Human Body-Fluid-Assisted Fracture of Zinc Alloys as Biodegradable Temporary Implants: Challenges, Research Needs and Way Forward

Alloys of magnesium, zinc or iron that do not contain toxic elements are attractive as construction material for biodegradable implants, i.e., the type of implants that harmlessly dissolve away within the human body after they have completed their intended task. The synergistic influence of mechanical stress and corrosive human body fluid can cause sudden and catastrophic fracture of bioimplants due to phenomena such as stress corrosion cracking (SCC) and corrosion fatigue (CF). To date, SCC and CF of implants based on Zn have scarcely been investigated. This article is an overview of the challenges, research needs and way forward in understanding human body-fluid-assisted fractures (i.e., SCC and CF) of Zn alloys in human body fluid.

Long-term in vivo degradation of Mg-Zn-Ca elastic stable intramedullary nails and their influence on the physis of juvenile sheep

The use of bioresorbable magnesium (Mg)-based elastic stable intramedullary nails (ESIN) is highly promising for the treatment of pediatric long-bone fractures. Being fully resorbable, a removal surgery is not required, preventing repeated physical and psychological stress for the child. Further, the osteoconductive properties of the material support fracture healing. Nowadays, ESIN are exclusively implanted in a non-transphyseal manner to prevent growth discrepancies, although transphyseal implantation would often be required to guarantee optimized fracture stabilization. Here, we investigated the influence of trans-epiphyseally implanted Mg-Zinc (Zn)-Calcium (Ca) ESIN on the proximal tibial physis of juvenile sheep over a period of three years, until skeletal maturity was reached. We used the two alloying systems ZX10 (Mg-1Zn-0.3Ca, in wt%) and ZX00 (Mg-0.3Zn-0.4Ca, in wt%) for this study. To elaborate potential growth disturbances such as leg-length differences and axis deviations we used a combination of in vivo clinical computed tomography (cCT) and ex vivo micro CT (μCT), and also performed histology studies on the extracted bones to obtain information on the related tissue. Because there is a lack of long-term data regarding the degradation performance of magnesium-based implants, we used cCT and μCT data to evaluate the implant volume, gas volume and degradation rate of both alloying systems over a period of 148 weeks. We show that transepiphyseal implantation of Mg-Zn-Ca ESIN has no negative influence on the longitudinal bone growth in juvenile sheep, and that there is no axis deviation observed in all cases. We also illustrate that 95 % of the ESIN degraded over nearly three years, converging the time point of full resorption. We thus conclude that both, ZX10 and ZX00, constitute promising implant materials for the ESIN technique.

The combined effect of zinc and calcium on the biodegradation of ultrahigh-purity magnesium implants

Magnesium (Mg)-based implants are promising candidates for orthopedic interventions, because of their biocompatibility, good mechanical features, and ability to degrade completely in the body, eliminating the need for an additional removal surgery. In the present study, we synthesized and investigated two Mg-based materials, ultrahigh-purity ZX00 (Mg-Zn-Ca; <0.5 wt% Zn and <0.5 wt% Ca, in wt%; Fe-content <1 ppm) and ultrahigh-purity Mg (XHP-Mg, >99.999 wt% Mg; Fe-content <1 ppm), in vitro and in vivo in juvenile healthy rats to clarify the effect of the alloying elements Zn and Ca on mechanical properties, microstructure, cytocompatibility and degradation rate. Potential differences in bone formation and bone in-growth were also assessed and compared with state-of-the-art non-degradable titanium (Ti)-implanted, sham-operated, and control (non-intervention) groups, using micro-computed tomography, histology and scanning electron microscopy. At 6 and 24 weeks after implantation, serum alkaline phosphatase (ALP), calcium (Ca), and Mg level were measured and bone marrow stromal cells (BMSCs) were isolated for real-time PCR analysis. Results show that ZX00 implants have smaller grain size and superior mechanical properties than XHP-Mg, and that both reveal good biocompatibility in cytocompatibilty tests. ZX00 homogenously degraded with an increased gas accumulation 12 and 24 weeks after implantation, whereas XHP-Mg exhibited higher gas accumulation already at 2 weeks. Serum ALP, Ca, and Mg levels were comparable among all groups and both Mg-based implants led to similar relative expression levels of Alp, Runx2, and Bmp-2 genes at weeks 6 and 24. Histologically, Mg-based implants are superior for new bone tissue formation and bone in-growth compared to Ti implants. Furthermore, by tracking the sequence of multicolor fluorochrome labels, we observed higher mineral apposition rate at week 2 in both Mg-based implants compared to the control groups. Our findings suggest that (i) ZX00 and XHP-Mg support bone formation and remodeling, (ii) both Mg-based implants are superior to Ti implants in terms of new bone tissue formation and osseointegration, and (iii) ZX00 is more favorable due to its lower degradation rate and moderate gas accumulation.

Implant degradation of low-alloyed Mg-Zn-Ca in osteoporotic, old and juvenile rats

Implant removal is unnecessary for biodegradable magnesium (Mg)-based implants and, therefore, the related risk for implant-induced fractures is limited. Aging, on the other hand, is associated with low bone-turnover and decreased bone mass and density, and thus increased fracture risk. Osteoporosis is accompanied by Mg deficiency, therefore, we hypothesized that Mg-based implants may support bone formation by Mg²⁺ ion release in an ovariectomy-induced osteoporotic rat model. Hence, we investigated osseointegration and implant degradation of a low-alloyed, degrading Mg-Zn-Ca implant (ZX00) in ovariectomy-induced osteoporotic (Osteo), old healthy (OH), and juvenile healthy (JH) groups of female Sprague Dawley rats via in vivo micro-computed tomography (µCT). For the Osteo rats, we demonstrate diminished trabecular bone already after 8 weeks upon ovariectomy and significantly enhanced implant volume loss, with correspondingly pronounced gas formation, compared to the OH and JH groups. Sclerotic rim development was observed in about half of the osteoporotic rats, suggesting a prevention from foreign-body and osteonecrosis development. Synchrotron radiation-based µCT confirmed lower bone volume fractions in the Osteo group compared to the OH and JH groups. Qualitative histological analysis additionally visualized the enhanced implant degradation in the Osteo group. To date, ZX00 provides an interesting implant material for young and older healthy patients, but it may not be of advantage in pharmacologically untreated osteoporotic conditions. STATEMENT OF SIGNIFICANCE: Magnesium-based implants are promising candidates for treatment of osteoporotic fractures because of their biodegradable, biomechanical, anti-bacterial and bone regenerative properties. Here we investigate magnesium‒zinc‒calcium implant materials in a rat model with ovariectomy-induced osteoporosis (Osteo group) and compare the related osseointegration and implant degradation with the results obtained for old healthy (OH) and juvenile healthy (JH) rats. The work applied an appropriate disease model for osteoporosis and focused in particular on long-term implant degradation for different bone conditions. Enhanced implant degradation and sclerotic rim formation was observed in osteoporotic rats, which illustrates that the setting of different bone models generates significantly modified clinical outcome. It further illustrated that these differences must be taken into account in future biodegradable implant development.

Atomic structure evolution related to the Invar effect in Fe-based bulk metallic glasses

The Invar effect is universally observed in Fe-based bulk metallic glasses. However, there is limited understanding on how this effect manifests at the atomic scale. Here, we use in-situ synchrotron-based high-energy X-ray diffraction to study the structural transformations of (Fe_71.2B₂₄Y_4.8)₉₆Nb₄ and (Fe_73.2B₂₂Y_4.8)₉₅Mo₅ bulk metallic glasses around the Curie temperature to understand the Invar effect they exhibit. The first two diffraction peaks shift in accordance with the macroscopically measured thermal expansion, which reveals the Invar effect. Additionally, the nearest-neighbor Fe–Fe pair distance correlates well with the macroscopic thermal expansion. In-situ X-ray diffraction is thus able to elucidate the Invar effect in Fe-based metallic glasses at the atomic scale. Here, we find that the Invar effect is not just a macroscopic effect but has a clear atomistic equivalent in the average Fe–Fe pair distance and also shows itself in higher-order atomic shells composed of multiple atom species.

There is limited understanding of the Invar effect at atomic scale. Here the authors show that the Invar effect is not only a macroscopic effect, but also has a clear atomistic equivalent in the average distance of Fe–Fe pair as well as higher-order atomic shells composed of multiple atom species.

3D nanoscale analysis of bone healing around degrading Mg implants evaluated by X-ray scattering tensor tomography

The nanostructural adaptation of bone is crucial for its biocompatibility with orthopedic implants. The bone nanostructure also determines its mechanical properties and performance. However, the bone's temporal and spatial nanoadaptation around degrading implants remains largely unknown. Here, we present insights into this important bone adaptation by applying scanning electron microscopy, elemental analysis, and small-angle X-ray scattering tensor tomography (SASTT). We extend the novel SASTT reconstruction method and provide a 3D scattering reciprocal space map per voxel of the sample's volume. From this reconstruction, parameters such as the thickness of the bone mineral particles are quantified, which provide additional information on nanostructural adaptation of bone during healing. We selected a rat femoral bone and a degrading ZX10 magnesium implant as model system, and investigated it over the course of 18 months, using a sham as control. We observe that the bone's nanostructural adaptation starts with an initially fast interfacial bone growth close to the implant, which spreads by a re-orientation of the nanostructure in the bone volume around the implant, and is consolidated in the later degradation stages. These observations reveal the complex bulk bone-implant interactions and enable future research on the related biomechanical bone responses. STATEMENT OF SIGNIFICANCE: Traumatic bone injuries are among the most frequent causes of surgical treatment, and often require the placement of an implant. The ideal implant supports and induces bone formation, while being mechanically and chemically adapted to the bone structure, ensuring a gradual load transfer. While magnesium implants fulfill these requirements, the nanostructural changes during bone healing and implant degradation remain not completely elucidated. Here, we unveil these processes in rat femoral bones with ZX10 magnesium implants and show different stages of bone healing in such a model system.

X-ray Diffraction Computed Nanotomography Applied to Solve the Structure of Hierarchically Phase-Separated Metallic Glass

The structure of matter at the nanoscale, in particular that of amorphous metallic alloys, is of vital importance for functionalization. With the availability of synchrotron radiation, it is now possible to visualize the internal features of metallic samples without physically destroying them. Methods based on computed tomography have recently been employed to explore the local features. Tomographic reconstruction, while it is relatively uncomplicated for crystalline materials, may generate undesired artifacts when applied to featureless amorphous or nanostructured metallic alloys. In this study we show that X-ray diffraction computed nanotomography can provide accurate details of the internal structure of a metallic glass. We demonstrate the power of the method by applying it to a hierarchically phase-separated amorphous sample with a small volume fraction of crystalline inclusions, focusing the X-ray beam to 500 nm and ensuring a sub-micrometer 2D resolution via the number of scans.

Structural relaxation in layered, non-stoichiometric Fe7S8

In this study, we investigate the kinetics of the enantiotropic solid-solid β-transition in Fe7S8 pyrrhotite, which presents a prominent example of a metal-nonmetal compound with layered crystal structure. The low-temperature (4C) and high-temperature (1C) modifications differ in their crystallographic unit-cell dimension, vacancy distribution, and magnetic ordering in the crystal lattice. Fast differential scanning calorimetry (FDSC) reveals that cooling of the paramagnetic 1C phase below the transformation temperature Tβ = 597 K, which is also the Curie temperature, generates a metastable phase that transforms into the ferrimagnetic 4C phase with high vacancy order upon further annealing below Tβ. Upon fast cooling, the low-temperature modification shows an energetically excited phase with higher entropy that relaxes towards the equilibrated pyrrhotite polymorph. The kinetics of the superheating and the structural relaxation as obtained from FDSC experiments provide deeper insight into the stability of Fe7S8 polymorphs. This may pave a new path to decipher in detail the kinetics of solid-solid phase transformations and the long-term lifespan of defects in Earth and synthetic materials.

Unconventional magnetization textures and domain-wall pinning in Sm-Co magnets

Some of the best-performing high-temperature magnets are Sm–Co-based alloys with a microstructure that comprises an \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {Sm}_2\hbox {Co}_{17}$$\end{document}Sm2Co17 matrix and magnetically hard \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {SmCo}_5$$\end{document}SmCo5 cell walls. This generates a dense domain-wall-pinning network that endows the material with remarkable magnetic hardness. A precise understanding of the coupling between magnetism and microstructure is essential for enhancing the performance of Sm–Co magnets, but experiments and theory have not yet converged to a unified model. Here, transmission electron microscopy, atom probe tomography, and nanometer-resolution off-axis electron holography have been combined with micromagnetic simulations to reveal that the magnetization state in Sm–Co magnets results from curling instabilities and domain-wall pinning effects at the intersections of phases with different magnetic hardness. Additionally, this study has found that topologically non-trivial magnetic domains separated by a complex network of domain walls play a key role in the magnetic state by acting as nucleation sites for magnetization reversal. These findings reveal previously hidden aspects of magnetism in Sm–Co magnets and, by identifying weak points in the microstructure, provide guidelines for improving these high-performance magnetic materials.

Structured nanoscale metallic glass fibres with extreme aspect ratios

Micro- and nanoscale metallic glasses offer exciting opportunities for both fundamental research and applications in healthcare, micro-engineering, optics and electronics. The scientific and technological challenges associated with the fabrication and utilization of nanoscale metallic glasses, however, remain unresolved. Here, we present a simple and scalable approach for the fabrication of metallic glass fibres with nanoscale architectures based on their thermal co-drawing within a polymer matrix with matched rheological properties. Our method yields well-ordered and uniform metallic glasses with controllable feature sizes down to a few tens of nanometres, and aspect ratios greater than 10¹⁰. We combine fluid dynamics and advanced in situ transmission electron microscopy analysis to elucidate the interplay between fluid instability and crystallization kinetics that determines the achievable feature sizes. Our approach yields complex fibre architectures that, combined with other functional materials, enable new advanced all-in-fibre devices. We demonstrate in particular an implantable metallic glass-based fibre probe tested in vivo for a stable brain-machine interface that paves the way towards innovative high-performance and multifunctional neuro-probes.

A lean magnesium-zinc-calcium alloy ZX00 used for bone fracture stabilization in a large growing-animal model

Over the last decade, demand has increased for developing new, alternative materials in pediatric trauma care to overcome the disadvantages associated with conventional implant materials. Magnesium (Mg)-based alloys seem to adequately fulfill the vision of a homogeneously resorbable, biocompatible, load-bearing and functionally supportive implant. The aim of the present study is to introduce the high-strength, lean alloy Mg‒0.45Zn‒0.45Ca, in wt% (ZX00), and for the first time investigate the clinical applicability of screw osteosynthesis using this alloy that contains no rare-earth elements. The alloy was applied in a growing sheep model with osteotomized bone (simulating a fracture) and compared to a non-osteotomy control group regarding degradation behavior and fracture healing. The alloy exhibits an ultimate tensile strength of 285.7 ± 3.1 MPa, an elongation at fracture of 18.2 ± 2.1%, and a reduced in vitro degradation rate compared to alloys containing higher amounts of Zn. In vivo, no significant difference between the osteotomized bone and the control group was found regarding the change in screw volume over implantation time. Therefore, it can be concluded that the fracture healing process, including its effects on the surrounding area, has no significant influence on degradation behavior. There was also no negative influence from hydrogen-gas formation on fracture healing. Despite the proximal and distal screws showing chronologically different gas release, the osteotomy showed complete consolidation. STATEMENT OF SIGNIFICANCE: Conventional implants involve several disadvantages in pediatric trauma care. Magnesium-based alloys seem to overcome these issues as discussed in the recent literature. This study evaluates the clinical applicability of high-strength lean Mg‒0.45Zn‒0.45Ca (ZX00) screws in a growing-sheep model. Two groups, one including a simulated fracture and one group without fracture, underwent implantation of the alloy and were compared to each other. No significant difference regarding screw volume was observed between the groups. There was no negative influence of hydrogen-gas formation on fracture healing and a complete fracture consolidation was found after 12 weeks for all animals investigated.

Multistep Crystallization and Melting Pathways in the Free-Energy Landscape of a Au-Si Eutectic Alloy

Crystals do eventually melt if they are heated to their characteristic melting point. However, this is practically only the case for high-temperature stable crystals, whereas low-temperature metastable crystals generally transform, before melting, into a more stable solid during heating. Here, it is illustrated that low-temperature crystals can, however, be melted via fast differential scanning calorimetry (FDSC), even in metallic systems where nucleation and growth kinetics are rapid. For a Au-Si eutectic alloy, various metastable and stable solid states, i.e., (Au-α), (Au-β), γ, and (Au-Si), which form under well-controlled conditions and melt at high heating rates by preventing the metastable-to-stable solid phase transition, are isolated. It is demonstrated that Au81.4Si18.6 can fully melt at various temperatures, i.e., 294 °C, 312 °C, 352 °C, and 363 °C, with differing melting enthalpies ranging from 6.52 to 9.83 kJ mol-1. The melting and crystallization paths of the metastable solids are determined by constructing an energy-temperature diagram. This approach advances the general understanding of nucleation in metallic and other systems, and is expected to contribute to the detailed understanding of thermophysical phenomena that occur at spatially reduced dimensions and/or short time scales, for example in thin-film deposition, nanomaterials production, or additive manufacturing.

Towards refining microstructures of biodegradable magnesium alloy WE43 by spark plasma sintering

Microstructural refinement of magnesium (Mg) alloys is beneficial for mechanical and corrosion properties, both of which are critical for their successful application as temporary implant materials. One method of achieving a refined microstructure is through rapid solidification via gas-atomization-powder production. In this study we investigated spark plasma sintering (SPS) as a potential processing method for maintaining this refined microstructure while achieving a range of porosities up to full densification. We characterized the microstructural evolution as a function of sintering temperature from 250 to 450 °C for the alloy WE43 using multi-scale correlative microscopy techniques, including light microscopy and scanning and transmission electron microscopy-based methods. The spatial distribution of the two major alloying elements, neodymium (Nd) and yttrium (Y), was determined and the intermetallic phases they form identified using energy dispersive X-ray spectroscopy in conjunction with electron diffraction. The gas-atomized powder microstructure consists of Mg-rich dendrites and a percolating interdendritic Mg-Nd-Y ternary phase with structure Mg₁₄Nd₂Y, surrounded by a high Nd and Y content in solid solution. This microstructure is maintained up to a sintering temperature of 350 °C, while with higher sintering temperatures segregation of Nd and Y dominates. The percolating ternary phase breaks up into faceted globular precipitates with structure Mg₅Nd, which is isomorphous to Mg₁₄Nd₂Y. Y comes out of solution and migrates to previous powder-particle surfaces, possibly forming Y₂O₃. Sample densities ranged from 64 to 100% for sintering temperatures of 250 to 450 °C, respectively, and the grain size remained constant at about 10 µm. SPS is demonstrated to be an attractive alternative method for processing Mg alloys to a wide range of porosities and fine microstructures. The microstructural refinement achieved by SPS holds the potential for slow and homogeneous corrosion. STATEMENT OF SIGNIFICANCE: This study presents the impact spark plasma sintering (SPS) has on the microstructure of WE43, a magnesium alloy used for biodegradable implants. SPS is of great interest in this context as it is scalable, rapid, and has the potential for tuning density while maintaining a refined microstructure. The microstructure and density are explored from the gas-atomized powder to the densified material using electron microscopy and chemical mapping from the macro- to the nano-level. The insights gained reveal an original evolution of rare-earth element distribution with an isomorphous chemistry change, while the microstructure develops from the non-equilibrium state (powder) towards an equilibrium structure upon sintering. This study, including measurements of mechanical performance, sets the premises of SPS for the fabrication of Mg-based implants with tunable characteristics.

Biocorrosion Zoomed In: Evidence for Dealloying of Nanometric Intermetallic Particles in Magnesium Alloys

Biodegradable magnesium alloys generally contain intermetallic phases on the micro- or nanoscale, which can initiate and control local corrosion processes via microgalvanic coupling. However, the experimental difficulties in characterizing active degradation on the nanoscale have so far limited the understanding of how these materials degrade in complex physiological environments. Here a quasi-in situ experiment based on transmission electron microscopy (TEM) is designed, which enables the initial corrosion attack at nanometric particles to be accessed within the first seconds of immersion. Combined with high-resolution ex situ cross-sectional TEM analysis of a well-developed corrosion-product layer, mechanistic insights into Mg-alloys' degradation on the nanoscale are provided over a large range of immersion times. Applying this methodology to lean Mg-Zn-Ca alloys and following in detail the dissolution of their nanometric Zn- and Ca-rich particles the in statu nascendi observation of intermetallic-particle dealloying is documented for magnesium alloys, where electrochemically active Ca and Mg preferentially dissolve and electropositive Zn enriches, inducing the particles' gradual ennoblement. Based on electrochemical theory, here, the concept of cathodic-polarization-induced dealloying, which controls the dynamic microstructural changes, is presented. The general prerequisites for this new dealloying mechanism to occur in multicomponent alloys and its distinction to other dealloying modes are also discussed.

3D Printing of Salt as a Template for Magnesium with Structured Porosity

Porosity is an essential feature in a wide range of applications that combine light weight with high surface area and tunable density. Porous materials can be easily prepared with a vast variety of chemistries using the salt-leaching technique. However, this templating approach has so far been limited to the fabrication of structures with random porosity and relatively simple macroscopic shapes. Here, a technique is reported that combines the ease of salt leaching with the complex shaping possibilities given by additive manufacturing (AM). By tuning the composition of surfactant and solvent, the salt-based paste is rheologically engineered and printed via direct ink writing into grid-like structures displaying structured pores that span from the sub-millimeter to the macroscopic scale. As a proof of concept, dried and sintered NaCl templates are infiltrated with magnesium (Mg), which is typically highly challenging to process by conventional AM techniques due to its highly oxidative nature and high vapor pressure. Mg scaffolds with well-controlled, ordered porosity are obtained after salt removal. The tunable mechanical properties and the potential to be predictably bioresorbed by the human body make these Mg scaffolds attractive for biomedical implants and demonstrate the great potential of this additive technique.

Exceptional Strengthening of Biodegradable Mg-Zn-Ca Alloys through High Pressure Torsion and Subsequent Heat Treatment

In this study, two biodegradable Mg-Zn-Ca alloys with alloy content of less than 1 wt % were strengthened via high pressure torsion (HPT). A subsequent heat treatment at temperatures of around 0.45 T_m led to an additional, sometimes even larger increase in both hardness and tensile strength. A hardness of more than 110 HV and tensile strength of more than 300 MPa were achieved in Mg-0.2Zn-0.5Ca by this procedure. Microstructural analyses were conducted by scanning and transmission electron microscopy (SEM and TEM, respectively) and atom probe tomography (APT) to reveal the origin of this strength increase. They indicated a grain size in the sub-micron range, Ca-rich precipitates, and segregation of the alloying elements at the grain boundaries after HPT-processing. While the grain size and segregation remained mostly unchanged during the heat treatment, the size and density of the precipitates increased slightly. However, estimates with an Orowan-type equation showed that precipitation hardening cannot account for the strength increase observed. Instead, the high concentration of vacancies after HPT-processing is thought to lead to the formation of vacancy agglomerates and dislocation loops in the basal plane, where they represent particularly strong obstacles to dislocation movement, thus, accounting for the considerable strength increase observed. This idea is substantiated by theoretical considerations and quenching experiments, which also show an increase in hardness when the same heat treatment is applied.

Cation diffusion patterns across the magneto-structural transition in Fe7S8

Migration of atoms in solids during diffusion-dependent reactions is relatively fast and generally not directly recordable in experiments. Here we present an experimental framework that includes fast differential scanning calorimetry to resolve cation-migration paths in crystalline solids using the reversible magneto-structural transition of 4C to 1C pyrrhotite as a testbed. The transition between these two polymorphic Fe₇S₈ phases at about 600 K is a diffusive process of vacancies, respectively of Fe in octahedral interstitial sites within a hexagonal close-packed lattice of sulfur, and it coincides with the Curie temperature of 4C pyrrhotite. The Fe cations migrate along three kinds of diffusion paths, and their enthalpy contributions to the total reaction enthalpy are taken to define the diffusion patterns in the endothermic reaction and the exothermic back-reaction, respectively. Our experimental findings provide insight into the potential of diffusion patterns to disentangle ordering mechanisms in solids.

Existence of multiple critical cooling rates which generate different types of monolithic metallic glass

Via fast differential scanning calorimetry using an Au-based glass as an example, we show that metallic glasses should be classified into two types of amorphous/monolithic glass. The first type, termed self-doped glass (SDG), forms quenched-in nuclei or nucleation precursors upon cooling, whereas in the so-called chemically homogeneous glass (CHG) no quenched-in structures are found. For the Au-based glass investigated, the critical cooling and heating rates for the SDG are 500 K s⁻¹ and 20,000 K s⁻¹, respectively; for the CHG they are 4000 K s⁻¹ and 6000 K s⁻¹. The similarity in the critical rates for CHG, so far not reported in literature, and CHG’s tendency towards stochastic nucleation underline the novelty of this glass state. Identifying different types of metallic glass, as is possible by advanced chip calorimetry, and comparing them with molecular and polymeric systems may help to elaborate a more generalized glass theory and improve metallic glass processing.

Quantifying the complexity of glass formation is difficult because it usually requires cooling at enormous speeds. Here, the authors use fast differential scanning calorimetry to classify metallic glasses into two types, one with quenched-in nuclei and one without.

The relation between local structural distortion and the low-temperature magnetic anomaly in Fe7S8

Structural defects on an atomic level can crucially impact the magnetic properties of a material. We study this phenomenon by means of magnetometry and powder neutron diffraction on a stoichiometric, monoclinic pyrrhotite (Fe₇S₈), which is a classic omission structure with a magnetic anomaly at about 30 K. The initial structural distortion of the pyrrhotite at 300 K caused by the vacancy arrangement decreases upon cooling, and simultaneous to the magnetic anomaly the anisotropic contraction of the unit cell homogenizes the covalency of the Fe-Fe bonds with lengths less than 3.0 Å and the Fe-S-Fe bond angles. These changes on the atomic level affect the spin-orbit coupling and the super-exchange interactions in Fe₇S₈, and trigger the low-temperature magnetic anomaly within a crystallographically stable system.

Monopole-Induced Emergent Electric Fields in Ferromagnetic Nanowires

We predict that complete magnetization reversal in simple metallic ferromagnetic nanoparticles is directly linked to the pair creation of topological point defects in the form of hedgehog-antihedgehog pairs. These dynamical point defects move at exceptionally high speeds in excess of 1500  m/s, faster than any other known magnetic object. Their rapid motion generates unprecedented solenoidal emergent fields on the order of megavolts per meter, in analogy to the magnetic field of a moving electric charge, providing a striking example that a moving hedgehog constitutes an emergent magnetic monopole.

Monotropic polymorphism in a glass-forming metallic alloy

This study investigates the crystallization and phase transition behavior of the amorphous metallic alloy Au₇₀Cu_5.5Ag_7.5Si₁₇. This alloy has been recently shown to exhibit a transition of a metastable to a more stable crystalline state, occurring via metastable melting under strong non-equilibrium conditions. Such behavior had so far not been observed in other metallic alloys. In this investigation fast differential scanning calorimetry (FDSC) is used to explore crystallization and the solid-liquid-solid transition upon linear heating and during isothermal annealing, as a function of the conditions under which the metastable phase is formed. It is shown that the occurrence of the solid-liquid-solid transformation in FDSC depends on the initial conditions; this is explained by a history-dependent nucleation of the stable crystalline phase. The microstructure was investigated by scanning and transmission electron microscopy and x-ray diffraction. Chemical mapping was performed by energy dispersive x-ray spectrometry. The relationship between the microstructure and the phase transitions observed in FSDC is discussed with respect to the possible kinetic paths of the solid-liquid-solid transition, which is a typical phenomenon in monotropic polymorphism.

The influence of biodegradable magnesium implants on the growth plate

Mg-based biodegradable materials are considered promising candidates in the paediatric field due to their favourable mechanical and biological properties and their biodegrading potential that makes a second surgery for implant removal unnecessary. In many cases the surgical fixation technique requires a crossing of the growth plate by the implant in order to achieve an adequate fragment replacement or fracture stabilisation. This study investigates the kinetics of slowly and rapidly degrading Mg alloys in a transphyseal rat model, and also reports on their dynamics in the context of the physis and consecutive bone growth. Twenty-six male Sprague-Dawley rats received either a rapidly degrading (ZX50; n = 13) or a slowly degrading (WZ21; n = 13) Mg alloy, implanted transphyseal into the distal femur. The contralateral leg was drilled in the same manner and served as a direct sham specimen. Degradation behaviour, gas formation, and leg length were measured by continuous in vivo micro CT for up to 52 weeks, and additional high-resolution µCT (HRS) scans and histomorphological analyses of the growth plate were performed. The growth plate was locally destroyed and bone growth was significantly diminished by the fast degradation of ZX50 implants and the accompanying release of large amounts of hydrogen gas. In contrast, WZ21 implants showed homogenous and moderate degradation performance, and the effect on bone growth did not differ significantly from a single drill-hole defect.

STATEMENT OF SIGNIFICANCE

This study is the first that reports on the effects of degrading magnesium implants on the growth plate in a living animal model. The results show that high evolution of hydrogen gas due to rapid Mg degradation can damage the growth plate substantially. Slow degradation, however, such as seen for WZ21 alloys, does not affect the growth plate more than drilling alone, thus meeting one important prerequisite for deployment in paediatric osteosynthesis.

The influence of two common sterilization techniques on the corrosion of Mg and its alloys for biomedical applications

This paper studied the influence of two common sterilization techniques, ethylene oxide (EO) and gamma irradiation (GI), on the corrosion rate of four Mg-based materials in CO₂ -bicarbonate buffered Hanks' solution. The four materials were: high-purity (HP)-Mg, ZE41, ultra-high purity (XHP)-Mg, and XHP-ZX00. The corrosion rate was measured through mass loss (P_m ) and hydrogen evolution (P_H ). Two-way analysis of variance (ANOVA) was conducted to assess the effect of the sterilization techniques on the corrosion rates across the four materials. The ANOVA analyzed the variables of (1) material, (2) sterilization condition (EO, GI, and an unsterilized control group), and (3) the interaction between these two independent variables. Neither sterilization technique (EO and GI) significantly influenced the corrosion rate as measured by P_m (p < 0.84) nor P_H (p < 0.08). This result was consistent across the four materials tested, as there was no interaction between the test variables of material and sterilization condition for P_m (p < 0.49) or P_H (p < 0.27). As neither EO nor GI influenced the corrosion rates, either of these techniques warrants consideration for use on Mg-based medical implants and devices. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1907-1917, 2018.

Electron-band theory inspired design of magnesium-precious metal bulk metallic glasses with high thermal stability and extended ductility

Magnesium-based bulk metallic glasses (BMGs) exhibit high specific strengths and excellent glass-forming ability compared to other metallic systems, making them suitable candidates for next-generation materials. However, current Mg-based BMGs tend to exhibit low thermal stability and are prone to structural relaxation and brittle failure. This study presents a range of new magnesium-precious metal-based BMGs from the ternary Mg-Ag-Ca, Mg-Ag-Yb, Mg-Pd-Ca and Mg-Pd-Yb alloy systems with Mg content greater than 67 at.%. These alloys were designed for high ductility by utilising atomic bond-band theory and a topological efficient atomic packing model. BMGs from the Mg-Pd-Ca alloy system exhibit high glass-forming ability with critical casting sizes of up to 3 mm in diameter, the highest glass transition temperatures (>200 °C) of any reported Mg-based BMG to date, and sustained compressive ductility. Alloys from the Mg-Pd-Yb family exhibit critical casting sizes of up to 4 mm in diameter, and the highest compressive plastic (1.59%) and total (3.78%) strain to failure of any so far reported Mg-based glass. The methods and theoretical approaches presented here demonstrate a significant step forward in the ongoing development of this extraordinary class of materials.

Solid-solid phase transitions via melting in metals

Observing solid–solid phase transitions in-situ with sufficient temporal and spatial resolution is a great challenge, and is often only possible via computer simulations or in model systems. Recently, a study of polymeric colloidal particles, where the particles mimic atoms, revealed an intermediate liquid state in the transition from one solid to another. While not yet observed there, this finding suggests that such phenomena may also occur in metals and alloys. Here we present experimental evidence for a solid–solid transition via the formation of a metastable liquid in a ‘real' atomic system. We observe this transition in a bulk glass-forming metallic system in-situ using fast differential scanning calorimetry. We investigate the corresponding transformation kinetics and discuss the underlying thermodynamics. The mechanism is likely to be a feature of many metallic glasses and metals in general, and may provide further insight into phase transition theory.

Solid–solid phase transition via an intermediate liquid state has been identified in colloidal systems, but the universality of the phenomenon at atomic scales has not yet been proved. Pogatscher et al. observe a similar transition in a metallic glass system using fast differential scanning calorimetry.

Crystal-Like Rearrangements of Icosahedra in Simulated Copper-Zirconium Metallic Glasses and their Effect on Mechanical Properties

Indications of the Cu2Zr Laves phase are observed in MD simulations of amorphous Cu64Zr36 upon isothermal holding just above the glass transition temperature. The structural evolution towards Cu2Zr is accompanied by an increase in the fraction of Cu-centered icosahedra, which demonstrates that a large icosahedral fraction does not just indicate structural relaxation. The crystal-like regions generate an increase in strength and Young's modulus, and a stronger localized shear band. A universal relation between the fraction of full icosahedra and their interconnectivity is found, and both can be modified simultaneously via changes of temperature or strain.

Enhancement of the intrinsic fluorescence of adenine using aluminum nanoparticle arrays

This study demonstrates the metal-enhanced fluorescence of adenine using aluminum nanoparticle arrays in the deep UV range. It achieves the reproducible intensity enhancement of intrinsic fluorescence up to 80 on well-defined aluminum nanoparticle arrays at 257 nm excitation. In addition to a high signal enhancement, a strong modification of the fluorescence emission spectrum of adenine is observed. This study illustrates that the label-free detection of DNA bases and proteins that have low intrinsic fluorescence and absorption bands in the deep UV range can be facilitated using aluminum nanostructures.

Influence of trace impurities on the in vitro and in vivo degradation of biodegradable Mg-5Zn-0.3Ca alloys

The hydrogen evolution method and animal experiments were deployed to investigate the effect of trace impurity elements on the degradation behavior of high-strength Mg alloys of type ZX50 (Mg-5Zn-0.3Ca). It is shown that trace impurity elements increase the degradation rate, predominantly in the initial period of the tests, and also increase the material's susceptibility to localized corrosion attack. These effects are explained on the basis of the corrosion potential of the intermetallic phases present in the alloys. The Zn-rich phases present in ZX50 are nobler than the Mg matrix, and thus act as cathodic sites. The impurity elements Fe and Mn in the alloy of conventional purity are incorporated in these Zn-rich intermetallic phases and therefore increase their cathodic efficiency. A design rule for circumventing the formation of noble intermetallic particles and thus avoiding galvanically accelerated dissolution of the Mg matrix is proposed.

Towards deep-UV surface-enhanced resonance Raman spectroscopy of explosives: ultrasensitive, real-time and reproducible detection of TNT

We report ultrasensitive and label-free detection of 2,4,6-trinitrotoluene (TNT) deposited by drop coating using deep-ultraviolet surface-enhanced resonance Raman scattering (DUV-SERRS).

Effective temperature dynamics of shear bands in metallic glasses

We study the plastic deformation of bulk metallic glasses with shear transformation zone (STZ) theory, a physical model for plasticity in amorphous systems, and compare it with experimental data. In STZ theory, plastic deformation occurs when localized regions rearrange due to applied stress and the density of these regions is determined by a dynamically evolving effective disorder temperature. We compare the predictions of STZ theory to experiments that explore the low-temperature deformation of Zr-based bulk metallic glasses via shear bands at various thermal temperatures and strain rates. By following the evolution of effective temperature with time, strain rate, and temperature through a series of approximate and numerical solutions to the STZ equations, we successfully model a suite of experimentally observed phenomena, including shear-band aging as apparent from slide-hold-slide tests, a temperature-dependent steady-state flow stress, and a strain-rate- and temperature-dependent transition from stick-slip (serrated flow) to steady-sliding (nonserrated flow). We find that STZ theory quantitatively matches the observed experimental data and provides a framework for relating the experimentally measured energy scales to different types of atomic rearrangements.

Biodegradable Fe-based alloys for use in osteosynthesis: outcome of an in vivo study after 52 weeks

This study investigates the degradation performance of three Fe-based materials in a growing rat skeleton over a period of 1 year. Pins of pure Fe and two Fe-based alloys (Fe-10 Mn-1Pd and Fe-21 Mn-0.7C-1Pd, in wt.%) were implanted transcortically into the femur of 38 Sprague-Dawley rats and inspected after 4, 12, 24 and 52 weeks. The assessment was performed by ex vivo microfocus computed tomography, weight-loss determination, surface analysis of the explanted pins and histological examination. The materials investigated showed signs of degradation; however, the degradation proceeded rather slowly and no significant differences between the materials were detected. We discuss these unexpected findings on the basis of fundamental considerations regarding iron corrosion. Dense layers of degradation products were formed on the implants' surfaces, and act as barriers against oxygen transport. For the degradation of iron, however, the presence of oxygen is an indispensable prerequisite. Its availability is generally a critical factor in bony tissue and rather limited there, i.e. in the vicinity of our implants. Because of the relatively slow degradation of both pure Fe and the Fe-based alloys, their suitability for bulk temporary implants such as those in osteosynthesis applications appears questionable.

Diffusion on demand to control precipitation aging: application to Al-Mg-Si alloys

We demonstrate experimentally that a part-per-million addition of Sn solutes in Al-Mg-Si alloys can inhibit natural aging and enhance artificial aging. The mechanism controlling the aging is argued to be vacancy diffusion, with solutes trapping vacancies at low temperature and releasing them at elevated temperature, which is supported by a thermodynamic model and first-principles computations of Sn-vacancy binding. This "diffusion on demand" solves the long-standing problem of detrimental natural aging in Al-Mg-Si alloys, which is of great scientific and industrial importance. Moreover, the mechanism of controlled buffering and release of excess vacancies is generally applicable to modulate diffusion in other metallic systems.

Probing shear-band initiation in metallic glasses

In situ acoustic emission monitoring is shown to capture the initiation of shear bands in metallic glasses. A model picture is inferred from stick-slip flow in granular media such that the origin of acoustic emission is attributed to a mechanism of structural dilatation. By employing a quantitative approach, the critical volume change associated with shear-band initiation in a metallic glass is estimated to be a few percent only. This result agrees with typical values of excess free volume found in the supercooled liquid regime near the glass transition temperature.

High aspect ratio plasmonic nanostructures for sensing applications

We present an experimental and theoretical study of plasmonic modes in high aspect ratio nanostructures in the visible wavelength region and demonstrate their high performance for sensing applications. Ordered and well-defined plasmonic structures with various cross-sectional profiles and heights are obtained using a top-down fabrication process. We show that, compared to cylindrical nanorods, structures with split-ring resonator-like cross sections have great potential for powerful sensing due to a pronounced polarization dependence, strong field enhancement, structural tunability, and improved mechanical stability. The plasmonic structures under study exhibit high sensitivities, up to nearly 600 nm/RIU, and figures of merit above 20.

Interaction-induced partitioning and magnetization jumps in the mixed-spin oxide FeTiO3-Fe2O3

In this study we report on jumps in the magnetic moment of the hemo-ilmenite solid solution (x)FeTiO(3)-(1-x)Fe(2)O(3) above Fe(III) percolation at low temperature (T<3 K). The first jumps appear at 2.5 K, one at each side of the magnetization loop, and their number increases with decreasing temperature and reaches 5 at T=0.5 K. The jumps occur after field reversal from a saturated state and are symmetrical in the trigger field and intensity with respect to the field axis. Moreover, an increase of the sample temperature by 2.8% at T=2.0 K indicates the energy released after the ignition of the magnetization jump, as the spin-currents generated by the event are dissipated in the lattice. The magnetization jumps are further investigated by Monte Carlo simulations, which show that these effects are a result of magnetic interaction-induced partitioning on a sublattice level.

Magnetic metamaterials in the blue range using aluminum nanostructures

We report an experimental and theoretical study of the optical properties of two-dimensional arrays of aluminum nanoparticle in-tandem pairs. Plasmon resonances and effective optical constants of these structures are investigated, and strong magnetic response as well as negative permeability is observed down to 400 nm wavelength. Theoretical calculations based on the finite-difference time-domain method are performed for various particle dimensions and lattice parameters, and are found to be in good agreement with the experimental findings. The results show that metamaterials operating across the whole visible wavelength range are feasible.

On the in vitro and in vivo degradation performance and biological response of new biodegradable Mg-Y-Zn alloys

A design strategy deployed in developing new biodegradable Mg-Y-Zn alloys is summarized and the key factors influencing their suitability for medical applications are described. The Mg-Y-Zn alloys reveal microstructural features and mechanical characteristics expected to be appropriate for vascular intervention applications. The focus of this article lies in the evaluation of the degradation performance and biological response of the alloys with respect to their potential as implant materials (stents). The degradation characteristics analyzed by immersion testing and electrochemical impedance spectroscopy in simulated physiological media reveal slow and homogeneous degradation. In vitro cell tests using human umbilical vein endothelial cells indicate good cytocompatibility on the basis of the alloys' eluates (extracts). Animal studies carried out with pigs on Mg-2Y-1Zn (in wt.%) reveal an auspicious in vivo performance. Evaluation of preparations derived from implants in various types of tissues indicates homogeneous degradation and only limited gas formation during in vivo testing. The characteristics of the tissue reactions indicate good biocompatibility. The new Mg-Y-Zn alloys show an interesting combination of preferred microstructural, mechanical, electrochemical and biological properties, which make them very promising for degradable implant applications.

Design strategy for biodegradable Fe-based alloys for medical applications

The aim of this article is to describe a design strategy for the development of new biodegradable Fe-based alloys offering a performance considered appropriate for temporary implant applications, in terms of both an enhanced degradation rate compared to pure iron, and suitable strength and ductility. The design strategy is based on electrochemical, microstructural and toxicological considerations. The influence of alloying elements on the electrochemical modification of the Fe matrix and the controlled formation of noble intermetallic phases is deployed. Such intermetallic phases are responsible for both an increased degradation rate and enhanced strength. Manganese and palladium have been shown to be suitable alloying additions for this design strategy: Mn lowers the standard electrode potential, while Pd forms noble (Fe,Mn)Pd intermetallics that act as cathodic sites. We discuss the efficiency and the potential of the design approach, and evaluate the resulting characteristics of the new alloys using metal-physical experiments including electrochemical measurements, phase identification analysis and electron microscopy studies. The newly developed Fe-Mn-Pd alloys reveal a degradation resistance that is one order of magnitude lower than observed for pure iron. Additionally, the mechanical performance is shown to be adjustable not only by the choice of alloying elements but also by heat treatment procedures; high strength values >1400MPa at ductility levels >10% can be achieved. Thus, the new alloys offer an attractive combination of electrochemical and mechanical characteristics considered suitable for biodegradable medical applications.

Large-scale synthesis of defined cobalt nanoparticles and magnetic metal-polymer composites

We present a method where epsilon-cobalt nanoparticles with an average diameter of 4.5 nm can be synthesized in a controlled process and in significantly larger quantities than previously reported in the literature, based on the thermal decomposition of dicobaltoctacarbonyl in the presence of oleic acid and trioctylphosphine oxide (TOPO). Moreover, since the resulting particles are coated with an oleate layer, as shown by infrared (IR) spectroscopy, the colloids can be re-dispersed in organic solvents. These dispersions are suitable for the preparation of nanocomposites by a simple procedure, i.e. mixing of the cobalt dispersion with a polymer solution followed by casting and solvent evaporation. Magnetization measurements confirm the expected superparamagnetic behavior for both the cobalt nanoparticles and the metal-polymer nanocomposites.

MgZnCa glasses without clinically observable hydrogen evolution for biodegradable implants

Corrosion is normally an undesirable phenomenon in engineering applications. In the field of biomedical applications, however, implants that 'biocorrode' are of considerable interest. Deploying them not only abrogates the need for implant-removal surgery, but also circumvents the long-term negative effects of permanent implants. In this context magnesium is an attractive biodegradable material, but its corrosion is accompanied by hydrogen evolution, which is problematic in many biomedical applications. Whereas the degradation and thus the hydrogen evolution of crystalline Mg alloys can be altered only within a very limited range, Mg-based glasses offer extended solubility for alloying elements plus a homogeneous single-phase structure, both of which may alter corrosion behaviour significantly. Here we report on a distinct reduction in hydrogen evolution in Zn-rich MgZnCa glasses. Above a particular Zn-alloying threshold (approximately 28 at.%), a Zn- and oxygen-rich passivating layer forms on the alloy surface, which we explain by a model based on the calculated Pourbaix diagram of Zn in simulated body fluid. We document animal studies that confirm the great reduction in hydrogen evolution and reveal the same good tissue compatibility as seen for crystalline Mg implants. Thus, the glassy Mg(60+x)Zn(35-x)Ca5 (0 < or = x < or = 7) alloys show great potential for deployment in a new generation of biodegradable implants.

Electric and magnetic resonances in arrays of coupled gold nanoparticle in-tandem pairs

We present an experimental and theoretical study on the optical properties of arrays of gold nanoparticle in-tandem pairs (nanosandwiches). The well-ordered Au pairs with diameters down to 35 nm and separation distances down to 10 nm were fabricated using extreme ultraviolet (EUV) interference lithography. The strong near-field coupling of the nanoparticles leads to electric and magnetic resonances, which can be well reproduced by Finite-Difference Time-Domain (FDTD) calculations. The influence of the structural parameters, such as nanoparticle diameter and separation distance, on the hybridized modes is investigated. The energy and lifetimes of these modes are studied, providing valuable physical insight for the design of novel plasmonic structures and metamaterials.