The development and characterisation of liquid metal materials for antimicrobial applications
Creators: name: Cheeseman, Samuel
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 310607
subject: Nanobiotechnology
classificationCode: 401807
subject: Nanomaterials
classificationCode: 401810
subject: Nanoscale characterisation

description: Antimicrobial resistance (AMR) in pathogens is one of the most significant health issues of the 21st century. Infections caused by these resistant pathogens are responsible for a tremendous burden on healthcare systems internationally. Unfortunately, pathogens are gaining resistance much faster than the development of new antimicrobial drugs. As antimicrobial drugs, inclusive of antibiotics and antifungal drugs, become increasingly ineffective, the human population is set to lose its most successful tool to treat pathogenic infections, risking a return to a pre-antibiotics era, where common infections could be potentially fatal. In addition, microbial pathogens can also cause biofilm infections. Biofilms are communities of micro-organisms that adhere to a surface and become contained within a self-produced protective mucous matrix, making them more difficult to treat.

As an alternative to conventional antimicrobial drugs, this research explores the use of gallium liquid metal (LM) based strategies for treating pathogenic micro-organisms and microbial biofilms. Gallium and gallium alloys are liquid at room temperature and are believed to be biocompatible. The unique properties of gallium-based LMs have already been exploited for a wide range of applications, including material synthesis, soft electronics, and numerous biological applications. However, their use in antimicrobial applications has not been explored.

This thesis, initially investigates how LMs can be used for anti-biofilm applications. Towards this goal, magnetic nanoparticles are incorporated inside LM nano- and micro-droplets to generate magnetically responsive particles. Under a rotating magnetic field, these particles could be moved at high speeds, transforming their shape from smooth spheres to high-aspect-ratio rods, stars, and spheroids. These particles were able to remove single and multi-species biofilms from surfaces in vitro and kill the individual biofilm-causing cells. 

The adhesive and surface reactive properties of LM droplets were then utilized to develop and characterize a new method to generate antimicrobial coatings on textile materials. Specifically, the work focussed on an antimicrobial face mask application, utilizing a galvanic replacement reaction with copper to generate a Ga-Cu alloy coating. The resulting coating demonstrated superior adhesion properties to conventional Cu coating strategies and also enabled the face mask to possess broad-spectrum antimicrobial activity against pathogenic bacteria, fungi, and viruses.

Following these applications of gallium-based LMs, the work focussed on gaining a better fundamental understanding of the interactions taking place between LM droplets and cells. The cell-LM interface was examined with high-resolution electron microscopy, single-cell force spectroscopy atomic force microscopy (SCFS-AFM) and molecular dynamics (MD) simulations. For the first time, we characterized the interactions between LM droplets and cells, including bacterial, fungal and human cells. LM droplets were found to wrinkle when coming in contact with cells. It was proposed that the wrinkling of the LM droplet surface was a result of biophysical interactions and tensile stresses generated by multiple contact points between the two flexible colloids.

Finally, the cytotoxicity of Ga droplets was investigated at different concentrations towards the HaCaT human cell line. There is very little previous work to evaluate the cytotoxicity of Ga droplets. It was observed that high concentrations of Ga droplets can cause damage to the cell membrane and cause individual cells to appear misshapen. The presence of Ga droplets was also found to disrupt HaCaT cells’ capacity to form a confluent monolayer. This work provides an indication of the concentration at which Ga droplets become damaging for human cells, which will be useful information for a wide range of researchers and regulatory bodies.

Together, this work presents methods by which functionalized LM platforms can be used for antimicrobial and anti-biofilm applications, with their effectiveness being evaluated in vitro. This work will hopefully inspire the development of strategies for treating pathogenic microbes and their biofilms. Further, this work will also contribute to the fundamental understanding of the interactions taking place between LMs and a range of biological entities and considers their cytotoxicity, providing important insights towards the use of LMs for bio-applications more broadly.