G. Ziegler

In vitro -Osteoclastic Activity Studies on Surfaces of 3D Printed Calcium Phosphate Scaffolds:

Various biomaterials have been developed for the use as bone substitutes for bone defects. To optimize their integration and functionality, they should be adapted to the individual defect. Rapid prototyping is a manufacturing method to tailor materials to the 3D geometry of the defect. Especially 3D printing allows the manufacture of implants, the shape of which can be designed to fit the bone defect using anatomical information obtained from the patient. 3D printing of calcium phosphates, which are well established as bone substitutes, involves a sintering step after gluing the granules together by a binder liquid. In this study, we analyzed if and how these 3D printed calcium phosphate surfaces can be resorbed by osteoclast-like cells. On 3D printed scaffold surfaces consisting of pure HA and β-TCP as well as a biphasic mixture of HA and TCP the osteoclastic cell differentiation was studied. In this regard, cell proliferation, differentiation, and activation were analyzed with the monocytic cell line RAW 264.7. The results show that osteoclast-like cells were able to resorb calcium phosphate surfaces consisting of granules. Furthermore, biphasic calcium phosphate ceramics exhibit, because of their osteoclastic activation ability, the most promising surface properties to serve as 3D printed bone substitute scaffolds.

The chemical composition of synthetic bone substitutes influences tissue reactions in vivo: histological and histomorphometrical analysis of the cellular inflammatory response to hydroxyapatite, beta-tricalcium phosphate and biphasic calcium phosphate ceramics

Bone substitute material properties such as granule size, macroporosity, microporosity and shape have been shown to influence the cellular inflammatory response to a bone substitute material. Keeping these parameters constant, the present study analyzed the in vivo tissue reaction to three bone substitute materials (granules) with different chemical compositions (hydroxyapatite (HA), beta-tricalcium phosphate (TCP) and a mixture of both with a HA/TCP ratio of 60/40 wt%). Using a subcutaneous implantation model in Wistar rats for up to 30 days, tissue reactions, including the induction of multinucleated giant cells and the extent of implantation bed vascularization, were assessed using histological and histomorphometrical analyses. The results showed that the chemical composition of the bone substitute material significantly influenced the cellular response. When compared to HA, TCP attracted significantly greater multinucleated giant cell formations within the implantation bed. Furthermore, the vascularization of the implantation bed of TCP was significantly higher than that of HA implantation beds. The biphasic bone substitute group combined the properties of both groups. Within the first 15 days, high giant cell formation and vascularization rates were observed, which were comparable to the TCP-group. However, after 15 days, the tissue reaction, i.e. the extent of multinucleated giant cell formation and vascularization, was comparable to the HA-group. In conclusion, the combination of both compounds HA and TCP may be a useful combination for generating a scaffold for rapid vascularization and integration during the early time points after implantation and for setting up a relatively slow degradation. Both of these factors are necessary for successful bone tissue regeneration.

Conversion of Al2O3–SiO2 powder mixtures to 3:2 mullite following the stable or metastable phase diagram

A model system, composed of powder blends of amorphous isomorphic silica spheres, being 500 nm in diameter, and also monosized crystalline α-Al 2 O 3 powder, was investigated. Two different particle sizes of the corresponding alumina powder were employed: 300 nm and 2 μm. This particular assembly enabled a distinction between amorphous silica and crystalline alumina merely by their difference in particle morphology. The powder blends were sintered at temperatures between 1400 and 1700 C and the microstructure evolution was characterized by scanning (SEM) and transmission electron microscopy (TEM). It is worth noting that upon annealing at 1700°C, both microstructures were indistinguishable. However, depending on the Al 2 O 3 particle size, different conversion mechanisms were monitored. When using the 300 nm Al 2 O 3 powder, fast dissolution of alumina into the coalesced silica glass occurred, followed by homogeneous nucleation and growth of mullite within the glass. Utilizing 2 μm Al 2 O 3 particles, however, resulted in the formation of two Al-containing glasses (phase separation into a Si- and Al-rich glass). In this case, the transformation to mullite can be rationalized by the conversion of the metastable Al-rich transient glass into mullite, which forms an epitaxial, single crystalline coating on the host Al 2 O 3 particle. Therefore, depending on the initial Al 2 O 3 particle size, mullite formation follows either the stable or metastable phase diagram.

Relations between composition and microstructure of sialons

Abstract The relationship between composition and micro-structure of sialon ceramics was studied. Fully dense yttrium-containing α′- and α ′ + β ′-sialon ceramics were prepared by gas pressure sintering. Additionally, the effect of La 2 O 3 as a sintering additive was investigated. Experimental results reveal that the typical microstructure of these sialon ceramics consists of a crystalline phase of sialon(s) and a small fraction of amorphous phase remaining at grain boundaries. The composition of the amorphous phase is very similar, both in α′- and α ′ + β ′-sialon ceramics. However, the amount of the amorphous phase increases with increasing amount of oxides used in the starting mixtures. The α′- and α ′ + β ′-sialons exhibit differences in grain size and grain morphology. The addition of La 2 O 3 to the starting mixtures promotes the incorporation of yttrium ions into the crystal lattice and leads to the formation of crystal defects in the α′-grains.

Characterization and surface chemistry of uncoated and coated silicon nitride powders

Abstract Various Si3N4 powders, produced by different procedures, were characterized by imaging (TEM) and analytical methods (EDS, FT-IR, XPS) in the as-received state as well as after doping with a metal oxide (MgO). For the doping, an alternative procedure to the usual methods, was applied, which is based on soluble organometallic compounds. Analytical transmission electron microscopy combined with lateral resolution element analysis and XPS measurements was used for morphological, structural and analytical characterization. The distribution of the dopant was deduced from measurements of XPS sputter depth profiles. These investigations were supplemented by FT-IR measurements in order to determine qualitatively and semi-quantitatively the reactive groups on the particle surfaces of the as-received powders. For comparison, measurements were performed with Si3N4 powders which were doped by the above chemical procedure and by mechanical mixing. The results of the various characterization methods are interpreted in the form of a model display for surface reactions of organometallic doping reagents on the surfaces of ceramic particles. The results show that Si3N4 powders with high concentration of OH groups on their particle surface reveal very good distribution of the fluxing element (layer-like coating).

Organometallic precursors, an alternative route for the doping of silicon nitride powders?

Abstract Two silicon nitride powders prepared by diimide precipitation and direct nitridation were doped with 5 vol.% yttria using (i) a commercial Y2O3 powder and (ii) a soluble Y-organometallic compound as additive. The aim of this work was a direct comparison between those two doping methods. Homogeneity of additive distribution was characterized by Fourier transform infra-red spectroscopy (FT-IR), photo electron spectroscopy (XPS) and transmission electron microscopy (TEM). Depending on the doping technique used, no major difference in sintering behavior and microstructure evolution were monitored. However, chemical doping results in (i) an enhanced densification rate in the early stage of sintering, (ii) a slight shift of the densification rate maximum (ds/dt) to higher temperatures, and (iii) in a slightly finer final microstructure. Note that doping via organometallic precursors can only be successfully applied, when the powder particle surface reveals the presence of silanol groups, as in case of diimide starting powders. Electron microscopy, Organometallic dopants, Sintering, Si3N4

In situ carbon formation within a Si--Ti--C--O-fibre SiO2-glass composite

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