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Title: Design and fabrication of superhydrophobic and antimicrobial surfaces on AISI 316L stainless steel
Author: Cai, Yukui
ISNI:       0000 0004 8502 9426
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2019
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In a surgical procedure, the medical devices come into contact with blood, which increases the risk of the formation of blood clots, also known as thrombi. Antimicrobial resistance is another pressing issue in the healthcare field. With the increasing consumption of antimicrobial drugs, the threat of antimicrobial resistance is rising, and the global dimensions of this trend have been increasingly recognised. Some new strategies that utilise superhydrophobicity or antimicrobial properties to medical devices have garnered more attention and interest. Superhydrophobic surfaces have attracted extensive attention over the past decade, primarily due to their self-cleaning, corrosion resistance, anti-icing and drag reduction abilities. The ability to reduce blood adhesion is one of the critical benefits of these types of surfaces. Nanosecond pulsed laser ablation is considered to be a promising technique for the industrial fabrication of superhydrophobic structures due to its high efficiency and low-cost. In this PhD thesis, nanosecond pulsed laser-based ablation technology was developed to manufacture functional surfaces that have superhydrophobicity or antimicrobial properties on AISI 316L stainless steel. To achieve that goal, a deterministic design method was developed to design the dimensions of the microstructures to be fabricated by laser ablation in order to maximise superhydrophobicity. Then computational fluid dynamics (CFD) simulation was conducted to explore the underlying mechanism of superhydrophobicity and predict the hydrophobicity of the designed structures. The simulation results proved that the substrates trapped a large volume of air with high pressure at the bottom of the structures, which is critical to achieving stable superhydrophobicity. Moreover, the superhydrophobic substrate has greater potential energy and kinetic energy in the water droplet's impacting process, which helps explain its self-cleaning and lowadhesion properties. In the next step, the process and the product fingerprints are proposed for the first time to identify the correlations among the machining parameters, surface topography and functional performance (i.e. the contact angle of the laser ablated superhydrophobic surface on AISI 316L stainless steel) of the specimen. The dimensionless surface functional characterisation parameter Rhy (i.e. the average ratio of Rz to Rsm) has maximum values of Spearman and Kendall rank correlation coefficients with contact angle, which can be regarded as the product fingerprint. The laser pulse energy per unit area on the specimen (Is) represents the combined effect of the laser power, exposure time and pitch of the structure on the surface topography, and it is the best process fingerprint that can be used to control the product fingerprint Rhy. The threshold values of Rhy and Is are 0.41 and 536 J/mm2, respectively, ensuring the specimen's superhydrophobicity (contact angle larger than 150°) in the laser ablation process. Finally, two new hybrid processes based on laser ablation were developed to manufacture functional surfaces with anisotropic superhydrophobicity and antimicrobial properties. First, a sequential process of laser ablation and chemical etching (LA-CE) was proposed to produce ratchet-like microstructures on AISI 316L stainless steel. The experimental investigation concluded that the direction of the microstructures is the same as the direction of the laser beam feed. Moreover, the droplet easily rolls off the surface in the laser beam feed direction; however, it is pinned tightly in the opposite direction. This study was the first to use a single-step fabrication approach (StruCoat) to develop the antimicrobial surfaces based on laser ablation technology in order to generate the antimicrobial microstructures coated with silver nanoparticles (AgNPs) on AISI 316L stainless steel. The experimental results showed that silver nitrate with a molarity of 50 mmol at the laser power of 14 W, which resulted in AgNPs with a mean size of 480 nm, was the best processing condition for the chemical decomposition of silver nitrate micro drops. Furthermore, StruCoat helped increase the cooling rate of the substrate in the laser ablation process, resulting in a significant decrease in the material grain size (by 81%). Furthermore, antimicrobial efficacy testing also demonstrated the enhanced antimicrobial properties of StruCoat, with an 86.2% antimicrobial rate against Staphylococcus aureus, in comparison to the unmodified specimens.
Supervisor: Luo, Xichun ; Qin, Yi Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral