Sustainable Marine Coatings: Comparing the Costs, Benefits, and Impacts of Biocidal and Biocide-Free Paints

Abstract

Biofouling presents a major challenge for maritime industries, leading to the widespread use of copper-based biocidal coatings that, while effective, release harmful substances into marine environments. Biocide-free alternatives, such as silicone-based, hydrogel, and natural product-derived coatings, offer more sustainable solutions. This systematic review and meta-analysis assesses the functional, economic, and environmental performance of both coating types using PRISMA guidelines and literature from Scopus and ISI Web of Knowledge (2003–2025). Data from experimental, field, and modeling studies were synthesized, covering fouling intensity, coating durability, toxicity, cost-effectiveness, and regulatory compliance. Biocidal coatings generally performed better short-term, but biocide-free options showed comparable efficacy in some cases and clear environmental benefits. Although initial costs for biocide-free coatings are higher, they may yield savings over time. The meta-analysis found no significant differences in fouling or hydrodynamic performance, though quantitative evidence is limited. Research gaps remain, particularly in long-term studies, highlighting the need for standardized testing and lifecycle assessments to guide sustainable antifouling practices. The outcome of the review also showed that some evidence was excluded due to being in non-indexed sources. This highlights the importance of combining systematic and traditional review methods to ensure a more comprehensive assessment.

1. Introduction

Biofouling poses a persistent challenge in marine industries, contributing to increased hydrodynamic drag, higher fuel consumption, and elevated maintenance demands. Conventional biocidal antifouling coatings, such as those containing copper or copper pyrithione, have demonstrated effectiveness in controlling fouling organisms [1], yet they raise environmental concerns due to the leaching of toxic compounds into aquatic ecosystems [2]. To mitigate ecological risks, there is growing attention toward alternative antifouling strategies that maintain performance while reducing toxicity.

Biocide-free coatings, including silicone-based, hydrogel, and natural-extract-derived systems, represent a promising shift toward sustainable antifouling solutions. Studies have shown that zwitterionic cellulose nanocrystal coatings can achieve over 90% antifouling efficacy without toxic leachates [3], while PDMS-based systems incorporating marine plant extracts exhibit notable bioactivity against common fouling organisms [4]. These coatings not only minimize environmental impact but also align with regulatory pressures and market demand for eco-friendly maritime technologies.

Although numerous studies have evaluated antifouling coating performance, environmental risks, and economic aspects, the existing evidence remains fragmented across disciplines. For example, Lindholdt et al. [1] assessed long-term drag and biocide leaching, while Watermann et al. [2] examined chemical composition and ecological effects of non-toxic coatings. Experimental work by Ge et al. [3] and Oliva et al. [4] further explored innovative biocide-free materials, yet differences in design, context, and reporting limit cross-study comparisons. These gaps hinder decision-making regarding optimal antifouling strategies.

A systematic review is needed to synthesize existing knowledge, clarify trade-offs, and guide research and policy. Current literature lacks a multidisciplinary synthesis integrating marine biofouling control with ecological and economic outcomes. Such a review would enable structured comparison of coating types (e.g., copper-based vs. silicone-based), performance indicators (e.g., drag, fouling rates), and environmental impacts. Systematic reviews, when guided by PRISMA [5], enhance transparency, reproducibility, and evidence-based evaluation—key for advancing sustainable antifouling practices.

This review aims to systematically compare biocidal and biocide-free marine coatings by synthesizing data on performance, cost, and environmental impact. Biocidal coatings, such as copper pyrithione-based systems, have shown effective fouling control and hydrodynamic benefits [1] but are linked to metal release and ecological risks [2]. In contrast, biocide-free alternatives, including silicone-based and zwitterionic hybrid coatings, demonstrate variable antifouling efficacy and reduced toxicity [3,4], yet their long-term durability and cost-efficiency remain insufficiently compared across studies. The review will extract and analyze data on lifecycle costs, reapplication intervals, fouling prevention rates, and toxicity profiles. Aggregating such findings will support evidence-based guidance for stakeholders seeking effective and environmentally responsible antifouling technologies.

2. Materials and Methods

This study employed a systematic review approach to assess sustainable marine coatings by comparing the costs, benefits, and impacts of biocidal and biocide-free antifouling paints. Systematic reviews are structured, transparent, and reproducible methods for synthesizing existing research, aiming to minimize bias and provide comprehensive insights into a topic [5]. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) framework was used to ensure methodological rigor in literature selection, data extraction, and synthesis [5].

To ensure broad coverage of high-quality, peer-reviewed sources, we conducted searches in Scopus and the Web of Science (WoS), as these databases comprehensively index relevant scientific literature in marine science, material science, and environmental engineering. The search strategy was designed to capture studies examining both biocidal and biocide-free antifouling paints, considering their economic, environmental, and performance-related aspects. The search strings and respective queries for both Scopus and ISI Web of Knowledge, adapted to match each database’s syntax, are provided in Supplementary Materials.

The search was conducted on 24 February 2025, retrieving 93 documents from Scopus and 73 from ISI Web of Knowledge. Duplicate records were identified and removed. To ensure that only relevant studies were included in the systematic review, we conducted a two-step screening process at the title and abstract level before proceeding to full-text analysis. First, all retrieved studies were screened based on their titles, eliminating papers that were clearly irrelevant (e.g., studies on general ship coatings without antifouling focus, unrelated marine materials, or purely chemical synthesis research without application context).

Next, abstracts of the remaining studies were reviewed to determine whether they contained relevant information on biocidal or biocide-free antifouling paints, their performance, economic impact, environmental effects, or toxicity. Studies that lacked quantitative or qualitative insights into these aspects, such as those discussing antifouling principles without empirical data or solely focusing on paint formulation without application assessment, were excluded. Only studies that explicitly analyzed antifouling efficacy, economic feasibility, environmental sustainability, or durability were retained for full-text review.

The systematic review extracted both general and specific information from scientific publications to ensure a comprehensive comparison of biocidal and biocide-free antifouling paints (Supplementary Materials). Generic study details were collected, including the study title, authors, publication year, journal, study type (e.g., experimental, field study, modeling, meta-analysis), and funding source to assess potential biases. Additionally, the type of biocide-free coating (e.g., silicone-based, fluoropolymer) and the comparison group (e.g., biocidal paint, uncoated hull) were recorded to classify studies based on their research focus. Contextual details such as the study location, environmental conditions (temperature, salinity), ship type, and testing duration were also noted, particularly for field and experimental studies.

Key impact metrics were extracted in both qualitative and quantitative formats. Performance and environmental effects included antifouling efficacy (e.g., biofouling reduction over time), toxicity levels, and sustainability aspects (e.g., water quality impact, toxicity tests on marine organisms). Economic aspects such as lifecycle costs, maintenance needs, and fuel consumption impacts were also recorded. When quantitative data were available, specific measured indicators (e.g., percentage fouling reduction, cost savings per year) were extracted along with comparison groups, sample sizes (n), error values, and the type of error reported (e.g., standard deviation, variance, confidence interval). This ensured that effect sizes and statistical robustness could be evaluated, enabling a precise comparison of antifouling paint performance and sustainability across different studies.

For case studies reporting quantitative comparisons between biocidal and biocide-free coatings, standardized mean differences were calculated using Hedges’ g [6] to assess effect sizes. These analyses were stratified by broad study themes (e.g., efficacy, toxicity, economic performance) to account for disciplinary variation. All statistical analyses were performed in R using the metafor package [7].

3. Results and Discussion

The study applied a structured comparative framework to evaluate biocidal and biocide-free antifouling coatings across performance, economic, and environmental dimensions. A systematic review and meta-analysis were conducted using data from Scopus and ISI Web of Knowledge, ensuring comprehensive coverage. Included studies were categorized by coating type, study design (e.g., laboratory assays, field experiments, modeling), and measured indicators such as fouling intensity, toxicity, or cost metrics. Both qualitative descriptions and quantitative metrics were extracted to support an integrated analysis of trade-offs and effectiveness between coating types.

The dataset includes studies published between 2003 and 2025, reflecting two decades of research into antifouling coatings. Early work, such as Omae [8], focused on the transition from organotin to tin-free systems, while recent studies, including Ge et al. [3], explored advanced biocide-free technologies using nanomaterials.

Study locations span a wide geographical range, from tropical Brazil [9] to temperate estuaries in the UK [10] and fjords in Denmark [1]. Most studies describe testing under either marine or estuarine salinity conditions, with salinities ranging from ~1.2% in Roskilde Fjord to ~35 PSU in tropical bioassays. Temperature conditions were also varied, with several studies covering broad seasonal ranges or simulating temperate and tropical conditions.

Ship types, when applicable, include commercial vessels, naval ships, and leisure boats [8,11]. Laboratory bioassays and experimental field studies dominate the study types, while a subset of the literature consists of reviews or modeling studies. However, the very limited number of studies providing comparative quantitative data between biocidal and biocide-free coatings made it methodologically unfeasible to perform separate analyses by study type. Further subdivision of the already small dataset would have resulted in sample sizes too limited to support meaningful or representative conclusions. A wide variety of biocide-free paint types were reported, including silicone-based, fluoropolymers, hydrogels, and coatings with natural product extracts from marine algae [9,10]. These differences in setting, methodology, and material highlight the need for structured synthesis.

3.1. Comparing Biocidal vs. Biocide-Free Coatings

The literature highlights a notable gap in direct, quantitative comparisons between biocidal and biocide-free antifouling paints. While environmental and economic impacts of biocides are well documented, few studies offer statistically grounded evaluations of relative coating performance and longevity, complicating clear conclusions on their effectiveness under operational conditions. As sustainability becomes central to marine practices, biocide-free paints are increasingly seen as viable alternatives, but broader adoption requires targeted research on their long-term performance and practical viability.

Direct comparisons between traditional biocidal and biocide-free (or low-biocide) coatings show that the gap in fouling prevention has significantly narrowed. Several studies report that non-biocidal coatings can match copper-based paints. For example, silicone/fluoropolymer foul-release coatings performed comparably to copper paints in marine trials over several months [12]. Similarly, a low-copper paint with thymol matched the antifouling performance of a high-copper variant in barnacle coverage [13]. These findings suggest that well-formulated coatings with reduced biocide content can maintain equivalent antifouling efficacy.

However, differences remain in fouling type and maintenance. Biocidal paints maintain cleaner surfaces by continuously poisoning the hull, often resulting in species-poor communities. Biocide-free coatings may allow some slime or patchy growth, but this fouling is weakly attached and easily removed through routine cleaning. Environmentally, biocide-free solutions are favored. Biocidal paints reduce microbial diversity and promote biocide-tolerant species [14], while non-biocidal coatings support richer communities with non-harmful diatoms and bacteria forming slick biofilms. The absence of toxic leaching in non-biocidal coatings minimizes harm to surrounding ecosystems, unlike copper or organotin-based paints.

Still, “biocide-free” does not guarantee safety. Karlsson et al. [15] found that one such product was highly toxic, highlighting the need for critical evaluation. Overall, modern foul-release technologies can match biocidal paint performance in many cases [12,13]. When paired with periodic grooming, they offer effective fouling control without continuous toxin release. This progress supports the feasibility of transitioning to environmentally friendly antifouling solutions without sacrificing protection, benefiting both operational performance and ocean health. Although biocide-free coatings may allow initial settlement of weakly attached biofilms, several studies (e.g., [8,11]) report that these films are easily dislodged under hydrodynamic shear during vessel operation. This property, particularly observed in silicone-based foul-release systems, reduces the need for intensive manual cleaning and helps maintain hull performance with minimal maintenance, even over extended service intervals.

In order to summarize the scattered and unharmonized evidence from different papers and across multiple performance and impact dimensions, we semi-quantitatively assessed the comparative outcomes of biocidal and biocide-free coatings (

Table 1. Main types of antifouling coatings and their typical strengths and weaknesses, synthesized from the systematic review.

Coating Type and Examples	Typical Strengths	Typical Weaknesses
Traditional Biocidal Antifouling Paints, e.g., copper-based ablative paints (self-polishing co-polymers), zinc oxides	- Proven high efficacy against a broad range of fouling organisms (hard and soft fouling) by continuously releasing biocides. - Long history of use with well-understood performance; generally lower upfront cost than advanced new coatings. - Self-polishing types renew the active surface by slow erosion, maintaining antifouling effect and a smoother hull (reducing drag) over time.	- Environmental toxicity: leaches toxic biocides (copper, organotins, or booster biocides) into water, harming non-target marine life and accumulating in sediments. Under tightening regulations, many biocides are restricted or banned. - Coating layer wears out and requires regular reapplication (typically every 1–3 years), adding maintenance cost and downtime. - Only mitigates fouling to an extent—microfilm/slime still forms, especially during idle periods, so hulls can still accumulate slime and some fouling if vessel is stationary. Fouling that does occur may transport invasive species. - Optimal performance depends on usage: If the ship operates outside its intended profile (long lay-ups, low speed), heavy growth can still occur.
Silicone-Based and Fluoropolymer Foul-Release Coatings, e.g., PDMS silicone elastomers (Intersleek® 700, AkzoNobel, Amsterdam, Netherlands), fluorinated polymers (Intersleek 900, AkzoNobel, Amsterdam, Netherlands), other low-surface-energy non-toxic paints	- Non-toxic, biocide-free: do not poison marine life (environmentally benign). No harmful leaching, aligning with eco-friendly requirements. - Low surface energy and elasticity: organisms attach weakly; water flow can easily shear off fouling. A smooth, “slippery” hull surface means any growth is lightly held and detaches when the vessel is at speed. - This yields fuel savings by reducing drag—e.g., one study saw ~4–8% fuel consumption reduction in the first year with a non-toxic coating. - Commercially available alternatives to biocides; proven effective on fast, active vessels (e.g., naval ships, cruise liners) when operational profiles are suitable.	- Requires activity or cleaning: Optimal performance only at high speeds and sufficient water flow. During long idle periods or low speeds, slime and larvae can settle (no biocidal action to stop them), so fouling still grows if not periodically cleaned. - Fragile coating film: Silicone-based paints have poor adhesion to the hull and are soft, so they are susceptible to damage (scratching, peeling). They often need special primers and careful handling; mechanical abrasion (even aggressive cleaning) can ruin the coating. - High initial cost and application complexity—these coatings are expensive and usually require professional application in drydock (multi-layer systems). - Foul-release means organisms are not killed—they can be carried alive to new regions. Thus, if fouling is not removed promptly, there’s a higher risk of transporting invasive species with these coatings.
Hydrogel and Zwitterionic Polymer Coatings, e.g., hydrophilic hydrogel-based paints, PEG- or zwitterion-grafted surfaces, amphoteric polymer brushes	- Hydrophilic “slippery” surface: binds a layer of water, which resists protein and microbe adhesion—effectively reducing biofilm formation and initial fouling settlement. Zwitterionic polymers, for example, show excellent prevention of bacterial and algal biofilms by this mechanism. - No toxic biocides released; truly non-toxic approach (coating itself is benign), so minimal environmental impact. - Has shown strong fouling reduction in tests—preventing buildup of slime can also improve ship fuel efficiency. One study with a hydrogel-like coating noted a 4–8% drag reduction from fouling prevention. - Can be combined with foul-release matrices to get both effects. For instance, silicone–hydrogel hybrids or amphiphilic zwitterionic paints aim to retain durability while achieving ultra-low fouling adhesion.	- Durability issues: Many hydrophilic coatings (hydrogels, polymer brushes) have limited mechanical robustness—they can swell, soften or erode in seawater. Fast degradation or hydrolysis can undermine long-term performance. Balancing water-solubility (for antifouling function) with stability is challenging. - Adhesion and longevity challenges: Grafting or coating hydrophilic polymers onto hull surfaces is complex; coatings may delaminate or wear off. Some require specialized surface chemistry to attach, raising application cost and complexity. Nanoparticle additives can improve strength, but if they agglomerate, effectiveness drops. - Efficacy in real ocean conditions is not yet fully proven—many results come from lab or short field trials. Long-term field data are limited, so uncertainty remains about performance over multiple years and in varied waters. - Often high cost and scaling difficulties: the materials (e.g., PEG, zwitterionic monomers) and techniques involved are expensive. These coatings are mostly experimental or pilot-stage, not widely commercial.
Natural Compound-Based Coatings, e.g., paints with bio-based antifoulants like capsaicin, zosteric acid, seaweed extracts, enzyme coatings	- Bio-derived antifoulants: use natural chemicals (from plants, algae, marine organisms) instead of heavy metals. This reduces persistent pollution—they tend to biodegrade and do not bio-accumulate like copper. - Some natural compounds can specifically inhibit fouling processes (e.g., prevent larval attachment or spore germination) with low toxicity to non-target species. For example, coatings with certain algal extracts showed significant mussel attachment reduction without harming test organisms. - Lower environmental impact overall—by replacing copper or synthetic biocides, these coatings aim to meet environmental regulations and avoid broad-spectrum toxicity. Many natural agents are selective or work in synergy with the organism’s biology (often making them less harsh on ecosystems).	- Variable/Lower efficacy: Natural additives are often less potent or broad-spectrum than traditional biocides. A given extract may work well on some fouling species (or microfouling) but not others (e.g., “no effect on macrofouler larvae” was noted in some cases). Copper-free formulations with natural boosters sometimes still perform weaker than copper paint and may allow more growth. - Limited longevity: Bio-based compounds can degrade or leach out faster in water, meaning the antifouling effect might diminish within months. Frequent reapplication or higher loading might be needed to sustain performance. - Many proposals are still unproven at scale—effective largely in lab tests or short-term panels. Field validation is lacking for many natural-product coatings, and some extracts that showed promise in the lab failed to work in real seawater conditions. - Practical challenges: Sourcing and formulating natural compounds can be costly or difficult (some antifoulants come from rare or protected species, limiting availability). Also, “natural” does not guarantee safety—in high doses, these compounds can still affect non-target marine life, so regulatory approval can be an obstacle.
Hybrid and Amphiphilic Coatings (Combined-Strategy Coatings), e.g., amphiphilic polymers with both hydrophobic (fluorocarbon, silicone) and hydrophilic (PEG, zwitterion) segments; silicone/fluoropolymer hybrids; fouling-release coatings with tethered or nano-encapsulated biocides	- Multi-mechanism synergy: Hybrid coatings amalgamate different antifouling strategies to cover each other’s gaps. For instance, amphiphilic surfaces present both hydrophobic and hydrophilic domains, deterring a wide range of organisms—the hydrophobic parts make it hard for macrofoulers to stick, while hydrophilic parts resist microbes and slime adhesion. This broad-spectrum approach can outperform single-mechanism coatings in diverse fouling conditions. - Can achieve better overall performance than either component alone. Silicone–fluoropolymer hybrids, for example, combine silicone’s elasticity with fluoropolymer’s harder, lower-energy surface—resulting in a coating with lower surface energy (high water contact angle) and improved foul-release effectiveness compared to pure silicone. Similarly, adding a bit of a hydrophilic or biocidal component to a fouling-release matrix can reduce slime build-up without sacrificing low adhesion for larger organisms. - Often designed for improved durability or adhesion by blending materials. (For example, adding a polyurethane or epoxy element to a silicone can increase hardness and adhesion, addressing silicone’s mechanical weaknesses while retaining foul-release properties.) Some hybrids also incorporate reinforcement (nano-additives, fibers) to extend service life. - May allow lower toxin usage: Hybrid concepts include embedding minimal biocides or anti-microbials in an inert matrix—providing protection against specific fouling (e.g., algal spores) but with far less biocide leaching than a fully toxic paint. This controlled release or contact-kill approach can reduce environmental impact compared to traditional paints.	- Formulation complexity: Combining disparate materials can lead to compatibility issues—phase separation, poor bonding between components, or unpredictable degradation. Such coatings can suffer reduced long-term stability if the ingredients do not integrate well. Achieving a uniform, stable hybrid coating is scientifically challenging. - Higher production cost: Multi-component systems (fluorinated groups + silicone, or polymer blends with nano-additives, etc.) are often expensive to produce and apply. Extra processing steps and specialized ingredients raise costs, which can hinder widespread adoption. - If a hybrid includes any biocidal ingredient, it is not fully non-toxic—it may still leach some chemicals (albeit less), so it does not completely eliminate environmental concerns. For example, a “copper-free” hybrid that still contains a small amount of copper or biocide tethered in the coating is not truly biocide-free. Such coatings could face regulatory scrutiny similar to low-biocide paints. - Potential performance trade-offs: combining traits might dilute the “best” of each. An amphiphilic coating, for instance, might not achieve as ultra-low surface energy as a pure fluoropolymer or as high a hydration layer as a pure hydrogel, so it could still allow some fouling in challenging conditions. Optimization is needed for each hybrid, and some designs may end up being only marginal improvements at high complexity.
“Smart” Coatings (Self-Healing and Stimuli-Responsive), e.g., self-healing silicone-polyurethane that repairs scratches via dynamic bonds; light-responsive coatings that change wettability; enzyme or bacteria-triggered release coatings	- Self-repairing capability: Coatings with self-healing chemistries (e.g., reversible covalent bonds, microcapsules) can automatically fix minor damage. This preserves a continuous antifouling surface and prolongs the coating’s lifetime. For example, a PDMS-polyurethane elastomer with disulfide bonds achieved rapid self-healing (within minutes to hours) of scratches, maintaining its antifouling function. Such coatings can also double as anticorrosive layers by healing barrier defects. - Stimuli-responsive antifouling: Some smart coatings actively change properties in response to environmental triggers. A prototype silicone with photo-sensitive additives can reversibly switch surface stiffness and wettability under UV/visible light, enhancing its fouling release when needed. This kind of “on-demand” adaptation can help counter different fouling scenarios (e.g., become more hydrophilic to shed biofilm, then revert to hydrophobic). - Multi-functional and adaptive: Smart coatings often integrate multiple functions—e.g., foul-release + self-healing + anti-corrosion. These advanced materials aim to extend service intervals and maintain performance over time by either repairing damage or adjusting to conditions. They represent a cutting-edge approach aligned with the future trend of responsive, sensor-enabled hull coatings. - Early studies show that such designs can significantly improve both antifouling efficacy and durability. For instance, an amphiphilic self-healing silicone-based coating achieved strong adhesion to the substrate and resisted both bacterial biofilm and algal adhesion, while also being highly elastic and tough. This demonstrates the promise of marrying fouling resistance with mechanical resilience.	- High complexity and cost: Smart coatings rely on novel chemistries (e.g., special polymers, functional microcapsules, nano-sensors) that are currently expensive and complicated to manufacture. Scaling these lab concepts to practical, affordable marine paints is non-trivial. The added complexity also raises chances of something failing (e.g., an encapsulated agent not releasing at the right time). - Unproven long-term in real conditions: Most such coatings are at the R&D or prototype stage. Their performance over multiple years in saltwater, under abrasion, UV exposure, etc., is largely unknown. For example, one self-healing coating showed slower healing in seawater vs. air, and excessive amounts of healing agent led to reduced flexibility/adhesion of the coating. These sensitive trade-offs need real-world validation. - Potential trade-offs in material properties: Embedding self-healing or responsive components can affect the coating’s hardness, strength, or adhesion. There is often a balance between having dynamic, mobile components and maintaining a robust coating. Achieving a smart coating that is as tough as conventional paints is a challenge (though progress is being made with new chemistries). - Not yet commercially available: As of now, these are mostly experimental. Regulatory approval, cost-effectiveness, and ease of application remain to be addressed. Field trials are needed to ensure that the “smart” features deliver tangible antifouling benefits in practice and do not degrade quickly under operational stresses.
Photocatalytic Coatings (Light-Activated Antifouling), e.g., TiO2 or ZnO nanoparticle-infused paints, doped visible-light photocatalysts, graphitic carbon nitride additives; UV-illuminated surfaces	- Active biofilm inhibition: Photocatalytic coatings contain light-activated catalysts that generate reactive oxygen species (e.g., OH· radicals) under illumination. These radicals kill or inhibit microorganisms on the surface, preventing biofilm (slime) formation and early-stage fouling. In effect, the coating continuously self-disinfects when exposed to light. This approach can dramatically reduce bacterial fouling on illuminated surfaces. - Non-depleting, eco-friendly mechanism: Unlike biocides, the catalyst is not consumed in the reaction—it can function as long as light is available. There is no ongoing release of toxic chemicals into the water, making it an environmentally friendly strategy. Only fouling organisms and organics at the surface are targeted (via oxidation), with minimal persistent residues. - Can be combined with other coating features for synergy. For example, integrating photocatalytic particles into a superhydrophobic coating yielded a dual effect: the superhydrophobicity prevented many spores from attaching initially and the photocatalysis killed micro-organisms, together effectively preventing biofilm establishment. Such hybrid designs show significantly improved performance in tests.	- Needs light to work: A major limitation is the requirement for UV or high-intensity light. Ship hulls receive very little UV light underwater (especially on the bottom in deep or turbid water), so photocatalytic antifouling is mostly effective on sun-lit areas or stationary structures near the surface. In low-light conditions or at night, the coating provides no active antifouling effect. - Limited spectrum efficiency: Many photocatalysts (like standard TiO2) only activate under UV radiation. Visible-light-activated variants exist, but still typically need strong illumination to be effective. This dependence on environmental conditions makes consistent performance hard to ensure on a moving vessel. - Fouling can shield the light: If organisms do start to grow (e.g., during dark periods or in shaded spots), they can form a film that blocks light from reaching the catalyst, thus stopping the protective effect. The method is better at preventing initial settlement than removing established fouling. - Material and durability challenges: Incorporating photocatalytic nanoparticles into paint must be done carefully—agglomeration of particles can reduce effectiveness, and particles must be securely embedded to avoid leaching. Long-term exposure to UV can also degrade the binder resin of coatings if not formulated properly. To date, these coatings are largely experimental; scaling them up and ensuring they maintain antifouling action over years (despite uneven light exposure) are ongoing challenges.
Superhydrophobic Coatings (Micro-Textured Water-Repellent Surfaces), e.g., nanostructured silicone or fluoropolymer coatings with lotus-leaf effect; rough hydrophobic sprays creating >150° water contact angle; slippery lubricant-infused surfaces (SLIPS)	- Extreme water repellency = self-cleaning: Superhydrophobic surfaces are engineered to repel water so strongly (contact angles often 150–170°) that water beads up and rolls off, carrying away dirt and organisms. This prevents initial fouling adhesion—marine algal spores and bacteria have trouble attaching because the surface is effectively “dry” and any contaminants are swept off by rolling droplets or flow. In early-stage fouling trials, superhydrophobic coatings have shown drastically reduced biofilm accumulation. - Low adhesion strength: Even if some fouling does attach, the adhesion is extremely weak due to the tiny contact area. Organisms can be removed with minimal force (sometimes even by the motion of the ship or gentle wiping). The coating thus makes cleaning very easy and can maintain a smooth, low-drag state in between cleanings. - No biocides needed: This approach is purely physical—no toxic additives. It is inspired by natural surfaces (lotus leaves, shark skin) and can achieve antifouling without environmental harm from chemicals. Some superhydrophobic formulations also trap a layer of air when submerged, potentially reducing fluid friction (though this “air layer” may be temporary).	- Fragile microstructure: Superhydrophobic coatings rely on micro/nano-scale surface textures (and often special low-energy chemistries). These textures are easily damaged by mechanical wear, impact, or abrasion—a swipe with a brush or contact with a rough surface can destroy the surface topology and with it the water-repellent effect. Maintaining the microstructure in harsh marine service is difficult. - Contamination sensitivity: If the surface becomes contaminated by oils, sediments, or a biofilm matrix, it can lose its water repellency (water will wet the surface), at which point fouling can start attaching normally. In practice, marine oils or microbial secretions can negate the superhydrophobic effect, so these coatings may require frequent gentle cleaning to restore repellency. - Performance decay over time: Prolonged immersion tends to eventually penetrate or collapse the air-trapping surface features, especially under pressure at depth. Thus, the superhydrophobic effect may diminish over long deployments. Repeated wetting/drying cycles or exposure to rough conditions can further degrade performance. - Scaling and cost issues: Fabricating a robust superhydrophobic surface over an entire ship hull is not yet commercially feasible. The processes to create nano-textures can be complex and costly. Some approaches use coatings with particles or patterns, but ensuring uniformity and durability at large scale is challenging. As a result, these coatings are still mostly in the research or niche-prototype stage.
Hard Inert Coatings (Non-Toxic Hard Films), e.g., glass-flake reinforced epoxy, ceramic-epoxy blends, silicone-epoxy hybrids without biocide	- Highly durable protection: These coatings cure into a very hard, impact- and abrasion-resistant layer on the hull. They can withstand mechanical damage that would ruin softer coatings—even in ice-prone waters or with frequent in-water cleanings, they hold up well. Their toughness gives them a much longer service life (often 5–10 years) before reapplication, far exceeding typical biocidal paints. - Completely inert and non-toxic: Hard inert coatings contain no biocides; they do not release any harmful substances into the environment. This makes them environmentally safe during operation—the only environmental consideration is the biofouling that grows on them and how it is managed. - Tolerant of frequent cleaning: Since the coating itself does not polish away or dissolve, ships can be cleaned regularly (by divers or hull cleaning robots) without rapidly degrading the coating. In fact, the strategy with these coatings is to allow fouling to grow and then periodically clean it off. The coating’s hardness lets it survive repeated cleanings that would wear down conventional paints. - Simplified re-coating cycle: With no need to refresh biocide, these coatings do not rely on yearly re-painting. They serve more as a durable lining—which can be cost-effective long-term if an appropriate cleaning schedule is maintained instead of repainting. They are often used in niche applications like ice-breakers or vessels in sensitive waters where biocides are undesired, providing basic hull protection and fouling management via cleaning.	- No antifouling function unless cleaned: These coatings do not actively prevent biofouling—organisms will settle and thrive on the hull as if it were any unprotected surface. This can lead to very heavy fouling and severe drag penalties if not frequently removed. Essentially, the bonus is on maintenance: the coating just ensures the hull can handle that maintenance. - Requires committed cleaning schedule: Ships using hard inert coatings must be prepared for regular in-water cleaning (e.g., every few weeks or months depending on fouling rate) to avoid performance loss. This adds operational costs and logistics (dive teams or robotics), and cleaning in port may be restricted by local regulations (due to biosecurity or debris concerns). - If cleaning is neglected, the vessel will incur significant fuel penalties and risk spreading invasive species. The coating itself will not stop accumulation, so the ship’s efficiency can quickly deteriorate between cleanings. - Niche use and initial cost: These coatings are mostly favored for special cases (e.g., in very cold waters where biocides are less effective or where hulls see physical abuse). The application of glass-flake epoxies, for example, can be as costly and involved as other high-performance coatings. Without a fouling-release property, their benefit is purely in durability—making them unsuitable for many commercial ships unless a robust maintenance plan is in place. Some ports allow cleaning of these coatings without capture of debris since no biocides are released, but the biofouling still must be managed responsibly.

, Figure 1). The synthesis showed that biocidal paints generally excel in initial antifouling effectiveness, require less frequent cleaning, and often have lower upfront cost, making them attractive for short-term operational performance. However, these benefits come at the expense of environmental harm due to continuous biocide leaching and tightening regulatory constraints. In contrast, biocide-free coatings are far more environmentally benign, with minimal pollutant release, and can offer good durability. Their smoother surfaces may also lead to fuel savings over the long term, potentially offsetting higher initial costs and more frequent maintenance demands such as periodic cleaning of loosely attached slime. Notably, fouling on biocide-free coatings is typically soft and easily removed, whereas biocidal coatings maintain cleaner surfaces outright. This highlights that the optimal choice ultimately depends on the balance between environmental objectives, cost, and maintenance requirements.

3.2. Antifouling Performance

Modern antifouling coatings, including those with reduced or no traditional biocides, can match the fouling protection of conventional toxic paints. Field trials show that copper-free Econea-based paints perform as effectively as standard copper-based coatings on aquaculture nets [12]. Similarly, experimental formulations using natural biocides or encapsulated copper achieve comparable results to high-copper paints while significantly reducing metal content [13,16]. Many of these coatings offer broad-spectrum efficacy against both microfouling and macrofouling. For instance, protein-enhanced paint reduced barnacle cover by ~81% and nearly eliminated bryozoan growth compared to controls [13]. Silicone-based foul-release coatings, while allowing some initial settlement, facilitate easy removal due to weak adhesion, maintaining low drag and strong performance in both static and dynamic conditions [12]. Collectively, these studies confirm that well-formulated low-toxic coatings can reliably prevent biofouling, supporting a shift toward safer antifouling solutions without compromising effectiveness [12].

Biocide-free coatings also show promising results in reducing fouling biomass, a key measure of efficacy. In a recent field study, Elkady et al. [17] found that paints formulated with natural bioactives like gorgosterol and sarcophine significantly reduced fouling. The Coral paint series (1–4) achieved biomass levels of 0.014 g/cm², outperforming the commercial biocidal reference (Optima Antifouling, SeaCoat Technology, LLC, St. Petersburg, FL, USA), which ranged from 0.002 to 0.032 g/cm² depending on the formulation. These results position bio-based coatings as competitive, environmentally safer alternatives to traditional biocidal paints.

3.3. Longevity and Durability

Beyond initial efficacy, antifouling coatings must remain protective over extended service periods. Recent innovations improve longevity by controlling biocide release and enhancing material durability. For example, combining capsaicin and dichlofluanid produced a synergistic >99.9% bacterial reduction while slowing biocide leaching, prolonging activity and reducing reapplication needs [18]. Some biocide-free strategies also show long-term performance: a silicone-based paint with a tethered biocide maintained efficacy for 30 months with toxin release 10× lower than comparable coatings [19].

Durable binder technologies further extend coating lifespan. Hard, inert coatings such as epoxy with ceramic additives resist wear, remain effective for years, and shed >95% fewer microplastic particles than ablative paints [16]. Emerging technologies like self-replenishing or self-healing materials enhance longevity; one silicone-polyurethane coating repaired minor damage and sustained fouling resistance under prolonged marine exposure [19]. These advances suggest modern coatings can match or exceed the durability of traditional paints with minimal maintenance.

In addition to efficacy, these coatings show promising field durability, even if specific longevity metrics are not always detailed. Lower fouling rates imply reduced cleaning and maintenance, improving operational efficiency. Reduced biomass accumulation also likely improves hydrodynamics and may lower fuel consumption, although direct fuel savings remain to be quantified.

3.4. Adhesion and Surface Properties

Adhesion prevention in antifouling coatings is strongly influenced by surface properties such as surface energy, hydrophobicity, and amphiphilic design. Low-surface-energy materials, particularly silicone-based coatings like PDMS, reduce the strength of attachment for macrofoulers, enabling easier removal under mild hydrodynamic conditions [20]. Similarly, the incorporation of amphiphilic elements, such as in polyurethane coatings, enhances fouling-release performance by combining hydrophobic and hydrophilic segments to disrupt fouling adhesion mechanisms [14].

Surface modifications also contribute to sustained antifouling efficacy. For example, fluorinated and photocatalytic coatings exhibit both low surface energy and additional fouling-inhibitory effects via surface activation under light, as shown with carbon nitride-enhanced systems [21]. Thus, rather than relying on biocidal activity, controlling surface chemistry and topology provides an effective and environmentally responsible strategy for managing marine biofouling.

3.5. Controlled-Release and Hydrodynamic Strategies in Antifouling Coatings

Controlled-release strategies in antifouling coatings have gained prominence to enhance environmental safety and coating longevity. Innovations such as encapsulation and polymer matrix integration allow for modulated diffusion of biocides, reducing their immediate release into marine environments. For instance, the combination of capsaicin with dichlofluanid in a water-based acrylic resin system exhibited effective bioactivity, suggesting synergy between natural and synthetic agents under controlled-release conditions [18]. Similarly, studies on PDMS and polyurethane matrices revealed that structural design could govern biocide retention and minimize leaching [14], indicating that polymer chemistry plays a pivotal role in release dynamics.

Biocide retention through immobilization or diffusion control was also demonstrated in studies employing amphiphilic and silicone-based matrices. These coatings often exhibited negligible biocide leakage, attributed to their dense cross-linking or tailored surface chemistry [2]. In particular, the development of polythiourethane composites incorporating ZnO nanoparticles tethered to PDMS matrices showed promise for achieving both mechanical resilience and sustained antifouling activity [22]. Thus, sustainability and long-term efficacy can be achieved by shifting from passive release to engineered modulation.

Foul-release coatings (FRCs), primarily based on silicones or fluoropolymers, represent advanced marine antifouling technologies by reducing organism adhesion through surface energy manipulation rather than biocidal action. Studies indicate that such coatings can modify hydrodynamic properties by altering boundary layer characteristics and reducing skin friction [23,24]. For instance, Silva et al. [23] demonstrated that silicone-based coatings not only deter fouling but also exhibit favorable drag-reduction properties when applied to hull surfaces, contributing to improved fuel efficiency and decreased greenhouse gas emissions.

Further examination by Wendt [24] and Scardino and Fletcher [25] suggests that the smoothness and elasticity of FRCs help maintain laminar flow and reduce turbulence around submerged surfaces. These modifications in flow dynamics imply that FRCs could significantly influence the boundary layer structure, a factor with direct implications for vessel speed and energy consumption. Despite their efficacy, performance can vary with operational conditions, warranting continued research into long-term durability and hydrodynamic optimization in diverse marine environments.

3.6. Innovations and Emerging Materials

Antifouling research is flourishing with novel materials and strategies aiming to replace traditional toxins. One exciting avenue is biocide-free surface designs that physically resist fouling. For instance, amphiphilic coatings, which incorporate both hydrophobic and hydrophilic domains, have demonstrated enhanced resistance to algae and bacterial films by disrupting organism adhesion [19]. An experimental amphiphilic polyurethane reduced algal spore settlement by over 90% compared to standard silicone, while still allowing easy fouling release due to its low surface energy. Another strategy is non-leaching biocides, where biocidal molecules are covalently tethered or encapsulated in the coating. A silicone coating with immobilized quaternary ammonium silane achieved >80–90% biofilm reductions without measurable biocide release, thus attacking microbes on contact but keeping the environment safe [19]. Researchers are also borrowing from nature: a variety of natural product antifoulants have been identified from marine organisms and plants (e.g., macroalgal extracts, medetomidine, capsaicin). These compounds can inhibit larval settlement and algal growth effectively, often biodegrading after release. Synergistic formulations are another breakthrough, as combining two benign biocides can amplify efficacy. A recent dual-biocide coating (capsaicin + dichlofluanid) killed >99.9% of bacteria in lab tests and significantly prolonged antifouling activity by slowing biocide diffusion [18]. Advanced materials are enabling other creative solutions: photocatalytic coatings doped with semiconductors (like C₃N₄ or TiO₂) generate reactive species under light to destroy biofilms [18]. One such hybrid paint achieved ~98% bacterial kill under visible light, illustrating “on-demand” antifouling power. Stimuli-responsive polymers are being explored that change surface properties with light or temperature to deter fouling or shed attached growth. In addition, self-healing coatings have emerged, e.g., a silicone-polyurethane that can repair microcracks, ensuring the coating remains intact and antifouling over time [19]. Nanostructured additives (like ZnO or graphene derivatives) are used to impart micro-textures or catalytic activity that discourages organism settlement. The state-of-the-art in antifouling spans from bio-inspired chemistry to smart materials, all converging on the goal of foul-resistant, durable, and eco-friendly coatings. These emerging solutions show great promise in outcompeting traditional toxic paints on performance, while vastly reducing environmental footprint.

Growing ecological concerns over traditional biocides have intensified research into bioinspired, natural product-based antifouling strategies emphasizing chemical and physical mimicry. Seaweed-derived extracts demonstrate promising antifouling potential. For instance, Medeiros et al. [9] observed that extracts from species like Heterosiphonia gibbesii and Bryothamnion seaforthii significantly reduced mussel attachment in laboratory assays, indicating potent antifouling activity. Similarly, Hellio et al. [26] reported that marine algal extracts, particularly from Sargassum muticum, exhibited substantial inhibitory effects on macrofouling organisms, suggesting these natural compounds may serve as effective alternatives to synthetic biocides.

Complementing natural extracts, biomimetic strategies also offer viable antifouling approaches. As reviewed by Dafforn et al. [11], surface modifications inspired by marine organisms, such as the use of silicone and fluoropolymer coatings, reduce settlement by minimizing surface adhesion, without relying on toxic leachates. Omae [8] provided further support for the development of tin-free coatings, advocating for biodegradable and low-surface-energy materials as environmentally sound alternatives.

3.7. Toxicity and Environmental Impact

The environmental profile of antifouling paints remains a major concern. Older biocidal formulations, such as tributyltin (TBT) and high-copper paints, caused severe non-target effects, prompting the 2008 global TBT ban and increasing restrictions on copper use [11]. In contrast, non-biocidal foul-release coatings, such as silicones and fluoropolymers, leach fewer chemicals and exhibit lower toxicity to marine organisms [11]. Laboratory bioassays confirm that biocide-free coatings are generally non-toxic, while copper-based paints induce strong adverse effects [2].

Several studies further highlight the environmental and toxicological advantages of biocide-free antifouling technologies. Watermann et al. [2] found no biocide leaching (below detection limits, e.g., <0.005 mg/kg for organotins) and no toxicity to Mytilus edulis, based on short-term (48 h) tests, indicating minimal ecological risk. Similarly, Valente et al. [27] showed that hydrophobic deep eutectic solvents (HDES) derived from menthol and natural oils provided effective antifouling without environmental harm. Carteau et al. [19] introduced a biodegradable polymer-based coating with strong performance and low degradation impact, while Beyazkilic et al. [18] demonstrated that combining natural compounds like capsaicin with reduced conventional agents preserved efficacy and reduced toxicity. Together, these studies support the feasibility of innovative, low-impact formulations that align with life-cycle sustainability goals and reduce environmental burdens.

However, “biocide-free” does not guarantee safety. Hazardous leachates such as bisphenol A and nonylphenol have been detected in some coatings [2,10], and a silicone-based paint marketed as eco-friendly was more toxic to test species than certain copper paints [15]. Nonetheless, many new antifoulants are explicitly designed for environmental safety: natural product antifoulants degrade readily, and tethered biocides prevent leaching [19]. Experimental low-copper coatings have also achieved full protection while releasing ~90% less copper [13]. Collectively, these developments demonstrate that reducing antifouling toxicity is both necessary and achievable, with modern coatings significantly lowering environmental risks compared to traditional biocides.

3.8. Economic Considerations of Biocidal and Biocide-Free Antifouling Coatings

The economic implications of antifouling coatings span both initial application costs and long-term operational savings. Biocide-free coatings such as silicone-based or fluoropolymer alternatives generally involve higher upfront costs than traditional biocidal paints [8]. However, they offer extended service lifetimes, reduce reapplication frequency, and lower hull cleaning needs, significantly cutting maintenance expenses over time [11]. For example, field data indicate that biocide-free silicone foul-release systems cost approximately USD 200–250 per m² to apply, compared to USD 100–150 per m² for copper-based paints, but the extended service interval of up to 5 years offsets the initial investment. By minimizing biofouling and hydrodynamic drag, these coatings also enhance fuel efficiency, generating substantial cost savings across a vessel’s lifespan.

Biofouling increases frictional drag, leading to higher fuel consumption and operating costs. Even minor fouling can have measurable economic effects. For example, switching to a drag-reducing foul-release coating improved vessel speed by ~3.7% and reduced fuel use by ~11.7% compared to biocidal paint [1]. Translated into fuel costs, this corresponds to savings of approximately USD 150,000–300,000 annually for a Panamax-size vessel operating 250 days at sea, depending on bunker fuel prices. However, the study did not report contaminant emissions, leaving the extent of toxic releases uncertain; see Section 3.7 Toxicity and Environmental Impact for further discussion on the environmental effects of biocidal and biocide-free coatings. Similarly, eco-friendly coatings like amphiphilic polyurethanes [14] and HDES-based systems [27] provide effective fouling control while posing considerably lower environmental risks than conventional biocides.

In aquaculture, biofouling on nets and gear can account for 5–10% of production costs due to frequent cleaning and replacements [12]. Durable antifouling coatings reduce these burdens by extending maintenance intervals. A field trial showed that a novel coating kept fish nets clear for over six months, compared to frequent cleaning and swapping of uncoated nets [12], cutting labor, downtime, and equipment damage. Economic evaluation from the same trial estimated that reducing net changes from monthly to biannually saved approximately EUR 15,000–20,000 annually for a medium-sized aquaculture facility.

Regulatory and compliance factors also influence cost assessments. As biocidal paints face tightening restrictions, ship operators may incur compliance costs [11]. In contrast, biocide-free coatings avoid these burdens and may qualify for incentives under green shipping programs. For instance, vessels certified under green shipping schemes can obtain port fee reductions of 5–10%, representing annual savings of USD 50,000–100,000 for frequent callers at participating ports. Environmentally benign solutions thus offer favorable life-cycle economics when accounting for reduced fuel use, maintenance, and regulatory risks. While some next-generation coatings have higher initial costs, their overall cost-effectiveness benefits both shipping and aquaculture operations [1,12].

3.9. Regulatory Pressures and Policy Trends

Stricter environmental regulations have been a major driver of antifouling paint innovation. The 2008 global ban on tributyltin (TBT) due to its extreme toxicity marked a turning point, prompting the industry to move away from its most effective but harmful biocide [8,11]. Since then, increasing scrutiny of copper and synthetic booster biocides, especially in sensitive areas like marinas and aquaculture, has accelerated the shift toward safer alternatives. Elevated copper levels in sediments and harm to non-target species have led to calls for phasing out copper-based paints [11,28].

In response, researchers have developed non-toxic coatings and promoted gradual transition strategies. Sector-specific standards, such as those from the Aquaculture Stewardship Council, now limit antifouling toxins on farm nets [11], prompting innovations like a copper-free Econea coating that meets certification while effectively preventing net fouling [12]. Broader frameworks, including the EU Biocidal Products Regulation, further restrict which biocides can be used, reinforcing the trend toward benign solutions. Regulatory attention has also expanded to materials like PFAS in fluoropolymer and silicone coatings, though recent tests found no detectable PFAS leaching from biocide-free variants [11].

Evidence shows that strong regulatory action has reshaped industry practices and spurred the search for safer alternatives. Several studies advocate extending such frameworks to other persistent and toxic biocides [28]. However, regulation alone is not sufficient. Complementary policy tools, such as subsidies and green certifications, are needed to incentivize adoption and mitigate higher costs or performance uncertainties of innovative coatings [29]. As governments and international bodies increasingly favor coatings that balance efficacy with ecological safety, regulatory and market pressures together are fueling a shift toward environmentally benign antifouling technologies.

3.10. Meta-Analysis of Biocidal vs. Biocide-Free Coating Performance

A random-effects meta-analysis using Hedges’ g was conducted to quantify differences in performance between biocidal and biocide-free antifouling coatings. This method accounts for sampling variance across studies and allows pooled estimation of standardized effect sizes.

A total of six studies provided quantitative data, yielding 25 comparisons focused exclusively on two topics: fouling intensity and hull performance. The meta-analysis suggests that biocide-free paints tend to have slightly more fouling than biocidal paints on average (g = −0.86; 95% CI: −2.25 to 0.54), but with wide uncertainty crossing zero, indicating no statistically significant difference (p = 0.23). Similarly, differences in ship hydrodynamic performance (drag) between biocidal vs. biocide-free coatings are minimal and non-significant with very large uncertainty (g = −0.13; 95% CI: −11.15 to 10.89; p = 0.98). These findings indicate that although biocide-free paints may allow somewhat more fouling, their overall impact on ship performance appears comparable to biocidal alternatives (Figure 2).

3.11. Methodologies, Knowledge Gaps and Future Research

Recent studies highlight several innovations in antifouling technologies. A biocide-free coating showed 99% efficacy after 50 weeks of seawater immersion, relying on a corrosion-inhibitive, self-renewing matrix that reduces both corrosion and biofilm formation [30]. Smart coatings such as superhydrophobic, biomimetic, and nanocomposite systems disrupt fouling adhesion via engineered surface energies and are fabricated using sol-gel, electrospinning, or grafting methods [31]. Life-cycle assessment of another biocide-free coating estimated a 43.3-ton annual CO₂ reduction through improved fuel efficiency over a 25-year vessel lifespan [32]. However, challenges remain for xerogel and bionic coatings, especially regarding reproducibility and stability in dynamic marine conditions [33]. Polymer-based alternatives emphasize multi-stakeholder collaboration for durable, ecologically safe coatings, highlighting the need for coordinated efforts among scientists, industry, and regulators to overcome technical barriers and support market adoption [34]. Hybrid antifouling–fouling release strategies tailored to vessel profiles are recommended for broader applicability [35]. Routine grooming can extend the effectiveness of non-toxic coatings [36], while antifouling solutions for marine sensors call for optically compatible, fouling-degrading materials, including SLIPS technologies [37].

Despite progress, significant knowledge gaps and methodological inconsistencies persist in antifouling research. Variability in testing approaches, such as lab bioassays [9] versus field trials [1], hinders direct comparison across studies. Coating diversity, ranging from silicone-based systems [2] to hydrophobic eutectic solvents [27], further complicates analysis. This heterogeneity highlights the need for standardized testing protocols.

Because only a very small number of studies reported comparable quantitative data for biocidal and biocide-free coatings, there were not enough data points to perform statistically reliable subgroup analyses by moderators (e.g., study type or other relevant factors). Consequently, the meta-analysis is constrained to the small subset of studies that quantify both performance and environmental impact.

Dafforn et al. [11] identified critical knowledge gaps over a decade ago, many of which persist. For example, how well biocide-free coatings work over time is still uncertain, as their effectiveness can vary greatly between different types of marine conditions, such as water temperature, salinity, or fouling pressure. Few studies assess durability or economic implications over repeated applications [1], and consistent measurement of sub-lethal toxicity is lacking [27]. Comprehensive life-cycle assessments (LCAs) are also rare, limiting sustainability evaluations [19].

Researchers are addressing these gaps by refining test methods. Dynamic systems like rotating disks and flow channels better mimic vessel movement, revealing that fluid shear can reduce fouling compared to static panels [14]. Standardized toxicity assays have been developed to screen for hidden toxic effects undetectable by chemical analysis alone [2]. DNA studies of biofilms show that biocidal paints lower microbial diversity and change community composition, while biocide-free coatings support more diverse microbial communities [14]. These ecological shifts may influence fouling dynamics and broader food webs.

A major challenge remains the lack of standardized metrics and benchmarking protocols [25]. Differences in experimental design, substrates, and biological endpoints, even for similar coatings like silicones or polyurethanes, impede reproducibility and delay validation. Without harmonized evaluation criteria, promising technologies such as siloxane–polyurethane and xerogel coatings face adoption barriers. Reference standards are essential for reproducibility and commercialization.

Laboratory studies often miss real-world variability, as coating performance depends on factors like water chemistry, temperature, and local fouling pressure, yet many tests are short-term or limited to one location. Long-term, multi-site field trials are needed to predict operational lifespan reliably. Multi-factor impact assessments, e.g., combining data on toxicity, microplastic shedding, and behavioral effects on larvae, are still rare but necessary for holistic evaluation.

Future research should include standardized, long-term field tests in different marine environments [38], alongside economic assessments that include application, maintenance, and fuel savings. Hybrid approaches combining non-toxic coatings with physical methods like UV light or ultrasound offer promise without chemical release [39]. Standardized metrics for both efficacy and ecological safety will be essential for advancing sustainable antifouling technologies.

3.12. Limitations of the Database Search Scope

Although the systematic review using Scopus and ISI Web of Science (2003–2025) ensures transparency and reproducibility, it unavoidably leaves out some foundational and technically important studies. Several pre-2003 publications and non-indexed works contained key insights on biofouling prevention, economic trade-offs, and regulatory developments. For instance, historical analyses reported fuel penalties of 40–50% within six months due to fouling and near-total prevention by early TBT coatings over multi-year deployments [40,41]. Critical overviews of regulatory shifts and the rise of booster biocides were also omitted [42].

Economic models published before 2003 estimated substantial operational cost increases following TBT bans, with predictions of up to 30% higher fuel and maintenance costs if replacements were suboptimal [43,44]. Operational assessments of mitigation strategies such as in-water cleaning and dry-docking also appeared in grey literature sources not retrieved by the database search [45]. Performance evaluations outside peer-reviewed journals similarly went unaccounted. For example, silicone foul-release coatings were shown to reduce drag and fuel use by 10–15% versus TBT [46], and coating color alone influenced biofilm recruitment during early exposure phases [47].

Field studies showed that actual contamination levels were higher than laboratory predictions. Copper leaching was shown to reach 1.7–2.1 tons/year in certain marinas [48], while “tin-free” biocides such as diuron and Sea-Nine were detected in sediments at levels up to ~2600 ng/g, surpassing residual organotin concentrations [49]. Advanced laboratory methods also revealed that conventional tests may underestimate copper release rates [50].

Despite their value, most studies did not directly compare biocidal and biocide-free coatings. Thus, while outside the scope of the meta-analysis, their contribution remains contextual—enriching understanding of mechanisms, risks, and real-world outcomes without providing comparative efficacy estimates required for systematic review synthesis.

Grey literature (e.g., patents, technical reports) may contain valuable insights into proprietary formulations, long-term field performance, and industrial-scale testing. Although systematic reviews prioritize peer-reviewed evidence for transparency and repeatability, incorporating grey literature through structured approaches or expert elicitation could bridge evidence gaps and support more comprehensive evaluations.

In applied interdisciplinary fields such as antifouling technology, database limitations are well-recognized. They often miss older, regional, or practice-based literature critical for regulatory, ecological, and economic assessments. A complementary traditional review, using expert knowledge, reference mining, and inclusion of high-impact non-indexed sources, could address these blind spots and better integrate decades of practical experience. Combining systematic and narrative approaches would yield a fuller picture of antifouling coating performance across biological, economic, and environmental dimensions.

4. Conclusions

The study applied a structured comparative framework to assess biocidal and biocide-free antifouling coatings based on economic, environmental, and performance criteria. A systematic review and meta-analysis were conducted using data from Scopus and ISI Web of Knowledge. The included studies, published between 2003 and 2025, were categorized by coating type, study design (such as laboratory assays, field experiments, and modeling), and measured indicators including fouling intensity, toxicity, and cost.

Geographically, the studies spanned locations such as tropical Brazil, UK estuaries, and Danish fjords, with testing conducted in marine and estuarine salinity conditions and a range of temperatures. Coating performance was evaluated on various vessel types, including commercial, naval, and leisure ships. Study types predominantly included bioassays and field experiments, with some modeling and review studies. Biocide-free coatings reported included silicone-based, fluoropolymers, hydrogels, and those incorporating natural product extracts.

Biocide-free marine coatings show promising effectiveness in reducing fouling. Coatings using natural bioactive compounds like gorgosterol and sarcophine, particularly the Coral paint series, demonstrated significantly lower fouling biomass compared to a commercial biocidal coating. These results suggest that bio-based coatings can offer competitive biofouling resistance.

The economic impact of antifouling coatings involves a trade-off between initial application costs and long-term savings. Biocide-free coatings, such as silicone-based or fluoropolymer options, are more expensive to apply initially than traditional biocidal coatings. Studies highlight the favorable durability of biocide-free coatings under extended marine exposure, although precise longevity data are lacking. Moreover, these coatings are expected to reduce maintenance demands and enhance hydrodynamic performance, potentially lowering fuel consumption, though this effect has not been directly measured.

Regulatory considerations also play a key role: biocidal coatings face increasing restrictions, which may lead to compliance costs, while biocide-free alternatives avoid these issues and may benefit from environmental incentives. Importantly, biocide-free antifouling solutions can be more cost-effective over a vessel’s lifetime.

The lack of comparative quantitative data makes it challenging to draw solid conclusions about which type of paint performs better under specific conditions, highlighting a gap in current research that could benefit from more targeted, quantitative studies. It is expected that biocide-free paints have the potential to be used more widely in the future, especially as sustainability becomes a greater focus, but more research is needed to fully understand their long-term performance and viability.

While the systematic review ensured transparency, it excluded older and non-indexed sources that offer important context. These works provided insights on antifouling performance, regulatory trends, and operational impacts. Though not suitable for meta-analysis, they highlight key issues and potential solutions. Combining systematic and traditional reviews can offer a more comprehensive understanding of antifouling technologies.