Poly(2‐hydroxyethyl methacrylate‐co‐ethylene dimethacrylate) as a mouse embryonic stem cells support

A series of swellable ethylene dimethacrylate‐crosslinked poly(2‐hydroxyethyl methacrylate) (PHEMA) sheets of homogeneous (nonporous) structure or with different degrees of swelling and porosities was produced by bulk polymerization in either the absence or the presence of various diluents (porogens). Calculations performed by use of the solubility parameter δ of the reaction components indicate that the solvation conditions of the polymerization system change, depending on the solvating power of the diluent, which thus controls the porosity. Pore volume also seemed to be sensitive to the presence of the linear polymer diluent. Polystyrene (PS) showed, compared with poly(methyl methacrylate) (PMMA), a higher precipitating ability to form porous PHEMA sheets with an increased pore size because of its higher noncompatibility with newly formed crosslinked PHEMA. Given that PHEMA hydrogel is well known for its biocompatibility, it was used here as a potential carrier of cells in transplantation therapies. Attachment and growth of mouse embryonic stem (ES) cells on gelatin‐coated transparent PHEMA hydrogel substrates were examined. Two days after plating, survival and morphology of ES cells were largely similar on both PHEMA hydrogel sheets and in petri dishes as controls. This suggests that PHEMA hydrogels are likely candidates for application in transplantation therapies involving ES cells. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 425–432, 2003     

Polymeric hydrogels and supercritical fluids: The mechanism of hydrogel foaming

A novel method, the hydrogel foaming, is used in this work for the production of porous, polymer-based materials by processing with supercritical carbon dioxide (CO₂). This method is applied to crystalline hydrophilic polymers that, practically, exhibit no phase transition (melting or glass transition) below thermal decomposition temperature and, due to their crystallinity, do not absorb CO₂. Such polymers are mainly natural (semi)-crystalline polymers (e.g. chitosan, cellulose, etc.) for which the classical polymer foaming method with supercritical carbon dioxide is not applicable. The hydrogel foaming process (similar to classical polymer foaming) is applied to gelatin, chitosan, and gelatin/chitosan blend hydrogels that are physically crosslinked and may also be chemically crosslinked with glutaraldehyde vapour. After the foaming process, water is removed from the gels by mild freeze-drying leading to porous materials. Pore size control can be achieved by controlling different process parameters. Gelatin exhibits solubility in water up to high concentrations and forms thermoreversible hydrogels, rendering it a suitable choice for the investigation of the process mechanism. The mechanism of hydrogel foaming is explored on the basis of X-ray diffraction, calorimetry, rheology, sorption, Raman spectroscopy measurements and theoretical calculations with the NRHB (Non Random Hydrogen Bonding) equation-of-state model. The sorption and Raman spectroscopy measurements suggest that, besides dissolution in water (of the hydrogel), extensive CO₂ sorption by the polymer also occurs. Based on these results, a critical discussion is made and a mechanism for the hydrogel foaming is proposed.     

Thermal phase transition of poly(N‐propionylethyleneimine) hydrogel

This article presents the preparation of the hydrogel of poly(N‐propionylethyleneimine) and its interpenetrating polymer network (IPN) hydrogel containing polyacrylamide by means of γ‐ray radiation and a study of the phase transition temperature of these hydrogels. As a result, the hydrogel of the crosslinked poly(N‐propionylethyleneimine) exhibited swelling below and shrinking above the phase transition temperature (about 61°C), as well as the lower critical solution temperature (LCST) of the liner polymer–water system. The experiment also showed that the LCST of the IPN hydrogel could be adjusted by the incorporation of the second component polyacrylamide. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2457–2461, 1999     

Fabrication and characterization of novel macroporous cellulose–alginate hydrogels

Novel macroporous hydrogels were prepared by blending of cellulose and sodium alginate (SA) solution, and then cross-linking with epichlorohydrin. The resulting cellulose/SA hydrogels were characterized by solid-state ¹³C NMR, wide-angle X-ray diffraction (WXRD), thermo-gravimetric analysis (TGA), scanning electron microscopy (SEM), rheological measurement, dynamic mechanical analysis (DMA) and swelling test to evaluate their structure, interior morphology, gelation time, compressive modulus, and equilibrium swelling ratio. Our findings revealed that the cellulose acted as backbone in the hydrogels, whereas SA contributed to the higher equilibrium swelling ratio. The combination of cellulose having semi-stiff chains and SA containing –COOH groups in the cross-linking hydrogel created the macroporous structure. This work provided a new pathway for preparation of hydrogel with large porous structure through incorporation of stiff polymer as support of pore wall and acidic polysaccharide as expander of pore size because of high water-absorbency.     

Easy and green synthesis of reduced graphite oxide-based hydrogels

We report an environmentally-friendly and easy to scale-up route to synthesize reduced graphite oxide (RGO) hydrogel by simple reduction of exfoliated graphite oxide (GO) with excess vitamin C (VC). Mono-layer graphene sheets self-assembling into a well-defined and interconnected 3D porous network through π–π interaction during gelation can be seen by scanning electron microscopy and atomic force microscopy images. The RGO hydrogels were further functionalized and the corresponding RGO/carbon nanotube or RGO/noble metal hybrid hydrogels were obtained after similar reduction or co-reduction when carbon nanotubes were added to and stabilized with GO sheets or when noble metal precursors were added and incorporated with GO sheets. Rheological performance and electrical conductivities of these RGO-based hydrogels were also investigated in this study. The residual VC retained in these hydrogels as a biofunctional component can be gradually released in a diffusion-controlled manner, which may endow these RGO-based hydrogels with a biofunctionality. Because encapsulated bioactive VC simultaneously occurs with the formation of these assemblies, the resulting RGO-based hydrogels may have great potential in use as transdermal systems for controlled delivery of VC, tissue engineering, biosensors, etc.     

Highly stretchable hydrogels for sensitive pressure sensor and programmable surface patterning by thermal bubble inkjet technology

Hydrogels with tough strength, programmable deformation are crucial for their practical applications. In this work, we reported the preparation and the programmable shape deformations of highly stretchable hydrogel. The graphene oxide/polyacrylamide/sodium alginate composite hydrogel was prepared, its microstructure and mechanical properties were studied. An aqueous calcium solution was selectively printed onto the hydrogel surfaces using an inkjet printer, resulting in programmable deformation of the composite hydrogel by creating regions of swelling/deswelling when subjected to external stimulations. Next, we fabricated a pressure-capacitance hydrogel sensor to demonstrate its application. Furthermore, the deformation rate and extent of the hydrogels can be controlled by adjusting the printing pattern position, number, length, and calcium solution concentration. Finally, several complex 2D and 3D shapes were fabricated by printing appropriate patterns on one or both surfaces of the hydrogel sheets.     

Coating water-swellable polymer latexes by interfacial polymerization

Two techniques of coating water-swellable polymer latexes (hydrogels) with an inorganic or organic layer were developed, based on interfacial polymerization. The hydrogel latexes were prepared by inverse suspension polymerization. The first kind of interfacial polymerization is hydrolysis and polycondensation of tetraethyl orthosilicate (TEOS) or 3-(trimethoxysilyl) propylmethacrylate (TMS-PMA) on the surface of hydrogel latexes. The hydrochloric acid, which is the catalyst for this reaction, was previously loaded into the hydrogel latexes. The HCL-containing hydrogel latexes were then suspended in an organic solution of TEOS or TMS-PMA. The hydrolysis and condensation occur only at the surface of each hydrogel particle and the generated SiO₂ or SiO(PMA) network covers the hydrogel latexes. The second interfacial polymerization is polymerization of methylenedi-p-phenyl diisocyanate (MDI) and triethylene glycol (TEG) on the surface of hydrogel latexes. TEG was deposited on the hydrogel by suspending the hydrogel powder in a tetrahydrofuran (THF) solution of TEG and evaporating the THF. The TEG-deposited hydrogel powder was then suspended in an organic solution containing both MDI and the catalyst dibutyltin dilaurate (DBTL). The MDI and DBTL molecules react with the TEG molecules only at the surface of the hydrogel particles, and the polyurethane P(MDI-TEG) thus formed wraps the particles. Based on these two coating procedures, three kinds of materials were obtained: SiO₂ coated hydrogels, SiO-(PMA) coated hydrogels and P(MDI-TEG) coated hydrogels. Soft polyether brushes were grafted to the SiO-(PAM) coated latexes. The P(MDI-TEG) coated hydrogels were used as reservoirs for β-hydroethyltheophylline (β-HETP), and the release of the latter molecules from the coated hydrogels was investigated. © 1995 John Wiley & Sons, Inc.     

A Novel Method of Endotoxins Removal from Chitosan Hydrogel as a Potential Bioink Component Obtained by CO2 Saturation

The article presents a new approach in the purification of chitosan (CS) hydrogel in order to remove a significant amount of endotoxins without changing its molecular weight and viscosity. Two variants of the method used to purify CS hydrogels from endotoxins were investigated using the PyroGene rFC Enzymatic Cascade assay kit. The effect of the CS purification method was assessed in terms of changes in the dynamic viscosity of its hydrogels, the molecular weight of the polymer, microbiological purity after refrigerated storage and cytotoxicity against L929 cells based on the ISO 10993-5:2009(E) standard. The proposed purification method 1 (M1) allows for the removal of significant amounts of endotoxins: 87.9–97.6% in relation to their initial concentration in the CS hydrogel without affecting the solution viscosity. Moreover, the final solutions were sterile and microbiologically stable during storage. The M1 purification method did not change the morphology of the L929 cells.     

Viscoelastic behaviour of self‐assembling polyurethane and poly(vinyl alcohol)

The rheological behaviour of polyurethane (PU) and poly(vinyl alcohol) (PVA) was investigated in aqueous solution and the hydrogel state. The dependence of viscosity on polymer concentration is discussed. The formation of supramolecular structures induced by temperature increase or shear conditions was evidenced. In PU solutions, as temperature increases, a self‐assembling process occurs due to hydrogen bonding and hydrophobic interactions determining a thermoreversible hydrogel formation. In creep and recovery tests, the weak PU network presents high elasticity only at low shear stress (below 10 Pa); it recovers only 15%–20% of strain above 40 Pa and the hydrogel structure fails at high shear stress (above 150 Pa). Also, PU hydrogel is not able to recover its structure after being submitted to successive low and high deformations. In PVA solutions, a shear induced aggregation was observed at 37 °C. PVA hydrogels obtained by the freezing–thawing method present high elasticity and stability due to the strong polymer–polymer interactions established between the polymer chains. Physical networks based on PU/PVA mixtures synergistically combine the characteristics of the two polymers, showing high elasticity when a shear stress up to 3000 Pa is applied during the creep test followed by a fast recovery of the hydrogel structure after exhibiting successive levels of deformation (self‐healing ability). Therefore, these hydrogels are suitable materials for tissue engineering applications. © 2019 Society of Chemical Industry     

京尼平交联大豆蛋白/壳聚糖复合凝胶的控释

利用天然无毒的京尼平交联大豆蛋白(SB)和壳聚糖(CS)制备复合水凝胶(HD)并用作茶碱的控释载体。同时对其在模拟胃肠液和pH7.4缓冲液（PBS）中的控释特性进行了研究。结合扫描电镜和红外光谱以及核磁共振表征了复合凝胶的表观形态和结构。结果表明,复合水凝胶中大豆蛋白和壳聚糖通过京尼平发生了明显的交联作用,并呈现致密的片层结构。复合凝胶在模拟胃肠液和pH7.4PBS中均呈现溶胀现象,在模拟胃液中的溶胀度较低。而且凝胶在pH1.2模拟胃液中的释放量比模拟肠液和pH7.4PBS液中的低,并发现该凝胶具有pH响应,在120 h内可实现对茶碱的可控释放。因此,这种京尼平交联的复合凝胶具有作为药物在胃肠道中定向运送载体的潜力。     

Pluronics as crosslinking agents for collagen: novel amphiphilic hydrogels

A series of Pluronic samples (L61, L121, F68, F108) were investigated as collagen crosslinking agents to determine their ability to improve the Young's modulus of a collagen hydrogel, while simultaneously serving as surfactants for single‐walled carbon nanotubes (SWNTs). The crosslinked collagen matrices were prepared by blending type I bovine collagen with either Pluronics or SWNTs dispersed in an aqueous Pluronic solution and crosslinked utilizing carbodiimide chemistry. The resulting material was a crosslinked collagen hydrogel with sufficient mechanical strength to be manipulated and transferred without damaging the matrix. Differential scanning calorimetry confirmed a change in the denaturation temperature for hydrogels prepared using Pluronic or Pluronic/SWNT solutions. Water uptake analysis confirmed the crosslinked matrices to be hydrogels. These collagen hydrogels produced with Pluronics as the crosslinking agents exhibited a Young's modulus 3 to 9 times greater than collagen hydrogels produced in the absence of any crosslinking agent, regardless of polymer molecular weight. However, non‐covalent incorporation of SWNTs was not found to affect the Young's modulus of the resulting collagen hydrogels at the incorporation levels achieved with the Pluronics surfactants. Copyright © 2010 Society of Chemical Industry     

Size adjustment of spherical temperature-sensitive hydrogel beads by liquid–liquid dispersion using a Kenics static mixer

Temperature-sensitive hydrogel beads were prepared by Kenics static mixer technology. The temperature-sensitive monomer N,N-diethylacrylamide and photo-crosslinkable pre-polymer ENT were used as model hydrogel materials. Drop dispersion of high viscosity polymer material in low viscosity hexadecane was made using the static mixer. Drops of a solution of the mixed materials were rapidly photo-crosslinked by UV irradiation after mixing in the static mixer, and spherical hydrogel beads with narrow, normal size distribution were thus prepared. The Kenics static mixer is a useful device for the preparation of spherical beads of temperature-sensitive hydrogels. The Sauter Mean Diameter of the hydrogel beads swollen in deionized water at 293 K was measured. The experimentally determined dimensionless swollen hydrogel bead diameter was well correlated with the Weber number, degree of swelling and viscosity ratio. The effects of gelation and ENT addition on the bead size were evaluated from the degree of swelling. The correlation equation can be used for size adjustment of temperature-sensitive spherical hydrogel beads.     

Dual-responsive shape memory hydrogels with self-healing and dual-responsive swelling behaviors

Shape memory hydrogels (SMHs) can fix the hydrogels in a provisional shape and restore the initial shape under external stimulation. Herein, a dual-responsive shape memory hydrogel with dual-responsive swelling and self-healing properties is presented in this work. The SMHs were fabricated by one-step emulsion copolymerization of acrylic acid (AAc), acrylamide (AAm) and stearyl methacrylate (SMA). Sodium alginate (SA) was introduced as an interpenetrating polymer in the network. With ionic cross-linking between -COO⁻ and Fe³⁺ or saline-reinforced hydrophobic association, the hydrogels can be fixed in a provisional shape, which can be restored by immersing the hydrogels in vitamin C solution or pure water, respectively. When the as-prepared hydrogels were immersed in FeCl₃ solutions, additional ionic cross-linking between Fe³⁺ and -COO⁻ could be formed, thus constructing the dual physically cross-linked (DPC) network, which endows the hydrogels with excellent fracture stress (2.6 MPa) and toughness (5.47 MJ/m³). Besides, the reversible physical cross-linkings endowed the hydrogel with outstanding self-healing capability. Furthermore, the pH and saline responsive swelling properties of the SMHs are additional fantastic properties. Therefore, we believe that this simple strategy provides a great opportunity for the preparation of SMHs with multiple intellectual performances.     

Synthesis and swelling behavior of thermosensitive IPN hydrogels based on sodium acrylate and N‐isopropyl acrylamide by a two‐step method

A series of interpenetrating polymer network (IPN) hydrogels having higher swelling ratio (SR) and thermosensitivity were synthesized from sodium acrylate (SA) and N‐isopropyl acrylamide (NIPAAm) by a two‐step method. A series of the porous poly(sodium acrylate ‐co‐1‐vinyl–2‐pyrrolidone) [poly(SA‐co‐VP)], (SV), hydrogels were prepared from acrylic acid having 90% degree of neutralization and VP monomer in the first step. The second step is to immerse the SV dried gels into the NIPAAm solution containing initiator, accelerator, and crosslinker to absorb NIPAAm solution and then polymerized to form the poly(SA‐co‐VP)/poly(NIPAAm) IPN hydrogels (SVN). The effect of the different molar ratios of SA/VP and the content of NIPAAm on the swelling behavior and physical properties of the SVN hydrogels was investigated. Results showed that the SVN hydrogels displayed an obviously thermoreversible behavior when the temperature turns across the critical gel transition temperature (CGTT) of poly(NIPAAm) hydrogel. The pore diameter distributions inside the hydrogel also indicated that the pore sizes inside the SVN hydrogels were smaller than those inside the SV hydrogels. At the same time, the more proportion of SA was added into the hydrogel, the larger pore diameter of the SV hydrogel was formed. The results also showed that the SR decreased with an increase of the VP content in the SV hydrogel and more obviously decreased in the SVN hydrogels. The SVN networks also showed stronger shear moduli than SV hydrogels. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Poly(vinyl alcohol) membranes in wound-dressing application: microstructure,physical properties,and drug release behavior

Poly(vinyl alcohol) (PVA) hydrogel membranes were prepared through three different preparation methods including freeze-thawing (FT), solution casting (SC) followed by thermal annealing, and phase separation (PS). The prepared hydrogels were characterized by Fourier transform-infrared spectroscopy, X-ray diffractometry, and scanning electron microscopy. Nitrofurazone (NFZ) was then loaded in the hydrogels. FT and SC methods led to obtaining dense membranes, while PS method resulted in an asymmetric one. The effects of hydrogel preparation method on water absorption, gel fraction, water vapor and oxygen permeabilities, bacterial barrier, tensile properties, and drug release profiles were investigated. The water vapor permeability of the hydrogel prepared through PS method was about 1.5 times higher than those obtained through FT and SC methods. Gel formation in PS method is probably responsible for the highest degree of crystallinity, and consequently the maximum gel fraction for the corresponded membrane. The elongation-at-break for this membrane in wet state was 41% higher than that made by FT method and 18% greater than that of SC method. Membranes prepared by all three methods showed excellent barrier property against bacterial penetration during 1 week. The results showed that PS membrane could control the release of NFZ more effectively as compared with the other two samples.     

An autonomous self‐healing hydrogel based on surfactant‐free hydrophobic association

Self‐healing hydrogels are attractive for a variety of applications including wound dressings and coatings. This paper describes the facile preparation and characterization of an autonomous self‐healing hydrogel system comprising surfactant‐free hydrophobic associations. The hydrogel comprised a copolymer of benzyl methacrylate (B), octadecyl methacrylate (O), and methacrylic acid (MA). The hydrogels were prepared via a controlled dehydration procedure to achieve the formation of strong intermolecular hydrophobic associations of the octadecyl groups above a critical polymer concentration. Fractured hydrogels healed within 30 min without any external intervention. Increasing hydrogel polymer content from 31 wt % to 39 wt % resulted in a threefold increase in the shear modulus and 50% reduction of the relaxation time. Addition of 4 mM NaCl to a hydrogel of 31 wt % polymer content resulted in 2.5 times longer relaxation time and 24% decrease in shear modulus. The hydrogels swelled up water by up to four times its weight, which corroborates the robustness of the hydrophobic association crosslinks. The bulk properties of the hydrogels are discussed in terms of the hydrophobic associations of the O‐groups and the electrostatic interaction of the MA‐groups in the polymer chains. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44800.     

Novel metronidazole-loaded hydrogel as a gastroretentive drug delivery system

A metronidazole-loaded hydrogel was synthesized by free radical polymerization using dimethylaminoethyl methacrylate (DMAEMA) monomer and triethyleneglycol dimethacrylate (TEGDA) and methylene bisacrylamide (MBA) as cross-linkers. The DMAEMA hydrogels were cross-linked with 5 and 10% MBA or with 0.1, 0.5, 1 and 4% TEGDA as cross-linking agents. Ammonium persulfate and tetramethyl ethylene diamine were used as initiator and catalyst, respectively. The prepared hydrogels were characterized, and the effect of cross-linking agent content on the swelling behavior and in vitro drug release of hydrogels was investigated. The results of X-ray diffractometry, differential scanning calorimetry and Fourier transform infrared spectroscopy studies indicated that the prepared hydrogels possessed an amorphous morphology and there was not any interaction between the hydrogel polymers and metronidazole as drug, which resulted in the dependence of drug release on the physicochemical characteristics of hydrogel such as swelling, polymer erosion, and surface morphology. According to the results, the hydrogel containing 0.5% TEGDA which was prepared by freeze-drying method exhibited a porous structure with a high swelling ratio and displayed a sustained and complete drug release. It could be concluded that the hydrogel developed by this facile method is a good candidate with a potential for use in gastroretentive drug delivery systems.     

Controlled release and absorption of cetylpyridinium chloride using polymer hydrogels

Interest in the biocide cetylpyridinium chloride (CPC) has resurged based on new studies showing its effectiveness against a wide variety of pathogens. Hydroxyethyl methacrylate (HEMA)‐based hydrogels have been developed for the controlled release of CPC. Initial burst release of the biocide can be controlled and sustained release can be achieved for more than two weeks. The burst and sustained release can be adjusted by varying the amount of anionic monomer (AMPSA), crosslink content (DEGDMA), release media, hydrogel surface area, and CPC loading. After removing the CPC‐loaded hydrogel from solution and drying, the release of CPC can also be reactivated. Very interesting swelling behavior was observed for CPC‐loaded hydrogels due to the electrostatic and hydrophobic interactions between the polymer hydrogel and CPC. In addition, HEMA‐based hydrogels can be used to recover or absorb CPC from aqueous solutions. By increasing the amount of AMPSA in the HEMA‐based hydrogel, more CPC can be absorbed from solution. The absorption is also enhanced by agitating the solution. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Self-healing quadruple shape memory hydrogels based on coordination,borate bonds and temperature with tunable mechanical properties

Quadruple shape memory hydrogels were prepared by one-pot in situ copolymerization using acrylamide, acrylic acid, agar, and poly(vinyl alcohol). The hydrogels have multiple reversible shape memory based on the coordination bonds of poly(acrylic acid) with Fe3+, borate bonds based on poly(vinyl alcohol), and hydrogen bonds of agar and poly(vinyl alcohol). The hydrogel demonstrated tunable mechanical properties when the hydrogels immersed in different solutions for various lengths of time. After immersion in the ferric chloride solution, tensile stress and elastic moduli of the hydrogels were enhanced with increasing soaking time. After immersion in the borax solution, tensile stress first increased and then decreased with increasing soaking time. Due to the reversible effect of the borate bond, the hydrogel achieved ultra-fast self-healing. The hydrogel after immersion in borax solution could begin healing in 24 h and healed at 44 h. The tensile stress and tensile strain of the self-healing hydrogel increased when soaking time increased from 48 to 96 h, and tensile stress at healing times of 96 h was nearly as the same as that of the original hydrogel when compared with it. The combination of tunable mechanical properties, efficient recoverability and self-healing abilities coupled with facile preparation endowed the developed hydrogel a high potential for use in biomedical applications.     

Antiswelling and robust supramolecular polyurea hydrogels induced by the synergy of hydrogen bond and hydrophobic interaction for constructing durable underwater anti-oil-fouling coatings

Hydrogels are attractive materials for constructing underwater antifouling coatings on solid substrates. However, the application of hydrogel coatings usually faces the obstacles of complex preparation process and poor durability. Herein, we present a facile method to prepare durable hydrogel coatings on metal foils based on rationally designed supramolecular polyurea (PU) hydrogels. PU hydrogels are designed to be cross-linked with hydrogen bonds (H-bonds) and hydrophobic interactions in the hard segment domains by using dihydrazides with different alkyl spacer lengths ( (CH₂)_m ) as chain extender. The synergy of H-bond and hydrophobic interaction can stabilize H-bonds in water, as confirmed by Raman spectroscopy. As a result, PU hydrogels exhibit antiswelling capacity and robustness in both deionized water and seawater. Subsequently, PU hydrogel coatings on Cu/Al foils are prepared by convenient brush coating and subsequent swelling. The resulting hydrogel coatings exhibit excellent underwater anti-oil-adhesion and self-cleaning property, and are durable enough to withstand various static and dynamic damaging tests. The good durability of PU hydrogel coatings should be ascribed to the robust adhesion interface and excellent antiswelling capacity of PU hydrogels. The combination of facile preparation and good durability makes PU hydrogel coatings promising candidates for reliable underwater antifouling.