DYNAMIC SURFACES FOR URINARY CATHETERS: THE EFFECT OF POLY (GLYCEROL SEBACATE CITRATE) COMPOSITION ON DEGRADATION, ENCRUSTATION RESISTANCE AND BACTERIAL RESPONSE

Abstract

Encrustation and blockage of urinary catheters, caused by the formation of crystalline biofilms, has been a problem since their invention more than 90 years ago. This thesis addresses the issue via a biomaterials approach, specifically, utilising poly glycerol sebacate citrate (PGSC) produced in-house. The effects of citric acid concentration on the degradation characteristics, mechanical behaviour, hydrophilicity and encrustation resistance of bulk samples were investigated and the feasibility of coating a silicone catheter material with PGSC using oxygen plasma treatment was assessed. The effect of citric acid concentration on encrustation resistance properties and against E. coli and P. mirabilis infections was then analysed. To optimise the production of the PGSC, the effect of N2 atmosphere on the PGS prepolymers was evaluated with Fourier-transform infrared spectroscopy (FTIR). A static N2 atmosphere was found to cause the oxidation of glycerol to glyceraldehyde and hence, preparation in a flowing N2 atmosphere was adopted. PGSC with different citric acid concentrations (0.25, 0.5, 0.6, 0.7 and 0.75 molar ratio with respect to glycerol) were synthesised by dissolving PGS prepolymer and citric acid in ethanol, followed by curing. A possible mechanism for esterification was proposed where increasing citric acid concentration led to the activation of secondary alcohols on glycerol molecules. Degradation experiments in artificial urine (AU) and deionised water (DW) were undertaken and the degradation pH, percentage mass loss and absorption of media were measured as a function of citrate concentration. Encrustation resistance was measured in an environment simulating P. mirabilis infection by adding urease solution to AU and was found to increase with increasing citric acid concentration. PGSC with a citric acid molar ratio of 0.6 was found to be optimal in terms of balancing encrustation resistance and degradation rate. Tensile testing was conducted on PGSC samples with different citrate contents before- and during degradation in AU. Before degradation, PGSC 0.5 was found to have the highest stiffness, but the tensile strength and failure strain values increased with increasing citric acid concentration. During degradation, the stiffness of the polymers decreased gradually whereas the tensile strength and failure strain dropped sharply after the first day of degradation. Water contact angle measurements indicated that increasing polymer citrate content had minimal effect on the hydrophilicity of PGSC. When compared with silicone, although PGSC had a higher hydrophilicity the mechanical properties were found to be inferior. Based on the evaluation of FTIR data for the first 24 hours of degradation, it was possible to propose a mechanism by which the process occurred. Modulated differential scanning calorimetry (MDSC) also revealed a decrease in glass transition temperature with degradation suggesting a decrease in the degree of crosslinking. Silicone was successfully coated with PGSC 0.6 using an oxygen plasma and the coating thickness could be controlled by varying the prepolymer solution concentration. FTIR results revealed that increasing plasma treatment time increased surface hydrophilicity (as a result of removing methyl groups and increasing the presence of hydroxyl groups), but prolonged plasma treatment led to the formation of a brittle surface layer. When subsequently degraded in AU, the PGSC-coated silicone samples were found to reduce the AU pH successfully, but all coatings detached from the substrate after 7 days of degradation. The ability of PGSC to prevent bacterial growth and encrustation was then investigated. In a comparison test with a silicone catheter surface, PGSC 0.6 was found to offer superior inhibition of bacterial growth, impeding biofilm development and crystal formation for both E. coli and P. mirabilis. The effect of citrate concentration was considered, and it was found that PGSC 0.75 had the optimum behaviour in terms of suppression of colony-forming units (CFUs), bactericidal effects on E. coli, bacteriostatic effects on P. mirabilis and inhibitory effects on crystal and biofilm formation. The results presented in this thesis offer an improved understanding of PGSC synthesis and the effects of citrate content on degradation behaviour, encrustation resistance, mechanical properties, and hydrophilicity. The data obtained suggest that PGSC offers potential for anti-encrustation and anti-bacterial applications on urinary catheters.