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The formation of crack patterns in drying starch-water slurries is studied by means of in-situ radiography (measuring of the crack front velocity) and X-ray microtomography as an example of crack patterns driven by inhomogeneous shrinkage. The tomograms show the 3D crack networks forming columns with polygonal cross-sections. After crack initiation, the average crack spacing increases with growing depth, even if the crack front velocity is constant. A constant velocity is obtained by maintaining a constant evaporation rate using a feedback control. When the crack front has propagated at a constant evaporation rate over a distance of some millimeters, the average crack spacing approaches a stable value which depends on crack front velocity according to a power law. This relationship is compared to corresponding results of other authors and model predictions. The increase of crack spacing before stable values are achieved, is interpreted as a result of successive crack front instabilities. [ABSTRACT FROM AUTHOR]

Abstract: Soil deformation is a perpetual process in the pedosphere where besides physicochemical stresses primarily alternating hydraulic and mechanical stresses continuously re-arrange the configuration of solid particles. In this study we present a local strain analysis and changes in soil structure resulting from hydraulic and mechanical stresses based on X-ray microtomography data. Digital image reconstructions were used to quantify local structural pore space characteristics and local soil deformation by 3D morphological and correlation analysis of grayscale tomograms. Swelling and shrinkage resulted in a complex heterogeneous soil structure which proofed to be very stable when mechanical loads were applied. The mechanism of soil deformation for both structure formation by internal hydraulic stresses and structure degradation by external mechanical stresses were in both cases controlled by pre-existing (micro)-structures. Especially during wetting such structures served as a nucleus for subsequent structure evolution. The results demonstrate the potential of more detailed non-invasive micromechanical analysis of soil deformation processes which could improve the conceptual understanding of the physical behavior of soil systems. [Copyright &y& Elsevier]

Abstract: Repeated observations of enhanced bone growth around various degradable magnesium alloys in vivo raise the question: what is the major mutual origin of this biological stimulus? Several possible origins, e.g. the metal surface properties, electrochemical interactions and biological effects of alloying elements, can be excluded by investigating the sole bone response to the purified major corrosion product of all magnesium alloys, magnesium hydroxide (Mg(OH)2). Isostatically compressed cylinders of pure Mg(OH)2 were implanted into rabbit femur condyles for 2–6weeks. We observed a temporarily increased bone volume (BV/TV) in the vicinity of Mg(OH)2 at 4weeks that returned to a level that was equal to the control at 6weeks. The osteoclast surface (OcS/BS) was significantly reduced during the first four weeks around the Mg(OH)2 cylinder, while an increase in osteoid surface (OS/BS) was observed at the same time. At 6weeks, the OcS/BS adjacent to the Mg(OH)2 cylinder was back within the same range of the control. The mineral apposition rate (MAR) was extensively enhanced until 4weeks in the Mg(OH)2 group before matching the control. Thus, the enhanced bone formation and temporarily decreased bone resorption resulted in a higher bone mass around the slowly dissolving Mg(OH)2 cylinder. These data support the hypothesis that the major corrosion product Mg(OH)2 from any magnesium alloy is the major origin of the observed enhanced bone growth in vivo. Further studies have to evaluate if the enhanced bone growth is mainly due to the local magnesium ion concentration or the local alkalosis accompanying the Mg(OH)2 dissolution. [Copyright &y& Elsevier]

Abstract: The aim of this study was to investigate whether the extruded magnesium alloy LAE442 reacts in vivo with an appropriate host response and to investigate how an additional magnesium fluoride (MgF2) coating influences the in vivo corrosion rate. Forty cylinders were machined from extruded LAE442 and 20 of these were coated additionally with MgF2 and implanted into the medial femur condyle of adult rabbits. Synchrotron-radiation-based X-ray computed micro-tomography (SRμCT) was used to quantitatively analyse corrosion non-destructively in vivo and comparisons were made to magnesium degradation rates based on area measurements of the remaining metal on uncalcified sections. Blood concentrations of the alloying elements were measured below toxicological limits. The MgF2 layer was no longer detected after 4weeks of implantation by particle-induced gamma emission, and the MgF2 coating reduced the blood content of alloying elements during the first 6weeks of implantation with no elevated fluoride concentration in the adjacent bone. Histopathological examinations of liver showed in 9 out of 40 cases minimal infiltrations of heterophil granulocytes of unknown origin (5 LAE442, 4 LAE442+MgF2). The kidneys were mainly regular in structure. The synovial tissue showed a granular cell infiltration as a temporary observation in the LAE442+MgF2 group after 2weeks. No subcutaneous gas cavities were observed clinically and on postoperative X-rays in all animals. All specimens were scanned by SRμCT at 2, 4, 6 and 12weeks postoperatively before uncalcified sections were performed. All magnesium implants have been observed in direct bone contact and without a fibrous capsule. Localized pitting corrosion occurred in coated and uncoated magnesium implants. This study shows that the extruded magnesium alloy LAE442 provides low corrosion rates and reacts in vivo with an acceptable host response. The in vivo corrosion rate can be further reduced by additional MgF2 coating. [Copyright &y& Elsevier]