Modified polymer surfaces with antimicrobial properties

Abstract

Biofilms are a multispecies community of bacterial cells that can establish over time on biotic and abiotic surfaces. Biofilm growth and maturation on industrial polymer surfaces poses a serious challenge to public health, industrial manufacturing, oil and gas industries, food production and healthcare. As a result, there has been increasing interest in the development of easily synthesised low-cost antimicrobial polymer surfaces and coatings to inhibit, and/or perturb bacterial attachment and biofilm viability. This thesis was focused on modifying the surface of low cost commercial polymers, with high-density polyethylene (HDPE) being used as the model substrate. Surface modifications were performed using a range of approaches. These included different surface oxidative treatment methods in order to evaluate their intrinsic antimicrobial performance, and as an approach for improving the adhesion for other antimicrobial coatings being investigated. Tin-dioxide thin-films and low/medium molecular weight chitosan-zinc oxide nanocomposite coatings were also investigated and examined as possible antimicrobial surface coating technologies for polymer materials, primarily for HDPE. A key aim of this work was to evaluate microbial adhesion and biofilm viability at the initial stages of biofilm maturation (early-stage biofilm), with successful antimicrobial approaches then being further evaluated against a more mature biofilm system. This study confirms that the sulfuric-chromic acid surface oxidisation of HDPE and polypropylene surfaces showed marked reductions in bacterial adhesion and viability, with a decrease in attachment seen for both Gram-negative and positive bacterial species with increasing surface oxidisation treatment time periods from 0 to 60 minutes. In particular, extended sulfuric-chromic acid oxidisation treatment times of 30 and 60 minutes significantly compromised microbial viability, such that there was no agar growth of early-stage biofilm of Gram-negative organisms recovered from these surfaces when cultured, even after 3 months of aging this polymer material under standard laboratory conditions. The mechanism for this activity was ascribed to the formation of chromium esters which are known reactive intermediates in the sulfuric-chromic acid oxidisation of HDPE, and their surface presence was further confirmed using TOF-SIMS. Tin-dioxide nanoparticle coatings showed a reduction in the earlystage biofilm formation of Pseudomonas aeruginosa, irrespective of nanoparticle size of the tin-dioxide coating used, which was also observed when coatings were deployed on polypropylene and polycarbonate materials. A range of chitosan-zinc oxide composite thin-films were examined which exhibited minimal viability reduction against Escherichia coli early-stage biofilms. Therefore, this study could not affirm a significant antimicrobial performance for the chitosan-ZnO nanocomposite coatings on oxidised HDPE materials which was attributed to the loss of a key antimicrobial active functional group in chitosan, the primary amines. However, optimisation of the chitosan-ZnO formulations revealed some key trends, where increased concentrations of low molecular weight chitosan in the presence of zinc-oxide in 1% acetic acid, resulted in a 2.8-log reduction of Escherichia coli due to the presence of primary amine substituents, confirmed by a ninhydrin assay. Experiments performed against mature biofilms prepared in a 24-hour aqueous environment found no antimicrobial performance on all substrates and coatings tested.