Advanced Bioinspired Superhydrophobic Marine Antifouling Coatings

Abstract

Following the tributyl-tin antifouling coatings’ prohibition in 2003, global interest was directed toward non-toxic coatings as an eco-friendly alternative. Natural surfaces with superhydrophobicity exhibited exciting antifouling mechanisms. Efficient and eco-friendly antifouling coatings have been developed using bioinspired polymeric nanostructured composites. These superhydrophobic surfaces have rough topologies and low surface-free energies. Various organic/inorganic polymeric nanocomposites were developed for increasing fouling prevention by physical microfouling repulsion and chemical surface inertness. The biofouling costs and the difficulties of artificial antifouling coatings were also discussed in this chapter. It will introduce a cutting-edge research platform for next-generation antifouling surfaces for maritime navigation. This chapter aims to explain the evolution of superhydrophobic antifouling surfaces inspired by biological systems.

1. Introduction

Modern research has focused on the development of effective marine antifouling and superhydrophobic surfaces to mitigate the detrimental effects of marine biofouling on the shipping sector [1]. Natural biomimetic structures with low surface-free energy (SFE) can provide inspiration for the creation of polymer/inorganic antifouling coatings. The key antifouling design tools required an understanding of the surface characteristics of nanocomposites, such as polymeric structures and nanofillers’ morphology. Fouling represents a scary nightmare with dynamic issues that negatively affect marine navigation in both ecological and economic terms. Shipping contributes to almost 90% of all international trade. Shipping costs and environmental risks are just two of the negative ecological and economic effects of fouling adhesion [2]. Fouling increases fuel consumption and friction drag. This can lower hydrodynamic efficiency and ultimately the ship’s speed [3]. Increased CO2, NOx, and SOx emissions may result from fouling adherence (Figure 1), which raises shipping costs by around $12 billion annually [2, 3]. Traditionally, biocidal antifouling coatings were used to prevent biofouling. But such coatings have a severe impact on non-target species such as dolphins and fish, and shorten the time between dry-dockings [4]. Additionally, they may result in low fuel savings, environmental degradation, and harm to the maritime eco-system. In 2003, the International Maritime Organization (IMO) forbade the application of organo-tin antifouling paints due to these coatings’ detrimental effects. Recent advancements have focused on FR coatings as an eco-friendly alternative to biocidal fouling-prevention substances.

Figure 1.

The conditioned film, micro- and macro-fouling, and the issues brought on by fouling adherence on the ship hull are typical fouling phases that are graphically illustrated. An example of how fouling adhesion has a negative economic and ecological influence [1]. Copyright 2017; reprinted with Elsevier’s consent.

Surfaces with FR coatings are non-toxic, very mobile, and non-stick. Fluoropolymers and polysiloxanes, two types of FR resin, have been shown to perform effectively as self-cleaning materials that can stop physical fouling through an anti-adhesion mechanism. Fluorine atoms are tightly bound in fluoropolymer structures, which results in high stiffness and a lack of structural rotation along the polymeric backbone [5]. This stiffness also makes it difficult for connected biofouling strains to easily free themselves. Polysiloxanes (particularly polydimethylsiloxane (PDMS)) offer a more widely used antifouling resin as compared to fluoropolymers [6].

PDMS building blocks include flexibility in the structure, non-toxicity, water-repellency, fouling-resistance, and reduced SFE [7]. Through the silicone matrix’s well-dispersion of the nanofillers, the antifouling performance of PDMS was enhanced [8]. Superhydrophobic surfaces have made environmentally beneficial and cost-effective advancements in a variety of industries, including textiles, antifouling coatings, anti-icing, and anti-corrosion [9]. The ability of lotus leaves to self-clean is one of the earliest lessons we can learn from nature. These leaves have been studied for decades because of their exceptional water repellency due to their distinctive structure and extremely hydrophobic composition [10]. The lotus leaves Nelumbo nucifera represent well-known examples of superhydrophobic surfaces that naturally clean themselves because of their 20–40 μm epicuticular wax rough topology [11]. Superhydrophobicity is found in nature in the form of butterfly wings, cabbage, Indian cress plant leaves, and more [12]. Due to their rough micro/nanostructure, low SFE, superhydrophobic surfaces have contact angle hysteresis ≤5° and water contact angles (WCA) ≥150° [13].

In 1990s, the concept of the first artificial superhydrophobic surface was demonstrated [14]. Sensitive surface features can be safeguarded by superhydrophobicity [15]. The Wenzel and Cassie-Baxter models can be used to explain how roughness affects a solid surface’s non-wettability [16, 17]. The rise in the solid-liquid interface might increase surface roughness and hydrophobicity by trapping air in the surface grooves to produce superhydrophobic surfaces [18, 19, 20]. The resulting nanocomposite structures may have improved interfacial bonding due to economic, extra-high surface-area, and matrix-nanofiller interfacial bonding [21]. By evenly dispersing the nanofillers throughout the coating resin, it is possible to successfully build a superhydrophobic structure with outstanding substrate-coating adhesive forces. Rough topology superhydrophobic composites are possible when ceramic nanofillers and polymers are structurally controlled [22]. Superhydrophobicity can be produced by rough surfaces by trapping air in the ridges and grooves present beneath the water droplets. Biomimetic lithography, stamping, and etching processes were used to produce superhydrophobic nano-surfaces [23]. The creation of long-lasting antifouling nanostructures for the shipping industry was the main goal of the mature rostrum’s nanocomposite coatings.

2. Detrimental effects of biofouling on shipping

2.1 Biofouling mechanism

Based on the biofoulans’ size, type, and invasion time, biofouling (Figure 1) is commonly classified into three main categories: 1—first colonizers; 2—secondary; and 3—tertiary colonizers (soft and hard biofoulers) [24]. First and second colonizers are termed micro-biofoulers, whereas soft and hard biofoulers are generally termed macro-biofoulers [24, 25]. First colonizers are microorganisms that colonize invaded surfaces within hours. This microbial assemblage consists of bacteria, diatoms, unicellular microalgae, and cyanobacteria to form the so-called biofilm or slime [26].

In the early biofouling event (the first minutes), proteins, proteoglycans, and polysaccharides of different biological processes compose an organic-conditioning film consisting of chemical compound macromolecules to achieve wettable surfaces [27]. Planktonic bacteria, as one of the first colonizers, start the invasion through two stages within hours [28]. One of which is returnable, the so-called adsorption, and the other is unreturnable, the adhesion. The instant allure response caused by returnable adsorption attracts germs to the wet surface. Detrimental physical powers are known to control the bacterial adsorption process, including van-der-Waal forces, electrostatic interaction, and Brownian motion [29]. The so-called slime film, which is composed of a spreading bacterial lawn and extracellular polymeric bridges produced by sessile bacteria, is what forms the unreturnable phase.

Secondary colonizers (protozoans and macroalgal zoospores) start to emerge and colonize surfaces after the settlement of the first colonizers’ stage within a period of almost a week [26, 30]. The primary colonizers provide them with enough nutrient supply via different microbial metabolic processes and products. A general insulation pattern of biofilm formation over time may be the net result of mutual and continuous physical interactions between the primary and secondary colonizers. Such physical interactions include conspiracies, survival tournaments, mutual elimination, and limited breakups.

Tertiary colonizers are macro-biofoulers (soft and hard biofoulers). Macro-biofoulers with non-calcareous structures are termed soft macro-biofoulers such as sponges and ascidians (tunicates) [31], whereas hard biofoulers are those with external calcareous skeletons, such as mussels, oysters, crustacean barnacles, and tube worms [32]. Tertiary colonization takes place after 2 to 3 weeks of settlement of both primary colonization and secondary colonization. However, the macroscopic tertiary biofoulers attach themselves to the invaded surfaces to feed on the primary and secondary colonizers [33]. Along the process of colonization, macro-biofoulers strongly engage themselves with the invaded surfaces via their secretions, which provide them with enough chemical bonding and electrostatic interactions to assure no breakup from such surfaces.

2.2 Marine biofouling costs

Biofouling assemblages cause ubiquitous calamity and negative economic and ecological impacts, especially in marine environment [34]. Maritime sectors appear to be adversely influenced by both micro- and macro-biofouling namely ships submerged surfaces, vessels and internal seawater systems, marine renewable energy, oil and gas, and aquaculture sectors [34, 35, 36]. In their colonization, macro-biofoulers are responsible for an extra substantial economic cost because their hard-formed layers create typically optimum anaerobic conditions that accelerate the microbially induced metal corrosion process (anaerobic micro-biofoulers) [37, 38]. Hard macro-biofouling accounts for about 85% of annual economic impacts, whereas the total estimated penalties and fines of micro-biofouling and anaerobic microbially induced corrosion account for about 20 and 50% of all pipeline infrastructure failures [38]. The global estimation of the damage cost of microbially induced corrosion accounts for about 2.5 trillion USD annually [39]. Biofouling-induced damage costs are the net impacts of increased ship drag resistance, decreased velocity of the vessels, the hydrodynamic deficiencies of increased surface roughness due to the adhesion of biofouling communities, and, subsequently, exceeded biofouling management costs [40]. Major biofouling management regimes provide antifouling leaching biocidal coatings [38, 41], anti-adhesion (anti-biofilm) coatings [35], and self-cleaning foul-release coatings [35, 38]. Biofouling-induced impacts and management are summarized in Table 1.

Maritime sector	Biofouling	Impact	Management
Shipping industry Oil & gas Renewable energy Aquaculture	Macro- and micro-biofoulers	Reduction of propeller efficiency and increased surface roughness Increased drag resistance and hydrodynamic weight Decreased vessel’s velocity loss of functionality and performance Increased fuel consumption Reduction of air quality and increased hazardous emissions	Leaching antifouling biocides Antifouling coatings (self-cleaning and foul-release)
	Micro-biofoulers	Metallic corrosion (microbially induced corrosion)

Table 1.

Major maritime biofouling-induced impacts and management [26, 35, 40, 41].

3. Traditional methods to combat biofouling

The necessity to protect ship hulls against marine fouling invasion had been established for as long as ships are needed for transportation. The tale of antifouling paints started in ancient time with the complaints of different sailors with their wooden ship hulls from the heavy burden of various assemblages of marine organisms and seaweeds on the submerged surfaces. Such accumulation negatively affected the vessels as it affected the overall ship’s performance [42]. Such suffering forced the sailors to provide their wooden ships with different coverings such as asphalt, wax, tar, oil, tallow [43, 44]. In the ages of 700 B.C., the Phoenician and Carthaginian sailors began to apply pitch-copper and tallow-lead thin sheets as protective shields from the marine invaders [45]. In the ages after 500 B.C., Phoenicians applied oil-mixed arsenic and sulfur sheathing. Protection by asphalt, wax, tar, tallow, pitch, copper, and lead had been applied during the ages from 300 B.C. until the fifteenth century among Greeks, Romans, Vikings, and Columbus sailors [46]. In the early 50’s, organometallic paints such as tin, arsenic, and mercury started to emerge and encountered a development to introduce tributyltin (TBT)-based paints that showed amazing performance [23]. Later on, in 2001, TBT was banned by the IMO due to the severe toxicity to the marine environment [47]. During the period between 1960’s–1970’s and the early nineteenth century, copper sheathing of wood and steel ships had been commonly used and favored with different mixtures of copper sulfide, arsenic ore, iron dust, and zinc alloy. From the late period of 1970’s–1980’s to the mid-nineteenth century paints containing copper, zinc, and mercuric oxide had been applied with polymeric binder [36].

Considering the foregoing history, antifouling paints could be classified into different categories: 1—biocidal paints (as organotin compounds such as mercury, arsenic, and their compounds), 2—leaching paints (containing polymers with high molecular weight, such as epoxy, acrylates, and chlorinated rubber), 3—ablative paints (a mixture of biocide, such as iron, zinc oxide, arsenic, and mercury, incorporated with the coating resin), 4—self-polishing paints (acrylic or methacrylic copolymer-based paints such as TBT-based paints and tin-free paints based on copper, zinc, and silyl acrylate), 5—foul-release paints (their mode of action depends on prevention fouling adhesion via their smooth and low-energy surfaces that enable the hydrodynamic forces of water to wash off the fouling attachments, where the paint is based on the elastomers PDMS, the epoxy primer, and the oil additive to enhance the smooth performance), 6—engineered antifouling surfaces (physically, chemically, and topographically controlled surfaces inspired by different natural antifouling surfaces found around us in nature, such as shark and whale skin and molluscan shells).

4. Innovative approaches, such as superhydrophobic surfaces, to tackle biofouling

4.1 Eco-friendly FR coatings

Fouling refers to the accumulation of marine organisms, such as algae, barnacles, and mussels, on underwater surfaces such as ship hulls, offshore structures, and marine equipment [48, 49, 50]. This fouling can lead to increased drag, reduced fuel efficiency, and increased maintenance costs. The rise in fuel consumption inherently results in elevated transportation expenses for the shipping sector. About 2% of the world’s total energy is allocated to commercial shipping, encompassing a fleet of nearly 100,000 cargo vessels [51]. Consequently, the excessive fuel usage attributed to biofouling can lead to heightened CO₂ emissions, which is counterproductive in the pursuit of carbon neutrality. Furthermore, organisms adhering to vessel hulls can traverse international waters, reaching local marine ecosystems devoid of their natural predators. In such settings, they can perpetuate unchecked and potentially trigger ecological upheavals [52]. Current data indicates that governments and enterprises on a global scale expend over $15 billion each year to mitigate or manage the impacts of marine biofouling [53].

Traditional antifouling coatings typically contain toxic biocides, such as TBT, which was prohibited by the IMO in 2001 [54, 55], quaternary ammonium salts, tar, asphalt, toxic (arsenic), or heavy metals (silver, copper, and lead), which can have negative impacts on marine ecosystems [56, 57, 58]. Eco-friendly FR coatings can effectively prevent the attachment of marine organisms (such as algae, barnacles, and mollusks) to submerged surfaces while minimizing environmental impacts [59, 60]. Eco-friendly fouling-release coatings can provide an effective alternative to traditional antifouling coatings. These coatings work by creating a low-friction surface, preventing marine organisms from firmly attaching to the submerged surface. The reduced friction allows water flow to easily remove any loosely attached organisms, preventing the formation of a thick fouling layer. Eco-friendly FR coatings are designed to be non-toxic and environmentally friendly. They are typically made from non-biocidal materials, such as silicone or fluoropolymer-based polymers, which have low toxicity and minimal impact on marine life [61, 62]. These coatings are engineered to have a smooth and slippery surface, making it difficult for fouling organisms to establish a strong attachment. Eco-friendly FR coatings can provide effective fouling prevention while minimizing negative environmental impacts. By reducing the need for toxic biocides, these coatings can help protect marine ecosystems and contribute to sustainable marine practices [63, 64]. Recent developments are based on FR coatings, including silicone, modified fluorinated polymers, cross-linked coatings, amphiphilic copolymer coatings, hydrogel coatings, and biomimetic coatings [65, 66]. Additionally, environmentally friendly compositions comprising curable polysiloxane binders and non-ionic hydrophilic-modified polysiloxanes have been utilized [67, 68].

For example, a recent paper highlights the importance of designing coatings that are bactericidal to prevent fouling by bacteria, marine organisms, and proteins. It discusses the major strategies for combating surface fouling, including preventing biofoulants from attaching or degrading them, and the use of various techniques such as functionalization of surfaces with poly (ethylene glycol) or oligo (ethylene glycol), biocidal agents, polycations, enzymes, nanomaterials, and photoactive agents [69]. Various innovative technologies have been employed to improve these coatings, including the use of copolymers containing MQ silicone and acrylate [65, 70], charcoal-based graphene oxide-copper oxide [71], and the incorporation of phenylmethyl silicone oil as a lubricant [72].

The main problem of biofouling is the accumulation of marine organisms on submerged surfaces and its negative impacts on various industries, such as shipping, aquaculture, and energy production [73, 74, 75]. There is a need for effective antifouling coatings that can prevent or reduce biofouling, and for discussing the limitations of current coatings, such as toxicity, environmental concerns, and durability. This work introduces the concept of FR coatings, which prevent biofouling by reducing the adhesion strength of marine organisms to the surface Also, it describes the advantages and challenges of these coatings. Overall, the introduction sets the context and motivation for the research presented in the paper and highlights the importance of developing new FR coatings based on innovative materials and formulations. The paper discusses the development of FR coatings based on copolymers containing MQ silicone and acrylate, and the preparation of FR paints based on these copolymers with the incorporation of non-reactive phenylmethylsilicone oil [70]. The coatings showed good hydrophobicity, mechanical properties, and adhesion strength, with high oil leaching efficiency and excellent antifouling performance. The paper also provides insights into the effects of copolymer composition on the properties of the coatings, which can guide the design of new coatings with tailored properties.

It is important to note that the effectiveness of FR coatings can vary depending on factors such as the specific coating formulation, water temperature, vessel speed, specific marine environmental conditions, and the type of fouling organisms present. Ongoing research and development are crucial to improving the performance and longevity of these eco-friendly coatings. Further studies and field trials are necessary to evaluate the long-term effectiveness, durability, and economic viability of eco-friendly FR coatings. However, the hypothesis suggests that these coatings hold promise as a more sustainable alternative to traditional antifouling coatings, benefiting both marine industries and the environment. FR coatings work on the principle of creating a slippery or low-energy surface that makes it difficult for organisms to adhere to the substrate.

4.2 Bioinspired strategies for antifouling applications

Biofouling is a complicated, dynamic phenomenon that affects both the environment and the economy on a worldwide scale. Through collaborative interdisciplinary efforts in marine biology, polymer science, and engineering, bio-inspired tactics have been harnessed to pave the way for advancing the next era of antifouling marine coatings [76, 77]. Bioinspired antifouling strategies aim to reduce the environmental impact of conventional antifouling coatings by utilizing sustainable and nature-inspired solutions. However, it is important to note that while these approaches hold promise, they may have limitations in certain environments or applications. Ongoing research and development are focused on optimizing the performance and durability of bioinspired antifouling coatings. There have been several bioinspired antifouling coatings that have been successfully developed and tested. These coatings draw inspiration from nature’s strategies to prevent fouling and have shown promise in reducing or preventing the attachment of marine organisms [76, 77, 78, 79].

Adhering to the principles of nature and drawing insights are the natural world form the foundational principles of bionic technology. Within this framework, antifouling approaches can be categorized into two main domains: chemical and mechanical. (i) Numerous aquatic and terrestrial organisms possess the ability to secrete natural substances that combat bacterial infections and biofouling [78, 79]. As an illustration, terpenoid and halogenated compounds sourced from algae serve as notable examples, and capsaicin produced from chili peppers [80, 81], also has antifouling action [82]. (ii) Mechanical antifouling approaches: Within this realm, scientists have identified intrinsic surface textures spanning from nanometer to millimeter dimensions across various organisms, including mangrove foliage, lotus leaves (Nelumbo nucifera), shells, sharkskin, and corals [83]. These natural topographies operate mechanically to diminish the prevalence of biofouling [84, 85]. The main bioinspired approach is the surface roughness and texture of marine organisms that have evolved rough or textured surfaces that discourage fouling attachment. Bioinspired antifouling coatings mimic these features to create surfaces that are less favorable for the attachment and growth of fouling organisms.

Also, the microtopography of some organisms that have tiny ridges and scales called denticles that reduce drag and discourage fouling, such as shark skin [84, 85]. Coatings with microtopography can mimic these structures to create a slippery surface that makes it difficult for organisms to settle and grow. These coatings, known as “sharkskin coatings” or “riblet coatings,” have been applied to ship hulls and have shown promising results in reducing drag and fouling attachment.

Schumacher et al. [86, 87] successfully recreated shark epidermal riblets on a PDMS elastomer to establish a microstructured surface. The architecture featured rib structures forming periodic rhombuses with dimensions of 2 mm width, lengths ranging from 4 to 16 mm, and 2-mm spacing. Notably, the outcomes revealed a significant decline in spore settlement on the engineered surface in contrast to smooth surfaces, manifesting an impressive effective reduction of up to 77%. Moreover, the surface exhibited the capability to minimize bacterial adhesion and suppress the formation of biofilms. Thus, these shark-inspired surfaces exhibit the potential to proficiently counteract microbial sedimentation and impede biofilm establishment.

Some lotus leaves and other plants have self-cleaning properties due to their waxes and hydrophobic surfaces with micro−/nano-structures that repel water and contaminants [88, 89]. To date, these principles are being applied to create self-cleaning antifouling coatings that shed water and debris, reducing the attachment of organisms [90]. As an illustration, Selim and colleagues effectively prepared a range of nanostructured coatings with a lotus-inspired configuration. This innovative approach involved utilizing in situ techniques to combine vinyl-terminated PDMS with YeAl₂O₃ nanorod composites. These composites, composed of single-crystal nanorods, are easily synthesized and possess regulated structures and shapes [91, 92]. Consequently, the resultant surface inherits superhydrophobic attributes, self-cleaning attributes, and noteworthy antifouling properties.

Furthermore, certain marine organisms use hydrophobic (water-repellent) or hydrophilic (water-attracting) surfaces, produce natural anti-adhesives or compounds, produce protective mucus layers, or can change their surface properties in response to environmental cues to control fouling. Some marine organisms have biocidal activity and produce compounds that are toxic to fouling organisms. Researchers are studying these properties and compounds to develop environmentally friendly antifouling coatings that mimic their effects, create a physical barrier, and can switch between fouling-resistant and fouling-release states, deterring fouling attachment or making it easier to clean. Additionally, researchers are exploring the use of naturally occurring substances such as enzymes and proteins to create antifouling coatings that interfere with the attachment and growth of fouling organisms.

Unlike mobile species like sharks and dolphins that can actively remove attached fouling organisms, sedentary species such as sponges and corals lack this ability. However, despite their stationary nature, corals manage to maintain clean and tidy skin surfaces. Certain sponges and corals have intricate structures that create turbulent water flow, preventing fouling attachment. Coatings inspired by these macrotextures have been developed, creating surfaces with complex patterns and features that make it difficult for fouling organisms to attach. For example, previous research has demonstrated that corals employ a variety of antifouling mechanisms [91, 92]. These strategies include the release of natural antifoulants, the utilization of skin with low surface energy, the emission of fluorescence, the shedding of old skin layers, and the swinging tentacles. Last but not least, biomimetic surface coatings directly replicate the surface features and properties of specific marine organisms known for their fouling resistance.

4.2.1 Superhydrophobicity in nature

During the 1990s, researchers in the fields of biology and materials science initiated investigations into naturally occurring superhydrophobic surfaces [93]. Over the past few years, scientists have introduced superhydrophobic coatings characterized by remarkable water repellency for automotive, marine, and medical sectors [94, 95]. The emergence of superhydrophobic nanocoatings represents a pivotal approach to achieving surface superhydrophobization, leading to remarkable alterations in both surface and interfacial domains. Numerous techniques have been established to create superhydrophobic surfaces, encompassing approaches such as layer-by-layer assembly, sol-gel method, photolithography, electrodeposition, electrospinning, and 3D printing [96]. While multiple methodologies exist, those founded on simple chemical reactions hold particular appeal due to their straightforward procedures, economical nature, and potential for large-scale production.

On a superhydrophobic surface, the water droplet forms a nearly spherical shape with minimal contact area, and it tends to slide off the surface easily due to the combination of surface texture and low surface energy [97]. A superhydrophobic surface is characterized by a high water contact angle (WCA) exceeding 150° [98, 99]. Superhydrophobicity, or the ability to repel water and maintain extremely high contact angles with water droplets, is observed in various natural phenomena. Here are a few examples of superhydrophobicity in nature:

An illustrative instance of superhydrophobicity is the surface of a lotus leaf. Lotus leaves boast remarkable water repellency and self-cleaning capability due to the presence of hierarchical micro−/nanostructures and hydrophobic waxes on their surfaces, a phenomenon referred to as the “lotus effect.” Insect wings, such as those of Morpho butterflies, similarly demonstrate characteristics of the “lotus effect” [93]. The wing surfaces feature intricate micro- and nanostructures that scatter light, giving the wings their vibrant colors. These structures also create a hydrophobic surface, causing water droplets to bead up and roll off the wings. Additionally, water striders are insects that can walk on the water surface due to their hydrophobic legs. The legs are covered with tiny hairs that trap air and create a cushioning effect, preventing the insect from breaking the water’s surface tension. This adaptation allows water striders to move effortlessly on the water without sinking.

Selim et al. introduced a self-cleaning solution utilizing elastomeric PDMS and nanomagnetite composites [100]. The composite surface demonstrated superior antifouling efficacy compared to the PDMS, PDMS/nanomagnetite, and control composites, as observed through diatom and bacterial fouling assessments. Furthermore, Selim and his colleagues explored the effects of nanostructured surfaces and the distribution of GO decorated with TiO₂ nanorod (NR) fillers on the FR and superhydrophobic properties of silicone nanocoatings [101]. By integrating 1 wt% GO/TiO₂ hybrid fillers, they achieved enhancements in surface roughness and non-wettability while reducing surface-free energy. Consequently, they developed an improved model for superhydrophobic nanocomposite coatings, enabling sustained surface attributes suitable for prolonged marine applications. These examples from nature inspire the development of superhydrophobic coatings and materials in extensive applications, such as drag reduction, antifouling, self-cleaning surfaces, anti-icing coatings, oil/water separation, anticorrosion purposes, and waterproof textiles. By emulating the structures and properties found in these natural systems, scientists and engineers aim to create functional and efficient superhydrophobic materials for a range of practical applications [102, 103, 104, 105].

4.2.2 Characterization of the superhydrophobic surfaces

The fabricated superhydrophobic materials are subjected to various tests to evaluate their hydrophobic properties. Contact angle measurements are commonly used to quantify the degree of water repellency, with higher contact angles indicating greater hydrophobicity. Other tests may include nanoscale roughness, water droplet roll-off angles, coating thickness, durability assessments, mechanical stability, and resistance to contamination [106].

The water contact angle (WCA) measures the angle between a water droplet and the surface. A WCA greater than 150° indicates superhydrophobicity. Several factors, including surface roughness, surface energy, and the level of cleanliness, influence the contact angle [107, 108]. When a liquid effectively wets the surface (referred to as a wetting liquid or hydrophilic surface), the static contact angle assumes a value within the range of 0⁰ ≤ θ ≤ 90⁰ degrees. Conversely, when a liquid does not wet the surface (termed a hydrophobic surface), the contact angle falls within the range of 90⁰ < θ ≤ 180⁰ degrees (Figure 2).

Figure 2.

(a) Example of the contact angles created by sessile liquid drops on a uniformly flat solid surface; (b) a droplet’s non-wetting behavior on solid substrates, including Wenzel’s and Cassie-Baxter models.

In 1805, Thomas Young introduced the foundational concepts of contact angle and wettability [109]. Presently, the contact angle measurement technique stands as the most prevalent approach employed to assess the wettability of a surface [110, 111]. The static contact angle (θ) is defined as the angle formed at the point where the gas-liquid interface intersects the three-phase boundary, represented by a tangent line.

The Wenzel state manifests when the interactions between the liquid and the solid are high, resulting in the complete filling of surface voids by the liquid droplet [112]. Meanwhile, when the voids are occupied by air, the liquid droplet resides on both the solid surface and the air. This state is called the Cassie-Baxter state. In contrast to Wenzel’s model, the Cassie-Baxter model exhibits a slightly reduced contact angle hysteresis and an increased advancing contact angle. Consequently, the Cassie-Baxter model is more extensively employed across various surfaces [113, 114, 115].

4.3 Bioinspired antifouling nanocomposite coatings

Numerous plants and animals have evolved surfaces with exceptional antifouling performance in nature [85, 116, 117, 118]. Strong attempts have been made to create coatings that resemble these natural antifouling surfaces because they serve as inspiration. In recent years, six main bioinspired strategies have been used to tackle biofouling: (i) micro−/nano-rough coatings: Some creatures, including the lotus and shark [85, 118], have developed micro/nanostructured surfaces to resist biofouling. (ii) Natural antifouling agents: One effective method to stop biofouling is to employ bioactive substances that are emitted by particular organisms, like sponges and corals [119]. Hydrogels resemble mucus. In other cases, fish and amphibians (like frogs) use the epidermal mucus to counter biofouling [120]. (iii) Slippery liquid-infused porous surfaces (SLIPS): Microstructures can be found on the skin of earthworms and pitcher plants [121]. They differ from the lotus, though, in that liquid is present and trapped in the microstructures. These interfaces, known as SLIPS, are made to be nonstick and can therefore be employed to fend off fouling organisms [122]. (iv) Some marine creatures molt epithelium to refresh their surface [123], which aids in clearing their surfaces of fouling organisms. Dolphin skin has a rubber-like feel and can generate an unsteady surface when subjected to turbulent flow [124, 125]. Fouling organisms have a hard time settling on a surface that is so unstable. Because of the shedding effect and the unstable surface, the surface is dynamic in marine environments. (v) Zwitterionic coatings: Bilayers of lipids in living things contain phosphotidylcholine units, which have zwitterions made up of two organisms with opposing charges. Researchers investigated the antifouling potential of phosphatidylcholine’s analogues and specifically created zwitterionic polymers, as a result of their blood clotting-preventing capabilities [85, 126, 127].

These bioinspired techniques exhibit none of TBT’s harmful environmental effects and have excellent antifouling properties; therefore, they have great potential for use in the future. The main bioinspired tactics used to prevent biofouling are highlighted in this chapter, which also provides new and intriguing results from nature. These bioinspired methodologies are also discussed in terms of their cutting-edge materials, production methods, pertinent antifouling processes, current issues, and potential future developments. The advantages of nanomaterials include their high surface area-to-volume ratio and economic advantage. The mechanical robustness and surface non-wettability are improved thanks to nanofillers’ promotion of mechanical strength and roughness. Because nanofillers typically have inherent antifouling activity and because rough surfaces will entrap air more than smooth surfaces, they can enhance the antifouling characteristics of surfaces [128, 129].

To create these antifouling coatings, a variety of nanofillers, including Cu₂O [130], Ag [131], MnO₂ [132], and graphene oxide [133, 134] were added to polymeric surfaces. These coatings are affordable, effective, lasting, harmless, and friendly to the environment. It is becoming more common to create bioinspired antifouling coatings using nanocomposite methods for innovative and large-scale production. Natural antifoulants have been shown to have a number of antifouling mechanisms; however, there is insufficient information to definitively pinpoint the molecular pathways. Natural antifoulants’ mechanisms are still poorly understood [135]. There is still much to learn about how natural antifoulants work to prevent fouling.

4.4 Marine antifouling biomimetic surface microtopographies

Numerous maritime animals possess fouling-resistance qualities, which were affected by the surface’s texture roughness, wettability, surface energy, etc. [136, 137]. Since no biocides are released, there is less potential danger to marine ecosystems when duplicating the natural antifouling surface microstructures. So, one of the upcoming research trends is biomimetic microstructure coating, which has received a lot of attentions lately [138, 139]. Rough microstructures may have an antifouling mechanism that reduces the surface area where fouling organisms come into contact with the surface [140, 141]. Surface roughness has a stronger fouling-resistance [142, 143]. When the structure is an order of magnitude smaller than the fouling organism, the number of attached foulers grows but adhesion strength decreases [144]. Gorgonians, shark skin, echinoderms, and other biomimicking antifouling surfaces were examined by Scardino and de Nys [145]. Terrestrial wildlife also offers priceless models, so inspiration is not confined to marine critters.

A number of physical fabrication techniques, such as picosecond laser texturing, electron beam lithography, hot embossing, ion beam lithography, photolithography, and soft lithography, were evaluated in order to replicate the biological surface microtopographies [146, 147]. Soft mold replication is the most common and straightforward method for recreating biological microstructures on surfaces coated with synthetic materials. The elastomer sheet converts into a negative replica of the biological sample after curing and is removed from the mold. The negative replica is then secured to a plate and used as a mold. The specimen’s positive replica is obtained after curing and separation [148]. The negative replica is covered with the precursor and curing agent. On the surface of lotus leaves, waxes and microstructures have been discovered [149, 150, 151, 152]. A layer of air can be trapped between a drop of liquid and the surface of a leaf by the hydrophobic epicuticular waxes and microstructures (Cassie-Baxter state). Both characteristics produce surface self-cleaning on lotus leaves with low CAH and stop moisture from seeping into the surface [153, 154]. Every micropapillary on the surface of the lotus leaf was found to be covered in nanostructures [155]. The micro−/nano-hierarchical composite architectures allow lotus leaves to be extremely hydrophobic and low sticky [156, 157]. When used in marine vessels, the microstructures can therefore have a dual effect.

Numerous fabrication processes have been used to create micro−/nanostructured surfaces thanks to the development of cutting-edge technologies [158]. These techniques include additive manufacturing (3D printing) [159], mechanical micromachining [160], self-assembly [161], etching procedures [162], electrostatic approaches [163], nanocomposite approaches [144], deposition approaches [164], and soft lithography [164]. Although there may be certain disadvantages, these techniques have a bright future when used in actual manufacturing. A successful FR coating of PDMS/β-SiC nanocomposite with 153° WCA and micro-roughness was prepared (Figure 3) [165]. Additionally, a silicone/ZnO nanostructured composite with micro/nano-roughness and surface superhydrophobicity was developed (Figure 4) [166]. PDMS enhanced with TiO₂/SiO₂ nanocomposite was created [167] for photocatalytic and self-cleaning outcomes. AFM examination of the produced well-dispersed silicone/TiO₂@SiO₂ (0.5 wt.%) indicated surface roughness and displayed superhydrophobic antifouling performance.

Figure 3.

Hydrosilation-cured silicone/β-SiC nanostructured composite as antifouling and superhydrophobic surface. The TEM of β-SiC nanowires and the AFM of the FR-causing PDMS/β-SiC composite are also included. The created nanocomposite underwent a 3-month field trial in seawater, and the results showed excellent fouling-prevention [165]. Copyright 2018 was adopted with springer Nature’s consent.

Figure 4.

Illustration of the silicone/ZnO nanostructured composite film [166] (copyright 2019, after permission from Elsevier).

Improved superhydrophobicity and fouling-inhibition properties may also be a result of this nano−/micro-surface and low SFE [112, 168]. Two recently created nanocomposites of PDMS/RGO and PDMS/GO-γ-AlOOH coating were compared as superhydrophobic FR materials [169]. Compared to PDMS/RGO nanocomposite, the nanocoating of PDMS/GO-γ-AlOOH (3 wt.%) composite displayed better FR performance under various microfouling stressors. The well-dispersed silicone/GO-γ-AlOOH (3 wt.%) could achieve the lowest biodegradability of 1.6%. It has microbial endurability against gram-negative and gram-positive bacterial strains with 97.94%, and 86.42%, respectively.

Avicennia marina/silver was employed to reduce GO and obtain GOH@Ag hybrid as reported by Soleimani et al. [170] (Figure 5). In PDMS-based coatings, GO, GOH@Ag, and CNTs were all utilized as nanofillers. The GOH@Ag (0.5 wt.%) showed the greatest performance with SFE = 16 mN/m, root mean square roughness (RMS) = 103 nm, and WCA = 118.8°. The production of PDMS/GOH@Ag nanocomposite was achieved by the combination of Ag and A. marina to the graphene nanocomposite coatings.

Figure 5.

The PDMS/GOH@Ag (0.5 wt.%) nanofillers for antifouling and FR nanocoatings [170]. Copyright 2021, reproduced with permission from Elsevier.

A surface with a micro−/nano-roughness, low SFE (12.06 mN/m), high WCA (158° ± 2°), and high FR efficiency was created by PDMS/GO-Fe₃O₄ (1 wt.%) nanocomposite as reported by Selim et al. [171] (Figure 6). It showed the lowest biodegradability percentage when tested against Kocuria rhizophila (2.047%), Pseudomonas aeruginosa (1.961%), and Candida albicans (1.924%).

Figure 6.

Schematic illustration for the PDMS/GO-Fe₃O₄ (1 wt.% nanofillers) for superhydrophobic antifouling nanocomposite coatings [171]. Copyright 2023, reprinted with Elsevier’s permission.

5. Conclusions

The intriguing characteristics of naturally occurring superhydrophobic surfaces have inspired the creation of new superhydrophobic antifouling nanocoatings. Surface non-wettability and FR structure were significantly influenced by surface heterogeneity and rough topology. Graphene is a practical substance for producing superhydrophobic surfaces due to its distinct physicochemical features. Water-repellent antifouling marine coatings have gained much interest for their advantages in both the economy and the environment. Numerous characteristics of graphene nanocomposites, including antifouling, self-cleaning, and anti-corrosion, are available at a low price. Graphene nanostructured surfaces have outstanding antifouling and anticorrosion features. The potential of nanoscale filler tectonics was illustrated for the fabrication of structurally folded nanocomposite materials. Self-cleaning and antifouling graphene nanocomposites have been produced and are being used as a new trend in antifouling applications in marine navigation. A variety of hybrid graphene composite materials are being researched for maritime coatings due to their superior surface properties. Well-dispersed graphene-based nanocoatings can exhibit outstanding physico-mechanical and non-wetting properties. Benefits of these nanocomposites include coating behavior that prevents fouling, facility, economics, and toughness. The fabrication of PDMS/nanofiller composites can result in FR nanostructures with improved self-cleaning capabilities. These multifunctional FR nanocomposites will be commercially available in the near future, saving tens of billions of dollars in annual fouling costs.

Acknowledgments

This work was supported by the Open Fund of the Key Laboratory of Science and Technology for National Defence (China) and the Egyptian Petroleum Research Institute (EPRI), Cairo, Egypt. Also, it was supported by the financial support from the National Natural Science Foundation of China (Grant No: 52150410401) and the post-doctoral research fellowships of Dalian Institute of Chemical Physics, Chinese Academy of Sciences, CAS, Dalian, 116023, China.

Declaration of competing interest

The authors declared that they have no conflict of interest.