Nanocomposite zirconium nitride/silver coatings to combat external bone fixation pin infections

Wickens, David John (2014) Nanocomposite zirconium nitride/silver coatings to combat external bone fixation pin infections. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

External bone fixation provides rehabilitation for severely broken limbs. The nature of the pins compromises the skin at the entry point and creates an interface that is prone to infection by opportunistic pathogens. Pin tract infections possess one of the highest infection rates of implanted devices. Infection induces further physical and psychological stress to the patient as well as increasing healthcare costs. If the infection can be prevented there is the potential to reduce patient distress, related healthcare costs and the overuse of antibiotics. Using magnetron sputtering, zirconium nitride/ silver coatings were co-deposited onto medical grade 316L stainless steel. Combining the hard wearing properties of ZrN and the broad spectrum antimicrobial properties of silver, these nanocomposite coatings displayed the potential to combat pin tract infections. The coatings were characterised in terms of surface topography, morphology, chemical composition, physicochemistry, antimicrobial efficacy and bacterial retention towards Staphylococcus aureus and Staphylococcus epidermidis. The Ti-ZrN/Ag coatings displayed an increase in nanotopography in comparison to the underlying stainless steel but no increase in microtopography was observed. The silver was deposited as particles in the coatings and the silver content increased linearly with an increase in magnetron power. The most antimicrobial coating was not necessarily the one with the most silver. The Ti-ZrN-Ag coatings were tested against two of the most commonly isolated bacteria from pin tract infections; Staphylococcus aureus and Staphylococcus epidermidis. The Ti-ZrN/Ag coatings displayed an effective contact kill towards both microorganisms, with the silver causing multiple bacterial physiological changes, reducing bacterial respiration and compromising the cell membranes. An increase in silver content displayed an increase in short term antimicrobial efficacy (< 1 hour) whereas following 24 hours contact time (humid conditions) no bacteria survived on the surfaces containing silver, regardless of the concentration. To replicate an in vivo environment, the Ti-ZrN/Ag coatings were characterised in the presence of a blood conditioning film. The underlying topography of the surfaces were found to have an effect on the physicochemical properties of the adsorbed conditioning film. The surface chemistry also affected the adsorption of the conditioning film, with differences in surface morphology between the stainless steel and ZrN in comparison to the ZrN/Ag coatings. The short term antimicrobial activity was reduced when the surfaces were dry (similar to the environment the pin will encounter outside the body), however keeping the environment humid (similar to the environment the pin will encounter inside the body) the coatings demonstrated an antimicrobial effect over 24 hours. Multifractal analysis was used to provide a method to quantify bacterial dispersion and density by analysing the micrographs of retained bacterial cells on the ZrN/Ag surfaces. Multifractal analysis found that with increasing silver content the densities of the retained S. aureus cells remained similar whereas the S. epidermidis decreased. S. aureus demonstrated a more heterogeneous dispersion and S. epidermidis a more homogeneous cell spread. Using this quantitative data acquisition method on the retention behaviour of bacteria displays potential to rapidly assess the surface – bacteria interfacial characteristics of biomaterials, thus, aiding improvement of surface characteristics of an implant at early research and development stages. These results demonstrate the importance of thorough characterisation, microbiological and conditioning film testing when designing new coatings. This work has demonstrated that Ti-ZrN/Ag coatings have the potential to provide an actively antimicrobial pin coating in situ.