Abstract Biocompatibility is a key issue in the development of new implant materials. In this context, a novel class of biodegrading Mg implants exhibits promising properties with regard to ...inflammatory response and mechanical properties. The interaction between Mg degradation products and the nanoscale structure and mineralization of bone, however, is not yet sufficiently understood. Investigations by synchrotron microbeam x-ray fluorescence (μXRF), small angle x-ray scattering (μSAXS) and x-ray diffraction (μXRD) have shown the impact of degradation speed on the sites of Mg accumulation in the bone, which are around blood vessels, lacunae and the bone marrow. Only at the highest degradation rates was Mg found at the implant–bone interface. The Mg inclusion into the bone matrix appeared to be non-permanent as the Mg-level decreased after completed implant degradation. μSAXS and μXRD showed that Mg influences the hydroxyl apatite (HAP) crystallite structure, because markedly shorter and thinner HAP crystallites were found in zones of high Mg concentration. These zones also exhibited a contraction of the HAP lattice and lower crystalline order.
The formation and growth of interior fatigue cracks have been studied thoroughly during the last decades, due to the high practical importance of accurate predictions about the fatigue lives of ...structures and machine components. This is of special interest in the VHCF regime, for which the ultrasonic fatigue method is especially useful. Fracture mechanical values and lifetimes can be measured at constant and variable amplitudes. Endurance limits and ΔK thresholds can also be determined very quickly (e.g. 1010 cycles within a few days). Three different materials were studied. For chromium steel, internal crack formation and a fracture mechanical analysis of its growth were carried out. For a specially manufactured Mg alloy, microstructural features leading to interior cracks were analysed. Finally, for polycrystalline high‐purity copper, the conditions for interior‐crack formation and growth were determined. The utilization of FE‐SEM microscopy assisted in interpreting the prevailing mechanisms responsible for endurance limits, lifetimes and fatigue‐crack growth thresholds.
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We report on the long-term effects of degrading magnesium implants on bone tissue in a growing rat skeleton using continuous in vivo micro-Computed Tomography, histological staining ...and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Two different magnesium alloys—one rapidly degrading (ZX50) and one slowly degrading (WZ21)—were used to evaluate the bone response and distribution of released Mg and Y ions in the femur of male Sprague-Dawley rats. Regardless of whether the alloy degrades rapidly or slowly, we found that bone recovers restitutio ad integrum after complete degradation of the magnesium implant. The degradation of the Mg alloys generates a significant increase in Mg concentration in the cortical bone near the remaining implant parts, but the Mg accumulation disappears after the implant degrades completely. The degradation of the Y-containing alloy WZ21 leads to Y enrichment in adjacent bone tissues and in newly formed bone inside the medullary space. Locally high Y concentrations suggest migration not only of Y ions but also of Y-containing intermetallic particles. However, after the full degradation of the implant the Y-enrichment disappears almost completely. Hydrogen gas formation and ion release during implant degradation did not harm bone regeneration in our samples.
Magnesium is generally considered to be one of the most attractive base materials for biodegradable implants, and many magnesium alloys have been optimized to adjust implant degradation. Delayed degradation, however, generates prolonged presence in the organism with the risk of foreign body reactions. While most studies so far have only ranged from several weeks up to 12months, the present study provides data for complete implant degradation and bone regeneration until 24months, for two magnesium alloys (ZX50, WZ21) with different degradation characteristics. μCT monitoring, histological staining and LA-ICP-MS illustrate the distribution of the elements in the neighboring bony tissues during implant degradation, and reveal in particular high concentrations of the rare-earth element Yttrium.
High cycle fatigue properties of high-pressure die-cast magnesium alloys AZ91 hp, AM60 hp, AE42 hp, AS21 hp and of similarly produced cast aluminium alloy AlSi9Cu3 have been investigated. Ultrasonic ...fatigue tests up to 10
9 cycles show mean fatigue limits of approx. 38–50 MPa (magnesium alloys) and 75 MPa (AlSi9Cu3) in the tested casting condition. Fatigue cracks initiated at porosity in 98.5% of the samples. Considering porosity as initial cracks, specimens fail, if critical stress intensity amplitude,
K
cr is exceeded.
K
cr of the magnesium alloys range from 0.85±0.05 to 1.05±0.05 MPa√m, and 1.85±0.10 MPa√m was found for AlSi9Cu3. Below
K
cr, fatigue cracks may initiate at porosity, however, do not propagate until failure. Using
K
cr, the statistical distribution of defects is linked to the fracture probability at different stress amplitudes.
Aim of this study was to evaluate the response of bone to novel biodegradable polymeric composite implants in the femora of growing rats. Longitudinal observation of bone reaction at the implant site ...(BV/TV) as well as resorption of the implanted pins were monitored using in vivo micro-focus computed tomography (µCT). After 12, 24 and 36 weeks femora containing the implants were explanted, scanned with high resolution ex vivo µCT, and the surface roughness of the implants was measured to conclude on the ingrowth capability for bone tissue. Scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX) were used to observe changes on the surface of Polyhydroxybutyrate (PHB) during degradation and cell ingrowth. Four different composites with zirconium dioxide (ZrO2) and Herafill® were compared. After 36 weeks in vivo, none of the implants did show significant degradation. The PHB composite with ZrO2 and a high percentage (30%) of Herafill® as well as the Mg-alloy WZ21 showed the highest values of bone accumulation (increased BV/TV) around the implant. The lowest value was measured in PHB with 3% ZrO2 containing no Herafill®. Roughness measurements as well as EDX and SEM imaging could not reveal any changes on the PHB composites׳ surfaces. Biomechanical parameters, such as the adhesion strength between bone and implant were determined by measuring the shear strength as well as push-out energy of the bone-implant interface. The results showed that improvement of these mechanical properties of the studied PHBs P3Z, P3Z10H and P3Z30H is necessary in order to obtain appropriate load-bearing material. The moduli of elasticity, tensile strength and strain properties of the PHB composites are close to that of bone and thus promising. Compared to clinically used PLGA, PGA and PLA materials, their additional benefit is an unchanged local pH value during degradation, which makes them well tolerated by cells and immune system. They might be used successfully for personalized 3D printed implants or as coatings of rapidly dissolving implants.
•Tested PHBs for biodegradable bone implants do not degrade within 36 weeks in vivo.•No or almost no accumulation of bone tissue and bone–cell ingrowth could be detected.•Low adhesive strength of bone-implant interface and no change over time was found.•Increasing the implant-surface roughness is suggested to stimulate bone–cell adhesion.•Alternative applications of specified PHB composites are proposed.
•Structural features on all scales give insight into different ways of fatigue response.•Ultrasonic-fatigue is most appropriate for studying structural changes in the VHCF regime.•Polycrystalline ...copper is used to study basic mechanisms of fatigue behavior.•Different SEM techniques are applied and correlated with S-N results.•Material properties play a more important role than loading frequency for its VHCF response.
Predicting life times of structures, machines and their parts as well as very small electronic devices for medical purposes is especially difficult for very low stress amplitudes and very high numbers of cycles. The development of the ultrasonic-fatigue method resulted in a reliable testing technique for VHCF loading. Some of its properties are described in this paper. To offer an explanation for the somewhat controversial ultrasonic-fatigue properties reported in the literature, S-N tests were performed at ∼19kHz and 20Hz on polycrystalline copper – a nominally homogenous and ductile material. Moreover, plastic-strain measurements were performed. In addition, micro-structural features and their changes in the high and very-high cycle regimes are reported.
Position-resolved small-angle X-ray scattering was used to investigate the nanostructure of the wood cell wall in two softwood species (Norwegian spruce and Scots pine) and two hardwood species ...(pedunculate oak and copper beech). The tilt angle of the cellulose fibrils in the wood cell wall versus the longitudinal cell axis (microfibril angle) was systematically studied over a wide range of annual rings in each tree. The measured angles were correlated with the distance from the pith and the results were compared. The microfibril angle was found to decrease from pith to bark in all four trees, but was generally higher in the softwood than in the hardwood. In Norwegian spruce, the microfibril angles were higher in late wood than in early wood; in Scots pine the opposite was observed. In pedunculate oak and copper beech, low angles were found in the major part of the stem, except for the very first annual rings in pedunculate oak. The results are interpreted in terms of mechanical optimization. An attempt was made to give a quantitative estimation for the mechanical constraints imposed on a tree of given dimensions and to establish a model that could explain the general decrease of microfibril angles from pith to bark.
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Understanding the implant–bone interaction is of prime interest for the development of novel biodegrading implants. Magnesium is a very promising material in the class of biodegrading ...metallic implants, owing to its mechanical properties and excellent immunologic response during healing. However, the influence of degrading Mg implants on the bone nanostructure is still an open question of crucial importance for the design of novel Mg implant alloys. This study investigates the changes in the nanostructure of bone following the application of a degrading WZ21 Mg implant (2wt% Y, 1wt% Zn, 0.25wt% Ca and 0.15wt% Mn) in a murine model system over the course of 15months by small angle X-ray scattering. Our investigations showed a direct response of the bone nanostructure after as little as 1month with a realignment of nano-sized bone mineral platelets along the bone–implant interface. The growth of new bone tissue after implant resorption is characterized by zones of lower mineral platelet thickness and slightly decreased order in the stacking of the platelets. The preferential orientation of the mineral platelets strongly deviates from the normal orientation along the shaft and still roughly follows the implant direction after 15months. We explain our findings by considering geometrical, mechanical and chemical factors during the process of implant resorption.
The advancement of surgical techniques and the increased life expectancy have caused a growing demand for improved bone implants. Ideally, they should be bio-resorbable, support bone as long as necessary and then be replaced by healthy bone tissue. Magnesium is a promising candidate for this purpose. Various studies have demonstrated its excellent mechanical performance, degradation behaviour and immunologic properties. The structural response of bone, however, is not well known. On the nanometer scale, the arrangement of collagen fibers and calcium mineral platelets is an important indicator of structural integrity. The present study provides insight into nanostructural changes in rat bone at different times after implant placement and different implant degradation states. The results are useful for further improved magnesium alloys.
The fatigue strength of an oil-tempered Si–Cr steel for valve springs (JIS G3561, SWOSC-V) was investigated. Smooth specimens without residual surface stresses were fatigued with two kinds of ...ultrasonic fatigue testing machines to clarify the fatigue properties up to very high numbers of cycles (giga cycle regime) under axial and torsional loading. The maximum inclusion size in the critical volume of a specimen predicted by the extremal statistics is 7.9
μm. Although scatter is somewhat large, the
S–
N data could be approximated by linear lines in double logarithmic plots for both loading conditions up to the giga cycle regime. The ratio of fatigue strength under torsional and axial loading at the same number of stress cycles is about 0.68 and is almost constant even in the giga cycle regime. Cracks were initiated from the specimen surface under tension–compression as well as torsion loading. No specimen showed crack initiation from the interior. Inclusions and granular facet areas could not be observed. Under torsional loading, cracks initiated either perpendicular or parallel to the longitudinal direction of the specimens. After shear crack propagation to a crack length of about 30
μm, crack branching and mode I propagation took place. The size of characteristic defects calculated on the basis of the propagation threshold of long cracks is much larger than the inclusion size calculated by the extremal statistics.
ABSTRACT
Spray‐formed hypereutectic aluminium silicon alloy DISPAL® S232–T6x is cycled with variable amplitude at ultrasonic frequency up to the very high cycle fatigue (VHCF) regime under fully ...reversed tension–compression loading. The Powder Metallurgy alloy is tested using a Gaussian cumulative frequency distribution of load cycles, and lifetimes are compared with constant amplitude data. Miner calculation delivers mean damage sums between 0.5 and 0.9 for mean lifetimes between 8 × 107 and 1.6 × 1010 cycles, respectively. Cracks are initiated at voids, at inclusions or at distributed inhomogeneities (porous areas or oxides) at the surface or in the interior. In situ analysis of vibration properties indicates that cracks are formed and start growing from the beginning of fatigue cycling, even if failure occurs in the very high cycle fatigue regime. Crack initiation stage is negligible. Lifetime prediction calculation is performed using an adapted Paris‐law and considering lifetime as cycles necessary to propagate an initial crack to failure. Measured and predicted mean lifetimes differ by factor 0.4–1.0. Large crack‐initiating defects strongly reduce the fatigue lifetimes, which is successfully covered in the crack propagation model.
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