Antibacterial Activity of Nanostructured Zinc Oxide Tetrapods.

Zinc oxide (ZnO) tetrapods as microparticles with nanostructured surfaces show peculiar physical properties and anti-infective activities. The aim of this study was to investigate the antibacterial and bactericidal properties of ZnO tetrapods in comparison to spherical, unstructured ZnO particles. Additionally, killing rates of either methylene blue-treated or untreated tetrapods and spherical ZnO particles for Gram-negative and Gram-positive bacteria species were determined. ZnO tetrapods showed considerable bactericidal activity against Staphylococcus aureus, and Klebsiella pneumoniae isolates, including multi-resistant strains, while Pseudomonas aeruginosa and Enterococcus faecalis remained unaffected. Almost complete elimination was reached after 24 h for Staphylococcus aureus at 0.5 mg/mL and Klebsiella pneumoniae at 0.25 mg/mL. Surface modifications of spherical ZnO particles by treatment with methylene blue even improved the antibacterial activity against Staphylococcus aureus. Nanostructured surfaces of ZnO particles provide active and modifiable interfaces for the contact with and killing of bacteria. The application of solid state chemistry, i.e., the direct matter-to-matter interaction between active agent and bacterium, in the form of ZnO tetrapods and non-soluble ZnO particles, can add an additional principle to the spectrum of antibacterial mechanisms, which is, in contrast to soluble antibiotics, depending on the direct local contact with the microorganisms on tissue or material surfaces.

Evaporation kinetics in highly porous tetrapodal zinc oxide networks studied using in situ SRµCT.

Tetrapodal zinc oxide (t-ZnO) is used to fabricate polymer composites for many different applications ranging from biomedicine to electronics. In recent times, macroscopic framework structures from t-ZnO have been used as a versatile sacrificial template for the synthesis of multi-scaled foam structures from different nanomaterials such as graphene, hexagonal boron nitride or gallium nitride. Many of these fabrication methods rely on wet-chemical coating processes using nanomaterial dispersions, leading to a strong interest in the actual coating mechanism and factors influencing it. Depending on the type of medium (e.g. solvent) used, different results regarding the homogeneity of the nanomaterial coating can be achieved. In order to understand how a medium influences the coating behavior, the evaporation process of water and ethanol is investigated in this work using in situ synchrotron radiation-based micro computed tomography (SRµCT). By employing propagation-based phase contrast imaging, both the t-ZnO network and the medium can be visualized. Thus, the evaporation process can be monitored non-destructively in three dimensions. This investigation showed that using a polar medium such as water leads to uniform evaporation and, by that, a homogeneous coating of the entire network.

Role of structural specificity of ZnO particles in preserving functionality of proteins in their corona.

Reconfiguration of protein conformation in a micro and nano particle (MNP) protein corona due to interaction is an often-overlooked aspect in drug design and nano-medicine. Mostly, MNP-Protein corona studies focus on the toxicity of nano particles (NPs) in a biological environment to analyze biocompatibility. However, preserving functional specificity of proteins in an NP corona becomes critical for effective translation of nano-medicine. This paper investigates the non-classical interaction between insulin and ZnO MNPs using a classical electrical characterization technique at GHz frequency with an objective to understand the effect of the micro particle (MP) and nanoparticle (NP) morphology on the electrical characteristics of the MNP-Protein corona and therefore the conformation and functional specificity of protein. The MNP-Protein corona was subjected to thermal and enzymatic (papain) perturbation to study the denaturation of the protein. Experimental results demonstrate that the morphology of ZnO particles plays an important role in preserving the electrical characteristics of insulin.

Development of Polythiourethane/ZnO-Based Anti-Fouling Materials and Evaluation of the Adhesion of Staphylococcus aureus and Candida glabrata Using Single-Cell Force Spectroscopy.

The attachment of bacteria and other microbes to natural and artificial surfaces leads to the development of biofilms, which can further cause nosocomial infections. Thus, an important field of research is the development of new materials capable of preventing the initial adhesion of pathogenic microorganisms. In this work, novel polymer/particle composite materials, based on a polythiourethane (PTU) matrix and either spherical (s-ZnO) or tetrapodal (t-ZnO) shaped ZnO fillers, were developed and characterized with respect to their mechanical, chemical and surface properties. To then evaluate their potential as anti-fouling surfaces, the adhesion of two different pathogenic microorganism species, Staphylococcus aureus and Candida glabrata, was studied using atomic force microscopy (AFM). Our results show that the adhesion of both S. aureus and C. glabrata to PTU and PTU/ZnO is decreased compared to a model surface polydimethylsiloxane (PDMS). It was furthermore found that the amount of both s-ZnO and t-ZnO filler had a direct influence on the adhesion of S. aureus, as increasing amounts of ZnO particles resulted in reduced adhesion of the cells. For both microorganisms, material composites with 5 wt.% of t-ZnO particles showed the greatest potential for anti-fouling with significantly decreased adhesion of cells. Altogether, both pathogens exhibit a reduced capacity to adhere to the newly developed nanomaterials used in this study, thus showing their potential for bio-medical applications.

Polydimethylsiloxane Microdomains Formation at the Polythiourethane/Air Interface and Its Influence on Barnacle Release.

In this study, polydimethylsiloxane (PDMS)/polythiourethane (PTU) composite reinforced with tetrapodal shaped micro-nano ZnO particles (t-ZnO) was successfully produced by a versatile, industrially applicable polymer blending process. On the surface of this composite, PDMS is distributed in the form of microdomains embedded in a PTU matrix. The composite inherited not only good mechanical properties originating from PTU but also promising fouling-release (FR) properties due to the presence of PDMS on the surface. It was shown that the preferential segregation of PDMS domains at the polymer/air interface could be attributed to the difference in the surface free energy of PDMS and PTU. The PDMS microdomains at the PTU/air interface significantly reduced the barnacle adhesion strength on the composite. Both the pseudo- and natural barnacle adhesion strength on the composite was approximately 0.1 MPa, similar to that on pure PDMS. The pseudo-barnacle adhesion on reference surfaces AlMg3 and PTU reached approximately 4 and 6 MPa, respectively. Natural barnacles could not be removed intact from AlMg3 and PTU surfaces without breaking the shell, indicating that the adhesion strength was higher than the mechanical strength of a barnacle shell (approximately 0.4 MPa). The integrity of PDMS microdomains was maintained after 12 months of immersion in seawater and barnacle removal. No surface deteriorations were found. In short, the composite showed excellent potential as a long-term stable FR coating for marine applications.

Development and Characterization of Mechanically Durable Silicone-Polythiourethane Composites Modified with Tetrapodal Shaped ZnO Particles for the Potential Application as Fouling-Release Coating in the Marine Sector.

Ecological considerations strongly necessitate the development of environmentally friendly antifouling paints. A promising alternative to biocide containing antifouling paints are fouling-release coatings, which are non-toxic and designed to prevent permanent attachment of marine organisms to the surface, due to their low surface energy. However, these coatings suffer from insufficient mechanical properties, which make them unsuitable for mechanically stressed surfaces e.g., on ship hulls. To overcome those obstacles, polydimethylsiloxane (PDMS)-polythiourethane (PTU) composites modified with tetrapodal shaped micro-nano ZnO particles (t-ZnO) were produced and characterized by evaluating the surface energy, mechanical properties, and fouling-release performance. Among all variations, PTU/1 wt.% PDMS composites with 1 wt.% t-ZnO particles possess superior properties for applications as fouling-release coatings for maritime purposes.

Sacrificial Template Synthesis and Properties of 3D Hollow-Silicon Nano- and Microstructures.

Novel three-dimensional (3D) hollow aero-silicon nano- and microstructures, namely, Si-tetrapods (Si-T) and Si-spheres (Si-S) were synthesized by a sacrificial template approach for the first time. The new Si-T and Si-S architectures were found as most temperature-stable hollow nanomaterials, up to 1000 °C, ever reported. The synthesized aero-silicon or aerogel was integrated into sensor structures based on 3D networks. A single microstructure Si-T was employed to investigate electrical and gas sensing properties. The elaborated hollow microstructures open new possibilities and a wide area of perspectives in the field of nano- and microstructure synthesis by sacrificial template approaches. The enormous flexibility and variety of the hollow Si structures are provided by the special geometry of the sacrificial template material, ZnO-tetrapods (ZnO-T). A Si layer was deposited onto the surface of ZnO-T networks by plasma-enhanced chemical vapor deposition. All samples demonstrated p-type conductivity; hence, the resistance of the sensor structure increased after introducing the reducing gases in the test chamber. These hollow structures and their unique and superior properties can be advantageous in different fields, such as NEMS/MEMS, batteries, dye-sensitized solar cells, gas sensing in harsh environment, and biomedical applications. This method can be extended for synthesis of other types of hollow nanostructures.

Complex shaped ZnO nano- and microstructure based polymer composites: mechanically stable and environmentally friendly coatings for potential antifouling applications.

Since the prohibition of tributyltin (TBT)-based antifouling paints in 2008, the development of environmentally compatible and commercially realizable alternatives is a crucial issue. Cost effective fabrication of antifouling paints with desired physical and biocompatible features is simultaneously required and recent developments in the direction of inorganic nanomaterials could play a major role. In the present work, a solvent free polymer/particle-composite coating based on two component polythiourethane (PTU) and tetrapodal shaped ZnO (t-ZnO) nano- and microstructures has been synthesized and studied with respect to mechanical, chemical and biocompatibility properties. Furthermore, antifouling tests have been carried out in artificial seawater tanks. Four different PTU/t-ZnO composites with various t-ZnO filling fractions (0 wt%, 1 wt%, 5 wt%, 10 wt%) were prepared and the corresponding tensile, hardness, and pull-off test results revealed that the composite filled with 5 wt% t-ZnO exhibits the strongest mechanical properties. Surface free energy (SFE) studies using contact angle measurements showed that the SFE value decreases with an increase in t-ZnO filler amounts. The influence of t-ZnO on the polymerization reaction was confirmed by Fourier transform infrared-spectroscopy measurements and thermogravimetric analysis. The immersion tests demonstrated that fouling behavior of the PTU/t-ZnO composite with a 1 wt% t-ZnO filler has been decreased in comparison to pure PTU. The composite with a 5 wt% t-ZnO filler showed almost no biofouling.

Dry friction of microstructured polymer surfaces inspired by snake skin.

The microstructure investigated in this study was inspired by the anisotropic microornamentation of scales from the ventral body side of the California King Snake (Lampropeltis getula californiae). Frictional properties of snake-inspired microstructured polymer surface (SIMPS) made of epoxy resin were characterised in contact with a smooth glass ball by a microtribometer in two perpendicular directions. The SIMPS exhibited a considerable frictional anisotropy: Frictional coefficients measured along the microstructure were about 33% lower than those measured in the opposite direction. Frictional coefficients were compared to those obtained on other types of surface microstructure: (i) smooth ones, (ii) rough ones, and (iii) ones with periodic groove-like microstructures of different dimensions. The results demonstrate the existence of a common pattern of interaction between two general effects that influence friction: (1) molecular interaction depending on real contact area and (2) the mechanical interlocking of both contacting surfaces. The strongest reduction of the frictional coefficient, compared to the smooth reference surface, was observed at a medium range of surface structure dimensions suggesting a trade-off between these two effects.

Friction behavior of a microstructured polymer surface inspired by snake skin.

The aim of this study was to understand the influence of microstructures found on ventral scales of the biological model, Lampropeltis getula californiae, the California King Snake, on the friction behavior. For this purpose, we compared snake-inspired anisotropic microstructured surfaces to other microstructured surfaces with isotropic and anisotropic geometry. To exclude that the friction measurements were influenced by physico-chemical variations, all friction measurements were performed on the same epoxy polymer. For frictional measurements a microtribometer was used. Original data were processed by fast Fourier transformation (FFT) with a zero frequency related to the average friction and other peaks resulting from periodic stick-slip behavior. The data showed that the specific ventral surface ornamentation of snakes does not only reduce the frictional coefficient and generate anisotropic frictional properties, but also reduces stick-slip vibrations during sliding, which might be an adaptation to reduce wear. Based on this extensive comparative study of different microstructured polymer samples, it was experimentally demonstrated that the friction-induced stick-slip behavior does not solely depend on the frictional coefficient of the contact pair.

Development and regeneration ability of the wax coverage in Nepenthes alata pitchers: a cryo-SEM approach.

The morphogenesis of the composite epicuticular wax coverage and regeneration ability of the upper wax layer in Nepenthes alata pitchers were studied using a cryo-scanning electron microscopy. Examination of pitchers of different ages revealed six stages in the wax coverage development. In the first stage, wax crystals resemble those found recently in mature pitches of N. dicksoniana and N. ventricosa. Platelets of the upper wax layer originate from broadened tips of stalks during the last developmental stage. Contrary to previous hypotheses, we found that wax crystals of both layers as well as the stalks connecting them are oriented perpendicularly to the pitcher wall. No changes in the height of the wax coverage were detected in 4-8 weeks after mechanical removal of the upper wax layer from mature pitchers on plants. This indicates that the wax coverage in N. alata pitchers is unable to regenerate.

Comparative Cryo-SEM and AFM studies of hylid and rhacophorid tree frog toe pads.

Cryo-scanning electron microscopy (cryo-SEM) and atomic force microscopy (AFM) offer new avenues for the study of the morphology of tree frog adhesive toe pads. Using these techniques, we compare toe pad microstructure in two distantly related species of tree frog, Litoria caerulea, White (Hylidae) and Rhacophorus prominanus, Smith (Rhacophoridae), in which the toe pads are considered to be convergent. AFM demonstrates the extraordinary similarity of both surface microstructures (largely hexagonal epithelial cells surrounded by deep channels) and nanostructures (an array of nanopillars, ca. 350 nm in diameter, all with a small dimple at the apex). The cryo-SEM studies examined the distribution of the fibrillar cytoskeleton within the different layers of the stratified toe pad epithelium, demonstrating that the cytoskeletal elements (keratin tonofilaments) that lie at an angle to the surface are relatively poorly developed in L. caerulea, clearly so in comparison to R. prominanus. Cryo-SEM also enabled the visualization of the fluid layer that is critical to a toe pad's adhesive function. This was achieved by examination of the frozen fluid residues left behind after removal of a toe within the cryo-SEM's experimental chamber. Such 'toeprints' demonstrated the presence of a wedge of fluid surrounding each toe pad, as well as fluid filling the channels that surround each epithelial cell. Cryo-SEM was used to examine epithelial cell shape. In a sample of 582 cells, 59.5% were hexagonal, the remainder being mainly pentagonal (23.1%) or heptagonal (16.1%). The distribution of differently-shaped cells was not random, but was not associated with either pad curvature or the distribution of mucous pores that provide fluid for the frogs' wet adhesion mechanism. Our main finding, the great similarity of toe pad structure in these two species, has important implications for biomimetics, for such convergent evolution suggests a good starting point for attempts to develop adhesives that will function in wet conditions.