Cooperative stiffening of flexible high aspect ratio nanostructures impart mechanobactericidal activity to soft substrates

Understanding how bacteria interact with surfaces with micrometer and/or sub-micrometer roughness is critical for developing antibiofouling and bactericidal topographies. A primary research focus in this field has been replicating and emulating bioinspired nanostructures on various substrates to investigate their mechanobactericidal potential. Yet, reports on polymer substrates, especially with very high aspect ratios, have been rare, despite their widespread use in our daily lives. Specifically, the role of a decrease in stiffness with an increase in the aspect ratio of nanostructures may be consequential for the mechanobactericidal mechanism, which is biophysical in nature. Therefore, this work reports on generating bioinspired high aspect ratio nanostructures on poly(ethylene terephthalate) (PET) surfaces to study and elucidate their antibacterial and antibiofouling properties. Biomimetic nanotopographies with variable aspect ratios were generated via maskless dry etching of PET in oxygen plasma. It was found that both high and low-aspect ratio structures effectively neutralized Gram-negative bacterial contamination by imparting damage to their membranes but were unable to inactivate Gram-positive cells. Notably, the clustering of the soft, flexible tall nanopillars resulted in cooperative stiffening, as revealed by the nanomechanical behavior of the nanostructures and validated with the help of finite element simulations. Moreover, external capillary forces augmented the killing efficiency by enhancing the strain on the bacterial cell wall. Finally, experimental and computational investigation of the durability of the nanostructured surfaces showed that the structures were robust enough to withstand forces encountered in daily life. Our results demonstrate the potential of the single-step dry etching method for the fabrication of mechanobactericidal topographies and their potential in a wide variety of applications to minimize bacterial colonization of soft substrates like polymers.

Exploring the potential of cold plasma treatment followed by zinc-priming for biofortification of buckwheat sprouts

Increasing the concentration of an element in edible produce (i.e., biofortification) can mitigate the element deficiency in humans. Sprouts are small but popular part of healthy diets providing vitamins and essential elements throughout the year. Element composition of sprouts can easily be amended, e.g., by soaking the grains in element-rich solution before germination (grain-priming). In addition, pre-treatment of grains to improve element translocation from the solution into the grain may further enhance the element concentration in the sprout. Cold plasma technique could provide such solution, as it increases wettability and water uptake of grains. Grains of common buckwheat (Fogopyrum esculentum Moench) were pre-treated/ untreated with cold plasma and soaked in ZnCl₂ solution/pure water. Germination tests, α-amylase activity, grain hydrophilic properties and water uptake were assessed. Element composition of grain tissues and of sprouts was assessed by micro-particle-induced-X-ray emission and X-ray fluorescence spectroscopy, respectively. Grain-priming increased Zn concentration in shoots of common buckwheat sprouts more than five-times, namely from 79 to 423 mg Zn kg^-1 dry weight. Cold plasma treatment increased grain wettability and water uptake into the grain. However, cold plasma pre-treatment followed by grain-priming with ZnCl₂ did not increase Zn concentration in different grain tissues or in the sprouts more than the priming alone, but rather decreased the Zn concentration in sprout shoots (average ± standard error: 216 ± 6.13 and 174 ± 7.57 mg Zn kg^-1 dry weight, respectively). When the fresh weight portion of whole sprouts (i.e., of roots and shoots) was considered, comparable average requirements of Zn, namely 24.5 % and 35 % for adult men and women would be satisfied by consuming cold plasma pre-treated and not pre-treated grains. Potential advantages of cold plasma pre-treatment need to be tested further, mainly to optimize the duration of soaking required to produce Zn-enriched sprouts.

Hydrophobic Recovery of Plasma-Hydrophilized Polyethylene Terephthalate Polymers

Oxygen plasma is widely used for enhancing the wettability of numerous polymers, including polyethylene terephthalate (PET). The treatment with plasma containing oxygen will cause surface functionalization with polar functional groups, which will, in turn, improve the wettability. However, the exact mechanisms leading to the hydrophilic or even super-hydrophilic surface finish are still insufficiently explored. The wettability obtained by plasma treatment is not permanent, since the hydrophobic recovery is usually reported. The mechanisms of hydrophobic recovery are reviewed and explained. Methods for suppressing this effect are disclosed and explained. The recommended treatment which assures stable hydrophilicity of PET samples is the treatment with energetic ions and/or vacuum ultraviolet radiation (VUV). The influence of various plasma species on the formation of the highly hydrophilic surface finish and stability of adequate wettability of PET materials is discussed.

In vitro screening procedure for characterization of thrombogenic properties of plasma treated surfaces

Estimation of thrombogenic surface properties is an important aspect of hemocompatibility studies. To improve our understanding of interaction between blood and biomaterial surfaces, there is a need to employ standardized methods that are both effective and efficient. This contribution details a systematic approach for the in vitro analysis of plasma modified polymer surfaces and human blood platelet interaction, following the recently introduced ISO 10933-4 guidelines. A holistic multistep process is presented that considers all aspects of testing procedure, including blood collection, platelet function testing, and incubation parameters, right through to a comparison and evaluation of the different methods and analysis available. In terms of detection and analysis, confocal light microscopy is shown to offer many advantages over the widely used scanning electron microscopy technique; this includes simpler, less-invasive sample preparation, and less time-consuming analysis procedure. On the other hand, as an alternative to microscopy techniques, toxicology sulforhodamine B based assay (TOX assay) was also evaluated. It has been shown that the assay could be used for rapid estimation of relative concentration of blood platelets on the surface of plasma treated materials, especially when samples do not allow the implementation of microscopy techniques.

Cicada-Wing-Inspired Self-Cleaning Antireflection Coatings on Polymer Substrates

The cicada has transparent wings with remarkable self-cleaning properties and high transmittance over the whole visible spectral range, which is derived from periodic conical structures covering the wing surface. Here we report a scalable self-assembly technique for fabricating multifunctional optical coatings that mimic cicada-wing structures. Spin-coated two-dimensional non-close-packed colloidal crystals are utilized as etching masks to pattern subwavelength-structured cone arrays directly on polymer substrates. The resulting gratings exhibit broadband antireflection performance and superhydrophobic properties after surface modification. The dependence of the cone shape and size on the antireflective and self-cleaning properties has also been investigated in this study.