Topographic cues guide the attachment of diatom cells and algal zoospores.

Surface topography plays a key role in the colonization of substrata by the colonizing stages of marine fouling organisms. For the innovation of marine antifouling coatings, it is essential to understand how topographic cues affect the settlement of these organisms. In this study, tapered, spiked microstructures and discrete honeycombs of varying feature dimensions were designed and fabricated in order to examine the influence of topography on the attachment of zoospores of the green macroalga Ulva linza and cells of the diatom (microalga) Navicula incerta. Contrasting results were obtained with these two species of algae. Indeed, the preferred location of cells of N. incerta was dominated by attachment point theory, which suggested a positive correlation between the density of cells adhering and the amount of available attachment points, while the settlement of spores of U. linza was mainly regulated by both Wenzel roughness and local binding geometry.

Bio-inspired hierarchical micro- and nano-wrinkles obtained via mechanically directed self-assembly on shape-memory polymers.

Inspired by complex multi-functional leaf and petal surfaces, we introduce a mechanically directed self-assembly process to create linearly oriented micro- and nanosized surface wrinkles in an all-polymer bi-layer system based on a shape-memory polymer substrate. By systematically investigating the influence of coating thickness and substrate programming strain on wrinkle period and height, we reveal how to control the structure size from a few hundred nanometers up to several microns. As a parameter unique to shape memory polymers, we demonstrate that the temperature during the recovery process can also be utilized to tailor the structure dimensions. Furthermore, we advance the method with a second structuring step to mimic the hierarchically structured petal surfaces of tulips and daisies. The presented structuring method provides a large-scale, mold-free, and very cost-effective way for the full-polymer fabrication of micro and sub-microstructures with adjustable structure size and intrinsic irregularity.

Bioinspired Superhydrophobic Highly Transmissive Films for Optical Applications.

Inspired by the transparent hair layer on water plants Salvinia and Pistia, superhydrophobic flexible thin films, applicable as transparent coatings for optoelectronic devices, are introduced. Thin polymeric nanofur films are fabricated using a highly scalable hot pulling technique, in which heated sandblasted steel plates are used to create a dense layer of nano- and microhairs surrounding microcavities on a polymer surface. The superhydrophobic nanofur surface exhibits water contact angles of 166 ± 6°, sliding angles below 6°, and is self-cleaning against various contaminants. Additionally, subjecting thin nanofur to argon plasma reverses its surface wettability to hydrophilic and underwater superoleophobic. Thin nanofur films are transparent and demonstrate reflection values of less than 4% for wavelengths ranging from 300 to 800 nm when attached to a polymer substrate. Moreover, used as translucent self-standing film, the nanofur exhibits transmission values above 85% and high forward scattering. The potential of thin nanofur films for extracting substrate modes from organic light emitting diodes is tested and a relative increase of the luminous efficacy of above 10% is observed. Finally, thin nanofur is optically coupled to a multicrystalline silicon solar cell, resulting in a relative gain of 5.8% in photogenerated current compared to a bare photovoltaic device.

Sub-Micron Replication of Fused Silica Glass and Amorphous Metals for Tool-Based Manufacturing.

The growing importance of submicrometer-structured surfaces across a variety of different fields has driven progress in light manipulation, color diversity, water-repellency, and functional enhancements. To enable mass production, processes like hot-embossing (HE), roll-to-roll replication (R2R), and injection molding (IM) are essential due to their precision and material flexibility. However, these processes are tool-based manufacturing (TBM) techniques requiring metal molds, which are time-consuming and expensive to manufacture, as they mostly rely on galvanoforming using templates made via precision microlithography or two-photon-polymerization (2PP). In this work, a novel approach is demonstrated to replicate amorphous metals from fused silica glass, derived from additive manufacturing and structured using hot embossing and casting, enabling the fabrication of metal insets with features in the range of 300 nm and a surface roughness of below 10 nm. By partially crystallizing the amorphous metal, during the replication process, the insets gain a high hardness of up to 800 HV. The metal molds are successfully used in polymer injection molding using different polymers including polystyrene (PS) and polyethylene (PE) as well as glass nanocomposites. This work is of significant importance to the field as it provides a production method for the increasing demand for sub-micron-structured tooling in the area of polymer replication while substantially reducing their cost of production.

3D direct laser writing of nano- and microstructured hierarchical gecko-mimicking surfaces.

Applying 3D direct laser writing, artificial hierarchical gecko-type structures are designed and fabricated down to nanometer dimensions. In this way, the elastic modulus and the length scale of the gecko's setae are very closely matched. Direct laser writing is a very flexible rapid prototyping method allowing the fabrication of arbitrary nanostructures. Since the parameters of the structures can be easily changed, this technique is perfect for design studies of dry adhesives. Measuring the adhesional forces by atomic force microscopy, the influence of several design parameters like density, aspect ratio, and tip-shape on dry adhesion performance are systematically examined. In this way, it is revealed that hierarchy is favorable for artificial gecko-inspired dry adhesives made of stiff materials on the nanometer scale.

Roll-to-roll fabrication of superhydrophobic pads covered with nanofur for the efficient clean-up of oil spills.

Superhydrophobic surfaces, which self-clean through rinsing with water, have gained significant importance during the last decades. A method to fabricate such a surface featuring the lotus effect, solely through structuring, is hot pulling of a polymer surface. This technique provides the so-called nanofur, which consists of a polymer surface densely covered with a polymeric fur of extremely thin hair-like structures. Here, we present a continuous roll-to-roll process for the fabrication of a thin polymeric film covered with nanofur from polypropylene. Our process enables structuring of large areas of the order of square meters using industry standard machinery. This opens up many possible applications for nanofur that could previously not be realized because of the limitations of conventional hot embossing regarding structurable area. The structured film is subsequently processed into an exemplary product, that is, so-called nanopads; polymeric sandwiches of polypropylene film covered with nanofur and filled with an oil-absorbing material. These are well-suited for the cleanup of small oil spills.

Synergies between Surface Microstructuring and Molecular Nanopatterning for Controlling Cell Populations on Polymeric Biointerfaces.

Polymeric biointerfaces are already being used extensively in a wide set of biomedical devices and systems. The possibility of controlling cell populations on biointerfaces may be essential for connecting biological systems to synthetic materials and for researching relevant interactions between life and matter. In this study, we present and analyze synergies between an innovative approach for surface microstructuring and a molecular nanopatterning procedure of recent development. The combined set of techniques used may be instrumental for the development of a new generation of functional polymeric biointerfaces. Eukaryotic cell cultures placed upon the biointerfaces developed, both before and after molecular patterning, help to validate the proposal and to discuss the synergies between the surface microstructuring and molecular nanopatterning techniques described in the study. Their potential role in the production of versatile polymeric biointerfaces for lab- and organ-on-a-chip biodevices and towards more complex and biomimetic co-culture systems and cell cultivation set-ups are also examined.

Soft-Lithography of Polyacrylamide Hydrogels Using Microstructured Templates: Towards Controlled Cell Populations on Biointerfaces.

Polyacrylamide hydrogels are interesting materials for studying cells and cell-material interactions, thanks to the possibility of precisely adjusting their stiffness, shear modulus and porosity during synthesis, and to the feasibility of processing and manufacturing them towards structures and devices with controlled morphology and topography. In this study a novel approach, related to the processing of polyacrylamide hydrogels using soft-lithography and employing microstructured templates, is presented. The main novelty relies on the design and manufacturing processes used for achieving the microstructured templates, which are transferred by soft-lithography, with remarkable level of detail, to the polyacrylamide hydrogels. The conceived process is demonstrated by patterning polyacrylamide substrates with a set of vascular-like and parenchymal-like textures, for controlling cell populations. Final culture of amoeboid cells, whose dynamics is affected by the polyacrylamide patterns, provides a preliminary validation of the described strategy and helps to discuss its potentials.

Combining Wax Printing with Hot Embossing for the Design of Geometrically Well-Defined Microfluidic Papers.

A simple, efficient, and repeatable combination of wax printing and hot embossing is reported. This combination yields microfluidic channels in paper, where fluid transport driven by paper-intrinsic capillary forces takes place inside the noncompressed areas, whereas embossed and wax-bonded areas serve as hydrophobic barriers laterally confining the fluid flow. Lab-made paper sheets first coated with a hydrophobic wax were hot-embossed with a tailor-made metal stamp. Both paper-intrinsic (e.g., grammage, fiber type) and paper-extrinsic parameters (e.g., embossing force) were studied for their influence on the geometry of the embossed structures and the resulting redistribution of the wax within the paper matrix. Embossing of wax-printed paper at temperatures above the wax melting point was completed within 15 s. Cotton linters papers required higher embossing forces than eucalyptus papers, which can be explained by their different intrinsic mechanical properties. In summary, both paper-intrinsic and paper-extrinsic parameters were found to have strong impact on resolution and reproducibility of the channels. All in all, the approach yields microfluidic channels in a fast and robust and reproducible manner with comparably low constrains on the precision of manufacturing parameters, such as embossing time, force, or temperature. Most importantly, embossing greatly reduces the lateral spreading of the wax as seen with melting approaches and therefore allows for a much higher feature density than the latter.

Glassomer-Processing Fused Silica Glass Like a Polymer.

Fused silica glass is one of the most important high-performance materials for scientific research, industry, and society. However due to its high chemical and thermal resistance as well as high hardness, fused silica glass is notoriously difficult to structure. This work introduces Glassomer, a solid nanocomposite, which can be structured using polymer molding and subtractive technologies at submicrometer resolution. After polymer processing Glassomer is turned into optical grade fused silica glass during a final heat treatment. The resulting glass has the same optical transparency as commercial fused silica and a smooth surface with a roughness of a few nanometers. This work makes high-performance fused silica glass components accessible to high-throughput fabrication technologies and will enable numerous optical, photonic and medical applications in science and industry.

Bio-inspired, large scale, highly-scattering films for nanoparticle-alternative white surfaces.

Inspired by the white beetle of the genus Cyphochilus, we fabricate ultra-thin, porous PMMA films by foaming with CO2 saturation. Optimising pore diameter and fraction in terms of broad-band reflectance results in very thin films with exceptional whiteness. Already films with 60 µm-thick scattering layer feature a whiteness with a reflectance of 90%. Even 9 µm thin scattering layers appear white with a reflectance above 57%. The transport mean free path in the artificial films is between 3.5 µm and 4 µm being close to the evolutionary optimised natural prototype. The bio-inspired white films do not lose their whiteness during further shaping, allowing for various applications.

Adaptable bioinspired special wetting surface for multifunctional oil/water separation.

Inspired by the multifunctionality of biological surfaces necessary for the survival of an organism in its specific environment, we developed an artificial special wetting nanofur surface which can be adapted to perform different functionalities necessary to efficiently separate oil and water for cleaning accidental oil spills or separating industrial oily wastewater. Initial superhydrophobic nanofur surface is fabricated using a hot pulling method, in which nano- and microhairs are drawn out of the polymer surface during separation from a heated sandblasted steel plate. By using a set of simple modification techniques, which include microperforation, plasma treatment and subsequent control of storage environment, we achieved selective separation of either water or oil, variable oil absorption and continuous gravity driven separation of oil/water mixtures by filtration. Furthermore, these functions can be performed using special wetting nanofur made from various thermoplastics, including biodegradable and recyclable polymers. Additionally, nanofur can be reused after washing it with organic solvents, thus, further helping to reduce the environmental impacts of oil/water separation processes.

Liquid Glass: A Facile Soft Replication Method for Structuring Glass.

Liquid glass is a photocurable amorphous silica nanocomposite that can be structured using soft replication molds and turned into glass via thermal debinding and sintering. Simple polymer bonding techniques allow the fabrication of complex microsystems in glass like microfluidic chips. Liquid glass is a step toward prototyping of glass microstructures at low cost without requiring cleanroom facilities or hazardous chemicals.

Tacky cyclic olefin copolymer: a biocompatible bonding technique for the fabrication of microfluidic channels in COC.

Cyclic olefin copolymer (COC) is widely used in microfluidics due to its UV-transparency, its biocompatibility and high chemical resistance. Here we present a fast and cost-effective solvent bonding technique, which allows for the efficient bonding of protein-patterned COC structures. The bonding process is carried out at room temperature and takes less than three minutes. Enzyme activity is retained upon bonding and microstructure deformation does not occur.

Tunable Diffractive Optical Elements Based on Shape-Memory Polymers Fabricated via Hot Embossing.

We introduce actively tunable diffractive optical elements fabricated from shape-memory polymers (SMPs). By utilizing the shape-memory effect of the polymer, at least one crucial attribute of the diffractive optical element (DOE) is tunable and adjustable subsequent to the completed fabrication process. A thermoplastic, transparent, thermoresponsive polyurethane SMP was structured with diverse diffractive microstructures via hot embossing. The tunability was enabled by programming a second, temporary shape into the diffractive optical element by mechanical deformation, either by stretching or a second embossing cycle at low temperatures. Upon exposure to the stimulus heat, the structures change continuously and controllable in a predefined way. We establish the novel concept of shape-memory diffractive optical elements by illustrating their capabilities, with regard to tunability, by displaying the morphing diffractive pattern of a height tunable and a period tunable structure, respectively. A sample where an arbitrary structure is transformed to a second, disparate one is illustrated as well. To prove the applicability of our tunable shape-memory diffractive optical elements, we verified their long-term stability and demonstrated the precise adjustability with a detailed analysis of the recovery dynamics, in terms of temperature dependence and spatially resolved, time-dependent recovery.

Bioinspired air-retaining nanofur for drag reduction.

Bioinspired nanofur, covered by a dense layer of randomly distributed high aspect ratio nano- and microhairs, possesses superhydrophobic and air-retaining properties. Nanofur is fabricated using a highly scalable hot pulling method in which softened polymer is elongated with a heated sandblasted plate. Here we investigate the stability of the underwater air layer retained by the irregular nanofur topography by applying hydraulic pressure to the nanofur kept underwater, and evaluate the gradual changes in the air-covered area. Furthermore, the drag reduction resulting from the nanofur air retention is characterized by measuring the pressure drop across channels with and without nanofur.

Liquid polystyrene: a room-temperature photocurable soft lithography compatible pour-and-cure-type polystyrene.

Materials matter in microfluidics. Since the introduction of soft lithography as a prototyping technique and polydimethylsiloxane (PDMS) as material of choice the microfluidics community has settled with using this material almost exclusively. However, for many applications PDMS is not an ideal material given its limited solvent resistance and hydrophobicity which makes it especially disadvantageous for certain cell-based assays. For these applications polystyrene (PS) would be a better choice. PS has been used in biology research and analytics for decades and numerous protocols have been developed and optimized for it. However, PS has not found widespread use in microfluidics mainly because, being a thermoplastic material, it is typically structured using industrial polymer replication techniques. This makes PS unsuitable for prototyping. In this paper, we introduce a new structuring method for PS which is compatible with soft lithography prototyping. We develop a liquid PS prepolymer which we term as "Liquid Polystyrene" (liqPS). liqPS is a viscous free-flowing liquid which can be cured by visible light exposure using soft replication templates, e.g., made from PDMS. Using liqPS prototyping microfluidic systems in PS is as easy as prototyping microfluidic systems in PDMS. We demonstrate that cured liqPS is (chemically and physically) identical to commercial PS. Comparative studies on mouse fibroblasts L929 showed that liqPS cannot be distinguished from commercial PS in such experiments. Researchers can develop and optimize microfluidic structures using liqPS and soft lithography. Once the device is to be commercialized it can be manufactured using scalable industrial polymer replication techniques in PS--the material is the same in both cases. Therefore, liqPS effectively closes the gap between "microfluidic prototyping" and "industrial microfluidics" by providing a common material.

Fabrication of polymeric microfluidic devices with tunable wetting behavior for biomedical applications.

We demonstrate the fabrication of microchannels with specific fluidic behavior due to micro- and/or nanostructures on the surfaces. With a combination of hot embossing and microthermoforming it is possible to produce microchannels with specific surface properties. These surface properties are highly dependent on the micro- and nanostructures embossed into the material. Different structure sizes and geometries where examined by contact angle measurements. Here the dependency of diameter and pitch of the structures on the contact angle is examined as well as the material impact. These results enable the fabrication of highly specific surfaces tunable to an application.

Characterization of a chip-based bioreactor for three-dimensional cell cultivation via Magnetic Resonance Imaging.

We describe the characterization of a chip-based platform (3(D)-KITChip) for the three-dimensional cultivation of cells under perfusion conditions via magnetic resonance imaging (MRI). Besides the chip, the microfluidic system is comprised of a bioreactor housing, a medium supply, a pump for generating active flow conditions as well as a gas mixing station. The closed circulation loop is ideally suited for a characterization via MRI since the small bioreactor setup with active perfusion, driven by the pump from outside the coils, not only is completely MRI-compatible but also can be transferred into the magnetic coil of an experimental animal scanner. We have found that the two halves of the chip inside the bioreactor are homogeneously perfused with cell culture medium both with and without cells inside the 3(D)-KITChip. In addition, the homogeneity of perfusion is nearly independent from the flow rates investigated in this study, and furthermore, the setup shows excellent washout characteristics after spiking with Gadolinium-DOTA which makes it an ideal candidate for drug screening purposes. We, therefore, conclude that the 3(D)-KITChip is well suited as a platform for high-density three-dimensional cell cultures, especially those requiring a defined medium flow and/or gas supply in a precisely controllable three dimensional environment, like stem cells.