Reinhard Krüger

Abstract: Hydrogels are fascinating materials with high water content and low surface friction that can be tailored for numerous applications. However, their practical application is often hampered by an intrinsic mechanical weakness. Strategies for mechanically strong hydrogels have been developed, with double network (DN) hydrogels reaching remarkable compression resistance, although so far with loss of these properties under repeated compression. In this study we present DN hydrogels composed of cross-linked six arm star-shaped poly(ethylene oxide-stat-propylene oxide) (sPEOPO) primary networks and polyacrylamide (PAAm) as secondary network. These hydrogels possess high water content (>90 w/w%) and high compression strength of up to 5.6 MPa. Most importantly, they show a fully reversible behaviour in repeated loading�unloading experiments with 1 MPa maximal stress, which we attribute to the partial healing capacity of the primary network and the overall macroporous morphology of the gels.

Abstract: Calcium phosphate cements are brittle biomaterials of low bending strength. One promising approach to improve their mechanical properties is reinforcement with fibers. State of the art degradable reinforced composites contain fibers made of polymers, resorbable glass or whiskers of calcium minerals. We introduce a new class of composite that is reinforced with degradable magnesium alloy wires. Bending strength and ductility of the composites increased with aspect ratio and volume content of the reinforcements up to a maximal bending strength of 139±41MPa. Hybrid reinforcement with metal and polymer fibers (PLA) further improved the qualitative fracture behavior and gave indication of enhanced strength and ductility. Immersion tests of composites in SBF for seven weeks showed high corrosion stability of ZEK100 wires and slow degradation of the magnesium calcium phosphate cement by struvite dissolution. Finally, in vitro tests with the osteoblast-like cell line MG63 demonstrate cytocompatibility of the composite materials.

Abstract: Bonding strength between fibers and matrices of ceramic matrix composites (CMCs) is of great importance for the fracture behavior and can be adjusted by appropriate fiber coatings. In this study, the continuous coating of SiBNC fibers with hexagonal boron nitride (h-BN) by CVD from TMB in an atmospheric pressure reactor is presented. Prior to fiber coating, the effects of deposition parameters, e.g., deposition temperature, concentration of the precursor and ammonia addition, and gas-flow rate are investigated on flat Si substrates, and the results are utilized for the fiber coating. Ammonia concentration has no influence on the deposition rate. A maximum deposition rate of 0.044�kg�m�2�h�1 for static conditions and 0.040�kg�m�2�h�1 for continuous conditions can be achieved, which is comparable to the literature. The depositions are characterized for their composition and structure. Smooth homogeneous coatings with a thickness of 180�250�nm are found on the fibers. High-resolution transmission electron microscopy (HRTEM) of the coated SiBNC fibers reveals that the hexagonal BN with 5�nm basal planes is mainly randomly oriented.

Abstract: Calcium phosphate cements (CPC) are well-established materials for the repair of bone defects with excellent biocompatibility and bioactivity. However, brittleness and low flexural/tensile strength so far restrict their application to non-load bearing areas. Reinforcement of CPC with fibers can substantially improve its strength and toughness and has been one major strategy to overcome the present mechanical limitations of CPC. Fiber reinforced calcium phosphate cements (FRCPC) thus bear the potential to facilitate the use of degradable bone substitutes in load bearing applications. This review recapitulates the state of the art of FRCPC research with focus on their mechanical properties and their biological evaluation in vitro and in vivo, including the clinical data that has been generated so far. After an overview on FRCPC constitutes and processing, some general aspects of fracture mechanics of reinforced cementitious composites are introduced, and their importance for the mechanical properties of FRCPC are highlighted. So far, fiber reinforcement leads to a toughness increase of up to two orders of magnitude. FRCPC have extensively been examined in vitro and in vivo with generally good results. While first clinical products focus on the improved performance of FRCPC with regard to secondary processing after injection such as fixation of screws and plates, first animal studies in load bearing applications show improved performance as compared to pure CPCs. Aside of the accomplished results, FRCPC bear a great potential for future development and optimization. Future research will have to focus on the selection and tailoring of FRCPC components, fiber-matrix compatibilization, integral composite design and the adjusted degradation behavior of the composite components to ensure successful long term behavior and make the composites strong enough for application in load bearing defects.

Abstract: Amorphous sol�gel-derived fibers of 82 mol% Al2O3�18 mol% Y2O3 were synthesized for structural and sintering investigations. Three sets of samples were fabricated by treatment up to 500°C in nitrogen (�N2500�), water vapor (�H2O500�), or an atmosphere that changed from evaporated nitric acid (up to 300°C) to water vapor (�HNO3300·H2O500�). All other synthesis parameters were unchanged. Residues of carbon or nitrogen in the oxide composition were analyzed. Nitrogen sorption and 27Al magic angle spinning nuclear magnetic resonance experiments were performed to investigate porosity and Al ion coordination of the xerogel fibers. Crystallization and densification of these pretreated samples were characterized using differential thermal analysis/TG and SEM at 1300°�1700°C. A high nucleation temperature and a vermicular morphology of large corundum grains were observed for the samples N2500 and HNO3300·H2O500. These samples could not be densified completely. Corundum crystallization at a lower temperature, and the formation of small compact grains was found for fibers H2O500, which could be sintered to full density. The individual crystallization and sintering behaviors are ascribed to the respective structures in the amorphous intermediate products. The amount of mesopores, internal surface area, and the Al coordination are regarded as the main controlling factors. A low content of residual carbon alone is not sufficient for good sintering results.

Abstract: The aim of this work is the coating of SiBNC fibers with hexagonal boron nitride (h-BN) for ceramic composite applications. h-BN is deposited from borazine in a vertical, hot-wall, atmospheric pressure (AP)CVD system. The effects of deposition parameters (e.g., temperatures, reactant concentrations, and gas velocity) on the deposition rates are investigated using Si(100) substrates statically placed at various positions in the reactor. The highest deposition rates are observed at 1090�°C. It is found that ammonia is necessary for a satisfying coating deposition and for low oxygen incorporation, though high concentrations decreased the deposition rate. The coating structures are characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. Based on the results of static h-BN deposition on Si substrates, h-BN is continuously and homogenously coated on SiBNC multifilaments with a fiber draw speed of 5�m h�1. The thickness of the coating on SiBNC fibers is 180�200�nm as measured by SEM. The h-BN coatings from our experiments have no influence on the mechanical strength of the SiBNC fibers.

Abstract: Via sol�gel processing metal�organic fibers were produced and dried up to 140 °C. For these gel fibers the influence of a treatment in different atmospheres was investigated for the temperature range of 200�850 °C. The atmospheres were nitrogen, water vapor, evaporated nitric and hydrochloric acid and evaporated hydrogen peroxide. In the presence of moisture and especially with acidic moisture fibers were transformed almost completely to their oxide composition (82 mol% Al2O3·18 mol% Y2O3). In these inorganic amorphous structures considerable differences were observed on several structural levels. On the atomic scale, the coordination of Al ions was investigated by 27Al MAS NMR and skeletal density by He-pycnometry. Porosity in the nm scale was characterized by N2-sorption. As a macroscopic effect of different treatment atmospheres, the longitudinal shrinkage was observed. For fibers treated at 500 °C the relative shrinkage varied by 100% (comparing water vapor and nitrogen atmosphere). No simple correlation between the release of organic constituents, the formation of porosity and the shrinkage could be found. These aspects were controlled by the rigidity of the inorganic network against atomic reconstitution. The kind of atmosphere was found to be an effective parameter to control various aspects of the xerogel structure.

Abstract: Ceramic granules of PZT (Pb(Zr53,Ti47)O3) and fibers of SKN-doped PZT (1 Mol-% Sr(K0.25Nb0.75)O3 substitution in PZT) were fabricated from sol�gel precursors. The transformation of the metal�organic gel to the oxide and its crystallization behavior was investigated by thermogravitmetry, and XRD. The formation of pyrochlore or perovskite phase was sensitive to the preceeding heat treatment conditions in moist atmosphere (termed vapor thermolysis). Lower thermolysis temperatures resulted in the preferred formation of metastable pyrochlore phase. Higher thermolysis temperatures favored the formation of perovskite. Investigation of sintered materials revealed a superior sintering activity for samples with pronounced intermediate pyrochlore content whereas low-pyrochlore samples remained highly porous after sintering. For constant chemical precursors the vapour thermolysis conditions (220�280 C) siginificantly affected crystallization and sintering behavior of the PZT and SKN-PZT material.

Abstract: Mg-phosphate ceramics have aroused growing interest as bone replacement materials due to their ability to degrade under physiological conditions. To mimic cancelous bone and to promote tissue repair mechanisms a highly macroporous structure with open cells is desired. In this study trimagnesium phosphate (farringtonite, Mg3(PO4)2) and struvite ((NH4)Mg(PO4)·6H2O) scaffolds were developed as open cell foams using the Schwarzwalder-Somers technique and optimized for pore size and mechanical performance. Polyurethane (PU) foam (20-80 ppi) was used as a template. For the optimization of the farringtonite scaffolds, ppi number of the PU foam as well as the technique that was used to remove excess slurry were varied. Sample characterization was done by SEM, XRD and compression testing. For best results were obtained using 60 ppi PU foams leading to a compressive strength of 58 kPa (90 % porosity). Farringtonite scaffolds were modified by either polymer infiltration or transformation into struvite with an ammonium phosphate solution. The pore macrostructure was retained for both of these processes and a reduction of porosity was observed. The microstructure of struvite foams was significantly altered showing larger and more facetted crystals than farringtonite. Mechanical properties substantially improved by transformation into struvite to 730 kPa (68 % porosity). Cytocompatibility was tested using osteoblasts and fibroblasts. Cell number and cell activity (WST) were tested over a period of 3 to 13 days. Farringtonite foams showed a tendency for higher cell numbers than struvite, while the WST activity was similar. Infiltration of farringtonite with PLGA approximately doubled cell number compared to pure farringtonite. In conclusion macroporous Mg-phosphate foams have been successfully produced. Compressive strength of the foams was drastically improved by optimization of pore fineness, transformation to struvite and infiltration with PLGA. The open porous structure was retained and the materials showed good cytocompatibility.

Abstract: In recent time Mg-phosphate ceramics have gained growing in- terest as biodegradable bone replacement materials. To mimic the structure of cancelous bone and to promote tissue repair mecha- nisms a highly porous structure with open cells is desired. Tri-magnesium phosphate (Mg3(PO4)2) and struvite ((NH4)Mg(PO4)2�6H2O) scaffolds were developed in this study as open cell foams using the foam replica technique. The ceramic scaffolds were investigated for their mechanical performance, phase com- position and cytocompatibility. Reticulated polyurethane (PU) foam (30 ppi pore size) was used as template. The PU foam samples were dip coated with a ceramic slurry consisting of Mg3(PO4)2�8H2O powder in distilled water, TWEEN 20 as a surfactant and NH4OH (pH~11). After the first infiltration excess slurry was removed by compressing the foams to 10% of their original height in a rolling process. By this only the struts of the PU scaffold were coated with slurry to obtain an open cell structure. Single coated green bodies were sintered at 1050 °C to burn out the PU foam and consolidate the structure. By repeating of the infiltration, blowing out the excess slurry by an air gun and a second sintering at 1050°C, the compression strength was improved and cracks were filled as analyzed by SEM. Sintered re-infiltrated foams showed a diametric shrinkage of 66%. The relative density was 3.8%. The open cell structure of the PU templates was almost completely retained. XRD analysis revealed farringtonite (Mg3(PO4)2) as the only crystalline phase. Soaking the produced Mg3(PO4)2 scaffolds in ammonium phosphate solution ((NH4)3PO4) lead to successive transformation to struvite accompanied with mass increase of 88% (24h). The transformation kinetics was investigated by XRD. Mechanical properties were analyzed by compression testing. Compression strength increased from 6.8 kPa after the first infiltration step to 14.1 kPa after the second infiltration and sintering. Transformation into struvite caused a further increase of strength to 31.1 kPa. Cracks observed in the farringtonite foams by SEM could no longer be found in the struvite foams. The final scaffolds were tested for cytocompatibility using fibroblast cells. Cell number and cell activity (WST) were tested after 3 to 14 days. Cell activity and cell size increased on both materials with time. Cell number only increased on struvite, indicating an improved cytocompatibility compared to farringtonite.