Plasma assisted surface treatments for surface sterilization and for deposition of thin film with antibacterial and virucidal thin film

Abstract

The presence of harmful microorganisms in human health has become of great concern, due to the variety of infections and diseases. Rapid antibiotic resistance further worsens the situation. Antimicrobial thin films have the potential to provide a long-lasting antimicrobial protection. NTP can be used to polymerize certain monomers on the material surface creating a stable film. The functions and properties of the thin film are related to the kind of monomers used. In the first section of this thesis, NTP was used to polymerize three organic precursor monomers that have antimicrobial properties (2-ethyl-2-oxazoline, Terpinene-4-ol and (Dimethyl amino) ethyl methacrylate) and one organic-inorganic monomer that has antimicrobial properties (Bis (dimethyl amino) dimethyl silane). The substrates used in the polymerization process were conductive substrates (stainless steel and silicon wafer) and non-conductive substrates (HDPE and TPU). The effect of changing the NTP operating parameters on the chemical composition of the plasma deposited thin films was also investigated by FTIR. The results showed that increasing the argon carrier gas flow rate or the argon working gas flow rate beyond the optimum value resulted in the decrease in the intensity of the functional groups. Pulsing the NTP during the treatment process was observed to guarantee congruent transfer of the target functional groups to the substrates and therefore generally higher peaks intensity compared to the continuous mode. The results also showed that generally higher peaks intensity was observed when compressed nitrogen was used as the type of cooling gas as compared to when compressed air was used as the type of cooling gas. The results showed that increasing the distance between the substrate and the plasma jet nozzle up to but not exceeding the optimum distance was observed to increase the peaks intensity. Finally, the results showed that increasing the plasma power beyond the optimum power value was observed to reduce the peaks intensity due to monomer fragmentation. The WCA of the 2-ethyl-2-oxazoline thin film, Terpinene-4-ol thin film, and (Dimethyl ethyl) dimethyl silane thin film indicated that they were all hydrophilic while the WCA of Bis(dimethyl amino) dimethyl silane thin film indicated that it was hydrophobic. The antibacterial activity of the thin films was investigated against two Gram-positive bacteria (Streptococcus pyogenes and Staphylococcus aureus cells) and two Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa cells). The antiviral activity of the thin films was investigated against enveloped Sars Covid 2 like virus. The antibacterial tests showed that all the thin films exhibited high antibacterial activity (greater than 90 % bacterial reduction) compared to the control. The antibacterial tests also showed that the thin films can be classified as contact-active surface. The antiviral activity of the thin films was not fully determined potentially due very weak forces of interactions. The excellent antibacterial properties of the thin films allow their potential application in microbiological protection materials and related fields. The utilization of NTP to sterilize surfaces containing pathogenic microorganisms has gained appreciable attention in the past decades due to their widely documented superior advantages over other methods. In the second section of this thesis, NTP was used to sterilize E. Coli cells. The cell adhesion and the cellular membrane integrity of the plasma-treated E. Coli was then imaged by SEM. E. Coli adhesion presents no problems on HDPE and TPU polymer surfaces, while being more dispersed on stainless steel and absent on silicon wafer substrate. Cumulative damages by the NTP treatment overrode the stress tolerance of E. Coli cells, eventually completing their death. Therefore, NTP has great potential for current and future practical use in decontamination and sterilization fields.