Scientific publications

The feathers in the train of the peacock serve not for flying but for sexual display. They are long, slender beams loaded in bending by their own weight. An outer circular conical shell, the cortex, is filled by a closed foam of 7.6% relative density, the medulla, both of feather keratin. Outer diameter and thickness of the cortex decrease linearly from the body toward the tip. This self-similar geometry leads to a division of labor. The cortex (longitudinal Young's modulus 3.3 GPa, transverse modulus 1 GPa) provides 96% of the longitudinal strength and bending rigidity of the feather. The medulla (Young's modulus 10 MPa) provides 96% of the transverse compressive rigidity. Fracture stress of the cortex, both longitudinal and transverse, is 120 MPa.

The twelve membered and eighteen membered cyclosiloxazanes [Me2SiOSiMe2N(H)SiMe2O]2 (3) and [Me2SiOSiMe2N(H)SiMe2O]3 (4) have been obtained as colourless liquids from the reaction of 1,5-Dichlorohexamethyltrisiloxane with ammonia at-50 °C without any solvent. For compound 3 and 4, the yield is 30 % and 43 %, respectively. The treatment of 3 with two equivalents of n-butyllithium in n-hexane leads in almost quantitative yield (93 %) to the dilithiumsilylamide [Me2SiOSiMe2N(Li)SiMe2O]2 (5). Compound 5 can be isolated at room temperature and recrystallized from benzene without any decomposition or contraction of the ring size. An X-ray structure determination on [Me2SiOSiMe2N(Li)SiMe2O]2 (5) reveals a waved crown-like twelve membered Si6N2O4-cycle. Each of the two lithium ions is coordinated by two nitrogen and two oxygen atoms leading to a square pyramid, in which the metal occupies the apex. The two square-pyramidal coordination figures are located above and below the waved crown-like Si6N2O4-ring sharing a common edge by the atoms N(1) and N(2). All synthesised compounds have been characterised by spectroscopic methods. Because of the lower solubility of the dilithiumsilylamide 5, only a 1H-NMR could be applied for further characterisation in solution.

Nature has developed reversibly adhesive surfaces whose stickiness has attracted much research attention over the last decade. The central lesson from nature is that "patterned" or "fibrillar" surfaces can produce higher adhesion forces to flat and rough substrates than smooth surfaces. This paper critically examines the principles behind fibrillar adhesion from a contact mechanics perspective, where much progress has been made in recent years. The benefits derived from "contact splitting" into fibrils are separated into extrinsic/intrinsic contributions from fibril deformation, adaptability to rough surfaces, size effects due to surface-to-volume ratio, uniformity of stress distribution, and defectcontrolled adhesion. Another section covers essential considerations for reliable and reproducible adhesion testing, where better standardization is still required. It is argued that, in view of the large number of parameters, a thorough understanding of adhesion effects is required to enable the fabrication of reliable adhesive surfaces based on biological examples.

ZO-1 is a peripheral protein that plays a central role in the macromolecular assembly of tight junctions by interacting with integral proteins (occludin, claudins, JAMs) of the membrane of adjoining cells, with the actin cytoskeleton, and with nuclear factors. Human ZO-1 is expressed in all epithelia and some specialized endothelia as variable amounts of two related isoforms, which originate from the alternatively spliced mRNA transcripts α+ and α- and whose specific differential role is still unknown. Moreover, little is known about the timing of expression of ZO-1 isoforms at the protein and mRNA level. This study shows that during growth of freshly plated Caco-2 cells the α+/α- ratio increased as a result of simultaneous increase of α+ and decrease of α-. Differences in the isoform ratio also correlated with differences in epithelium differentiation. This was determined by aminopeptidase-N measurements of cells grown on conventional substrates and on modified, micro/nano-patterned surfaces. A comparable shift of ZO-1 isoforms was not observed in other tumour cell lines of non-intestinal origin (A549, Calu-3). Pancreatic stem cells propagated without exogenous differentiation stimuli display a slight, stable prevalence of the α- isoform. Of the intestinal cell lines examined (Caco-2 and T84) only Caco-2 cells displayed a dramatic shift in isoform expression. This suggests that this tumour cell line retains to a higher degree a developmental program related with the dynamic of enterocytic differentiation in vivo.

Gecko inspired adhesives are surfaces with many microscale pillars that form Van der Waals forces with other surfaces. They differ from conventional tape in that adhesion is reversible and has the potential for switchability. These properties make gecko adhesives interesting for various biomedical applications. The two objectives of this project were to investigate the formation of biofilms on such surfaces and how the surfaces affect cell development. The developmental stages of the model organism Dictyostelium discoideum were observed by time lapse photography using light and environmental scanning electron microscopy. This study shows that micropatterned surfaces can be used as a biophysical tool to interfere with multicellular tissue formation in multiple ways.

The solvothermal reactions of Ti(OiPr)4 in alcohol using ionic liquid as additive were investigated. In the presence of [BMIM][Cl], [BMIM][Br], [BMIM][NTf2], [BMIM][SO3Me], [BMIM][SO4Me], or [BMIM][OTf] (BMIM = 1-Butyl-3-methylimidazolium), pure anatase nanoparticles were obtained. The controlled hydrolysis of Ti(OiPr)4 in the presence of ionic liquids to form titanium oxo clusters plays a key role in the formation of anatase nanostructures, and ionic liquids can be repeatedly used to synthesise anatase nanoparticles. However, in the presence of [BMIM][PF6], [BMIM]2[Ti(OH)6] was obtained by an anion exchange reaction.

The hydrolysis reactions of Ti(OiPr)4 in the presence of different ionic liquids using ethanol as solvent were investigated. The reaction in the presence of [BMIM][BF4] gave rise to the anion exchange reaction product [BMIM]2[Ti(OH)6] in moderate yield. On the contrary, Ti7O4(OEt)20 was obtained in ionic liquids with other anions. The anion exchange reaction has a broad capacity for group functionalization. It can be used to synthesize both imidazolium and pyridium [Ti(OH)6]2− salts. In contrast, under the similar reaction conditions, the reactions of zirconium alcoholate gave rise to [Zr(OH)5]− or [Zr(OH)6]2− intermediates depending on the counter cations. Under the solvothermal reaction conditions, [PF6]− based ionic liquids can also undertake the similar anion exchange reactions.

Tin oxide (SnO2) nanowires grown by chemical vapor deposition were modified by Ar/O2 plasma treatment through preferential etching of the lattice oxygen atoms, which produced nonstoichiometric surface compositions that imparted a manyfold higher sensitivity toward gas absorption on such surfaces. Microstructures of as-grown and plasma-treated SnO2 nanowires confirmed the gradual change in the chemical composition and morphologies. Surficial disorder caused by the bombardment of argon and oxygen ions present in the plasma was visible as a disordered overlayer in high-resolution TEM micrographs, when compared to single crystalline s-grown SnO2 nanowires. Gas-sensing experiments on modified SnO2 nanostructures showed higher sensitivity for ethanol gas at lower operating temperatures and exhibited an improved transduction response toward changing gas atmospheres, attributed to the increased concentration or oxygen vacancies on the surface or SnO2 nanowires. Modulation of surface chemistry was also supported by photoluminescence and X-ray photoemission spectroscopy studies.

The branched tripodal chloro-methyl-siloxanes of the general formula tBuSi[ {OSiMe2}yOSiMe3-xCl3]3 [x = 0-3; y = 0-2] were synthesized, starting with tert-Butyl-trisilanol (1). The treatment of 1 with the chloro-methyl-silanes (Me3-xSiClx+1) (x = 0-3) in the presence of triethylamine leads to the compounds tBuSi(OSiMe2Cl)3 (2), tBuSi(OSiMeCl2)3 (3) and tBuSi(OSiCl3)3 (4), The siloxanes 2-4 are colourless oily liquids, which can be purified by distillation, Their yields decrease with the number of chloro substituents. In the reaction of compound 2 with three equivalents of water the silantriol tBuSi(OSiMe2OH)3 (5) is generated which is used to create the branched tripodal chloro-methyl-siloxanes tBuSi(OSiMe2OSiMe3)3 (6), tBuSi(OSiMe2OSiMe2Cl)3 (7), tBuSi(OSiMe2OSiMeCl2)3 (9) and tBuSi(OSiMe2OSiCl3)3 (10). Compound (7) is only a side product with a yield of 25 %. The cyclic tBuSi[{(OSiMe2)2Cl}(OSiMe2)3O] (8) can be isolated and characterised. The transformation of the compound tBuSi(OSiMe2OSiMe2Cl)3 (7) into the trisilanol tBuSi(OSiMe2OSiMe2OH)3 (11) allows to prepare the tripodale siloxane tBuSi(OSiMe2OSiMe2OSiMe3)3 (12) in good yields. The reaction of tBuSi(OSiMe2Cl)3 (2) with tert-butyl trisilanol 1 leads to the formation of bicyclic tBuSi(OSiMe2O)3SitBu (13). An X-ray structure determination on 13 reveals a [3.3.3]-bicycle with, a C3 axis, which crystallizes in the cubic crystal system in the space group Pa3̄¯. The reported compounds 2-13 were characterised by NMR- and IR spectroscopy (5, 11) and show correct elemental analyses. The 29SiNMR-data of the compounds show interesting trends with respect to the Si-O chain length and the chloro substistuents.