Investigating Nanostructured Titanium Surfaces for Advanced Antimicrobial Applications

Hospital acquired infections of antimicrobial resistant pathogens are the leading cause of patient readmission. Pathogens, including bacteria, fungi, parasites and viruses, are regularly transferred via surfaces to compromised individuals. Microbes, specifically bacteria and fungi, are responsible for implant associated infections and with the rise of antibiotic resistance, these infections are becoming increasingly difficult to treat. This is due to the overuse, over prescription and misuse of antimicrobial pharmaceuticals, including antibiotics and antifungals, that microbes have built inherent resistance to the mechanism of action. Without a new strategy to treat infections, what was once easy to treat can become life-threatening or potentially fatal infection. Prevention in the transfer of bacteria and fungi is an important aspect of infection control especially across surfaces. Once adhered to a surface, microbes are able to form a biofilm: a self-sustaining colony within a secreted mucous matrix. The protective coating makes it more difficult for the body’s immune response and antimicrobial drugs to combat the biofilm infection. The search for alternatives to prevent biofilm formation led to the discovery of naturally occurring, antifouling and antimicrobial surfaces on cicada and dragonfly wings. Biomimetic surfaces that possess similar nanostructured topography as these insect wings have been engineered to physically rupture microbial cells and the dimensions of the structures can be customised based on fabrication techniques. Firstly, the adhesive properties of bacteria and fungi and the effects of how the membrane composition changes the response to a nanostructured titanium surface are discussed. Specifically, this fundamental study probed the inherent differences in microbial cell membrane and the response of each cell atop three surface morphologies. Bacterial and fungal cell membranes were probed via single-cell force spectroscopy to elucidate the elasticity and force required to rupture the membrane, properties that were dependent on membrane composition. Rigidity and ease of rupture effected the rate of microbial death leading to the conclusion that there is no one-size fits all approach when it comes to synthesising antimicrobial structures. Since bacterial and fungal infections are commonly found at the site of newly inserted orthopaedic implants, it was appropriate to investigate the resilience of these structures over longer incubation periods. Bacterial and fungal strains were investigated to understand whether the presence of previously inactivated cells atop the nanostructures mask the surface’s ability to continue to be antimicrobial between 1 and 7 days. It was found that while the nanostructures can initially combat attaching cells, as the interaction continues, the contents of the cells and the extracellular matrix conceals the structures which provided a smooth surface for incoming cells to form a biofilm. In the previous studies, the surface of titanium was modified via hydrothermal etching to impart nanostructures. These structures prevent colonisation of a cell and increase antimicrobial efficacy compared to unmodified titanium surfaces. However, these surfaces had no effect on the cells that did not directly contact the nanostructures and a secondary mode of action was added to increase the effectiveness. Silver nanoparticles were deposited on these surfaces and increased the antimicrobial efficacy against bacterial and fungal species. Finally, as implant manufacturing begins to advance into a customisable and ‘just-in-time’ healthcare model, it is important to investigate whether newly printed forms of titanium are able to undergo the same forms of modification to achieve similar levels of antimicrobial efficacy as commercially pure titanium. Selective laser melting was utilised to print titanium at various angles, effecting how the laser melts the metallic powder and resulted in a difference in the roughness caused by the partially melted particles on the surface. Here, adhesion of bacteria and fungi were investigated for the various surface roughnesses and then the surfaces were nanostructured and cell adhesion examined again to understand if the same antimicrobial structures can be transferred from commercial to additively manufactured titanium. Together, this work presents the advancement of antimicrobial, nanostructured titanium as a method to mitigate and reduce the presence of live bacteria and fungi on a surface. In addition, fundamental studies provide deeper understanding into the interactions between strains of microbes and nanostructured surfaces to contribute to the improvement of nanostructured surfaces for broader microbiological mitigation applications.<p></p>