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Chapter 13
Biomimetic Anchors for Antifouling and Antibacterial Polymeric Coatings Li Qun Xu,*,1 Koon-Gee Neoh,2 and En-Tang Kang*,1 1Institute
for Clean Energy and Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing, People’s Republic of China 400715 2Department of Chemical and Biomolecular Engineering, National University of Singapore, Kent Ridge, Singapore 117576 *E-mail:
[email protected]. *E-mail:
[email protected].
Biofouling is a complex and dynamic problem with global impact on human health, economy and environment. The design and construction of antifouling and antibacterial coatings have been a persistent research effort. A variety of functional polymeric coatings have been developed for combating biofouling. The development of mussel- and tea stains-inspired surface chemistry has led to the implementation of biomimetic strategies for the deposition of antifouling and antibacterial polymer brush coatings. This report describes the application of biomimetic anchors, in conjugation with the advances in surface-initiated controlled radical polymerization methods, for the deposition or immobilization of functional polymeric materials in the construction of antifouling and antibacterial coatings.
1. Introduction Biofouling has always been a serious and costly problem for biomedical devices and implants, biosensors, textiles, food processing and storage, water purification systems, and maritime industries (1). Biofouling in biomedical © 2018 American Chemical Society Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Materials and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
devices and implants is the major cause of infections, which account for a large portion of treatment failures and even death (2, 3). Fouling of food contacting surfaces during processing has a significant impact on the operating efficiency and increases the likelihood of biofilm formation and contamination (4, 5). In the case of maritime industries, biofouling causes high drag resistance with increased fuel consumption, maintenance costs and greenhouse gases emissions. In addition, marine biofouling could create a corrosive environment, resulting in microbiologically-influenced corrosion (MIC) of metals and failure of maritime structures (6, 7). The deposition of the functional polymer coatings depends to a large extent on the surface anchoring strategies. Traditional methods, including silanization, nitrene addition, diazonium radical chemistry, ozone, plasma and electro-beam treatments, electrografting, photografting, phosphate-metal oxide and thiol-metal complexation, autocatalytic plating, as well as ion implantation, have been develop to deposit the polymeric materials on the substrate surfaces (6, 8–11). In recent years, biomimetic anchors based on mussel adhesive-inspired dopamine and tea stains-inspired polyphenols have gained considerable scientific interest in the design of functional polymer brush coatings (12–14). Thus, this chapter focuses on the recent development of antifouling and antibacterial polymer brush coatings via biomimetic anchors and controlled radical polymerization techniques.
2. Biomimetic Anchors Recently, the adhesives of biological organisms have inspired new approaches to surface chemistry. Of particular interest is the protein-based adhesives found in marine mussel Mytilus edulis. The essential component of mussel foot protein has been identified as the unusual amino acid 3,4-dihydroxyphenylalaine (Dopa). Several studies have shown that the catecholic units of Dopa residues in the adhesive play vital roles in their surface adhesions (15). Dopamine, one of the Dopa derivatives, can undergo self-polymerization under mild alkaline conditions, resulting in the deposition of polydopamine films on virtually almost any material surface (13). Catecholic compounds can form bidentate coordination and hydrogen bonds to metal and oxide surfaces. Thus, functional surfaces can be created by employing polydopamine and catecholic compounds as the anchors for the introduction of antifouling and antibacterial polymers onto the substrate surfaces (16, 17). Tea has been consumed for centuries. Interestingly, tea drinking always leaves stains on the tea cup. Polyphenols with high dihydroxyphenyl and trihydroxyphenyl contents have been identified to play an important role in the tea staining. Tannic acid (TA), one of the naturally abundant polyphenols, can serve as the sole coating precursors for the formation of polyphenol coatings on substrate surfaces (12). The TA coating can also be accomplished by the deposition of TA-metal complexes (14). The as-formed TA coatings are reactive and can be used to conjugate antifouling and antibacterial polymers through 234 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Materials and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
various chemical or physical interactions. In addition, TA-containing compounds or polymers can be anchored directly on the substrate surfaces, enabling the construction of antifouling and antibacterial polymeric coatings in a single step (18, 19).
3. Antifouling and Antibacterial Polymer Coatings Antifouling and antibacterial polymeric coatings are usually prepared from hydrophilic, amphiphilic and cationic polymers. It is hypothesized that the antifouling abilities of hydrophilic polymers are closely related to the surface hydration layers (20), as the bound water layers form physical and energetic barriers to resist protein adsorption and settlement of fouling organisms. Several hydrophilic polymers, such as poly(ethylene glycol) (PEG), hyperbranched polyglycerol (HPG), poly(vinyl alcohol) (PVA), poly(2-methyl-2-oxazoline) (PMOXA), poly(vinylpyrrolidinone) (PVP), polypeptoids, zwitterionic polymers and polysaccharides, have been developed as antifouling coatings. Many amphiphilic copolymers containing both hydrophilic and non-polar hydrophobic segements have also been used to construct the antifouling coatings (6). The non-polar hydrophobic segments tend to self-assemble on the surfaces to form micro-domains, which are advantageous to antifouling performance due to their low surface free energy (21). Thus, both the hydrophilic and hydrophobic segments can contribute to the antifouling effect of amphiphilic copolymer coatings. Cationic polymers with quaternary ammonium groups could disrupt the cell wall and/or cytoplasmic membranes, resulting in bacterial death. After deposition of the cationic polymers, the substrate surfaces could exhibit antibacterial properties (22). In general, the antifouling and antibacterial polymers and coatings can be categorized into non-ionic, amphiphilic, zwitterionic and cationic polymers, and polysaccharides. 3.1. Non-ionic Polymers Antifouling polymers should be hydrophilic and electrically neutral. Among them, PEG is by far the most frequently used polymer (23). The antifouling performances of surface-immobilized PEG are attributed to the large exclusion volume, high chain mobility and steric hindrance effects of the highly hydrophilic layers (24). Methyl-PEG succinimidyl succinate was reacted with Dopa derivatives via the N-hydroxysuccinimde (NHS) ester-mediated derivitization. Merthyl-PEGs end-functionalized with 1-3 Dopa moieties (mPEG-Dopa1, mPEG-Dopa2, and mPEG-Dopa3) were obtained. These mPEG-Dopa polymers can be adhered on the titanium oxide (TiO2) surfaces in a rapid and essentially irreversible process. Quantitative analysis of X-ray photoelectron spectroscopy spectra suggested that mPEG-Dopa could displace the hydroxyl groups on the TiO2 surface via the formation of charge-transfer complexes, providing a plausible mechanism for Dopa-mediated surface anchoring. The adsorption of serum protein on the pristine and modified TiO2 surfaces were examined by 235 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Materials and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
spectroscopic ellipsometry (ELM) and optical waveguide lightmode spectroscopy (OWLS). ELM revealed the thickness of adsorbed protein on the pristine TiO2 surface is approximately 60 Å after exposure to serum for 15 min. After deposition of mPEG-Dopa3 for 30 min, the thickness of adsorbed protein was less than the sensitivity limit of the technique (< 0.5 Å). OWLS revealed that the masses of serum protein on the pristine and mPEG-Dopa3-coated TiO2 surfaces were 250 and
