Atomic force microscopy studies of bioprocess engineering surfaces - imaging, interactions and mechanical properties mediating bacterial adhesion

The detrimental effect of bacterial biofilms on process engineering surfaces is well documented. Thus, interest in the early stages of bacterial biofilm formation; in particular bacterial adhesion and the production of anti-fouling coatings has grown exponentially as a field. During this time, Atomic force microscopy (AFM) has become an essential tool for the evaluation of bacterial adhesion. Due to its versatility AFM offers not only insight into the topographical landscape and mechanical properties of the engineering surfaces, but elucidates, through direct quantification the topographical and biomechnical properties of the foulants The aim of this paper is to collate the current research on bacterial adhesion, both theoretical and practical, and outline how AFM as a technique is uniquely equipped to provide further insight into the nanoscale world at the bioprocess engineering surface

Paraffin Wax Crystal Coarsening: Effects of Strain and Wax Crystal Shape

We developed a model of paraffin wax crystal coarsening that well describes our experimental results and allows the behavior of the paraffin films to be predicted on the basis of the extracted kinetic parameters. Wax crystalline films were evaporated on different substrates (silicon wafer, glass slide, thin layer of gold on silicon), thermally treated at different temperatures (25–60 °C), and investigated by powder X-ray diffraction, high-resolution scanning electron microscopy, and optical confocal imaging of the surfaces. A preferred (110) crystal orientation of all deposited wax films, independent of substrate type, was observed from the start and increased during heat treatment. The change in preferred orientation was accompanied by changes in crystal morphology and shape, resulting in surface nanoroughening. We modeled the process as the coarsening of oriented C₃₆H₇₄ crystal islands driven by the decrease in total surface energy. Coarsening kinetics was controlled by diffusion of single molecular chains along the substrate. Evolution of nanoroughness during annealing time was well described by a surface coarsening law, <i>H</i>_<i>r</i> ∼ <i>t</i>^1/4. Two additional factors influenced the evolution rate: strains accumulated in wax crystals during deposition, and the initial crystal shape diverged from the shape at equilibrium. Both factors lowered activation energy and effectively shortened coarsening time

Insect attachment on crystalline bioinspired wax surfaces formed by alkanes of varying chain lengths

The impeding effect of plant surfaces covered with three-dimensional wax on attachment and locomotion of insects has been shown previously in numerous experimental studies. The aim of this study was to examine the effect of different parameters of crystalline wax coverage on insect attachment. We performed traction experiments with the beetle Coccinella septempunctata and pull-off force measurements with artificial adhesive systems (tacky polydimethylsiloxane semi-spheres) on bioinspired wax surfaces formed by four alkanes of varying chain lengths (C36H74, C40H82, C44H90, and C50H102). All these highly hydrophobic coatings were composed of crystals having similar morphologies but differing in size and distribution/density, and exhibited different surface roughness. The crystal size (length and thickness) decreased with an increase of the chain length of the alkanes that formed these surfaces, whereas the density of the wax coverage, as well as the surface roughness, showed an opposite relationship. Traction tests demonstrated a significant, up to 30 fold, reduction of insect attachment forces on the wax surfaces when compared with the reference glass sample. Attachment of the beetles to the wax substrates probably relied solely on the performance of adhesive pads. We found no influence of the wax coatings on the subsequent attachment ability of beetles. The obtained data are explained by the reduction of the real contact between the setal tips of the insect adhesive pads and the wax surfaces due to the micro- and nanoscopic roughness introduced by wax crystals. Experiments with polydimethylsiloxane semi-spheres showed much higher forces on wax samples when compared to insect attachment forces measured on these surfaces. We explain these results by the differences in material properties between polydimethylsiloxane probes and tenent setae of C. septempunctata beetles. Among wax surfaces, force experiments showed stronger insect attachment and higher pull-off forces of polydimethylsiloxane probes on wax surfaces having a higher density of wax coverage, created by smaller crystals

Three-Dimensional Triple Hierarchy Formed by Self-Assembly of Wax Crystals on CuO Nanowires for Nonwettable Surfaces

Novel hierarchical surfaces combining paraffin wax crystals and CuO nanowires are presented. We demonstrate a bioinspired hierarchical wax on nanowire (NW) structures having high water and ethylene glycol repellence. In general, vertically grown nanowire arrays can provide a superhydrophobic surface (SHS) due to extremely high surface roughness but cannot repel ethylene glycol. In this paper, C₃₆H₇₄ and C₅₀H₁₀₂ waxes are thermally evaporated on the surface of CuO NWs, forming highly ordered, three-dimensional (3D) hierarchical structures via self-assembly of wax crystals. These two and three level hierarchical structures provide perfect self-cleaning characteristics, with water contact angles (CAs) exceeding 170°. Furthermore, C₃₆H₇₄ and C₅₀H₁₀₂ wax crystals assembled perpendicularly to the longitudinal NW axis form a re-entrant (that is, a multivalued surface topography) curvature enabling high repellence to ethylene glycol (EG) with CAs exceeding 160°. We analyze the wettability dependence on wax crystal size and structure for the optimization of nonwettable hierarchical structured surfaces