Günter Ziegler

The common occurrence of furan fatty acids in plants

The observation that F-acids (1) occur in rat chow initiated a search for F-acids in human diet. We observed that the amount of F-acids with a pentyl side chain in α-position taken up with a one-day diet correlates well with the amount of excreted degradation products, the pentyl urofuran acids (2), (3) and (4). Therefore it can be concluded that F-acids with a pentyl side chain are not produced in the human body but are introduced through the diet. The origin of F-acids carrying an α-propyl side chain is less clear. The amount of propyl-urofuran acids (2) and (3) excreted in urine was found in one case out of three to be five times higher than the amount of F-acids carrying a propyl group in α-position taken up by the diet. Therefore, it can presently not be excluded that a portion of the propyl F-acids is produced by the body.F-acids found in human food are mainly introduced into the body by vegetables and fruits. F-acids were found also in birch leaves in considerable amounts, as well as in grasses, dandelion and clover leaves. Thus, we can conclude that F-acids are common constituents of plants.

Structural characterisation of silicon carbonitride ceramics derived from polymeric precursors

Abstract Polymeric precursors with tailored structures were prepared from functionalised chlorosilanes. Pyrolysis under inert atmospheres led to amorphous Si–C–N–(H) ceramics at 1000°C. Further heat treatment caused the transformation into the thermodynamically stable crystalline phase assemblage. The structural changes associated with crosslinking, pyrolysis and crystallisation were studied by characterising the solid intermediates between 300 and 1600°C applying 29 Si and 13 C solid state nuclear magnetic resonance (NMR) spectroscopy. In addition, Fourier transformed infrared (FTIR) spectroscopy, X-ray diffraction (XRD), thermoanalytical techniques and density measurements were employed. The combination of these methods and the use of polymeric precursors with tailored structures pointed to a correlation of the polymer architecture with the structure of the amorphous ceramic material.

Characterisation of the free-carbon phase in precursor-derived SiCN ceramics

Abstract Different polymeric precursors with varying carbon contents were prepared by ammonolysis of functionalised chlorosilanes. Pyrolysis under inert atmospheres at 1000 °C led to amorphous Si–C–N–(H) ceramics. Further heat treatment caused the transformation into the thermodynamically stable crystalline phase assemblage. The structural changes, especially those of the excess carbon, were studied by characterising the solid intermediates via solid state magic angle spinning (MAS) nuclear magnetic resonance spectroscopy (NMR). In addition, Raman spectroscopy, electron spin resonance spectroscopy (ESR), microwave conductivity measurements and chemical analysis were employed. Combination of all these methods provides a comprehensive picture of the formation and of the behaviour of the free-carbon phase present in the polymer-derived ceramics.

Structural characterization of intermediate species during synthesis of Al2O3-aerogels

Abstract Alumina sols in the system (Al(OC 4 H 9 sec ) 3 /H 2 O/HNO 3 ) were synthesized based on a well known method. Structural changes during sol–gel transition, aging of the gel and water–acetone exchange were studied, and the effect of these parameters on the structure and morphology of the aerogels was investigated. The alumina aerogels were prepared by supercritical drying with CO 2 . Structural changes during the various synthesis and processing steps were followed by Raman and Fourier transform infrared spectroscopy, 27 Al nuclear magnetic resonance, and rheological measurements. Additionally, pore morphology of the aerogels was investigated by BET analysis and scanning electron microscopy observations. In the sol alumina polycations with four- and sixfold coordination were detected by 27 Al NMR. During sol–gel transition at 403 K a growth of the boehmite crystallites as well as a growth of the network was observed. The gel consists of an amorphous network with tetra- and octahedrally coordinated Al-atoms and boehmite crystallites which are embedded in an amorphous phase. During aging the number of network points in the gel increases by reactions between the amorphous phase and the crystallites or between the crystallites. Additionally, the crystallite size increases at the expense of the amorphous phase. For the calcination residue of 13.7 wt% Al 2 O 3 the pore structure of the aerogels is controlled by two effects: at the beginning of aging by the growth of the network, for long aging times by the irregular growth of boehmite crystallites.

Fiber-reinforced composites with polymer-derived matrix : processing, matrix formation and properties

Carbon and SiC fiber-reinforced ceramic matrix composites were prepared via infiltration of fiber preforms using the polymer infiltration technique and polymer pyrolysis. Suitable silazane (SiCN) precursors with appropriate thermosetting behavior, viscosity and ceramic yield were synthesized, starting from functionalized chlorosilanes. Microstructural development and fracture behavior was studied after various infiltration and pyrolysis cycles. Residual stresses induced during processing were evaluated. Mechanical and thermo-physical properties of the composites with polymer-derived matrix, i.e. 3-pt bending strength and thermal expansion coefficients (CTE), were measured dependent on reinfiltration cycles and fiber orientation. The oxidation resistance was investigated. Specific pyrolyzed samples were infiltrated via silicon melts in order to enhance corrosion and wear resistance.

Transmission electron microscopy of microstructures in ceramic materials

Abstract Based on selected examples from the area of Si3N4 ceramics, the value of utilizing transmission electron microscopy (TEM) as a technique to study ceramic microstructures as well as a characterization tool for the development of new materials is demonstrated. In the field of ‘new ceramics’, one Si3N4-based composite is discussed, which was processed via pyrolysis of liquid precursors (polysilazanes). Moreover, it is shown that TEM in general can helpfully accompany ceramic processing techniques. This applies to the characterization of ceramic starting powders as well as to the study of densified materials. The investigation of Si3N4 powders, in particular the influence of the addition of sintering aids via organometallic precursors, which leads to a homogeneous distribution of sintering additives in the powder compact, in contrast to the use of metal oxide powders, is shown. The variation of micro-structures during the densification process of liquid assisted sintering is also demonstrated. The most common application of the TEM technique is to characterize dense ceramic components in the as-processed (as-sintered) state. Post-sintering heat treatment can initiate secondary phase crystallization. However, the very important aspect of microstructure integrity at elevated temperatures, e.g. the stability of microstructures under severe service conditions, is also addressed. Emphasis is placed on the fact that ceramic microstructures, which are typically thought to be rather stable, can undergo serious microstructural changes when temperature and stress is applied simultaneously, which strongly limits potential applications of these materials.

Fabrication of Tailored Hydroxyapatite Scaffolds: Comparison between a Direct and an Indirect Rapid Prototyping Technique

In the last few years new fabrication methods, called rapid prototyping (RP) techniques, have been developed for the fabrication of hydroxyapatite scaffolds for bone substitutes or tissue engineering applications. With this generative fabrication technology an individual tailoring of the scaffold characteristics can be realised. In this work two RP techniques, a direct (dispense-plotting) and an indirect one (negative mould technique), are described by means of fabricating hydroxyapatite (HA) scaffolds for bone substitutes or bone tissue engineering. The produced scaffolds were characterised, mainly regarding their pore and strut characteristics. By these data the performance of the two fabrication techniques was compared. Dispense-plotting turned out to be the faster technique while the negative mould method was better suited for the fabrication of exact pore and strut geometries.

A new biodegradable bone wax substitute with the potential to be used as a bone filling material

A novel biomaterial consisting of the biodegradable compounds hydroxyapatite, chitosan and starch was developed. Hydroxyapatite particles were modified in a layer-to-layer reaction with oxidised starch and deacetylated chitosan. In this manner, two kinds of particles with different surfaces were generated, one featuring deacetylated chitosan, the other one exhibiting oxidised starch at the outermost layer. Mixed together, these two kinds of particles form a homogeneous powder, which can be transformed into a paste by adding water. In contact with water the amino groups of deacetylated chitosan and the aldehyde groups of oxidised starch form a Schiffs base bonding the particles together. Particle size distribution of powder modified with polysaccharides was characterised by using a CILAS granulometer. Burn-out experiments carried out by TG-MS provided the organic/inorganic ratio. The morphology of the particles was analysed by SEM. The cytotoxicity of the new composite was examined in vitro by culturing MC3T3 mouse fibroblasts directly on the new material. The viability of MC3T3 cells was largely unaffected, and cells proliferated well on the surface of the new substance indicating excellent biocompatibility. In view of its mechanical stability, the new composite may also be used as a bone filling material in orthopedic surgery.

Overcoming hypoxia in 3D culture systems for tissue engineering of bone in vitro using an automated, oxygen-triggered feedback loop

Tissue engineering is an attractive approach to heal bony defects. However, three-dimensional cell-scaffold constructs display uneven oxygen supply resulting in inhomogeneous tissue quality. We assessed different strategies to improve oxygen supply in vitro. Scaffolds with differing inner surface were seeded with preosteoblastic cells and cultivated either statically or in perfusion bioreactors. Oxygen concentration and pH were measured in the center of the scaffolds. An inductive feedback mechanism was build to increase bioreactor pump speed according to the oxygen concentrations measured within the scaffolds. While pH remained stable, oxygen concentration decreased significantly under static conditions within the cell-seeded scaffolds. Reducing the scaffolds’ inner surface as well as increasing perfusion speeds in bioreactors resulted in improved oxygen supply. We conclude that improving oxygen supply to three dimensional culture systems for bone tissue engineering is feasible in an automated manner. Culture conditions have to be adapted to each cell-scaffold system individually.

Influence of Phase Composition on Degradation and Resorption of Biphasic Calcium Phosphate Ceramics

In this study the degradation behaviour of pure hydroxyapatite (HA), pure tricalcium phosphate (β-TCP) and four biphasic calcium phosphate ceramics was studied to gain information about the influence of the phase composition on this property with the aim to tailor individually adapted bone substitute materials. The chemical dissolution of each ceramic composition was measured by its release of calcium ions into a buffered solution. With decreasing HA content in the ceramics the degradation rate increased. Cell experiments were carried out with stimulated osteoclast-like RAW 264.7 cells. Using biochemical, genetic and microscopic analysis, the differentiation of the cells on the ceramic samples was studied. The monocytic precursor cells differentiated into osteoclast-like cells on all ceramics. The strongest cell differentiation into osteoclast-like cells was found on ceramics with HA/β-TCP ratios of 80/20, 60/40 and 40/60. Cells on these ceramics had many nuclei and the largest cell size. As a result of resorption, lacunas were found on all ceramics except β-TCP. All these experimental results proved the influence of the phase composition on degradation and resorption of calcium phosphate ceramics. Biphasic calcium phosphate ceramics with HA/β-TCP ratios of 80/20 and 60/40 exhibited the most promising properties to serve as synthetic bone substitute materials because for integration in the physiological bone remodeling process the implanted bone substitute materials should have optimized dissolution and resorption properties.