (skins of killer an analogue of this exposure at the waterline whales, pilot whales, dolphins and porpoises) of the splash zone. Marine organisms are able through to fish (e.g., sharks), crustaceans to synthesize new compounds to maintain the (e.g., crabs) and molluscs (e.g., mussels and antifouling effect, but such an option is not oysters). One of the most investigated model available to the paint formulator. surfaces from the marine world is the shark’s skin, as sharks require low drag in order to Despite the wide range of promising natural develop high speeds during prey catching. products deriving from algae, they are Indeed, the shark’s skin is characterized by becoming less effective models for bio- repetitive microstructured diamond shaped inspired antifouling technologies against ridges which may help by reducing the drag biofouling. This is mainly because developing through the increase of the water flow along antifouling compounds from marine natural the shark’s skin as it swims. Effectively, products is considerably restricted by the this allows the shark to move faster, while regulatory environment for chemicals in preventing biofouling accumulation. These coatings. Also, secondary metabolites are riblet structures can range from 30 μm to 300 complex molecules and the active compounds μm according to the species. A technology

Example of biofouling growth on buoys that support instruments for oceanographic work. inspired by the shark’s skin, named Sharklet of seaweed (Ulva) by 62%. Observations of AFTM, was achieved by designing similar shark skin patterns showed that surfaces with patterns using silicone (polydimethylsiloxane) less points of attachment were more successful at a variety of sizes that were scaled down in preventing settlement. Although these are from the ones found in nature. Interestingly, very promising results, the patterns were some of the designs managed to reduce optimized to the size ranges of the specific barnacle larvae settlement by 97% and spores biofouling organisms tested. This illustrates

32 The Journal of Ocean Technology, Vol. 9, No. 4, 2014 Copyright Journal of Ocean Technology 2014 the difficulty in taking biofouling as a whole, creating a self-adhesive foil mounted with as organism sizes range from a couple of millions of micro-fibres that vibrate constantly micrometres (bacteria) to a few hundreds of by the water’s movement. This movement, micrometres (larvae and spores). Nevertheless, mimicking thorn-looking fibres observed on ongoing investigation of shark skin pattern marine living surfaces, appears to discourage sizes and combinations may show improved settlement of macrofoulers such as barnacles, performance in the future. mussels and algae. These last two fibre- inspired approaches may provide a non-toxic Oysters and mussels have also been antifouling solution to the long-standing the focus of research for their potential biofouling issue; however, there is no clear antifouling properties deriving from their understanding of the effect on the vessel’s surface properties. These are attributed to drag when covered with fibres and whether microstructures formed by the top layer of increased friction eventually underpins the the shells, the periostracum, which has been beneficial antifouling effects. shown to inhibit biofouling. Interestingly, juvenile oysters with intact periostacum Biomimetics: A Few Examples from the Land appeared to inhibit boring by other organisms, Another desirable characteristic for an while older oysters (more than one year old) antifouling coating system is a self-cleaning did not prevent boring which was related to effect. In nature, this is observed in the eroded periostracum. The same principle was lotus leaf with its superhydrophobic and observed for mussels which were found to low adhesion nature. Specifically, the lotus form straight and sigmoidal micro-ridges on leaf exhibits hierarchical micro-scale bumps their shells. The replication of the mussel’s covered with waxy nanostructures. This surface texture has been attempted and tested texture/wax combination allows air pockets against several biofouling organisms in the to form at the solid-liquid interface, offering a lab. Interestingly, the results were very similar self-cleaning effect aided by rain drops taking to those found for shark’s skin; that is, both with them the dirt as they smoothly roll off microfoulers and macrofoulers exhibited the leaf. A recent attempt to use the combined increased settlement and attachments on effect of low surface energy and self-cleaning surfaces with a high number of attachment characteristics lead to the investigation of rice points. By developing topographies with leaves and butterfly wings. In rice leaves, a lowest number of attachment points, the self-cleaning effect is used to avoid fouling settlement and growth of larvae and spores of that would otherwise prevent photosynthesis. fouling organisms can be decreased and the For butterflies, a similar principle allows release of fouling from surfaces increased. maintenance of wing colouration and flight control, achieved by the observed hierarchical Recently, a bio-inspired paint has been scales and microgrooves on their wings. developed that is flocked with fibres These two systems appeared successful mimicking the seal’s coat (SealCoat). This against bacterial colonization in laboratory paint is claimed to remain foul-free from experiments (especially for the rice leaf), macrofoulers for up to five years; however, no making them interesting antifouling candidates published literature provides scientific data to which however have yet to be tested in real-life confirm this. Although SealCoat exploits an conditions of the sea. interesting concept, it has to be noted that seals intensively groom their hairs when out of the Conclusions and Future Directions water, therefore mechanically removing dirt off I would like to end this essay by their coat. Moving away from the traditional acknowledging the fact that most marine paint application, a new bio-inspired approach, organisms utilize a combination of called Thorn-D®, challenges biofouling by mechanical, chemical, and physical

Copyright Journal of Ocean Technology 2014 The Journal of Ocean Technology, Vol. 9, No. 4, 2014 33 34 The Journal ofOcean Technology, V ol . 9, N 9, . o . 4, 2014 4, . Copyright Journal of Ocean Copyright Technology 2014

Example of biofouling growth on a docked vessel in Venice, Italy.

Maria Salta Maria Salta

Mimicking the shark’s skin (left insertion), the bio-inspired Sharklet™ has been developed (right insertion) – adapted from Scardino and de Nys [2011]. Biofouling, Vol. 27, pp. 73-86. mechanisms to deter biofouling. For instance, the most promising results are often temporal, some species will use surface renewal to lasting a month or less when tested in the field. shed their top surface layer and along with However, as Dr. Andew Scardino brilliantly it the fouling organisms (e.g., algae), while put it: “Nature has successfully managed this metabolites are being released by their crusts to challenge and we should graciously accept chemically enhance their defence. This kind of guidance, and be inspired, in understanding observation has made it clear that a combined and developing the most sustainable marine approach such as topography corroborated technologies to control biofouling.” u by non-toxic chemistry will lead to a more successful antifouling technology, ideally Maria Salta is a marine against a range of biofouling organisms. An biologist (focusing on important aspect in antifouling research is environmental microbiology) and oceanographer working to gain a better understanding of the overall on projects ranging from biofouling processes which are poorly coral conservation to understood. Specifically, biofilm formation is plankton ecology and the first step towards establishing the entire ecophysiological responses biofouling community; therefore, considerable of microorganisms to high effort should be placed in investigating these CO2 levels. Over the past microorganisms. Recent advances in molecular seven years, Dr. Salta has been funded by the European tools can uncover unprecedented information Defence Agency and the European Union to work on the on biofilm dynamics and attempt to answer discovery of environmentally friendly antifouling coatings by exploring and mimicking natural systems. Having questions such as “which bug sticks to which been based in the Engineering Sciences Department at antifouling surface.” This will eventually the University of Southampton for seven years, she has lead us to better manage novel antifouling been applying her knowledge in marine natural systems technologies. to conduct translational research by fabricating novel bio- inspired antifouling surfaces. Dr. Salta has recently been Despite repeated efforts, finding a universal assigned the post of lecturer in Environmental Microbiology physical antifoulant may be a hard task and (University of Portsmouth) where she will continue focusing on bio-inspired solutions to environmental issues.