Designing environmentally benign modified silica resin coatings with biomimetic textures for antibiofouling

Abstract

Siloxane modified acrylic resin coatings with positive and negative replication textures were successfully fabricated by biomimicking the surface structures of natural lotus leaf and white crab shell via a replication method. The physical and chemical properties of the as-prepared coatings were systematically characterized by FT-IR spectroscopy, SEM, AFM and contact angle measurements. Moreover, the antifouling (AF) property of the biomimetic textured surfaces was tested via the settlement assay with two microalgae of different sizes. The results indicated that the micromastoids and microdimples of the lotus leaf and white crab shell could significantly inhibit the settlement of microalgae. Both biomimetic textured coatings with positive and negative lotus leaf morphology can reduce 73% attachment of Closterium and 74% attachment of Navicula. Both biomimetic textured coatings with positive and negative white crab shell morphology could reduce over 65% attachment of Closterium and Navicula. Different antifouling mechanisms of the biomimetic textured coatings were analyzed based on three key factors, including surface wettability, morphology, and algae size.

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Introduction

Marine biofouling, such as the settlement of sessile microorganisms including animals and plants on artificial surfaces subjected to an aquatic environment, has become a widespread problem in the maritime industry for both military and commercial vessels and industrial equipments.^1–4 It is well known that marine biofouling has an enormous harmful impact such as increased fuel consumption, decreased vessel speed, elevated dry-dock cleaning expenses, loss of hull strength and biocorrosion. Usually, the marine biofouling can be reduced markedly or resisted by the use of traditional toxic antifouling coatings.^5,6 However, biofouling control using toxic antifouling coatings will result in significant adverse environmental effects. Regulations passed at the Antifouling Systems Convention by the International Maritime Organization (IMO) in October 2001 banned the presence of tributyltin (TBT) paints on ships from 2008.^7,8 In addition, the use of biocidal paints such as copper paints is becoming more regulated and limits on copper release have been established.⁹ Thus, it is immensely desirable to develop highly efficient, broad-spectrum, environmentally friendly antifouling coatings.

Though living in the ocean, the skin of many marine organisms, such as the shark, whale and dolphin, do not display any biofouling at all. Multiple antifouling strategies against marine organisms may consist of the combinations of physicochemical, mechanical, and topographic mechanisms that have significant impacts on the interactions between the marine organisms and the surface. Inspired by these interesting phenomena, people have begun to take advantage of structural coatings for anti-biofouling. A number of antifouling (AF)/fouling release (FR) materials free of biocides have been investigated in recent years, especially those that influence AF/FR surfaces with controlled micro-/nano-textures. Biomimicking surfaces have been successfully fabricated by replicating the natural microtextures of gorgonian echinoderms, marine mammal skin, and Sharklet skin; moreover, these biomimetic surfaces exhibited excellent fouling resistance.^10–12 Schumacher and Carman et al.^13,14 fabricated the microtopographies, which contained hexagonally packed 2 μm diameter circular pillars with a 2 μm spacing; 2 μm sized ridges separated by 2 μm wide channels; and 10 μm equilateral triangles combined with 2 μm diameter circular pillars, and the Sharklet AF™, which consists of 2 μm ribs with the lengths of 4, 8, 12, and 16 μm. The results indicated that all the topographies significantly reduced spore settlement compared to the smooth surface. Uniform surfaces of either 2 μm diameter circular pillars or 2 μm wide ridges reduced the pore settlement by 36% or 31%, respectively. A novel multi-feature topography consisting of 2 μm diameter circular pillars and 10 μm equilateral triangles reduced spore settlement by 58%. The largest reduction in spore settlement, 77%, was obtained with the Sharklet AF™ topography. The Sharklet AF™ is not the only bio-inspired surface that was investigated for its antifouling properties. Karen Wooley's research team,^15–20 from Washington University, focused on the replication of dolphin skin. They discovered that dolphin skin presented an unstable fluff layer, which was distinct from the shark skin pattern. The textured coatings made by biomimicking the dolphin skin proved to have the ability to inhibit the adhesion of fouling organisms. Moreover, this study broke the traditional view that rough surfaces do not have antifouling properties. Baum et al.²¹ examined the skin of the pilot whale and discovered that the skin displayed an average nanorough surface characterized by a pattern of nanoridge-enclosed pores; the average pore size (approximately 0.20 μm²) was below the size of most marine biofouling organisms. Furthermore, they suggested a new model for the short-term self-cleaning abilities of dolphins based on the nanoroughness of their skin surface. Larger biofoulers settling in the laminar boundary layer were excluded from the surface by the texture and could be flushed away by a turbulent water flow or removed by the detachment forces of an air–liquid interface during breathing intervals or when the dolphin jumps. Smaller adhesive molecules were excluded from the surface by the properties of the gel. Although plenty of research on microtextured coatings fabricated by biomimicking marine organism surfaces, such as sharks, dolphins and whales, have been carried out, there is a new question worth thinking about. Sharks, dolphins and whales move pretty quickly, so that fouling organisms can be washed out under the action of water flow. Moreover, mucus secreted from their skin surface can inhibit the adhesion of fouling organisms.²¹ However, little work has been focused on biomimicking replication textures of the static or low-speed organisms such as the lotus leaf and white crab.

It should be pointed out that the chemical composition of the coating plays an important role in terms of AF/FR. Pure organic silicon polymers, such as poly(dimethylsiloxane) (PDMS), have been proven to display excellent AF/FR performance under suitable hydrodynamic conditions and low surface energy, and they are considered as non-toxic and environmentally friendly marine coatings.^22,23 However, silicon polymers have some drawbacks, such as low mechanical strength, poor adhesion strength with substrates and a high curing temperature. The siloxane modified acrylic resin shows good mechanical properties, such as good weather resistance, oxidation resistance and sintering, and have AF/FR performance to a certain extent.^24–26

In this study, siloxane modified acrylic resin coatings with positive and negative biomimetic textures replicated from the lotus leaf and white crab shell were developed, which combined the contributions of both the micro/nano-structures and the low surface energy property. The effect of microstructures on the antifouling properties with two types of common microalgae, Closterium and Navicula, was investigated. The surface topography and wettability of the replicas were also measured by relevant instrumentation. The anti-fouling mechanisms of textured coatings were analyzed based on three key factors, including surface wettability, morphology, and algae size. In this study, we aimed to develop a new type of coating with biomimetic textures replicated from lotus leaf and white crab shell, which possesses excellent antifouling performance.

Materials and methods

Materials

SYLGARD® 184 silicone elastomer (Dow Corning Corporation), a transparent PDMS (polydimethylsiloxane), was used as the masterplate material for topographical modification due to its good elastic, non-toxic and reproducible properties. The elastomer was prepared by hand mixing (5 min) ten parts of the resin and one part of the curing agent by weight. The mixture was degassed under a vacuum drying oven for 30 min and allowed to cure for 12 h at 70 °C.

Hydroxyl-containing silicone modified acrylic resin (SMAR) (SKD002, Shanghai Zhaoyu Technology Limited Company, China) was employed to produce the micro/nano-scale textured surfaces, including the lotus leaf and white crab shell. The target coatings were fabricated by mixing three parts of SMAR, two parts of curing agent HDI (hexamethylene diisocyanate trimer, Bayer Corporation, Germany) and one part of dimethylbenzene by weight. First, the mixture was degassed under a vacuum drying oven at ambient temperature for 30 min and then cured at 70 °C for 12 h.

Biological originals, including the lotus leaf and white crab shell, were used in this study for fabricating biomimetic surfaces. Two types of common microalgae, Closterium (Chlorophyta) and Navicula (Bacillatiophyta), were acquired from East China Sea. The length of Closterium and Navicula was 45–55 μm and 10–12 μm, respectively, and both their widths were 3–4 μm (Fig. 1). Growth conditions for these two types of algae were 12 : 12 h light : dark cycles at 25 °C. Both were supplemented with Guillard's F/2 medium. Artificial seawater is a mixture of NaCl (25 g L⁻¹), MgCl₂ (3.5 g L⁻¹), MgSO₄ (2.5 g L⁻¹), KCl (0.7 g L⁻¹), K₂HPO₄ (0.1 g L⁻¹), NaHCO₃ (0.1 g L⁻¹), NaNO₃ (0.3 g L⁻¹), H₃BO₃ (2.68 mg L⁻¹), EDTA (0.8 mg L⁻¹), MnCl₂·4H₂O (2.5 mg L⁻¹), ZnSO₄·7H₂O (0.22 mg L⁻¹), CuSO₄·5H₂O (0.074 mg L⁻¹), FeSO₄·7H₂O (0.19 mg L⁻¹) and CaCl₂ (0.3 g L⁻¹).

Fig. 1 Optical images of Closterium (a) and Navicula (b).

Preparation of biomimetic coatings with the positive and negative replicas of lotus leaf and white crab shell

The schematic procedure of biomimicking the positive and negative structures of lotus leaf and white crab shell on the SMAR via a PDMS template is shown in Fig. 2. The fresh lotus leaf or white crab shell was used as the initial natural template. First, PDMS was mixed with a curing agent (10 : 1 by weight) and then degassed under a vacuum drying oven at ambient temperature for 30 min to remove bubbles. Then, the mixture was poured over the lotus leaf or white crab shell and heated at 70 °C for 12 h in a vacuum chamber. The PDMS template with negative textures was obtained and used to produce a positive SMAR replica. Similarly, the SMAR mixture was degassed under vacuum in a drying oven at ambient temperature for 30 min to remove bubbles. Then, the mixture was coated on the PDMS template with a negative texture and heated at 70 °C for 12 h. Finally, the textured SMAR coatings were peeled off directly from the PDMS template and the positive textures were transferred to the coatings. The negative replica was fabricated in a similar way.

Fig. 2 The schematic for the creation of textured coatings from the original creatures.

Characterization methods

Morphology observation. The micromorphologies of lotus leaf and white crab shell were characterized by means of atomic force microscopy (AFM, AIST-NT, CETR, USA) and scanning electron microscopy (SEM, Hitachi S-4800, Japan). SEM images were obtained using a secondary electron detector with an accelerating voltage of 5.00 kV at different magnifications. All samples had dimensions of 1 cm × 1 cm and were coated with gold for 2 min to obtain a 30 nm layer of evaporated Au.

Contact angle measurements. The static water contact angles of the biomimetic textured coatings were measured according to the sessile droplet method using a drop shape analysis system (Data Physics OCA20, Germany) with a computer-controlled liquid dispensing system. A deionized water droplet with a volume of 2 μL was employed as the source for the measurements at 20 ± 1 °C and a relative humidity of 65 ± 1%. The contact angle data listed was the average of three replicates for each textured sample and each sample was tested five times.

Algae settlement assay

Closterium and Navicula settlement assays were conducted with 85 mm × 25 mm glass microscope slides adhered with smooth coatings and topographically modified SMAR coatings with a 15 × 25 mm area via double-sided tape.

First, the glass microscope slides with samples were immersed in a beaker filled with 20 mL of filtered seawater and 20 mL of algae culture suspension. The cell density of Closterium and Navicula culture suspension was approximately 2 × 10⁶ cells mL⁻¹, and all of the samples, including uniformly smooth coatings, negative replicas and positive replicas of lotus leaf and white crab shell, were tested three times. Then, the algae were left to statically settle for 7 days in a biochemical incubator. The growth condition for these two types of algae was a 12 : 12 h light : dark cycle at 25 °C. Finally, all the coatings adhered with algae were rinsed by dipping into a new beaker of artificial seawater three times to remove unattached Closterium and Navicula before being set in a 2% glutaraldehyde seawater mixture for 15 min.^27,28

The quantity of algae was counted using a Dimension 3100v Laser Scanning Confocal Microscope (LSCM) analysis system. The images of algae settled on the biomimetic coatings were recorded in 10 random fields of view per 0.64 mm² area on each replicate sample.

Each textured coatings antifouling property was evaluated by the reduction ratio (R_r) of algae density compared to the smooth coating.

D_s, the algae density of the smooth coating; D_t, the algae density of the textured coating.

Results and discussion

FT-IR analysis

FT-IR spectra were obtained with a Fourier transform infrared spectrometer (FTIR, Thermo Nicolette 6700, America) for samples in the form of thin film KBr disks. Hydroxyl-containing silicone modified acrylic resin (SMAR) reacted with the curing agent, hexamethylene diisocyanate trimer (HDI), in the proportion of 3 : 2 by weight, and then the cured resin (C-SMAR) was obtained. As shown in Fig. 3, IR spectra collected from the surfaces of the C-SMAR and SMAR are dominated by peaks centered at 1130, 1380, and 1730 cm⁻¹, which are assigned to Si–O–C asymmetric stretching vibration, –CH₃ symmetric stretching vibration, and C=O symmetric stretching vibration, respectively. In the C-SMAR spectra, there is no –OH stretching vibration peak (3510 cm⁻¹) compared to the SMAR spectra, and the –NCO stretching vibration peak (2270 cm⁻¹) is evidently weakened compared to the HDI spectra. All of these peak signals demonstrated that SMAR reacted with HDI and cured successfully.

Fig. 3 IR spectra of the HDI, SMAR and C-SMAR.

Surface morphology and wettability

Scanning electron microscopy (SEM, Hitachi S-4800, Japan) and atomic force microscopy (AFM, AIST-NT, CETR, USA) were performed on the specimens. We prepared the positive and negative morphologies of the lotus leaf and white crab shell on the siloxane modified acrylic resin coatings according to the method shown in Fig. 2. Fig. 4 shows the typical SEM images of the positive and negative replicas of the lotus leaf and white crab shell. Many micro-papillaes were found on the positive replica of lotus leaf in a random distribution with diameters ranging from 6 to 8 μm (Fig. 4a–c). The distance between two adjacent papillae ranged from 9 to 11 μm, but there are few papillae whose spacing was less than 3 μm or more than 15 μm. Accordingly, the negative replica of lotus leaf exhibited many micro-pits with a random arrangement on the surface (Fig. 4d–f). Similarly, the diameters of the pits changed between 6 and 8 μm, and the pits interval distance was about 10 μm. SEM images of the positive replica of white crab shell show many protuberant agnails with a regular arrangement close to a close-packed hexagonal structure (Fig. 4h). The size of the protuberant agnail was basically identical and their bottom diameters were about 1.5 μm (Fig. 4i). The distances between two agnails ranged from 5.5 to 7.5 μm. For the negative replica of white crab shell, the microstructure was similar to the positive replica surface except that the protuberant agnail was replaced by micro-pits (Fig. 4j–l).

Fig. 4 (a)–(c) Positive replica of lotus leaf, (d)–(f) negative replica of lotus leaf, (g)–(i) positive replica of white crab shell, (j)–(l) negative replica of white crab shell. Magnification increases from left to right for each replica.

3D images of the biomimetic textured coatings replicated from lotus leaf and white crab shell are shown in Fig. 5. The height of papillae and the depth of pits on the positive replica and negative replica of lotus leaf surface were both 8 μm (Fig. 5a and b). The height of the protuberant agnail on the surface with the positive white crab shell replica was about 4 μm. However, the depth of the pits of the white crab shell negative replica does not correspond to the height of protuberant agnail. The reason might be that SMAR did not completely cover the protuberant agnail when it was poured onto the PDMS template and the depth was limited to 1 μm.

Fig. 5 3D images of textured coatings ((a and b) positive replica and negative replica of lotus leaf, (c and d) positive replica and negative replica of white crab shell).

Apparent water contact angles of all the samples are shown in Fig. 6. It was obvious that the contact angles of water droplets residing on the biomimetic coatings were much larger than that of the water droplet residing on the smooth coating. On the basis of hydrophobic models, including the Wenzel and Cassie models, it can be concluded that the solid–liquid interface was changed by the microstructure and the wettability of the textured coatings was weakened. Simultaneously, due to the micro-papillaes or pits, there was air trapped in the textures so that the solid–liquid interface was changed to the composite interface of the solid–liquid and air–liquid interfaces.

Fig. 6 The contact angles of water droplets residing on the coatings ((a) smooth coating; (b) positive replica of lotus leaf; (c) negative replica of lotus leaf; (d) positive replica of white crab shell; (e) negative replica of white crab shell).

Antifouling properties

The antifouling properties of biomimetic textured coatings with the lotus leaf and white crab shell morphologies were investigated via the settlement assays of Closterium and Navicula. Under the laser scanning confocal microscope (LSCM, Dimension 3100v), green fluorescence could be observed from the attached algae (Fig. 7). Fluorescence microscopy images were obtained in 10 random fields of view per 0.64 mm² area on each replicate sample. It can be seen that the biomimetic textured coatings, both positive and negative replicas of lotus leaf and white crab shell, showed an obvious lower algae settlement compared with the smooth coatings. These coatings significantly inhibited the settlement of Closterium and Navicula. Micro/nano-structures on the surface played a very important role in deterring the biofouling.

Fig. 7 Fluorescent images of Closterium (a–e) and Navicula (a′–e′) settled on the textured coatings ((a and a′) smooth coating; (b and b′) the positive replica of lotus leaf; (c and c′) the negative replica of lotus leaf; (d and d′) the positive replica of white crab shell; (e and e′) the negative replica of white crab shell).

Fig. 8 reveals the settlement amount of attached algae on different samples after 7 days. It can be seen from Fig. 8a that both the positive and negative replica of the lotus leaf reduced about 73% attachment of Closterium. The reason might be that both the papillae and pits can reduce the adhesion strength and contact area of Closterium upon coating. Thus, for Closterium, it is not easy to attach onto the surface. In addition, the settlement amount of Closterium on the positive and negative replicas was similar. In addition, the size of Closterium (length of 45–55 μm) was much greater than the size of the texture (papillae and pits, 6–8 μm in diameter, 9–11 μm spacing). Accordingly, the different shape of the texture relatively affected the amount of attached algae only to a slight degree. Moreover, these two textured coatings possessed similar hydrophobicity and water contact angles (CA) are 112.8 ± 1° (positive replica) and 113.5 ± 1° (negative replica). Closterium adhesion behavior may be similar to the wetting behavior of droplets to a large extent. As a result, the antifouling effect of the positive and negative replicas of lotus leaf for Closterium is similar.

Fig. 8 Settlement data of Closterium and Navicula on different biomimetic coatings.

In Fig. 8b, the positive and negative replicas of lotus leaf show a significant reduction of Navicula compared to the smooth coating and the reduction ratios were 54% and 74%, respectively. Obviously, the anti-Navicula ability of the negative replica was superior to the positive replica. For the negative replica, there were micropits whose diameters were about 7.5 μm. The length of Navicula was 10–12 μm, which was larger than the pit diameter. Thus, an “air cushion” would be formed between the pit and Navicula (Fig. 9a). The “air cushion” can reduce the adhesion area, which can decrease the adhesion strength, and then reduce the amount of Navicula to a large extent. On the surface with papillae spaced less than 10 μm, some Navicula (length of 10–12 μm) attached to the surface in the form of a “bridge” reduces the contact area between the Navicula and surface. This form of attachment reduced the number of Navicula to a certain degree. However, the area between some adjacent pillars (spaced 9–11 μm) was suitable to settle for the Navicula, which can generate the attachment of “mosaic” (Fig. 9b). Therefore, the amount of Navicula on the negative replica was less than that on the positive replica.

Fig. 9 Attached sketch of Navicula on the textured coatings relication of lotus leaf ((a) negative replica, (b) positive replica).

As shown in Fig. 8c, the positive and negative replicas of white crab shell can reduce the amount of attached Closterium in a similar degree (reduction ratios of 68% and 65%, respectively). Their antifouling mechanism was similar to that of textured coatings with positive and negative lotus leaf morphologies. The protuberant agnail and pit on the surface could reduce the adhesion strength and contact area of Closterium, and the size of Closterium (length of 45–55 μm) was greater than that of the texture (protuberant agnails and pits, 1.5 μm in diameter with 6.5 μm spacing). At the same time, both the positive and negative replica of white crab shell possessed the desirable hydrophobicity (CA: 103.1 ± 1° and 95.8 ± 1°). Although the morphologies were different, the reduction ratio of Closterium adhesion was similar.

Fig. 8d reveals the settlement amount of Navicula on the biomimetic textures with the positive and negative replicas of white crab shell. The biomimetic textures of both the positive and negative replicas can reduce about 70% of the attachment of Navicula. These two types of coatings with different morphologies presented a perfect anti-Navicula property using a mechanism which was similar to that of lotus leaf. On the surface with pits 1.5 μm in diameter, there were “air cushions” between the pit and Navicula. For the surface with protuberant agnails spaced 6.5 μm apart, most of the Navicula (length of 10–12 μm) was bridged on the top of the agnails. Thus, the reduction ratios of attached Navicula on these two types of coatings were similar. Moreover, the anti-Navicula property of the positive replica of white crab shell was superior to that of the positive replica of lotus leaf. There were several reasons underlying this phenomenon. Protuberant agnails on the positive replica of white crab shell were spaced about 6.5 μm and were arranged regularly with a nearly close-packed hexagonal structural arrangement. There were fewer areas suitable for Navicula to embed between protuberant agnails. Therefore, the number of attached Navicula on the positive replica of the white crab shell was less than that on the positive replica of lotus leaf.

The surface with pillars, pits or protuberant agnails all showed excellent antifouling capability towards resisting Closterium, which was significantly different from that of Navicula. Researchers have presented that microscale topography alters surface wettability^28–30 and wettability influences bioadhesion.^31,32 Previous results confirmed that the organism's responses were governed by the same underlying thermodynamic principles as wettability when its size was much bigger than the surface feature size.¹⁴ Closterium (length of 45–55 μm) was too large to rest between textures, and it should bridge on two or more pillars or pits. Considering Closterium settling on a textured surface of pits, bridging is similar to the air pocket state described by the Cassie–Baxter relation or alternatively, conforming similar to Wenzel behaviour. For Closterium, there was a general trend that the amount of attached Closterium seemed to be reduced with coatings possessing a larger water contact angle.

Conclusions

The positive and negative replicas of lotus leaf and white crab shell were fabricated by biomimicking the surface structures using replication technology. The antifouling properties of the various as-prepared coatings against Closterium and Navicula were systematically investigated. The anti-fouling mechanisms of the textured coatings were analyzed based on three key factors, including surface wettability, morphology, and algae size. For Closterium, there was a strong correlation between the antifouling properties and wettability, and the morphology had little effect on the anti-fouling performance of the coating under the premise of hydrophobicity. For Navicula, there was a strong correlation between the antifouling properties and the surface morphologies of the textures. The “bridge” and “air cushion” effect from the textures could effectively reduce the attachment of Navicula, and the “mosaic” effect from the texture or the area between the textures increased the attachment of Navicula.

Acknowledgements

We express our thanks to the National Key Basic Research Program of China (973) (2014CB643305), the National Natural Science Foundation of China (51202263 & 51335010), the Zhejiang Province Public Welfare Program (2014C31154) and the Innovative Team Project of Zhejiang Province (2011R50006).

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