Modulation of barnacle (Balanus amphitrite Darwin) cyprid settlement behavior by sulfobetaine and carboxybetaine methacrylate polymer coatings

Measurement of the Adhesion Force of a Living Sessile Organism on Antifouling Coating Surfaces Prepared with Polysulfobetaine-Grafted Particles

An antifouling polymer brush-like structure was fabricated by a simple and versatile dip-coating method of sulfobetaine containing copolymer-grafted silica nanoparticles (SiNPs) and alkyl diiodide cross-linkers. Surface-initiated atom transfer radical copolymerization of 3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate (MAPS) and N,N-dimethylaminoethyl methacrylate (DMAEMA) was carried out from initiator-immobilized SiNPs to give poly(MAPS-co-DMAEMA)-grafted SiNPs (MAPS/DMAEMA = 9/1, mol/mol) with diameters of 150-170 nm. The SiNP-g-copolymer/2,2,2-trifluoroethanol solution was dip-coated on silicon and glass substrates. Successive treatment with 1,4-diiodobutane in methanol gave a hydrophilic cross-linked coating film for the SiNP-g-copolymer. The cross-linked particle brushes did not peel off from the substrate even after washing with water in an ultrasonic cleaner despite the simple physical absorption of the SiNP-g-copolymer on the substrate surface. The adhesion force of the tentacle of a living barnacle cyprid on a glass surface covered with the cross-linked SiNP-g-copolymer was directly measured by scanning probe microscopy in seawater. The coating film exhibited extremely low adhesion to the cypris larva in the seawater, expecting this to be an effective antifouling property.

Evaluation of lipophosphoramidates-based amphiphilic compounds on the formation of biofilms of marine bacteria

The bacteriostatic and/or bactericidal properties of few phosphoramide-based amphiphilic compounds on human pathogenic bacteria were previously reported. In this study, the potential of two cationic (BSV36 and KLN47) and two zwitterionic (3 and 4) amphiphiles as inhibitors of marine bacterial growth and biofilm formation were investigated. Results showed that the four compounds have little impact on the growth of a panel of 18 selected marine bacteria at a concentration of 200 µM, and up to 700 µM for some bacterial strains. Interestingly, cationic lipid BSV36 and zwitterionic lipids 3 and 4 effectively disrupt biofilm formation of Paracoccus sp. 4M6 and Vibrio sp. D02 at 200 µM and to a lesser extent of seven other bacterial strains tested. Moreover, ecotoxicological assays on four species of microalgae highlighted that compounds 3 and 4 have little impact on microalgae growth with EC50 values of 51 µM for the more sensitive species and up to 200 µM for most of the others. Amphiphilic compounds, especially zwitterionic amphiphiles 3 and 4 seem to be promising candidates against biofilm formation by marine bacteria.

Zwitterionic Biomaterials

The term "zwitterionic polymers" refers to polymers that bear a pair of oppositely charged groups in their repeating units. When these oppositely charged groups are equally distributed at the molecular level, the molecules exhibit an overall neutral charge with a strong hydration effect via ionic solvation. The strong hydration effect constitutes the foundation of a series of exceptional properties of zwitterionic materials, including resistance to protein adsorption, lubrication at interfaces, promotion of protein stabilities, antifreezing in solutions, etc. As a result, zwitterionic materials have drawn great attention in biomedical and engineering applications in recent years. In this review, we give a comprehensive and panoramic overview of zwitterionic materials, covering the fundamentals of hydration and nonfouling behaviors, different types of zwitterionic surfaces and polymers, and their biomedical applications.

The movement and settlement behaviour of cyprids of Balanus reticulatus on the surfaces of the titanium alloys

The motion paths of Balanus reticulatus cyprids were similar on all the titanium alloys surfaces. On the parallel grinding surfaces, the temporary attachment duration and the settlement ratio of the cyprids were influenced by the roughness and the composition of the surfaces and correlated positively. The surface roughness could also change the contact area and the numbers of the attachment points of the cyprids in the similar pattern. Consequently, the roughness and the composition of the surfaces regulated the cyprid settlement by the temporary attachment duration. The cross grinding increased the temporary attachment duration but drastically decreased the settlement ratio to 0 compared to the parallel grinding, possibly due to the voids and the drastic decrease of the contact area and the numbers of the attachment points of the cyprids on the cross grinding surface, respectively. The cross grinding therefore significantly reduced the cyprid settlement compared to the parallel grinding.

Functional nanomaterials, synergisms, and biomimicry for environmentally benign marine antifouling technology

Marine biofouling remains one of the key challenges for maritime industries, both for seafaring and stationary structures. Currently used biocide-based approaches suffer from significant drawbacks, coming at a significant cost to the environment into which the biocides are released, whereas novel environmentally friendly approaches are often difficult to translate from lab bench to commercial scale. In this article, current biocide-based strategies and their adverse environmental effects are briefly outlined, showing significant gaps that could be addressed through advanced materials engineering. Current research towards the use of natural antifouling products and strategies based on physio-chemical properties is then reviewed, focusing on the recent progress and promising novel developments in the field of environmentally benign marine antifouling technologies based on advanced nanocomposites, synergistic effects and biomimetic approaches are discussed and their benefits and potential drawbacks are compared to existing techniques.

Amphiphilic Nitroxide-Bearing Siloxane-Based Block Copolymer Coatings for Enhanced Marine Fouling Release

The buildup of organic matter and organisms on surfaces exposed to marine environments, known as biofouling, is a disruptive and costly process affecting maritime operations. Previous research has identified some of the surface characteristics particularly suited to the creation of antifouling and fouling-release surfaces, but there remains room for improvement against both macrofouling and microfouling organisms. Characterization of their adhesives has shown that many rely on oxidative chemistries. In this work, we explore the incorporation of the stable radical 2,2,6,6-tetramethylpipiderin-1-oxyl (TEMPO) as a component in an amphiphilic block copolymer system to act as an inhibitor for marine cements, disrupting adhesion of macrofouling organisms. Using polystyrene-b-poly(dimethylsiloxane-r-vinylmethysiloxane) block copolymers, pendent vinyl groups were functionalized with TEMPO and poly(ethylene glycol) to construct an amphiphilic material with redox active character. The antifouling and fouling-release performance of these materials was investigated through settlement and removal assays of three model fouling organisms and correlated to surface structure and chemistry. Surfaces showed significant antifouling character and fouling-release performance was increased substantially toward barnacles by the incorporation of stable radicals, indicating their potential for marine antifouling applications.

Amphiphilic Zwitterionic Acrylate/Methacrylate Copolymers for Marine Fouling-Release Coatings

Methacrylate and acrylate monomers are popular building blocks for antifouling (AF) and fouling-release (FR) coatings to counteract marine biofouling. They are used in various combinations and often combined into amphiphilic materials. This study investigated the FR properties of amphiphilic ethylene glycol dicyclopentenyl ether acrylate (DCPEA) and the corresponding methacrylate (DCPEMA) blended with 5 wt % zwitterionic carboxybetaine acrylate (CBA) and the corresponding methacrylate (CBMA). A series of (co)polymers with different acrylate/methacrylate compositions were synthesized and tested against the attachment of the diatom Navicula perminuta and in short-term dynamic field exposure experiments. The more hydrophobic methacrylate DCPEMA homopolymer outperformed its acrylate counterpart DCPEA. Incorporated zwitterionic functionality of both CBMA and CBA imparted ultralow fouling capability in the amphiphilic polymers toward diatom attachment, whereas in the real ocean environment, only the employment of CBMA reduced marine biofouling. Moreover, it was observed that CBA-containing coatings showed different surface morphologies and roughnesses compared to the CBMA analogues. Particularly, a high impact was found when acrylic CBA was mixed with methacrylic DCPEMA. While the wettability of the coatings was comparable, investigated methacrylates in general exhibited superior fouling resistance compared to the acrylates.

Anti-biofouling properties of poly(dimethyl siloxane) with RAFT photopolymerized acrylate/methacrylate surface grafts against model marine organisms

Biofouling of man-made surfaces by marine organisms is a global problem with both financial and environmental consequences. However, the development of non-toxic anti-biofouling coatings is challenged by the diversity of fouling organisms. One possible solution leverages coatings composed of diverse chemical constituents. Reversible addition-fragmentation chain-transfer (RAFT) photopolymerization was used to modify poly(dimethylsiloxane) (PDMSe) surfaces with polymeric grafts composed of three successive combinations of acrylamide, acrylic acid, and hydroxyethyl methacrylate. RAFT limited conflicting variables and allowed for the effect of graft chemistry to be isolated. While all compositions enhanced the anti-biofouling performance compared with the PDMSe control, the ternary, amphiphilic copolymer was the most effective with 98% inhibition of the attachment of zoospores of the green alga Ulva linza, 94% removal of cells of the diatom Navicula incerta, and 62% removal of cells of the bacterium Cellulophaga lytica. However, none of the graft compositions tested were able to mitigate reattachment of adult barnacles, Amphibalanus amphitrite.

Controlled radical polymerization of hydrophilic and zwitterionic brush-like polymers from silk fibroin surfaces

Bombyx mori silk fibroin is a fibrous protein whose tunable properties and biocompatibility have resulted in its utility in a wide-variety of applications, including as drug delivery vehicles, wound dressings, and tissue engineering scaffolds. Control of protein and cell attachment is vital to the performance of biomaterials, but silk fibroin is mostly hydrophobic and interacts nonspecifically with cells and proteins. Silk functionalised with hydrophilic polymers reduces attachment, but the low number of reactive sites makes achieving a uniform conjugation a persistent challenge. This work presents a new approach to grow brush-like polymers from the surface of degradable silk films, where the films were enriched with hydroxyl groups, functionalised with an initiator, and finally reacted with acrylate monomers using atom transfer radical polymerisation. Two different routes to hydroxyl enrichment were investigated, one involving reaction with ethylene oxide (EO) and the other using a two-step photo-catalysed oxidation reaction. Both routes increased surface hydrophilicity, and hydrophilic monomers containing either uncharged (poly(ethylene glycol), PEG) pendant groups or zwitterionic pendant groups were polymerised from the surfaces. The initial processing of the films to induce beta sheet structures was found to impact the success of the polymerizations. Compared to the EO modified or unmodified silk surfaces, the oxidation reaction resulted in more polymer conjugation and the surfaces appear more uniform. Mesenchymal stem cell and protein attachment were the lowest on polymers grown from oxidised surfaces. PEG-containing brush-like polymers displayed lower protein attachment than surfaces conjugated with PEG using a previously reported "grafting to" method, but polymers containing zwitterionic side chains displayed both the lowest contact angles and the lowest cell and protein attachment. This finding may arise from the interactions of the zwitterionic pendant groups through their permanent dipoles and is an important finding because PEG is susceptible to oxidative damage that can reduce efficacy over time. These modified silk materials with lower cell and protein attachments are envisioned to find utility when enhanced diffusion around surfaces is required, such as in drug delivery implants.

Colloidal assembly of polydisperse particle blends during drying

In this work, we synthesize a polydisperse aqueous colloidal system composed of small and large zwitterionic particles, as well as medium sized standard acrylic particles. By assembling these dispersions into films by drying, we show using atomic force microscopy (AFM) how their top surfaces can be mostly covered by zwitterionic groups for a wide range of evaporation rates. We probe underneath the top film surface using Fourier-transform infrared (FTIR) spectroscopy - attenuated total reflection (ATR), observing that the content in zwitterionic particles of the film upper layer increases for faster evaporation rates. We show how polydisperse systems hold great potential to overcome the evaporation rate dependence of size segregation processes in drying colloidal blends, and we provide further insights into the assembly mechanisms involved. Polydisperse blends enhance the robustness of such processes for application in coatings and other soft products where evaporation rate can not be tuned.

Amphiphilic Dicyclopentenyl/Carboxybetaine-Containing Copolymers for Marine Fouling-Release Applications

Zwitterionic materials received great attention in recent studies due to their high antifouling potential, though their application in practical coatings is still challenging. Amphiphilic polymers have been proven to be an effective method to combat fouling in the marine environment. This study reports the incorporation of small amounts of zwitterionic carboxybetaine methacrylate (CBMA) into hydrophobic ethylene glycol dicyclopentenyl ether acrylate (DCPEA). A new set of copolymers with varying amphiphilicities was synthesized and coated on chemically modified glass substrates. The antifouling capabilities were assessed against the diatom Navicula perminuta and multiple species in the field. Unsurprisingly, high diatom densities were observed on the hydrophobic control coatings. The integration of small zwitterionic contents of only ∼5 wt % was already sufficient to rapidly form a hydrophilic interface that led to a strong reduction of fouling. Ultralow fouling was also observed for the pure zwitterionic coatings in laboratory experiments, but it failed when tested in the real ocean environment. We noticed that the ability to absorb large amounts of water and the diffuse nature of the interphase correlate with the adsorption of silt, which can mask the hydrophilic chemistries and facilitate the settlement of organisms. The amphiphilic coatings showed low fouling in dynamic short-term field exposures, which could be explained by the reduced tendency of the coatings for sediment adsorption.

Effects of crosslink density in zwitterionic hydrogel coatings on their antifouling performance and susceptibility to silt uptake

Hydrogel coatings effectively reduce the attachment of proteins and organisms in laboratory assays, in particular when made from zwitterionic monomers. In field experiments with multiple species and non-living material, such coatings suffer from adsorption of particulate matter. In this study, the zwitterionic monomer 3-[N-(2-methacryloyloxyethyl)-N,N-dimethylammonio] propanesulfonate (SPE) was copolymerized with increasing amounts of the photo-crosslinker benzophenon-4-yloxyethyl methacrylate (BPEMA) to systematically alter the density of crosslinks between the polymer chains. The effect of increasing crosslink density on the antifouling (AF) performance of the coatings was investigated in laboratory assays and fields tests. In both cases, the AF performance was improved by increasing the crosslinker content. The coatings reduced protein, diatom, and barnacle accumulation, and showed better resistance to biomass accumulation. The findings underline that the marine AF performance of hydrogel coatings does not only depend on the specific chemical structure of the polymers, but also on their physico-chemical properties such as rigidity and swelling.

Fabrication of low-fouling, high-loading polymeric surfaces through pH-controlled RAFT

Low-fouling and high-loading surfaces are increasingly important for biosensing and blood purification technologies. Selective and efficient target binding from complex media can be achieved with poly(carboxybetaine) (pCB) surfaces that consist of a dense brush layer to resist non-specific protein adsorption and a sparse “mushroom” upper layer for high-density capture agent immobilization (i.e. high-loading). We developed pH-controlled surface-reversible addition–fragmentation chain-transfer (S-RAFT) polymerization to simplify fabrication of multi-modal, low-fouling and high-loading pCB surfaces without the need for quenching or re-initiation steps, toxic transition metals or light irradiation. Multi-modal polymer layers were produced through partial polymer termination by temporarily raising the pH to aminolyse a fraction of dormant chain transfer agents (CTAs); remaining polymer chains with intact CTAs continued uninterrupted extension to create the “mushroom” upper layer. The multi-modal pCB surfaces were low-fouling towards proteins (<6.7 ng cm⁻²), and macrophages. Compared to mono-modal brush surfaces, multi-modal pCB surfaces were high-loading with 5-fold greater capture agent immobilization (e.g. antibody) and 4-fold greater target binding (e.g. biotin-fluorescein).

pH-Controlled surface-reversible addition–fragmentation chain-transfer (S-RAFT) polymerization yields a one-pot synthesis for bimodal polymeric surfaces for improved capture agent immobilization.

Anti-Biofouling Features of Eco-Friendly Oleamide-PDMS Copolymers

The biofouling of marine organisms on a surface induces serious economic damage. One of the conventional anti-biofouling strategies is the use of toxic chemicals. In this study, a new eco-friendly oleamide-PDMS copolymer (OPC) is proposed for sustainable anti-biofouling and effective drag reduction. The anti-biofouling characteristics of the OPC are investigated using algal spores and mussels. The proposed OPC is found to inhibit the adhesion of algal spores and mussels. The slippery features of the fabricated OPC surfaces are examined by direct measurement of pressure drops in channel flows. The proposed OPC surface would be utilized in various industrial applications including marine vehicles and biomedical devices.

Antifouling Properties of Dendritic Polyglycerols against Marine Macrofouling Organisms

Dendritic polyglycerols (PGs) were synthesized and postmodified by grafting of poly(ethylene glycol) (PEG) and polypropylene glycol (PPG) diglycidyl ether groups, and their antifouling and fouling-release properties were tested. Coating characterization by spectroscopic ellipsometry, contact angle goniometry, attenuated total internal reflection-Fourier transform infrared spectroscopy (ATR-FTIR), and atomic force microscopy showed brushlike morphologies with a high degree of microscale roughness and the ability to absorb large amounts of water within seconds. PGs with three different thicknesses were tested in laboratory assays against settlement of larvae of the barnacle Balanus improvisus and against the settlement and removal of zoospores of the alga Ulva linza. Very low coating thicknesses, e.g., 11 nm, reduced the settlement of barnacles, under static conditions, to 2% compared with 55% for an octadecyltrichlorosilane reference surface. In contrast, zoospores of U. linza settled readily but the vast majority were removed by exposure to a shear force of 52 Pa. Both PEG and PPG modification increased the antifouling properties of the PG films, providing a direct comparison of the ultralow fouling properties of all three polymers. Both, the modified and the nonmodified PGs are promising components for incorporation into amphiphilic fouling-resistant coatings.

Probing Surface Hydration and Molecular Structure of Zwitterionic and Polyacrylamide Hydrogels

A hydrogel is a hydrophilic cross-linked polymer network which can contain a large amount of water. Hydrogels with distinguished interfacial physical toughness were analyzed for their potential application as antifouling coating materials, utilizing sum frequency generation (SFG) spectroscopy as the interfacial analytical technique. The surface structures of one sulfobetaine (SBMA) zwitterionic hydrogel (ZWHG) and two polysaccharide hydrogels (PHGs) were probed in air; their interfacial structures with silica were examined using SFG in water and protein solutions, respectively. Both ZWHG and PHGs interfaces in water were dominated by strongly hydrogen-bonded water molecules, but the bonding strength associated with ZWHG was much stronger. Although all hydrogels experienced interfacial change in the presence of protein solutions, after cleaning, the zwitterionic hydrogel interface recovered almost completely while the other two hydrogels were subject to irreversible protein adsorption. Additionally, orientational analysis of ZWHG methyl groups in water was conducted and related to the superior hydrogen-bonding strength of water molecules at the ZWHG interface. The interfacial structures of hydrogel materials probed by SFG can be correlated to their antifouling properties. This research highlighted the critical role that hydrogen-bonding strength of interfacial water molecules play for antifouling applications.

Nonfreezable Water and Polymer Swelling Control the Marine Antifouling Performance of Polymers with Limited Hydrophilic Content

Zwitterionic chemical groups have well-documented resistance to marine fouling species when presented as homogeneous polymer brushes. These model formulations are not, however, suitable for practical fouling-control applications. It is presently unknown if a uniform film of zwitterions is required to elicit nonfouling character via the binding of interfacial water or if the incorporation of zwitterionic functionality into a more practical bulk polymer system will suffice. Here, copolymers of n-butyl methacrylate were synthesized with low incorporation levels (up to 20 mol %) of hydrophilic functionality, including zwitterionic moieties. Their antifouling (AF) properties were evaluated using barnacle cyprids (Balanus improvisus), diatom cells (Navicula incerta), and a multispecies biofilm. The laboratory assays revealed higher resistance of ionic copolymers toward cyprid settlement, which was attributed to their swelling and the presence of nonfreezable water molecules bound tightly to the polymer chains. Additionally, cells of N. incerta and the multispecies biofilm were removed more effectively on polymers containing sulfobetaine methacrylate and sulfopropyl methacrylate moieties. The results indicate that the presence of tightly bound interfacial water is not limited to model systems of pure hydrophilic homopolymers, but that this mechanism can also reduce the settlement and adhesion of fouling species via bulk copolymer systems with limited hydrophilic content. The swelling of polymers with hydrophilic content may also contribute to their AF efficacy, and such materials may therefore represent a route to translation of the well-documented nonfouling character of zwitterions into practical, industrially relevant coating formulations.

Brush Swelling and Attachment Strength of Barnacle Adhesion Protein on Zwitterionic Polymer Films as a Function of Macromolecular Structure

The exceptional hydration of sulfobetaine polymer brushes and their resistance toward nonspecific protein absorption allows for the construction of thin films with excellent antibiofouling properties. In this work, swollen sulfobetaine brushes, prepared by surface-initiated atom transfer radical polymerization of two monomers, differentiated by the nature of the polymerizable group, are studied and compared by a liquid-cell atomic force microscopy technique and spectroscopic ellipsometry. Colloidal AFM-based force spectroscopy is employed to estimate brush grafting density and characterize nanomechanical properties in salt water. When the ionic strength-induced swelling behaviors of the two systems are compared, the differences observed on the antipolyelectrolyte response can be correlated with the stiffness variation on brush compression, likely to be promoted by solvation differences. The higher solvation of amide groups is proposed to be responsible for the lower adhesion force of the barnacle cyprid's temporary adhesive proteins. The adhesion results provide further insights into the antibiofouling activity against barnacle cyprid settlement attributed to polysulfobetaine brushes.

Sediment challenge to promising ultra-low fouling hydrophilic surfaces in the marine environment

Hydrophilic coatings exhibit ultra-low fouling properties in numerous laboratory experiments. In stark contrast, the antifouling effect of such coatings in vitro failed when performing field tests in the marine environment. The fouling release performance of nonionic and zwitterionic hydrophilic polymers was substantially reduced compared to the controlled laboratory environment. Microscopy and spectroscopy revealed that a large proportion of the accumulated material in field tests contains inorganic compounds and diatomaceous soil. Diatoms adhered to the accumulated material on the coating, but not to the pristine polymer. Simulating field tests in the laboratory using sediment samples collected from the test sites showed that incorporated sand and diatomaceous earth impairs the fouling release characteristics of the coatings. When exposed to marine sediment from multiple locations, particulate matter accumulated on these coatings and served as attachment points for diatom adhesion and enhanced fouling. Future developments of hydrophilic coatings should consider accumulated sediment and its potential impact on the antifouling performance.

Dynamic surface antifouling: mechanism and systems

Marine biofouling is a global problem today. High efficiency and eco-friendly antifouling systems are in pressing need. In recent years, we have proposed the concept of dynamic surface antifouling (DSA). That is, a continuously changing surface can effectively prevent marine fouling organisms from landing and adhesion. Based on this strategy, we developed coatings with dynamic surfaces by using degradable polymers including polyester-polyurethane, modified polyester and poly(ester-co-acrylate). They exhibit tunable renewability, and excellent antifouling and mechanical performance. Moreover, the polymers can serve as carrier and controlled release systems of antifoulants so that they have long service life. This paper reviews the progress and trends in marine anti-biofouling, and presents the mechanism and systems of DSA.

Low-Fouling Thin Hydrogel Coatings Made of Photo-Cross-Linked Polyzwitterions

Although zwitterionic chemistries are among the most promising materials for producing nonfouling surfaces, their structural diversity has been low until now. Here, we compare the in vitro fouling behavior of a set of four systematically varied sulfa-/sulfobetaine-containing zwitterionic hydrogel coatings against a series of proteins and nonmotile as well as motile marine organisms as model foulers. The coatings are prepared by simultaneous photoinduced cross-linking and surface anchoring to elucidate the effect of the molecular structure of the zwitterionic moieties on their antifouling activity. Analogously prepared coatings of poly(butyl methacrylate) and poly(oligoethylene glycol methacrylate) serve as references. Photoreactive polymers are synthesized by the statistical copolymerization of sulfobetaine or sulfabetaine methacrylates and methacrylamides with a benzophenone derivative of 2-hydroxyethyl methacrylate and are applied as a thin film coating. While keeping the density of the zwitterionic and cross-linker groups constant, the molecular structure of the zwitterionic side chains is varied systematically, as is the arrangement of the ion pairs in the side chain by changing the classical linear geometry to a novel Y-shaped geometry. All of the polyzwitterions strongly reduce fouling compared to poly(butyl methacrylate). Overall, the sulfabetaine polyzwitterion coatings studied matches the high antifouling effectiveness of oligo(ethylene glycol)-based ones used as a control. Nevertheless, performances varied individually for a given pair of polymer and fouler. The case of the polysulfobetaines exemplifies that minor chemical changes in the polymer structure affect the antifouling performance markedly. Accordingly, the antifouling performance of such polymers cannot be correlated simply to the type of zwitterion used (which could be generally ranked as better performing or poorer performing) but is a result of the polymer's precise chemical structure. Our findings underline the need to enlarge the existing structural diversity of polyzwitterions for antifouling purposes to optimize the potential of their chemical structure.

Resistance of Zwitterionic Peptide Monolayers to Biofouling

Self-assembled monolayers (SAMs) are widely used in science and engineering, and recent progress has demonstrated the utility of zwitterionic peptides with alternating lysine (K) and glutamic acid (E) residues for antifouling purposes. Aiming at developing a peptide-based fouling-resistant SAM suitable for presentation of surface-attached pheromones for barnacle larvae, we have investigated five different peptide SAMs, where four are based on the EK motif, and the fifth was designed based on general principles for fouling resistance. The SAMs were formed by self-assembly onto gold substrates via cysteine residues on the peptides, and formation of SAMs was verified via ellipsometry, wettability, infrared reflection-absorption spectroscopy and cyclic voltammetry. Settlement of cypris larvae of the barnacle Balanus (=Amphibalanus) amphitrite, the target of pheromone studies, was tested. SAMs were also subjected to fouling assays using protein solutions, blood serum, and the bacterium Mycobacterium marinum. The results confirm the favorable antifouling properties of EK-containing peptides in most of the assays, although this did not apply to the barnacle larvae settlement test, where settlement was low on only one of the peptide SAMs. The one peptide that had antifouling properties for barnacles did not contain a pheromone motif, and would not be susceptible to degredation by common serine proteases. We conclude that the otherwise broadly effective antifouling properties of EK-containing peptide SAMs is not directly applicable to barnacles, and that great care must be exercised in the design of peptide-based SAMs for presentation of barnacle-specific ligands.

Correlative assays of barnacle cyprid behaviour for the laboratory evaluation of antifouling coatings: a study of surface energy components

Laboratory evaluation of antifouling coatings is underpinned by settlement studies with specific fouling organisms. Established methods provide insight into the likelihood of failure of a particular coating system, but can neglect the process of surface selection that often precedes attachment. The present approach for quantifying the exploratory behaviour of barnacle cypris larvae suggested that inspection behaviour can be a rapid and predictive proxy for settlement. Two series of xerogels with comparable total surface energy, but different dispersive and polar components, were evaluated. Settlement assays with three-day-old cyprids of Balanus improvisus demonstrated that while attachment was not linked directly to dispersive free energy, the composition of the xerogel was nevertheless significant. Behavioural analysis provided insight into the mechanism of surface rejection. In the case of a 50:50 PH/TEOS (phenyltriethoxysilane-based) xerogel vs a 50:50 TFP/TEOS (3,3,3-trifluoropropyltrimethoxysilane-based) xerogel, wide-searching behaviour was absent on the former.

Quantitative analysis of the complete larval settlement process confirms Crisp's model of surface selectivity by barnacles

For barnacle cypris larvae at the point of settlement, selection of an appropriate surface is critical. Since post-settlement relocation is usually impossible, barnacles have evolved finely tuned surface-sensing capabilities to identify suitable substrata, and a temporary adhesion system for extensive surface exploration. The pattern of exploratory behaviour appears complex and may last for several hours, imposing significant barriers to quantitative measurement. Here, we employ a novel tracking system that enables simultaneous analysis of the larval body movement of multiple individuals over their entire planktonic phase. For the first time, to our knowledge, we describe quantitatively the complete settlement process of cyprids as they explore and select surfaces for attachment. We confirm the 'classic' behaviours of wide searching, close searching and inspection that comprise a model originally proposed by Prof. Dennis Crisp FRS. Moreover, a short-term assay of cyprid body movement has identified inspection behaviour as the best indicator of propensity to settle, with more inspection-related movements occurring in conditions that also promote higher settlement. More than half a century after the model was first proposed by Crisp, there exists a precise method for quantifying cyprid settlement behaviour in wide-ranging investigations of barnacle ecology and applied studies of fouling management.

Mini-review: effect sizes and meta-analysis for antifouling research

It is widely recognised that findings from experimental studies should be replicated before their conclusions are accepted as definitive. In many research areas, synthesis of results from multiple studies is carried out via systematic review and meta-analysis. Some fields are also moving away from null hypothesis significance testing, which uses p values to identify 'significant' effects, towards an estimation approach concerned with effect sizes and confidence intervals. This review argues that these techniques are underused in biofouling and antifouling (AF) research and discusses potential benefits of their adoption. They enable comparison of test surfaces even when these are not tested simultaneously, and allow results from repeated tests on the same surfaces to be combined. They also enable the use of published data to explore effects of different variables on the functioning of AF surfaces. AF researchers should consider using these approaches and reporting results in ways that facilitate future research syntheses.

Exploring the Long-Term Hydrolytic Behavior of Zwitterionic Polymethacrylates and Polymethacrylamides

The hydrolytic stability of polymers to be used for coatings in aqueous environments, for example, to confer anti-fouling properties, is crucial. However, long-term exposure studies on such polymers are virtually missing. In this context, we synthesized a set of nine polymers that are typically used for low-fouling coatings, comprising the well-established poly(oligoethylene glycol methylether methacrylate), poly(3-(N-2-methacryloylethyl-N,N-dimethyl) ammoniopropanesulfonate) ("sulfobetaine methacrylate"), and poly(3-(N-3-methacryamidopropyl-N,N-dimethyl)ammoniopropanesulfonate) ("sulfobetaine methacrylamide") as well as a series of hitherto rarely studied polysulfabetaines, which had been suggested to be particularly hydrolysis-stable. Hydrolysis resistance upon extended storage in aqueous solution is followed by ¹H NMR at ambient temperature in various pH regimes. Whereas the monomers suffered slow (in PBS) to very fast hydrolysis (in 1 M NaOH), the polymers, including the polymethacrylates, proved to be highly stable. No degradation of the carboxyl ester or amide was observed after one year in PBS, 1 M HCl, or in sodium carbonate buffer of pH 10. This demonstrates their basic suitability for anti-fouling applications. Poly(sulfobetaine methacrylamide) proved even to be stable for one year in 1 M NaOH without any signs of degradation. The stability is ascribed to a steric shielding effect. The hemisulfate group in the polysulfabetaines, however, was found to be partially labile.

Synthesis and Characterization of Ultralow Fouling Poly(N-acryloyl-glycinamide) Brushes

The rational design of biomaterials with antifouling properties still remains a challenge, although this is important for many bench-to-bedside applications for biomedical implants, drug delivery carriers, and marine coatings. Herein, we synthesized and characterized poly(N-acryloylglycinamide) (polyNAGA) and then grafted poly(NAGA) onto Au substrate to form polymer brushes with well-controlled film stability, wettability, and thickness using surface-initiated atom transfer radical polymerization (SI-ATRP). The NAGA monomer integrates two hydrophilic amides on the side chain to enhance surface hydration, which is thought as a critical contributor to its antifouling property. The antifouling performances of poly(NAGA) brushes of different film thicknesses were then rigorously assessed and compared using protein adsorption assay from undiluted blood serum and plasma, cell-adhesive assay, and bacterial assay. The resulting poly(NAGA) brushes with a film thickness of 25-35 nm exhibited excellent in vitro antifouling ability to prevent unwanted protein adsorption (<0.3 ng/cm²) and bacterial and cell attachments up to 3 days. Molecular dynamics (MD) simulations further showed that two hydrophilic amide groups can interact with water molecules strongly to form a strong hydration layer via coordinated hydrogen bonds. This confirms a positive correlation between antifouling property and surface hydration. In line with a series of polyacrylamides and polyacrylates as antifouling materials synthesized in our lab, we propose that small structural changes in the pendent groups of polymers could largely improve the antifouling capacity, which may be used as a general design rule for developing next-generation antifouling materials.

Marine antifouling performance of polymer coatings incorporating zwitterions

Zwitterionic materials display antifouling promise, but their potential in marine anti-biofouling is still largely unexplored. This study evaluates the effectiveness of incorporating small quantities (0-20% on a molar basis) of zwitterions as sulfobetaine methacrylate (SBMA) or carboxybetaine methacrylate (CBMA) into lauryl methacrylate-based coatings whose relatively hydrophobic nature encourages adhesion of the diatom Navicula incerta, a common microfouling organism responsible for the formation of 'slime'. This approach allows potential enhancements in antifouling afforded by zwitterion incorporation to be easily quantified. The results suggest that the incorporation of CBMA does provide a relatively minor enhancement in fouling-release performance, in contrast to SBMA which does not display any enhancement. Studies with coatings incorporating mixtures of varying ratios of the cationic monomer [2-(methacryloyloxy)ethyl]trimethylammonium chloride and the anionic monomer (3-sulfopropyl)methacrylate, which offer a potentially lower cost approach to the incorporation of anionic and cationic charge, suggest these monomers impart little significant effect on biofouling.

Model Amphiphilic Block Copolymers with Tailored Molecular Weight and Composition in PDMS-Based Films to Limit Soft Biofouling

A set of controlled surface composition films was produced utilizing amphiphilic block copolymers dispersed in a cross-linked poly(dimethylsiloxane) network. These block copolymers contained oligo(ethylene glycol) (PEGMA) and fluoroalkyl (AF6) side chains in selected ratios and molecular weights to control surface chemistry including antifouling and fouling-release performance. Such properties were assessed by carrying out assays using two algae, the green macroalga Ulva linza (favors attachment to polar surfaces) and the unicellular diatom Navicula incerta (favors attachment to nonpolar surfaces). All films performed well against U. linza and exhibited high removal of attached sporelings (young plants) under an applied shear stress, with the lower molecular weight block copolymers being the best performing in the set. The composition ratios from 50:50 to 60:40 of the AF6/PEGMA side groups were shown to be more effective, with several films exhibiting spontaneous removal of the sporelings. The cells of N. incerta were also removed from several coating compositions. All films were characterized by surface techniques including captive bubble contact angle, atomic force microscopy, and near edge X-ray absorption fine structure spectroscopy to correlate surface chemistry and morphology with biological performance.

Automated tracking and classification of the settlement behaviour of barnacle cyprids

A focus on the development of nontoxic coatings to control marine biofouling has led to increasing interest in the settlement behaviour of fouling organisms. Barnacles pose a significant fouling challenge and accordingly the behaviour of their settlement-stage cypris larva (cyprid) has attracted much attention, yet remains poorly understood. Tracking technologies have been developed that quantify cyprid movement, but none have successfully automated data acquisition over the prolonged periods necessary to capture and identify the full repertoire of behaviours, from alighting on a surface to permanent attachment. Here we outline a new tracking system and a novel classification system for identifying and quantifying the exploratory behaviour of cyprids. The combined system enables, for the first time, tracking of multiple larvae, simultaneously, over long periods (hours), followed by automatic classification of typical cyprid behaviours into swimming, wide search, close search and inspection events. The system has been evaluated by comparing settlement behaviour in the light and dark (infrared illumination) and tracking one of a group of 25 cyprids from the water column to settlement over the course of 5 h. Having removed a significant technical barrier to progress in the field, it is anticipated that the system will accelerate our understanding of the process of surface selection and settlement by barnacles.

Modification of Silicone Elastomer Surfaces with Zwitterionic Polymers: Short-Term Fouling Resistance and Triggered Biofouling Release

We present a method for dual-mode-management of biofouling by modifying surface of silicone elastomers with zwitterionic polymeric grafts. Poly(sulfobetaine methacrylate) was grafted from poly(vinylmethylsiloxane) elastomer substrates using thiol-ene click chemistry and surface-initiated, controlled radical polymerization. These surfaces exhibited both fouling resistance and triggered fouling-release functionality. The zwitterionic polymers exhibited fouling resistance over short-term (∼hours) exposure to bacteria and barnacle cyprids. The biofilms that eventually accumulated over prolonged-exposure (∼days) were easily detached by applying mechanical strain to the elastomer substrate. Such dual-functional surfaces may be useful in developing environmentally and biologically friendly coatings for biofouling management on marine, industrial, and biomedical equipment because they can obviate the use of toxic compounds.

Classification of the pre-settlement behaviour of barnacle cyprids

Barnacle cyprids exhibit a complex swimming and exploratory behaviour on surfaces and settlement is a consequence of extensive surface probing and selection of suitable settlement sites. In this work, the behaviour of cyprids in their pre-settlement phase was studied by three-dimensional video stereoscopy. With this technique, three-dimensional trajectories were obtained that were quantitatively analysed. The velocity during vertical sinking of cyprids of Balanus amphitrite was used with a modified form of Stokes' law to calculate their mean body density. Furthermore, a classification of the swimming patterns allowed the extension of existing models describing cyprid locomotion and swimming behaviour. The patterns were characterized with respect to their occurrence, transition between patterns and their velocity distribution, and motions were identified that led to surface contacts. This analysis provides a classification framework, which can assist future attempts to identify behavioural responses of cyprids to specific settlement cues.

Probing nanoscale chemical segregation and surface properties of antifouling hybrid xerogel films

Over the past decade there has been significant development in hybrid polymer coatings exhibiting tunable surface morphology, surface charge, and chemical segregation-all believed to be key properties in antifouling (AF) coating performance. While a large body of research exists on these materials, there have yet to be studies on all the aforementioned properties in a colocalized manner with nanoscale spatial resolution. Here, we report colocalized atomic force microscopy, scanning Kelvin probe microscopy, and confocal Raman microscopy on a model AF xerogel film composed of 1:9:9 (mol:mol:mol) 3-aminopropyltriethoxysilane (APTES), n-octyltriethoxysilane (C8), and tetraethoxysilane (TEOS) formed on Al2O3. This AF film is found to consist of three regions that are chemically and physically unique in 2D and 3D across multiple length scales: (i) a 1.5 μm thick base layer derived from all three precursors; (ii) 2-4 μm diameter mesa-like features that are enriched in free amine (from APTES), depleted in the other species and that extend 150-400 nm above the base layer; and (iii) 1-2 μm diameter subsurface inclusions within the base layer that are enriched in hydrogen-bonded amine (from APTES) and depleted in the other species.

Polysulfone and polyacrylate-based zwitterionic coatings for the prevention and easy removal of marine biofouling

A series of polysulfone and polyacrylate-based zwitterionic coatings were prepared on epoxy-primed aluminum substrata and characterized for their antifouling (AF) and fouling-release (FR) properties towards marine bacteria, microalgae and barnacles. The zwitterionic polymer coatings provided minimal resistance against bacterial biofilm retention and microalgal cell attachment, but facilitated good removal of attached microbial biomass by exposure to water-jet apparatus generated hydrodynamic shearing forces. Increasing the ion content of the coatings improved the AF properties, but required a stronger adhesive bond to the epoxy-primed aluminum substratum to prevent coating swelling and dissolution. Grafted poly(sulfobetaine) (gpSBMA), the most promising zwitterionic coating identified from microfouling evaluations, enabled the removal of four out of five barnacles reattached to its surface without incurring damage to their baseplates. This significant result indicated that gpSBMA relied predominately on its surface chemistry for its FR properties since it was very thin (~1-2 µm) relative to commercial coating standards (>200 µm).

Barnacle cyprid motility and distribution in the water column as an indicator of the settlement-inhibiting potential of nontoxic antifouling chemistries

Testing of new coatings to control fouling frequently involves single-species laboratory bioassays. Barnacle cyprids are among the most widely used model organisms in marine biofouling research, and surfaces that inhibit their settlement are considered to be promising candidates for new coating concepts. An analysis of motility parameters (mean velocity and swimming area coefficient) and distribution of cyprids of Balanus amphitrite in different swimming regions in the vicinity of model surfaces (self-assembled monolayers) is presented. The data are correlated with the settlement preferences of cyprids on these surfaces. Cyprids were predominantly found in interfacial regions and the transition frequencies between swimming regions of different depths were determined.

Barnacle larvae exploring surfaces with variable hydrophilicity: influence of morphology and adhesion of "footprint" proteins by AFM

Interaction forces of adhesive proteins employed by cyprid larvae of Amphibalanus amphitrite for temporary attachment during surface exploration in marine fouling were studied by AFM force spectroscopy using chemically modified, reactive colloidal probes. The proteins were covalently attached to the surfaces of the probes by incubation in the protein deposits (footprints) left behind at the surface by the cyprids. This covalent coupling enabled robust and reproducible probing of adhesion of the attachment proteins to model surfaces with variable hydrophilicity. Three model monolayer surfaces were designed and prepared that exhibited different wettabilities derived from variations in the monolayer chemical composition. The morphology and size of cyprid protein deposits was imaged by AFM. The deposits showed larger area of spreading on more hydrophobic surfaces, whereas the overall volume of the secreted proteins exhibited no significant variation. Notable difference in adhesion forces was found among the surfaces by force spectroscopy, with substantially higher values measured on the hydrophobic surface (21 ± 2 nN) than that measured on the more hydrophilic surface (7.2 ± 1 nN). The same surfaces were also tested in laboratory essays. Rather surprisingly, no significant differences were found in values of fractional cyprid settlement among the surfaces studied, indicating that variations of surface wettability and adhesion strength of settlement proteins may be insufficient to explain settlement trends.

Sequence of Hydrophobic and Hydrophilic Residues in Amphiphilic Polymer Coatings Affects Surface Structure and Marine Antifouling/Fouling Release Properties

Amphiphilic polymers, specifically combinations of hydrophilic and hydrophobic residues, have been shown to be effective as antifouling materials against the algae Ulva linza and Navicula diatoms. Here we use the inherent sequence specificity of polypeptoids made by solid-phase synthesis to show that the sequence of hydrophilic (methoxy) and hydrophobic (fluorinated) moieties affects both antifouling and fouling release of U. linza. The platform used to test these sequences was a polystyrene-b-poly(ethylene oxide-co-allyl glycidyl ether) (PS-b-P(EO-co-AGE)) scaffold, where the polypeptoids are attached to the scaffold using thiol-ene click chemistry. The fluorinated moiety is very surface active and directs the surface composition of the polymer thin film. The position and number of fluorinated groups in the polypeptoid are shown to affect both the surface composition and antifouling properties of the film. Specifically, the position of the fluorinated units in the peptoid chain changes the surface chemistry and the antifouling behavior, while the number of fluorinated residues affects the fouling-release properties.