Abstract
Biofouling, also known as the unwanted attachment of protein, bacteria, and macro-organisms on materials surfaces, has resulted in large economic and ecological consequences in aqueous and marine environments. Biofouling generally creates a film that may cause serious microbial contamination problems, for example, in medical engineering, dentistry, pharmaceutical processes, bioprocessing, and food manufacturing. Stainless steel is used in a large variety of food and medical applications, where it is susceptible to adsorption of proteins and other materials on its surface, ultimately leading to biofouling. Frequent clean-in-place procedures are used to remove these fouling layers in commercial applications.
The intended purpose of this work is to form an understanding and develop a coating to achieve pristine, foulant-resistant stainless steel surfaces in aqueous proteinaceous or microbial environments such as those found in medical implants or in food processing plants. The coating should be inexpensive and non-toxic and must have the ability to inhibit the rate of adhered proteins and bacteria from a metal surface. A simple solution-based method will be used to deposit the coating, which would not require any adhesion promotor, heat treatment, or toxic chemicals.
Chapter 1 provides a general introduction and review of biofouling on surfaces and the various antifouling materials used to minimize it.
Chapter 2 describes the use of stainless steel nanoparticles and their functionalization with poly(ethylene glycol) phosphonic acid polymers. There is an emphasis for a response at the adsorption of the polymer at acidic pHs. However, experiments were also performed at other pHs from 3-11. A comparison was made between the adsorbed polymer film properties on stainless steel nanoparticles and chromium hydroxide particles. It also shows that chromium oxide is an appropriate and useful model for stainless steel.
Chapter 3 describes the synthesis of a range of antifouling polymers based on poly(ethylene glycol) and phosphate, phosphonate, or carboxylic acid monomers as an easily applied nano-coating for stainless steel substrate. The synthesis of polymers was based on the idea if multivalent attachment using copolymers may give more tenaciously bound films to a metal surface than monofunctional materials due to the additive effect of multiple binding sites.
Chapter 4 shows that highly tenacious films can be easily deposited by dipping the stainless steel substrate into a dilute aqueous solution of the copolymer, or by flowing a copolymer solution for a few minutes. This simple attachment method, which does not need solvents, annealing, or adhesion promoters looks to be very easy transferable into commercial applications.
Chapter 5 describes the interaction of a variety of proteins and skim milk on the polymer coated surfaces. It also covers the effect and role of grafting density and surface linkers on the antifouling efficacy of the polymer films. Phosphate-containing copolymers showed almost a 100 % reduction in binding affinity of proteins which was the main aim of the project. The role of surface linkers was also tested by rinsing the polymer films in various aqueous solutions. The films based on phosphate and phosphonic acid linkers resisted rinsing over many days.
Chapter 6 covers two aims of the project. Initially, the polymer films were assessed against two pathogenic bacteria i.e. Escherichia coli and Bacillus cereus. The biofilms were grown in vitro for several days on uncoated and polymer coated stainless steel surfaces. Phosphate and phosphonate coated surfaces exhibited strong inhibition of bacterial adhesion for both Escherichia coli and Bacillus cereus over several days and the formation of a biofilm was observed on uncoated surfaces. Other polymer coated surfaces containing carboxylic acids as surface linkers were also tested but results were not as good as for phosphate and phosphonic acids. The cytotoxicity of the polymer films was tested by several quantitative and qualitative methods, and all polymers revealed excellent non-toxicity which fulfill the aims of the project.
Chapter 7 describes the idea if phosphorus-containing monomers can be polymerized to prepare low-energy-surfaces. It covers the synthesis of a range of new hydrophobic fluorinated phosphorus-containing homo- and copolymers. The polymers were attached to the surface of stainless steel by simple treatment of dilute solutions in trifluoroacetic acid for 30 minutes. The polymer film stability was also determined after rinsing in pure water for 3 weeks. The ease of application and the stability of the films makes this a promising avenue for the synthesis of low surface energy metal surfaces.
Chapter 8 includes the final results and conclusion with future directions of the project.