Investigation of Biointerfaces of Opportunistic Pathogenic Candida albicans and Environmental Filamentous Aspergillus brasiliensis With Biomimetic Bactericidal Surfaces

In nature, the distinctive characteristics of the topography of the wings of flying insects have evolved to sustain their lifestyle and environmental conditions. The unique surface topographies of insect wings have inspired many fundamental investigations in areas such as aerodynamics, hydrodynamics, and self-cleaning materials. It has also been shown that insect wings exhibit antibacterial properties. Numerous biomimetic surfaces have been developed to capitalise on the inherent bactericidal properties of the insect wings; however, the antifungal effects of these surfaces remain unknown. This study aimed to determine whether nanostructured surfaces display antifungal properties, and if so, to understand the mechanism by which these properties occur. In addition, it was intended to explore the biointerface interactions occurring between surface nanotopography and fungi in the context of prevention of fungal biofilm formation.

Many fungal species such as yeast-like-fungi Candida albicans and environmental fungi Aspergillus brasiliensis are considerably harmful to vulnerable and immunocompromised individuals, children and seniors. Moreover, these invasive environmental fungi have a negative impact on agriculture and business, costing the United States more than US$21 billion in crop losses each year, and in particular, are considered as parasitic microorganisms, which can devastate infrastructure by causing corrosion or deterioration. Hence, there is an urgent need for research to determine whether nanostructured surface display antifungal properties, and if so, to understand the mechanism by which these properties occur.

First, through expanding the development of antifungal surfaces based on surface nanotopography, the impact of the titanium surface topology on the formation of Candida albicans biofilms has been investigated. Titanium is a much-used metal using for designing implantable devices in the medical industry. Titanium that has been modified to possess a micro-structured surface, called ‘black titanium’, has been shown to exhibit bactericidal activities against both Gram-positive and Gram-negative bacteria, however, its antifungal properties are still unknown. The distinctive properties of the surface nanoarchitecture, such as roughness, skewness, and kurtosis appeared to be critical in inhibiting Candida albicans attachment via inhibiting the development of biofilm-related phenotypes of Candida albicans cells. Particularly, skewness and kurtosis are essential measurements of a distribution's shape. Skewness relates to the degree of asymmetry in the distribution, while kurtosis indicates the degree of "peakiness" or "flatness" compared to the normal distribution. The outcomes of this study have paved the way for a further investigation of the mechanical properties of C. albicans when attaching on titanium surfaces. The observed changes in the mechanical characteristics of C. albicans cells imply that the unique surface topographical landscape may have caused physical environmental stress, which has a direct effect on the morphology and rigidity of Candida cells during attachment.

In parallel with the assessment of antifungal properties of black titanium, the potential biomedical applications of this nanostructured surface were also investigated. It was found that black titanium supported human adipose-derived stem cells (hASCs) growth and proliferation, while maintaining the stemness and osteogenic potential of the cells. It is demonstrated that these antimicrobial nanostructured titanium surfaces represent a promising support for hASCs.

Thus, the outcomes of this project significantly advanced our understanding of the mechanisms governing the interactions between fungi and nanostructured surfaces, to allow preventing the formation of yeast biofilms while supporting the growth and differentiation of stem cells.

Furthermore, the self-cleaning and antifungal capabilities of the superhydrophobic surfaces of damselfly Calopteryx haemorrhoidalis wings have been investigated. The correlation between the nanostructured pattern and the air entrapment at the micro- and nanoscale has been elucidated and it was found that fungal spores were unable to traverse the air-liquid interface. In contrast, it was confirmed that bacterial cells could cross the air-water barriers and were ruptured upon attachment to the nanopillar surface. These new findings will aid in the development of biomimetic anti-fouling surfaces that possess both bactericidal and antifungal properties. 

This work influenced the design of the experimental studies that followed. It has further been shown that biomimetic bactericidal nanostructured surfaces such as black silicon (bSi) can poses the antifungal effect against fungal conidia, or spores. Here, antifungal characteristics of nanostructured surfaces such as bSi have been thoroughly investigated to shed light to cell-surface interactions with nanomaterials. The mechanism of spore adhesion to nanostructured surfaces was elucidated by modulating the wettability of the surface.