Rapidly solidified metallic glass alloys for antibacterial touching surfaces

Villapún Puzas, Victor Manuel (2018) Rapidly solidified metallic glass alloys for antibacterial touching surfaces. Doctoral thesis, Northumbria University.

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Abstract

The emergence of antibiotic resistant bacteria and healthcare-acquired infections are regarded among the greatest socioeconomic threats of our time, affecting up to two million patients annually, of whom about 90,000 die. Hospital surfaces are a reservoir of pathogens, which can be correlated to 20-40% of nosocomial infections due to hand to hand or surface to hand cross contamination. Pure copper and Cu-based alloys display outstanding antimicrobial activity (i.e. 7-8 log10 reduction per hour), but their relatively poor mechanical properties restrict their use. Cu-rich metallic glasses and metallic glass composites can bring together the high biocidal performance of copper and the exceptional physiochemical properties of amorphous materials. This work aimed to understand which parameters influence the antimicrobial and wear behaviour of Cu-rich metallic glasses.

Analysis of composition (i.e. increasingly copper content) and volume fraction of crystalline phases in Cu-Zr-Al bulk metallic glass composites revealed that increasing both parameters led to higher wear and antimicrobial performance, reaching scratch hardness between 3-2 GPa and antimicrobial activity as high as 2.5 log10 reduction after 4 hours. The main parameter influencing antimicrobial activity and wear resistance was the volume fraction of crystalline phases, optimal when the percolation threshold (~50%) was reached, but not overcome. Experiments developed on as-cast and oxidised samples of different roughness, 70-600 nm, showed that, while superficial changes had a limited influence on the contact killing properties, oxidation enhanced their antimicrobial properties, by at least 1 log10 reduction, due to the formation of a complex multi-layered Cu-rich structure.

Coating large surfaces with thin films is an attractive manufacturing option for antimicrobial surfaces; thus, Cu-Zr films were deposited by Magnetron sputtering using different chamber pressures, 0.1 to 0.5 Pa, and substrate temperatures, from room temperature to 403 K. Here, lowering compactness improved the antimicrobial activity (i.e. from 0.2 to 2.3 log10 reduction after 4 hours). At the same time, the authenticity of standard antimicrobial testing protocols for hospital touch surfaces is questionable. “Dry” applications of bacteria to surfaces is more authentic than the “wet” applications used in standards; here no substantial differences were found in these experiments but dry applications resulted in more effective killing in the less compact films. A combination of careful bacterial recovery and imaging studies on both bulk and film samples suggested that contact killing occurs due to copper ion diffusion into the cell, rather than by extensive cell envelope degradation.

All aforementioned tests showed that it is possible to obtain tuneable materials with optimal antimicrobial and wear resistance for use in preventing bacterial transmission and healthcare-acquired infections.

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