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- ItemSwitchable Adhesion Surfaces with Enhanced Performance Against Rough Counterfaces(Basel : MDPI, 2016) Prieto-López, Lizbeth; Williams, JohnIn a recent study, we demonstrated that the pressurization of micro-fluidic features introduced in the subsurface of a soft polymer can be used to actively modify the magnitude of the adhesion to a harder counterface by changing its waviness or long wavelength undulations. In that case, both contacting surfaces had very smooth finishes with root-mean-square roughnesses of less than 20 nm. These values are far from those of many engineering surfaces, which usually have a naturally occurring roughness of between ten and a hundred times this value. In this work, we demonstrate that appropriate surface features, specifically relatively slender “fibrils”, can enhance the ability of a such a soft surface to adhere to a hard, but macroscopically rough, counterface, while still maintaining the possibility of switching the adhesion force from one level to another. Conversely, stiffer more conical surface features can suppress adhesion even against a smooth counterface. Examples of each form of topography can be found in the natural world.
- ItemModeling the contact mechanics of hydrogels(Basel : MDPI, 2019) Mueser, M.H.; Li, H.; Bennewitz, R.A computationally lean model for the coarse-grained description of contact mechanics of hydrogels is proposed and characterized. It consists of a simple bead-spring model for the interaction within a chain, potentials describing the interaction between monomers and mold or confining walls, and a coarse-grained potential reflecting the solvent-mediated effective repulsion between non-bonded monomers. Moreover, crosslinking only takes place after the polymers have equilibrated in their mold. As such, the model is able to reflect the density, solvent quality, and the mold hydrophobicity that existed during the crosslinking of the polymers. Finally, such produced hydrogels are exposed to sinusoidal indenters. The simulations reveal a wavevector-dependent effective modulus E*(q) with the following properties: (i) stiffening under mechanical pressure, and a sensitivity of E*(q) on (ii) the degree of crosslinking at large wavelengths, (iii) the solvent quality, and (iv) the hydrophobicity of the mold in which the polymers were crosslinked. Finally, the simulations provide evidence that the elastic heterogeneity inherent to hydrogels can suffice to pin a compressed hydrogel to a microscopically frictionless wall that is undulated at a mesoscopic length scale. Although the model and simulations of this feasibility study are only two-dimensional, its generalization to three dimensions can be achieved in a straightforward fashion. © 2019 by the authors.
- ItemRoll-to-roll manufacturing of micropatterned adhesives by template compression(Basel : MDPI, 2018) Yu, D.; Beckelmann, D.; Opsölder, M.; Schäfer, B.; Moh, K.; Hensel, R.; de, Oliveira, P.; Arzt, E.For the next generation of handling systems, reversible adhesion enabled by micropatterned dry adhesives exhibits high potential. The versatility of polymeric micropatterns in handling objects made from various materials has been demonstrated by several groups. However, specimens reported in most studies have been restricted to the laboratory scale. Upscaling the size and quantity of micropatterned adhesives is the next step to enable successful technology transfer. Towards this aim, we introduce a continuous roll-to-roll replication process for fabrication of high-performance, mushroom-shaped micropatterned dry adhesives. The micropatterns were made from UV-curable polyurethane acrylates. To ensure the integrity of the complex structure during the fabrication process, flexible templates were used. The compression between the template and the wet prepolymer coating was investigated to optimize replication results without structural failures, and hence, to improve adhesion. As a result, we obtained micropatterned adhesive tapes, 10 cm in width and several meters in length, with adhesion strength about 250 kPa to glass, suitable for a wide range of applications. © 2018 by the authors.
- ItemCombining soft polysilazanes with melt-shear organization of core–shell particles: On the road to polymer-templated porous ceramics(Basel : MDPI, 2019) Boehm, A.K.; Ionescu, E.; Koch, M.; Gallei, M.The preparation of ordered macroporous SiCN ceramics has attracted significant interest and is an attractive area for various applications, e.g., in the fields of catalysis, gas adsorption, or membranes. Non-oxidic ceramics, such as SiCN, own a great stability based on the covalent bonds between the containing elements, which leads to interesting properties concerning resistance and stability at high temperature. Their peculiar properties have become more and more important for a manifold of applications, like catalysis or separation processes, at high temperatures. Within this work, a feasible approach for the preparation of ordered porous materials by taking advantage of polymer-derived ceramics is presented. To gain access to free-standing films consisting of porous ceramic materials, the combination of monodisperse organic polymer-based colloids with diameters of 130 nm and 180 nm featuring a processable preceramic polymer is essential. For this purpose, the tailored design of hybrid organic/inorganic particles featuring anchoring sites for a preceramic polymer in the soft shell material is developed. Moreover, polymer-based core particles are used as sacrificial template for the generation of pores, while the preceramic shell polymer can be converted to the ceramic matrix after thermal treatment. Two different routes for the polymer particles, which can be obtained by emulsion polymerization, are followed for covalently linking the preceramic polysilazane Durazane1800 (Merck, Germany): (i) Free radical polymerization and (ii) atom transfer radical polymerization (ATRP) conditions. These hybrid hard core/soft shell particles can be processed via the so-called melt-shear organization for the one-step preparation of free-standing particle films. A major advantage of this technique is the absence of any solvent or dispersion medium, enabling the core particles to merge into ordered particle stacks based on the soft preceramic shell. Subsequent ceramization of the colloidal crystal films leads to core particle degradation and transformation into porous ceramics with ceramic yields of 18–54%. © 2019 by the authors.
- ItemNanopillar diffraction gratings by two-photon lithography(Basel : MDPI, 2019) Purtov, J.; Rogin, P.; Verch, A.; Johansen, V.E.; Hensel, R.Two-dimensional photonic structures such as nanostructured pillar gratings are useful for various applications including wave coupling, diffractive optics, and security features. Two-photon lithography facilitates the generation of such nanostructured surfaces with high precision and reproducibility. In this work, we report on nanopillar diffraction gratings fabricated by two-photon lithography with various laser powers close to the polymerization threshold of the photoresist. As a result, defect-free arrays of pillars with diameters down to 184 nm were fabricated. The structure sizes were analyzed by scanning electron microscopy and compared to theoretical predictions obtained from Monte Carlo simulations. The optical reflectivities of the nanopillar gratings were analyzed by optical microscopy and verified by rigorous coupled-wave simulations. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
- ItemInfluence of water on tribolayer growth when lubricating steel with a fluorinated phosphonium dicyanamide ionic liquid(Basel : MDPI, 2019) Urtis, L.A.; Arcifa, A.; Zhang, P.; Du, J.; Fantauzzi, M.; Rauber, D.; Hempelmann, R.; Kraus, T.; Rossi, A.; Spencer, N.D.This work aims to elucidate the role of environmental humidity on the tribological behavior of steel surfaces lubricated with an ionic liquid comprised of a fluorinated phosphonium cation-tributyl-3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-octyl-phosphonium-and a dicyanamide anion (i.e. N(CN) 2 - ). Ball-on-disk tribotests were carried out at room temperature and at various levels of relative humidity (RH). Water was found to be required to promote the formation of a tribofilm over the contact area. The reaction layer exhibited a patchy morphology, which resembles that observed formed with conventional antiwear additives such as ZnDTP. A surface-chemical analysis of the tribofilm indicated that the tribofilm is composed of fluorides, oxides, and phosphates, pointing to a stress-induced degradation of the ions and corrosion of the sliding counterparts, which is enabled by the presence of water at the sliding interface. © 2019 by the authors.
- ItemIn vitro entero-capillary barrier exhibits altered inflammatory and exosomal communication pattern after exposure to silica nanoparticles(Basel : MDPI, 2019) Kasper, J.Y.; Iris, Hermanns, M.; Kraegeloh, A.; Roth, W.; James, Kirkpatrick, C.; Unger, R.E.The intestinal microvasculature (iMV) plays multiple pathogenic roles during chronic inflammatory bowel disease (IBD). The iMV acts as a second line of defense and is, among other factors, crucial for the innate immunity in the gut. It is also the therapeutic location in IBD targeting aggravated leukocyte adhesion processes involving ICAM-1 and E-selectin. Specific targeting is stressed via nanoparticulate drug vehicles. Evaluating the iMV in enterocyte barrier models in vitro could shed light on inflammation and barrier-integrity processes during IBD. Therefore, we generated a barrier model by combining the enterocyte cell line Caco-2 with the microvascular endothelial cell line ISO-HAS-1 on opposite sides of a transwell filter-membrane under culture conditions which mimicked the physiological and inflamed conditions of IBD. The IBD model achieved a significant barrier-disruption, demonstrated via transepithelial-electrical resistance (TER), permeability-coefficient (Papp) and increase of sICAM sE-selectin and IL-8. In addition, the impact of a prospective model drug-vehicle (silica nanoparticles, aSNP) on ongoing inflammation was examined. A decrease of sICAM/sE-selectin was observed after aSNP-exposure to the inflamed endothelium. These findings correlated with a decreased secretion of ICAM/E-selectin bearing exosomes/microvesicles, as evaluated via ELISA. Our findings indicate that aSNP treatment of the inflamed endothelium during IBD may hamper exosomal/microvesicular systemic communication. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
- ItemA self-adhesive elastomericwound scaffold for sensitive adhesion to tissue(Basel : MDPI, 2019) Boyadzhieva, Silviya; Sorg, Katharina; Danner, Martin; Hensel, René; Fischer, Sarah C.L.; Schick, Bernhard; Wenzel, Gentiana; Arzt, Eduard; Kruttwig, KlausPressure sensitive adhesives based on silicone materials are used particularly for skin adhesion, e.g., the fixation of electrocardiogram (ECG) electrodes or wound dressings. However, adhesion to sensitive tissue structures is not sufficiently addressed due to the risk of damage or rupture. We propose an approach in which a poly-(dimethylsiloxane) (PDMS)-based soft skin adhesive (SSA) acts as cellular scaffold for wound healing. Due to the intrinsically low surface free energy of silicone elastomers, functionalization strategies are needed to promote the attachment and spreading of eukaryotic cells. In the present work, the effect of physical adsorption of three different proteins on the adhesive properties of the soft skin adhesive was investigated. Fibronectin adsorption slightly affects adhesion but significantly improves the cellular interaction of L929 murine fibroblasts with the polymeric surface. Composite films were successfully attached to explanted tympanic membranes. This demonstrates the potential of protein functionalized SSA to act as an adhesive scaffold in delicate biomedical applications.
- ItemMagnetic-responsive bendable nozzles for open surface droplet manipulation(Basel : MDPI, 2019) Prieto-López, L.O.; Xu, J.; Cui, J.The handling of droplets in a controlled manner is essential to numerous technological and scientific applications. In this work, we present a new open-surface platform for droplet manipulation based on an array of bendable nozzles that are dynamically controlled by a magnetic field. The actuation of these nozzles is possible thanks to the magnetically responsive elastomeric composite which forms the tips of the nozzles; this is fabricated with Fe3O4 microparticles embedded in a polydimethylsiloxane matrix. The transport, mixing, and splitting of droplets can be controlled by bringing together and separating the tips of these nozzles under the action of a magnet. Additionally, the characteristic configuration for droplet mixing in this platform harnesses the kinetic energy from the feeding streams; this provided a remarkable reduction of 80% in the mixing time between drops of liquids about eight times more viscous than water, i.e., 6.5 mPa/s, when compared against the mixing between sessile drops of the same fluids. © 2019 by the authors.
- ItemOut of the dark: Establishing a large-scale field experiment to assess the effects of artificial light at night on species and food webs(Basel : MDPI, 2015) Holzhauer, Stephanie I.J.; Franke, Steffen; Kyba, Christopher C.M.; Manfrin, Alessandro; Klenke, Reinhard; Voigt, Christian C.; Lewanzik, Daniel; Oehlert, Martin; Monaghan, Michael T.; Schneider, Sebastian; Heller, Stefan; Kuechly, Helga; Brüning, Anika; Honnen, Ann-Christin; Hölker, FranzArtificial light at night (ALAN) is one of the most obvious hallmarks of human presence in an ecosystem. The rapidly increasing use of artificial light has fundamentally transformed nightscapes throughout most of the globe, although little is known about how ALAN impacts the biodiversity and food webs of illuminated ecosystems. We developed a large-scale experimental infrastructure to study the effects of ALAN on a light-naïve, natural riparian (i.e., terrestrial-aquatic) ecosystem. Twelve street lights (20 m apart) arranged in three rows parallel to an agricultural drainage ditch were installed on each of two sites located in a grassland ecosystem in northern Germany. A range of biotic, abiotic, and photometric data are collected regularly to study the short- and long-term effects of ALAN on behavior, species interactions, physiology, and species composition of communities. Here we describe the infrastructure setup and data collection methods, and characterize the study area including photometric measurements. None of the measured parameters differed significantly between sites in the period before illumination. Results of one short-term experiment, carried out with one site illuminated and the other acting as a control, demonstrate the attraction of ALAN by the immense and immediate increase of insect catches at the lit street lights. The experimental setup provides a unique platform for carrying out interdisciplinary research on sustainable lighting.