Design of bio-inspired texturing treatments, via high power ultra-short pulse lasers, of ferritic stainless steel surfaces that inhibit the formation of bacterial biofouling on tools for hospital use and in the food industry

Bacteria are microorganisms that can be found in a wide variety of environments and locations, which one of their key characteristics is their high capacity to adhere to different surfaces. This adhesion process results in the formation of biofilms. The significance of these biostructures lies in the fact that the bacteria within them acquire new properties, such as increased resistance to antibiotics and unfavorable external conditions. Additionally, it is important to note that there is a growing number of bacteria that have developed antibiotic resistance due to their misuse, primarily due to the overuse. Both factors combined make biofilms that form spontaneously on surfaces very difficult to completely eliminate using traditional cleaning and disinfection methods, which are often not only ineffective but also slow and costly. This issue is of critical importance in several sectors, particularly in the food industry and healthcare environments. In the food industry, the environmental conditions are favorable for the proliferation of microorganisms. In healthcare settings, patients often have weakened immune systems or open wounds, which become potential infection sources. Therefore, it is crucial that these environments are kept as free from pathogenic microorganisms as possible, making it essential to prevent or at least minimize biofilm formation. Given this context, the scientific community is making significant efforts to design and produce antibacterial surfaces that hinder bacterial adhesion. Among the various techniques available for creating such surfaces, laser technology has emerged as particularly promising due to its numerous advantages, such as ease of scaling, environmental sustainability (as it produces no waste), and high reproducibility. For these reasons, this doctoral thesis has employed the aforementioned laser technique to develop surfaces with antibacterial properties inspired by the living surfaces, such as the leaf of the lotus flower. The material chosen for this study is EN 1.4016 ferritic stainless steel with bright finish, as it is widely used in both the food industry and healthcare environments, as well as in other applications like door handles and handrails, which are in frequent contact with human hands and therefore serve as significant reservoirs of microorganisms. Laser technology enables the generation of a variety of nanostructures, among which Laser-Induced Periodic Surface Structures (LIPSS) stand out. These quasi-periodic nanostructures consist of alternating ridges and valleys with a spatial period similar to or smaller than the laser wavelength used and an orientation dependent on the polarization of the laser. To generate the LIPSS, the average pulse fluence and the scanning speed were first studied to determine the optimal parameters for creating these nanostructures without damaging the surface (1D analysis). Following this study, in pursuit of generating LIPSS across the entire surface, the width of the laser lines was analyzed to estimate the optimal hatch distance (the spacing between lines) to ensure overlap and avoid gaps without LIPSS between consecutive lines, thus covering complete surfaces (2D analysis). The three variables studied -average fluence, scanning speed, and hatch distance- can be jointly examined using the Surface Density of Applied Energy (SDAE), which is calculated based on the number of pulses incident on an area, considering the energy of each pulse. This process was performed using lasers with different wavelengths: 1030 nm (infrared range), 513 nm (visible range, corresponding to the green color), and 343 nm (ultraviolet range). As a result, LIPSS with average periodicities of 890, 390, and 260 nm, respectively, were produced. Bacterial adhesion tests were initially performed using Escherichia coli as a model microorganism. In the first adhesion tests conducted on surfaces with LIPSS generated by the infrared laser, satisfactory results were achieved, with E. coli adhesion reduced by 45% compared to untreated surfaces. Physical-chemical characterization of the surfaces revealed that the textured samples evolved over time, with their contact angle (upon water droplet deposition) stabilizing after approximately two weeks, shifting from hydrophilic to hydrophobic behavior. This evolution is attributed to the adsorption of volatile organic compounds in the first few days after laser texturing, as confirmed by X-ray photoelectron spectroscopy (XPS) measurements, consistent with the literature. The initial tests were conducted on freshly prepared, unstable samples, so the tests were repeated after stabilization. The new tests with stable samples, prepared under the same conditions as before, yielded negative results in every case, with no reduction in bacterial adhesion. This is likely because E. coli has a size similar to the spacing between consecutive ridges, allowing the bacteria to fit into the valleys and facilitating surface colonization. Consequently, it was hypothesized that treatments using lasers with shorter wavelengths, which generate LIPSS with smaller spatial periods, could yield positive results in reducing bacterial adhesion by preventing bacteria from settling between ridges. The experiments confirmed this hypothesis, achieving nearly 30% reduction in adhesion on surfaces treated with the green laser and 35% on UV laser-treated surfaces. In both cases, these surfaces were also stable over time, as the contact angle evolution was independent of the laser wavelength used. Furthermore, it was observed that in all cases (IR-unstable, IR-stable, visible, and UV samples), E. coli adhesion increased with increasing SDAE. Atomic Force Microscopy (AFM) analysis indicated that higher incident energy results in deeper LIPSS (greater peak-to-valley height). Consequently, the deeper the LIPSS, the greater E. coli adhesion and subsequent biofilm formation, likely due to increased contact area between the bacteria and the surface, promoting interactions. Moreover, all these surfaces exhibit hydrophobic properties, though not excessively so, which according to the literature, favor the reduction of bacterial adhesion. Following the highly satisfactory results obtained with E. coli on UV LIPSS-covered surfaces, bacterial adhesion tests were conducted with Staphylococcus aureus and Salmonella spp. on these surfaces. Positive results were also obtained for these two microorganisms, with adhesion reductions of 43% and 70%, respectively. For S. aureus, due to its spherical shape, the depth of the LIPSS was not a key factor in adhesion, whereas for Salmonella spp., adhesion decreased as the LIPSS depth increased. Once surfaces capable of reducing bacterial adhesion were successfully developed, so the primary objective of this doctoral thesis was achieved, corrosion tests were performed to ensure that the stainless steel retained its corrosion resistance. Both pitting corrosion tests and salt spray chamber tests demonstrated that the corrosion resistance of the nanostructured samples was not worse than that of the untextured samples. In fact, in the pitting corrosion tests, the corrosion potential of the nanostructured samples was consistently slightly higher than that of the untextured ones, maintaining a certain linearity concerning the SDAE of each treatment. Additionally, both qualitative and quantitative analyses of surface bright were conducted post-laser treatment, as this is an essential feature of these polished samples. This was determined using Light Reflectance Value (LRV) measurements, a parameter that quantifies surface bright, showing that the bright loss after texturing was only around 10%, maintaining the specular behaviour. This phenomenon was also qualitatively observed. Apart from surface topography, already shown to be a key factor in bacterial adhesion, other surface properties that may also play an important role in the adhesion process were analyzed. Firstly, surface wettability was evaluated, and as previously mentioned, all samples exhibited hydrophobicity, which hinders bacterial adhesion. AFM was also used to determine the surface potential. Although no clear trend was observed, a potential difference was consistently found between the ridges and valleys of the LIPSS, with the ridges showing a higher potential. Since bacterial surfaces generally carry a net negative charge, they are more strongly attracted to the ridges, where the contact area between the bacteria and the material is smaller, thus hindering biofilm formation. To further investigate this phenomenon, ellipsometry measurements of the textured samples were performed in two configurations: with the incident beam parallel to the LIPSS and perpendicular to them. The first configuration provides information about the entire nanostructure, while the second focuses on the ridges, as they "shade" the valleys. The fit model for the ellipsometric angles, Psi and Delta, in the parallel configuration, reflects the metallic nature of the surface, typical of stainless steel. However, in the perpendicular configuration, the model suggests a dielectric character in the ridges, which could explain the potential difference. Additionally, from the fit models, the effective thicknesses of the measured layers can be obtained. In the parallel configuration, the values closely match the peak-to-valley heights obtained by AFM, as this configuration provides information about the entire surface. In the perpendicular configuration, the effective thicknesses are smaller, as expected, but it is noteworthy that for all treatments, the effective thickness represents approximately 60% of the total LIPSS depth. Finally, the ellipsometric models also allow the calculation of the dielectric constant of the surface. The value obtained from the model corresponding to the ridges (perpendicular configuration) is much closer to that of the bacterial surface than the value provided by the model for the entire surface (parallel configuration). This also indicates that bacteria are more likely to interact with the ridges, limiting contact to these areas. Electron microscopy images of the surfaces and LIPSS profiles were also obtained, supporting and validating the previous findings. In conclusion, the factors studied in detail (topography, periodicity and depth of LIPSS, wettability, surface potential, and dielectric constant values) suggest the presence of a synergistic effect among them, leading to the significant bacterial reductions achieved.

Author dc:creator

• Outón Porras, Javier

Advisors dc:contributor.advisor

• Blanco Ollero, Eduardo
• Matres Serrano, Victoria

Statement dc:rights

• Attribution-NonCommercial-NoDerivatives 4.0 Internacional

Licence dc:rights.uri

• http://creativecommons.org/licenses/by-nc-nd/4.0/

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/10498/38191

OAI identifier oai:identifier

oai:rodin.uca.es:10498/38191