Antibacterial Properties And Characterisation Of Diamond And Graphite Thin Films And Coatings For Biomedical Applications

The rise of bacteria resistant to a wide spectrum of antibiotics has been a concern for researchers in the last decades. Several strategies to replace antibiotics were investigated such as silver or copper-based materials to name only the most promising antibacterial agents. However, bacteria evolve fast. Structural and genetic resistance abilities appeared before copper and silver drawbacks of being toxic to mammalian cells were overcome with research. Thus, it is critical to find alternatives with higher intrinsic biocompatibility and less chance for bacteria to develop resistance mechanisms against them. Carbon-based, and especially diamond-based materials are very promising regarding these properties. Their antibacterial abilities as surface-modified materials, drug carriers, and biocompatible materials were already demonstrated at different scales. However, the mechanisms of action of antibacterial diamonds are not understood yet.

Hence, this PhD focused on understanding the influence of the carbon hybrids proportion in the diamond structure on their antibacterial properties. The results obtained with two model bacteria strains showed that the sp2/sp3 hybridised carbon ratio affected the antibacterial properties of diamond films. The higher the sp3 proportion, the better the antibacterial properties against both bacteria. The role of the spatial localisation of the electrons involved in sp3 hybridisation was discussed to explain these results.

Surface structure of diamond films showed promising abilities to kill Gram-negative bacteria in the literature but failed against Gram-positive bacteria. This work demonstrated that adapting the size of the surface structures to the Gram-positive bacteria by tailoring the roughness also increased the surface free energy of the diamond films and displayed antibacterial properties against both Gram-positive and Gram-negative bacteria.

Finally, the film deposition technique is also of prominent importance, and it modified several surface characteristics of the graphite films, explaining why CVD graphite coatings showed substantial antibacterial properties while PVD graphite coatings did not.