04 Fakultät Energie-, Verfahrens- und Biotechnik

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Institute: search.filters.UBSInstitut.Institut für Grenzflächenverfahrenstechnik und PlasmatechnologieSubject: search.filters.subject.540

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    Precision 3D‐printed cell scaffolds mimicking native tissue composition and mechanics
    (2020) Erben, Amelie; Hörning, Marcel; Hartmann, Bastian; Becke, Tanja; Eisler, Stephan A.; Southan, Alexander; Cranz, Séverine; Hayden, Oliver; Kneidinger, Nikolaus; Königshoff, Melanie; Lindner, Michael; Tovar, Günter E. M.; Burgstaller, Gerald; Clausen‐Schaumann, Hauke; Sudhop, Stefanie; Heymann, Michael
    Cellular dynamics are modeled by the 3D architecture and mechanics of the extracellular matrix (ECM) and vice versa. These bidirectional cell‐ECM interactions are the basis for all vital tissues, many of which have been investigated in 2D environments over the last decades. Experimental approaches to mimic in vivo cell niches in 3D with the highest biological conformity and resolution can enable new insights into these cell‐ECM interactions including proliferation, differentiation, migration, and invasion assays. Here, two‐photon stereolithography is adopted to print up to mm‐sized high‐precision 3D cell scaffolds at micrometer resolution with defined mechanical properties from protein‐based resins, such as bovine serum albumin or gelatin methacryloyl. By modifying the manufacturing process including two‐pass printing or post‐print crosslinking, high precision scaffolds with varying Young's moduli ranging from 7‐300 kPa are printed and quantified through atomic force microscopy. The impact of varying scaffold topographies on the dynamics of colonizing cells is observed using mouse myoblast cells and a 3D‐lung microtissue replica colonized with primary human lung fibroblast. This approach will allow for a systematic investigation of single‐cell and tissue dynamics in response to defined mechanical and bio‐molecular cues and is ultimately scalable to full organs.
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    Physical interactions strengthen chemical gelatin methacryloyl gels
    (2019) Rebers, Lisa; Granse, Tobias; Tovar, Günter E. M.; Southan, Alexander; Borchers, Kirsten
    Chemically cross-linkable gelatin methacryloyl (GM) derivatives are getting increasing attention regarding biomedical applications. Thus, thorough investigations are needed to achieve full understanding and control of the physico-chemical behavior of these promising biomaterials. We previously introduced gelatin methacryloyl acetyl (GMA) derivatives, which can be used to control physical network formation (solution viscosity, sol-gel transition) independently from chemical cross-linking by variation of the methacryloyl-to-acetyl ratio. It is known that temperature dependent physical network formation significantly influences the mechanical properties of chemically cross-linked GM hydrogels. We investigated the temperature sensitivity of GM derivatives with different degrees of modification (GM2, GM10), or similar degrees of modification but different methacryloyl contents (GM10, GM2A8). Rheological analysis showed that the low modified GM2 forms strong physical gels upon cooling while GM10 and GM2A8 form soft or no gels. Yet, compression testing revealed that all photo cross-linked GM(A) hydrogels were stronger if cooling was applied during hydrogel preparation. We suggest that the hydrophobic methacryloyl and acetyl residues disturb triple helix formation with increasing degree of modification, but additionally form hydrophobic structures, which facilitate chemical cross-linking.
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    Acid catalyzed cross‐linking of polyvinyl alcohol for humidifier membranes
    (2021) Michele, Andre; Paschkowski, Patrick; Hänel, Christopher; Tovar, Günter E. M.; Schiestel, Thomas; Southan, Alexander
    Polyvinyl alcohol (PVA) is a hydrophilic polymer well known for good film forming properties, high water vapor permeance JW, and low nitrogen permeance. However, depending on molar mass and temperature, PVA swells strongly in water until complete dissolution. This behavior affects the usability of PVA in aqueous environments and makes cross‐linking necessary if higher structural integrity is envisaged. In this work, PVA networks are formed by thermal cross‐linking in the presence of p‐toluenesulfonic acid (TSA) and investigated in a design of experiments approach. Experimental parameters are the cross‐linking period tc, temperature ϑ and the TSA mass fraction wTSA. Cross‐linking is found to proceed via ether bond formation at all reaction conditions. Degradation is promoted especially by a combination of high wTSA, tc and ϑ. Thermal stability of the networks after preparation is strongly improved by neutralizing residual TSA. Humidification membranes with a JW of 6423 ± 63.0 gas permeation units (GPU) are fabricated by coating PVA on polyvinyliden fluoride hollow fibers and cross‐linking with TSA. Summarizing, the present study contributes to a clearer insight into the cross‐linking of PVA in presence of TSA, the thermal stability of the resulting networks and the applicability as selective membrane layers for water vapor transfer.
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    PFG-NMR studies of ATP diffusion in PEG-DA hydrogels and aqueous solutions of PEG-DA polymers
    (2018) Majer, Günter; Southan, Alexander
    Adenosine triphosphate (ATP) is the major carrier of chemical energy in cells. The diffusion of ATP in hydrogels, which have a structural resemblance to the natural extracellular matrix, is therefore of great importance to understand many biological processes. In continuation of our recent studies of ATP diffusion in poly(ethylene glycol) diacrylate (PEG-DA) hydrogels by pulsed field gradient nuclear magnetic resonance (PFG-NMR), we present precise diffusion measurements of ATP in aqueous solutions of PEG-DA polymers, which are not cross-linked to a three-dimensional network. The dependence of the ATP diffusion on the polymer volume fraction in the hydrogels, φ, was found to be consistent with the predictions of a modified obstruction model or the free volume theory in combination with the sieving behavior of the polymer chains. The present measurements of ATP diffusion in aqueous solutions of the polymers revealed that the diffusion coefficient is determined by φ only, regardless of whether the polymers are cross-linked or not. These results seem to be inconsistent with the free volume model, according to which voids are formed by a statistical redistribution of surrounding molecules, which is expected to occur more frequently in the case of not cross-linked polymers. The present results indicate that ATP diffusion takes place only in the aqueous regions of the systems, with the volume fraction of the polymers, including a solvating water layer, being blocked for the ATP molecules. The solvating water layer increases the effective volume of the polymers by 66%. This modified obstruction model is most appropriate to correctly describe the ATP diffusion in PEG-DA hydrogels.
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    Coumarin-4-ylmethyl- and p-hydroxyphenacyl-based photoacid generators with high solubility in aqueous media: synthesis, stability and photolysis
    (2020) Adatia, Karishma K.; Halbritter, Thomas; Reinfelds, Matiss; Michele, Andre; Tran, Michael; Laschat, Sabine; Heckel, Alexander; Tovar, Günter E. M.; Southan, Alexander
    (Coumarin‐4‐yl)methyl (c4m) and p‐hydroxyphenacyl (pHP)‐based compounds are well known for their highly efficient photoreactions, but often show limited solubility in aqueous media. To circumvent this, we synthesized and characterized the two new c4m and pHP‐based photoacid generators (PAGs), 7‐[bis(carboxymethyl)amino]‐4‐(acetoxymethyl)coumarin (c4m‐ac) and p‐hydroxyphenacyl‐2,5,8,11‐tetraoxatridecan‐13‐oate (pHP‐t), and determined their solubilities, stabilities and photolysis in aqueous media. The two compounds showed high solubilities in water of 2.77 mmol L−1±0.07 mmol L−1 (c4m‐ac) and 124.66 mmol L−1±2.1 mmol L−1 (pHP‐t). In basic conditions at pH 9, solubility increased for c4m‐ac to 646.46 mmol L−1±0.63 mmol L−1, for pHP‐t it decreased to 34.68 mmol L−1±0.62 mmol L−1. Photochemical properties of the two PAGs, such as the absorption maxima, the maximum molar absorption coefficients and the quantum yields, were found to be strongly pH‐dependent. Both PAGs showed high stabilities s24h ≥95 % in water for 24 h, but decreasing stability with increasing pH value due to hydrolysis. The present study contributes to a clearer insight into the synthesis, solubilities, stabilities, and photolysis of c4m and pHP‐based PAGs for further photochemical applications when high PAG concentrations are required, such as in polymeric foaming.
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    Eclectic characterisation of chemically modified cell-derived matrices obtained by metabolic glycoengineering and re-assessment of commonly used methods
    (2020) Keller, Silke; Liedek, Anke; Shendi, Dalia; Bach, Monika; Tovar, Günter E. M.; Kluger, Petra J.; Southan, Alexander
    Azide-bearing cell-derived extracellular matrices (“clickECMs”) have emerged as a highly exciting new class of biomaterials. They conserve substantial characteristics of the natural extracellular matrix (ECM) and offer simultaneously small abiotic functional groups that enable bioorthogonal bioconjugation reactions. Despite their attractiveness, investigation of their biomolecular composition is very challenging due to the insoluble and highly complex nature of cell-derived matrices (CDMs). Yet, thorough qualitative and quantitative analysis of the overall material composition, organisation, localisation, and distribution of typical ECM-specific biomolecules is essential for consistent advancement of CDMs and the understanding of the prospective functions of the developed biomaterial. In this study, we evaluated frequently used methods for the analysis of complex CDMs. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and (immune)histochemical staining methods in combination with several microscopic techniques were found to be highly eligible. Commercially available colorimetric protein assays turned out to deliver inaccurate information on CDMs. In contrast, we determined the nitrogen content of CDMs by elementary analysis and converted it into total protein content using conversion factors which were calculated from matching amino acid compositions. The amount of insoluble collagens was assessed based on the hydroxyproline content. The Sircol™ assay was identified as a suitable method to quantify soluble collagens while the Blyscan™ assay was found to be well-suited for the quantification of sulphated glycosaminoglycans (sGAGs). Eventually, we propose a series of suitable methods to reliably characterise the biomolecular composition of fibroblast-derived clickECM.
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    Proton-conducting membranes for the artificial leaf
    (2023) Bosson, Karell; Tovar, Günter E. M. (Prof.)
    With the aim of producing proton conducting membranes with improved proton conductivity and mechanical properties, the poly(pentafluorostyrene)-b-(butyl acrylate) (PPFS-b-PBuA) system was investigated. The study mainly focuses on the influence of the forming polymer nanostructures on the conductivity properties of the membranes. A series of well-defined PPFS-b-PBuA block copolymers (BCPs) were synthesized via nitroxide-mediated controlled radical polymerization (NMP). Spontaneous self-assembly of the BCP element was induced via a targeted change in polymer composition. Moreover, by adjusting the molar composition via enrichment of one of the blocks after synthesis, controlled self-assembly of the BCPs was realized. This was done by combining the corresponding homopolymer with the block copolymer to form a polymer blend - one of the blocks mixed to the BCP. Forming such polymer blends expanded the range of available techniques for tailoring the morphology for desired applications. Sulfonation of BCPs for the preparation of proton-conducting membranes was carried out by a para-fluoro thiol "click" reaction using sodium 3-mercapto-1-propanesulfonate (SMPS). The accessibility of fluorine in the para position of the phenylene group of PPFS provides countless opportunities for polymer functionalization by nucleophilic substitution. After modification of BCP, the self-assembly ability was retained, and higher conductivities were obtained compared to random copolymers. In addition, complementary studies were conducted on the use of printing techniques for membrane upscaling and evaluation of their life cycle.
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    Differentiation of physical and chemical cross-linking in gelatin methacryloyl hydrogels
    (2021) Rebers, Lisa; Reichsöllner, Raffael; Regett, Sophia; Tovar, Günter E. M.; Borchers, Kirsten; Baudis, Stefan; Southan, Alexander
    Gelatin methacryloyl (GM) hydrogels have been investigated for almost 20 years, especially for biomedical applications. Recently, strengthening effects of a sequential cross-linking procedure, whereby GM hydrogel precursor solutions are cooled before chemical cross-linking, were reported. It was hypothesized that physical and enhanced chemical cross-linking of the GM hydrogels contribute to the observed strengthening effects. However, a detailed investigation is missing so far. In this contribution, we aimed to reveal the impact of physical and chemical cross-linking on strengthening of sequentially cross-linked GM and gelatin methacryloyl acetyl (GMA) hydrogels. We investigated physical and chemical cross-linking of three different GM(A) derivatives (GM10, GM2A8 and GM2), which provided systematically varied ratios of side-group modifications. GM10 contained the highest methacryloylation degree (DM), reducing its ability to cross-link physically. GM2 had the lowest DM and showed physical cross-linking. The total modification degree, determining the physical cross-linking ability, of GM2A8 was comparable to that of GM10, but the chemical cross-linking ability was comparable to GM2. At first, we measured the double bond conversion (DBC) kinetics during chemical GM(A) cross-linking quantitatively in real-time via near infrared spectroscopy-photorheology and showed that the DBC decreased due to sequential cross-linking. Furthermore, results of circular dichroism spectroscopy and differential scanning calorimetry indicated gelation and conformation changes, which increased storage moduli of all GM(A) hydrogels due to sequential cross-linking. The data suggested that the total cross-link density determines hydrogel stiffness, regardless of the physical or chemical nature of the cross-links.
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    Wetting, de-icing and anti-icing behavior of microstructured and plasma-coated polyurethane films
    (2019) Grimmer, Philipp E. S.; Hirth, Thomas (Prof. Dr. rer. nat.)
    Ice build-up on surfaces, for example on wings of airplanes or on rotor blades of wind turbines, impairs the functionality of transportation vehicles or technical systems and reduces their safety. Therefore, functional anti-ice surfaces are being researched and developed, which shall enable an easy removal or reduce the amount of ice on the surfaces at risk. The starting hypothesis for this work is that superhydrophobic polyurethane (PU) films with microstructure base diameters of 35 µm or more reduce the wetting by water, show a low ice adhesion for easy removal of ice and reduce or delay icing. Superhydrophobic PU films for passive anti- and de-icing were created by hot embossing and plasma enhanced chemical vapor deposition (PECVD). The hot embossing process as well as the plasma coating and etching processes were analyzed for the dependence of the surface characteristics on different process parameters. The functionalized PU films were characterized for their surface topography, surface chemistry, stability against erosion, wettability, ice adhesion and icing behavior. For comparison, the ice adhesion and icing behavior were examined on relevant technical materials (aluminum, titanium, copper, glass, epoxy resin of carbon fiber reinforced polymer and other fluoropolymers) and on some commercial anti-ice coatings. The PU films were chemically analyzed by IR spectroscopy. As the first process step for functionalization, microstructures of cylindrical, elliptical or linear shape were imprinted in PU films by a hot embossing technique with different ns-pulsed laser-drilled stamps and characterized by several microscopy methods. The microstructures had heights of 15 µm to 140 µm, diameters or widths of 35 µm to 300 µm and distances (pitch values) of 50 µm to 500 µm. The embossing process was analyzed and optimized in terms of the process parameters temperature, pressure, time, PU film release temperature and reproducibility of the microstructures. In a second functionalization step (PECVD) the microstructured surfaces were coated with thin, hydrophobic plasma polymers using different fluorocarbon precursors (CHF3, C3F6 and C4F8) or hexamethyldisiloxane (HMDSO). Different process parameters for plasma coating and etching (Ar or O2 plasmas) were used in order to create various nanoscale roughness values. Electron spectroscopy for chemical analysis (ESCA), spectroscopic ellipsometry and atomic force microscopy (AFM) were used for analysis of the chemical composition, the thickness and the nanoroughness of the plasma polymers. The functionalizations, especially the plasma coatings, were completely worn off by a UV/water weathering test (1000 h, X1a CAM 180 Test, SAE J-2527), but showed sufficient stability against sand erosion (DIN 52348), in a long-term outdoor test for 13.5 months and against fivefold repeated pull-off of ice. The silicone-like plasma coatings were more stable than the fluorocarbon plasma coatings. The wetting behavior of water was determined by static, advancing and receding contact angle measurements. Static contact angle measurements with diiodomethane (DIM) were made for determination of the surface free energies of the relevant surfaces. Advancing contact angles of over 150° and very low contact angle hysteresis values below 10° were reached on some of the cylindrically and elliptically structured PU samples with microstructure base diameters in the range of 35 µm to 50 µm. The measured water advancing contact angles did not reach the theoretical values of the Cassie-Baxter state. Starting from a mixed wetting state near Cassie-Baxter in case of the superhydrophobic PU surfaces, they approached the Wenzel state with an increasing pitch/diameter (P/d) factor. Fluorescence laser scanning microscopy images were taken of some microstructured, uncoated or plasma coated samples during the wetting by a water drop containing a fluorescent dye. These images show the Wenzel state or a mixed wetting state by visualization of the interface between the water droplet and the surface. A new icing test chamber and a test setup were developed for characterization of the ice adhesion and the icing behavior. The tensile ice adhesion was measured at -20 °C by pull-off of ice cylinders (highly purified water, (<0.056 µS/cm, diameter of 4 mm, similar to the diameter of large raindrops) and compared to the theoretical values and the wetting behavior. The technical material surfaces measured for comparison showed a high ice adhesion, which led to cohesive fractures especially on the metal surfaces, whereas some of the commercial anti-ice coatings showed lower ice adhesion values. The flat, plasma coated PU surfaces showed adhesive fractures with a reduced ice adhesion compared to the technical material surfaces and uncoated PU and revealed a good correlation of the ice adhesion with the wetting behavior of water (work of adhesion). On the other hand, the microstructured PU surfaces showed a greatly increased ice adhesion in comparison to the flat PU and technical material surfaces which was enhanced even further by the plasma coatings and did not correlate with the wetting behavior. The reason for this is the wetting transition from the Cassie-Baxter to the Wenzel state during the cooling or freezing process, leading to an increased ice-surface contact area and mechanical interlocking of the ice with the micro- and nanostructures. The freezing of water drops was examined in thermodynamic equilibrium (static experiment) and under quasi-steady conditions (dynamic experiment). In the static experiment, 15 µl water drops (corresponding to medium to large raindrops) at room temperature were dispensed onto a cold surface at a constant temperature of -20 °C. The freezing delay times, the crystallization times and the total freezing times were measured and compared to calculated expected values. On the flat samples, the freezing delay times could be extended by the plasma treatments. On the microstructured samples, the freezing (nucleation) could sometimes be delayed even further, but not always reproducible because of an unstable Cassie-Baxter state. In the dynamic experiment, 25 µl water drops (corresponding to large raindrops) were cooled down in quasi-steady conditions with the surface and the surrounding atmosphere by a constant, low cooling rate of 1 K/min while the water drop temperature was measured by an IR camera for determination of the surface-specific nucleation temperature and crystallization time. A lower nucleation temperature could be measured on the flat, plasma coated PU surfaces compared to uncoated PU and the hydrophilic glass and metal surfaces. The superhydrophobic PU surfaces did not show a further reduction of the nucleation temperature because of an unstable Cassie-Baxter state. The resulting measured nucleation temperatures were compared to the expected values calculated with an enhanced nucleation theory including a quasi-liquid interfacial layer of the ice nucleus and a Poisson process. Overall, it is shown that hot embossing and PECVD are useful processes for creating superhydrophobic PU surfaces with regard to a roll-to-roll process. The flat, plasma coated PU films show a reduced ice adhesion and lowered nucleation temperature compared to the relevant technical material surfaces. The microstructured, plasma coated PU films are far more water repellent than the flat, plasma coated PU surfaces or the other technical materials. However, the microstructures with base diameters of 35 µm or more and the nanoroughness of the plasma coatings cannot stabilize the Cassie-Baxter state of a freezing water drop enough for a low ice adhesion or a significant decrease of the nucleation temperature. These superhydrophobic PU films are therefore not more icephobic than the flat, plasma coated PU films. In the outlook, the reduction of the geometrical parameters of the microstructures (diameter D, distance P) and nanostructures (curvature radius R) of the surface functionalizations for lower ice adhesion values and nucleation temperatures is proposed.