Testurizzazione laser: una nuova strada verso la realizzazione di superfici antibatteriche

Abstract

Bacterial surface retention is a common and interesting problem, involving a wide variety of technological fields of application, including agrifood, biomedical devices, treatment of water, marine technologies, etc.. Despite its relevance and diffusion, the problem has never found a simple, straightforward, and unified picture, owing to the complexity of the basic processes, of a nature inherently at the border between different disciplines (physics, chemistry, biology, etc.), and to the occurrence of different length scales of interest, ranging from the nanoscopic to the macroscopic one. Nonetheless, scientists have tried to find practical solutions to tune the amount of surface bacterial biofilm. Recently a novel strategy, based on contrasting bacterial adhesion using surface roughness, has emerged as a promising approach to realize an effective and long-lasting treatment. Advantages with respect to conventional chemistry-based methods, entailing deposition of a biocide layer, are the absence of chemicals, leading to long-lasting properties, and the scale-up possibilities, well feasible thanks to the availability in the industrial framework of ultrafast laser sources operating at high fluence and large repetition rates. In the present thesis work, AISI 316L surfaces have been laser textured and assessed in terms of antibactericity. This work has been carried out within the frame of the TresClean (“High ThRoughput lasEr texturing of SelfCLEANing and antibacterial surfaces”) European Project (ICT-H2020), coordinated by the University of Parma, involving an international collaboration with ECOR Research (Schio, Italy), University of Stuttgart (Stuttgart, Germany), AlphaNov-Centre Technologique Optique & Lasers (Bordeaux, France), Raylase (Munich, Germany), BSH Electrodomésticos España (Zaragoza, Spain) and Modus, Research and Innovation (Dundee, United Kingdom). Various processing parameters and fabrication strategies were explored and correlation with antibacterial properties searched via bacterial assays based on ISO standards, carried out in a collaboration with the center ARTEST (Modena, Italy). Furthermore, the possibility of replicating the laser induced texture from metallic to polymeric substrates by injection molding, in the attempt to replicate the functional properties as well, has been investigated. The present state-of-the-art in the knowledge of the processes enhancing or inhibiting bacterial growth on surfaces textured at the micro- and nanoscale includes several pictures, such as the sheltering effect, where the bacterial cell is trapped inside a microstructured pattern, leading the adhesion probability to increase, and the fakir effect, a biomimetic-inspired mechanism where the presence of sharp nanostructures is expected to reduce contact area, hence adhesion probability. Such knowledge is, however, of mostly qualitative nature. In particular, detailed microscopic models able to describe dynamics and interaction. of bacterial cells in proximity of a rough surface, showing micro- and nanometric asperities, are not available. The present work aims at progressing in the related knowledge through the development of two essential pillars. On one hand, also within a collaboration with the Department of Physics “Enrico Fermi”, University of Pisa, Italy, a range of robust, sensitive, and spatially resolved diagnostics has been applied to the processed surfaces in order to reconstruct their local morphology. The morphological analysis revealed that, by varying the laser process parameters and the target crystallinity, different surface features can be obtained. To make the description more suitable for a straightforward implementation in an industrial context, a metrological analysis in the framework of ISO standards was made aimed at identifying a set of parameters suitable for classifying the obtained surfaces. On the other hand, a comprehensive computational effort was devoted to numerically simulate the dynamics and the behaviour of single bacterial cells in the planktonic state (i. e. immersed in a fluid flow, a common situation in food handling) placed in proximity of a rough surface. An original simulation approach was developed to this aim, that enabled highlighting the role of surface structures in enhancing or, on the contrary, in contrasting bacterial adhesion. Being the simulation focused onto the early stage of the bacterial contamination process, a physical model was used treating cell/surface interactions through suitable expressions of interaction potentials. The simulations were carried out within the framework of the High Performance Computing (HPC) facility of the University of Parma. The computational results led to predictions that will be useful in designing textured surfaces, with the aim of employing their roughness to tune the antibacterial behaviour. In addition, the simulations can represent a valid ally of the common experimental procedures to understand basic mechanisms in the interactions of bacteria with substrates during the early stage of biofilm formation.