Exploring Novel Polymer Coatings for Antimicrobial Applications

Abstract

The ideal remedy to resist bacterial contamination on surfaces is to avoid the initial attachment of bacteria. The combination of the advances of polymer science and the requirement for long-lasting sterilisation of surfaces gave birth to antimicrobial polymer coatings, which provide an effective approach to preventing the initial attachment of bacteria through biocidal or antibiofouling functions. To meet the requirements of biomedical applications, antimicrobial polymer coatings still need refinements on the basis of the interaction between bacteria and the materials. Concurrently, interdisciplinary studies have inspired novel designs of polymer coatings through introducing ordered patterns to improve their antimicrobial performance. These two points are the foundations of this thesis to explore three novel polymer coating systems focusing on their antimicrobial performance, i.e. antibiofouling, biocidal, and combined functionalities through surface patterning.For antibiofouling purposes, fluorinated polymers have been insufficiently used compared to superhydrophilic and zwitterionic polymer coatings, despite proven effective against the contamination by particles, proteins and marine organisms. It is worthwhile investigating the feasibility and efficacy of fluorinated polymers as antibiofouling materials. In Chapters 2 and 3, block and statistical copolymers of partly-fluorinated methacrylate monomer, 1H,1H-heptafluorobutyl methacrylate (HFBMA) and methyl methacrylate (MMA) were prepared and fabricated into thin films. Through thermal annealing, the migration of the HFBMA repeating units at the air-polymer interface diminished surface free energy. The decrease in surface energy was consistent with the increase of the composition of HFBMA, while the block copolymers showed a more efficient reduction in surface energy compared to statistical copolymers due to phase separation. Subsequently, the antibiofouling performance of the films against S. aureus and E. coli were examined, where stronger and more constant antibiofouling behaviour was observed for block copolymer coatings with higher compositions of HFBMA.The combination of antibiofouling and biocidal functionalities potentially enhances the efficacy of coatings. Coatings with surface patterning such as line/trench topographies in micro-scale are effective against the formation of biofilms. As one of novel designs, coatings with micro-scale pores in honeycomb patterns and their antimicrobial behaviour were systematically studied in Chapters 4 and 5. The honeycomb patterns were introduced to polystyrene-block-poly(4-vinlylpyridine) (PS-bP4VP) through “breath figures” method. The influences of different casting and chemical parameters on the size and regularity of the patterns were investigated, through which honeycomb-patterned films with a range of pore sizes were fabricated for subsequent biological tests. In the meantime, biocidal function was integrated into the honeycomb-patterned film by quaternising the P4VP blocks. The antibiofouling activity was dependent on the size of the pore relative to the shape and size of bacteria, while the biocidal function showed effectiveness against E. coli but insufficiency for S. aureus.Finally, a novel biocidal polymer with anilinium side functional groups was explored in Chapters 6 and 7. The novel polymethacrylamide, poly(N,N-dimethylaminophenylene methacrylamide) (PDMAPMA) was synthesised, and thoroughly characterised for its basic physicochemical properties. Subsequently, this novel material was tested against bacteria both in solution and as coatings. Quaternised PDMAPMA exhibited high effectiveness against S. aureus and E. coli in both forms, while E. coli was found more susceptible to the material following a kinetic biocidal study. At the same time, the charge density of PDMAPMA films, a basic and key character for antimicrobial polymer coatings, was adjusted by tuning the degree of quaternisation. An improvement of biocidal performance against both strains was observed as the charge density increased.Through the exploration of the novel polymer coating systems, i.e. antibiofouling fluorainted polymer coatings, biocidal anilinium polymer coatings, and honeycomb-patterned films combining dual functionalities, it is expected to broaden the current selection of effective materials and designs for the improvement of antimicrobial performance. Furthermore, the relationship between the polymer structures, the surface properties and the antimicrobial behaviour can be better understood for future investigations and optimisations of polymeric materials for biomedical applications.