Jens Nellesen

In vivo corrosion and corrosion protection of magnesium alloy LAE442

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 (MgF(2)) coating influences the in vivo corrosion rate. Forty cylinders were machined from extruded LAE442 and 20 of these were coated additionally with MgF(2) and implanted into the medial femur condyle of adult rabbits. Synchrotron-radiation-based X-ray computed micro-tomography (SRmicroCT) 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 MgF(2) layer was no longer detected after 4 weeks of implantation by particle-induced gamma emission, and the MgF(2) coating reduced the blood content of alloying elements during the first 6 weeks 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+MgF(2)). The kidneys were mainly regular in structure. The synovial tissue showed a granular cell infiltration as a temporary observation in the LAE442+MgF(2) group after 2 weeks. No subcutaneous gas cavities were observed clinically and on postoperative X-rays in all animals. All specimens were scanned by SRmicroCT at 2, 4, 6 and 12 weeks 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 MgF(2) coating.

Magnesium hydroxide temporarily enhancing osteoblast activity and decreasing the osteoclast number in peri-implant bone remodelling.

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 2-6 weeks. We observed a temporarily increased bone volume (BV/TV) in the vicinity of Mg(OH)(2) at 4 weeks that returned to a level that was equal to the control at 6 weeks. 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 6 weeks, 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 4 weeks 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.

In vitro and in vivo evaluation of biodegradable, open-porous scaffolds made of sintered magnesium W4 short fibres

A cytocompatible and biocompatible, degradable, open-porous, mechanically adaptable metal scaffold made of magnesium alloy W4 melt-extracted short fibres was fabricated by liquid phase sintering. Cylindrical samples (3×5 mm) of sintered W4 short fibres were evaluated under in vitro (L929, HOB, eudiometer, weight loss) and in vivo conditions (rabbits: 6 and 12 weeks). The in vitro corrosion environment (e.g., temperature, flow, composition of corrosion solution, exposure time) significantly influenced the corrosion rates of W4 scaffolds compared with corrosion in vivo. Corrosion rates under cell culture conditions for 72 h varied from 1.05 to 3.43 mm y(-1) depending on the media composition. Corrosion rates measured in eudiometric systems for 24 h were ~24-27 times higher (3.88-4.43 mm y(-1)) than corrosion in vivo after 6 weeks (0.16 mm y(-1)). Moreover, it was found that the cell culture media composition significantly influences the ionic composition of the extract by selectively dissolving ions from W4 samples or their corrosion products. A pilot in vivo study for 6 and 12 weeks demonstrated active bone remodelling, no foreign body reaction and no clinical observation of gas formation during W4 scaffold implantation. Long-term in vivo studies need to be conducted to prove complete degradation of the W4 scaffold and total replacement by the host tissue.

Effect of the addition of low rare earth elements (lanthanum, neodymium, cerium) on the biodegradation and biocompatibility of magnesium

Rare earth elements are promising alloying element candidates for magnesium alloys used as biodegradable devices in biomedical applications. Rare earth elements have significant effects on the high temperature strength as well as the creep resistance of alloys and they improve magnesium corrosion resistance. We focused on lanthanum, neodymium and cerium to produce magnesium alloys with commonly used rare earth element concentrations. We showed that low concentrations of rare earth elements do not promote bone growth inside a 750 μm broad area around the implant. However, increased bone growth was observed at a greater distance from the degrading alloys. Clinically and histologically, the alloys and their corrosion products caused no systematic or local cytotoxicological effects. Using microtomography and in vitro experiments, we could show that the magnesium-rare earth element alloys showed low corrosion rates, both in in vitro and in vivo. The lanthanum- and cerium-containing alloys degraded at comparable rates, whereas the neodymium-containing alloy showed the lowest corrosion rates.

Biocompatibility of rapidly solidified magnesium alloy RS66 as a temporary biodegradable metal

Biodegradable magnesium-based alloys are very promising materials for temporary implants. However, the clinical use of magnesium-based alloys is often limited by rapid corrosion and by insufficient mechanical stability. Here we investigated RS66, a magnesium-based alloy with extraordinary physicochemical properties of high tensile strength combined with a high ductility and a homogeneous grain size of ~1 μm which was obtained by rapid solidification processing and reciprocal extrusion. Using a series of in vitro and in vivo experiments, we analyzed the biodegradation behavior and the biocompatibility of this alloy. In vitro, RS66 had no cytotoxic effects in physiological concentrations on the viability and the proliferation of primary human osteoblasts. In vivo, RS66 cylinders were implanted into femur condyles, under the skin and in the muscle of adult rabbits and were monitored for 1, 2, 3, 4 and 8 weeks. After explantation, the RS66 cylinders were first analyzed by microtomography to determine the remaining RS66 alloy and calculate the corrosion rates. Then, the implantation sites were examined histologically for healing processes and foreign body reactions. We found that RS66 was corroded fastest subcutaneously followed by intramuscular and bony implantation of the samples. No clinical harm with transient gas cavities during the first 6 weeks in subcutaneous and intramuscular implantation sites was observed. No gas cavities were formed around the implantation site in bone. The corrosion rates in the different anatomical locations correlated well with the local blood flow prior to implantation. A normal foreign body reaction occurred in all tissues. Interestingly, no enhanced bone formation could be observed around the corroding samples in the condyles. These data show that RS66 is biocompatible, and due to its interesting physicochemical properties, this magnesium alloy is a promising material for biodegradable implants.

Microtomography of magnesium implants in bone and their degradation

Biodegradable metal implants for musculoskeletal and intravascular applications made of magnesium alloys have been shown to degrade in-vivo by corrosion. The in vivo corrosion of magnesium alloys has the potential to provide a new mechanism which will allow metal implants to be applied in musculoskeletal surgery as biodegradable implants. This would particularly be true if magnesium alloys with predictable in vivo corrosion rates could be developed. Since the magnesium corrosion process depends on the corrosive environment, the corrosion rates of magnesium alloys under standard in-vitro environmental conditions are not directly comparable to results obtained from an animal model. Synchrotron-radiation based microtomography (SRμCT) enabled us to investigate non-destructively the in vivo corrosion as well as the osteointegration at the implant-bone interphase at a high spatial resolution. Corrosion morphology and its metallurgical quantification of pit formation could be obtained. Since the alloying elements of magnesium alloys have significant importance for the degradation process in biological environments the biocompatibility depending on their local concentration and distribution has to be investigated. For this purpose we used element-specific SRμCT to show the spatial distribution without destroying the bone-implant interphase. The SRμCT setup at HASYLAB at DESY will be an excellent tool in the future to develop suitable magnesium alloys and magnesium implants for special medical applications.

Repetitive recombinant human bone morphogenetic protein 2 injections improve the callus microarchitecture and mechanical stiffness in a sheep model of distraction osteogenesis

Evidence suggests that recombinant human bone morphogenetic protein 2 (rhBMP-2) increases the mechanical integrity of callus tissue during bone healing. This effect may be either explained by an increase of callus formation or a modification of the trabecular microarchitecture. Therefore the purpose of the study was to evaluate the potential benefit of rhBMP-2 on the trabecular microarchitecture and on multidirectional callus stiffness. Further we asked, whether microarchitecture changes correlate with optimized callus stiffness. In this study a tibial distraction osteogenesis (DO) model in 12 sheep was used to determine, whether percutaneous injection of rhBMP-2 into the distraction zone influences the microarchitecture of the bone regenerate. After a latency period of 4 days, the tibiae were distracted at a rate of 1.25 mm/day over a period of 20 days, resulting in total lengthening of 25 mm. The operated limbs were randomly assigned to one treatment groups and one control group: (A) triple injection of rhBMP-2 (4 mg rhBMP-2/injection) and (B) no injection. The tibiae were harvested after 74 days and scanned by µCT (90 µm/voxel). In addition, we conducted a multidirectional mechanical testing of the tibiae by using a material testing system to assess the multidirectional strength. The distraction zones were tested for torsional stiffness and bending stiffness antero-posterior (AP) and medio-lateral (ML) direction, compression strength and maximum axial torsion. Statistical analysis was performed using multivariate analysis of variance (ANOVA) followed by students t-test and Regression analysis using power functions with a significance level of P<0.05. Triple injections of rhBMP-2 induced significant changes in the trabecular architecture of the regenerate compared with the control: increased trabecular number (Tb.N.) (treatment group 1.73 mm/1 vs. control group 1.2 mm/1), increased cortical bone volume fraction (BV/TV) (treatment group 0.68 vs. control group 0.47), and decreased trabecular separation (Tb.Sp.) (treatment group 0.18 mm vs. control group 0.43 mm). The analyses of the mechanical strength of regenerated bone showed significant differences between treatment group (A) and the control group (B). The bending stiffness anterior-posterior (treatment group 17.48 Nm vs. control group 8.3 Nm), medial-lateral (treatment group 18,9 Nm vs. control group 7.92 Nm) and the torsional stiffness (treatment group 41.17N/° vs. control group 16.41N/°) are significantly higher in the treatment group than in the control group. The regression analyses revealed significant non-linear relationships between BV/TV, TB.N., Tb.Sp. and all mechanical properties. Maximal correlation coefficients were found for the Tb.Sp. vs. the bending stiffness AP and ML with R2=0.69 and R2=0.70 (P<0.0001). There was no significant relation between Connectivity and the compression strength and the maximum axial torque. This study suggests that rhBMP-2 optimizes the trabecular microarchitecture of the regenerate, which might explain the advanced mechanical integrity of newly formed bone under rhBMP-2 treatment.

Tomographic Analysis and FE-Simulations of MMC-Microstructures under Load

Microstructural changes like micro deformation and damaging due to tensile load precede the macroscopical failure of a component. In order to contribute to the understanding of such processes, the microstructure of tensile test specimens was imaged by microtomography in the course of deformation. The specimens consist of particle reinforced metal matrix composites (the MMCs Cobalt/Diamond and Al/TiN) manufactured on a powder metallurgical route. Tomograms of a volume in the gauge length of the specimens were reconstructed from the projection data acquired at different deformation stages. Both polychromatic radiation of a microfocus X-ray tube and monochromatic synchrotron radiation were used for projection data acquisition. With the help of 3D data processing 3D surface nets were extracted from the tomograms which indicate the particle/matrix interface. These nets which are composed of triangles were afterwards optimized with respect to the shape of the triangles. Using the triangles as seeds a 3D FE-mesh without gaps consisting of tetrahedra was generated. 3D FE-simulations were carried out utilizing both arbitrary and realistic boundary constraints. Realistic conditions were derived from an iterative matching procedure of tomograms. The effect of finite element type (tetrahedron or hexahedron) on the simulated distribution of stresses was analyzed. The appearance and development of plastic zones in the metal matrix depending on externally applied displacements were studied in the simulations. The calculated peak stresses are compared with the loci of cracks found in the tomograms.

Influence of non-invasive X-ray computed tomography (XRCT) on the microbial community structure and function in soil.

In this study the influence of X-ray computed tomography (XRCT) on the microbial community structure and function in soils has been investigated. Our results clearly indicate that XRCT of soil samples has a strong impact on microbial communities and changes structure and function significantly due to the death of selected microbial groups as a result of the treatment.

Metal Matrix Composites Deposition in Twin Wire Arc Spraying Utilizing an External Powder Injection Composition

The powder injection parameters, the location of the injection port, as well as the metal matrix composites are important features, which determine the deposition efficiency and embedding behavior of hard materials in the surrounding matrix of the twin wire arc-spraying process. This study investigates the applicability of external powder injection and aims to determine whether the powder injection parameters, the location, and the material combination (composition of the matrix as well as hard material) need to be specifically tailored. Therefore, the position of the injection port in relation to the arc zone was altered along the spraying axis and perpendicular to the arc. The axial position of the injection port determines the thermal activation of the injected powder. An injection behind the arc, close to the nozzle outlet, seems to enhance the thermal activation. The optimal injection positions of different hard materials in combination with zinc-, nickel- and iron-based matrices were found to be closer to the arc zone utilizing a high-speed camera system. The powder size, the mass of the particle, the carrier gas flow, and the electric insulation of the hard material affect the perpendicular position of the radial injection port. These findings show that the local powder injection, the wetting behavior of particles in the realm of the molten pool as well as the atomization behavior of the molten pool all affect the embedding behavior of the hard material in the surrounded metallic matrix. Hardness measurement by means of nanoindentation and EDX analysis along transition zones were utilized to estimate the bonding strength. The observation of a diffusion zone indicates a strong metallurgical bonding for boron carbides embedded in steel matrix.