Michele V. Manuel

A study of a biodegradable Mg–3Sc–3Y alloy and the effect of self-passivation on the in vitro degradation☆

Magnesium and its alloys have been investigated for their potential application as biodegradable implant materials. Although properties of magnesium such as biocompatibility and susceptibility to dissolution are desirable for biodegradable implant applications, its high degradation rate and low strength pose a significant challenge. A potential way to reduce the initial degradation rate is to form a self-passivating protective layer on the surface of the alloy. Oxides with a low enthalpy of formation result in a strong thermodynamic driving force to produce oxide surfaces that are more stable than the native oxide (MgO), and possibly reduce the initial degradation rate in these alloys. In the present study a ternary Mg-3wt.% Sc-3wt.% Y alloy was investigated and its oxidation behavior studied. The effect of surface passivation on the in vitro degradation rate was studied and the degradation products identified. The results show that the oxide provided an initial degradation barrier and 24h oxidation resulted in a negligible degradation rate for up to 23 days. Furthermore, the degradation products of the alloy showed no significant toxicity to osteoblastic cells, and cell proliferation studies confirmed cell attachment and proliferation on the surface of the oxidized alloy.

The Effect of Substrate Microstructure on the Heat-Affected Zone Size in Sn-Zn Alloys Due to Adjoining Ni-Al Reactive Multilayer Foil Reaction

The rapid release of energy from reactive multilayer foils can create extreme local temperature gradients near substrate materials. In order to fully exploit the potential of these materials, a better understanding of the interaction between the substrate or filler material and the foil is needed. Specifically, this work investigates how variations in local properties within the substrate (i.e. differences between properties in constituent phases) can affect heat transport into the substrate. This can affect the microstructural evolution observed within the substrate, which may affect the final joint properties. The effect of the initial substrate microstructure on microstructural evolution within the heat-affected zone is evaluated experimentally in two Sn-Zn alloys and numerical techniques are utilized to inform the analysis.

Peri-implant tissue response and biodegradation performance of a Mg-1.0Ca-0.5Sr alloy in rat tibia.

Biodegradable magnesium (Mg) alloys combine the advantages of traditional metallic implants and biodegradable polymers, having high strength, low density, and a stiffness ideal for bone fracture fixation. A recently developed Mg-Ca-Sr alloy potentially possesses advantageous characteristics over other Mg alloys, such as slower degradation rates and minimal toxicity. In this study, the biocompatibility of this Mg-Ca-Sr alloy was investigated in a rat pin-placement model. Cylindrical pins were inserted in the proximal tibial metaphyses in pre-drilled holes orthogonal to the tibial axis. Implant and bone morphologies were investigated using μCT at 1, 3, and 6 weeks after implant placement. At the same time points, the surrounding tissue was evaluated using H&E, TRAP and Goldners trichrome staining. Although gas bubbles were observed around the degrading implant at early time points, the bone remained intact with no evidence of microfracture. Principle findings also include new bone formation in the area of the implant, suggesting that the alloy is a promising candidate for biodegradable orthopedic implants.

MR measurement of alloy magnetic susceptibility: towards developing tissue-susceptibility matched metals.

Magnetic resonance imaging (MRI) can be used to relate structure to function mapped with high-temporal resolution electrophysiological recordings using metal electrodes. Additionally, MRI may be used to guide the placement of electrodes or conductive cannula in the brain. However, the magnetic susceptibility mismatch between implanted metals and surrounding brain tissue can severely distort MR images and spectra, particularly in high magnetic fields. In this study, we present a modified MR method of characterizing the magnetic susceptibility of materials that can be used to develop biocompatible, metal alloys that match the susceptibility of host tissue in order to eliminate MR distortions proximal to the implant. This method was applied at 4.7T and 11.1T to measure the susceptibility of a model solid-solution alloy of Cu and Sn, which is inexpensive but not biocompatible. MR-derived relative susceptibility values of four different compositions of Cu-Sn alloy deviated by less than 3.1% from SQUID magnetometry absolute susceptibility measurements performed up to 7T. These results demonstrate that the magnetic susceptibility varies linearly with atomic percentage in these solid-solution alloys, but are not simply the weighted average of Cu and Sn magnetic susceptibilities. Therefore susceptibility measurements are necessary when developing susceptibility-matched, solid-solution alloys for the elimination of susceptibility artifacts in MR. This MR method does not require any specialized equipment and is free of geometrical constraints, such as sample shape requirements associated with SQUID magnetometry, so the method can be used at all stages of fabrication to guide the development of a susceptibility matched, biocompatible device.

The Effect of Indium Additions on Mg-Li and Mg-Li-Al Alloys

It is known that BCC Mg-Li alloys offer the opportunity to realize both high specific strength and good ductility in a light weight alloy. The commonest addition to this system is Al due to its solid solution-strengthening ability. This system is also known to precipitate a potent metastable phase that subsequently transitions to the equilibrium AlLi phase. There are other systems, Zn, Cd, and In, which are also known to precipitate a phase that is similar to the AlLi phase. Of these additions, the phase-evolution characteristics associated with adding In to Mg-Li and Mg-Li-Al alloys are largely unknown. This article seeks to understand the phase transformation, microstructural evolution, and mechanical behavior of In additions by systematically studying Mg-Li-In in contrast to Mg-Li-Al and Mg-Li-Al-In. This study represents an initial investigation of the Mg-Li-In system, while simultaneously determining Indium’s potential as a microalloying addition. It was found that in the compositional ranges under investigation, a metastable phase does form in each system, and at longer aging times, In3Li13 is found to precipitate. Commentary and insight are also provided with respect to precipitate nucleation and coarsening behavior.

The effect of Mg–Ca–Sr alloy degradation products on human mesenchymal stem cells

Abstract Biodegradable Mg alloys have the potential to replace currently used metallic medical implant devices, likely eliminating toxicity concerns and the need for secondary surgeries, while also providing a potentially stimulating environment for tissue growth. A recently developed Mg–Ca–Sr alloy possesses advantageous characteristics over other Mg alloys, having a good combination of strength and degradation behavior, while also displaying potentially osteogenic properties. To better understand the effect of alloy degradation products on cellular mechanisms, in vitro studies using human bone marrow‐derived mesenchymal stem cells were conducted. Ionic products of alloy dissolution were found to be nontoxic but changed the proliferation profile of stem cells. Furthermore, their presence changed the progress of osteogenic development, while concentrations of Mg in particular appeared to induce stem cell differentiation. The work presented herein provides a foundation for future alloy design where structures can be tailored to obtain specific implant performance. These potentially bioactive implants would reduce the risks for patients by shortening their healing time, minimizing discomfort and toxicity concerns, while reducing hospital costs.

Prediction and characterization of heat-affected zone formation in tin-bismuth alloys due to nickel-aluminum multilayer foil reaction

Reactive multilayer foils have the potential to be used as local high intensity heat sources for a variety of applications. Most of the past research effort concerning these materials have focused on understanding the structure-property relationships of the foils that govern the energy released during a reaction. To improve the ability of researchers to more rapidly develop technologies based on reactive multilayer foils, a deeper and more predictive understanding of the relationship between the heat released from the foil and microstructural evolution in the neighboring materials is needed. This work describes the development of a numerical model for the purpose of predicting heat affected zone size in substrate materials. The model is experimentally validated using a commercially available Ni-Al multilayer foils and alloys from the Sn-Bi binary system. To accomplish this, phenomenological models for predicting the variation of physical properties (i.e., thermal conductivity, density, and heat capacity) ...

Magnesium Technology 2014

Employing AZ3 IB Eco-Magnesium® (Eco-Mg) alloy in the European project Green Metallurgy (Green Metallurgy Project) process route provides the lowest carbon footprint since precursor materials. Chips produced from the machining phase can be used directly in the cold compaction step, and followed by direct extrusion to produce fully densified semifinished bars. These materials are of great interest in certain manufacturing sectors as they can impact future market scenarios based on the high rate of recycled material. Specifically, there are two key-points which can be considered sources of improvement: a) Eco-Mg alloys contribute to drastically reduce of the Global Warming Potential (GWP) of the entire process route as recycled chips have been used as feedstock material; b) Eco-Mg are less expensive materials, and therefore of interest to the automobile sectors for cost-driven lightweight components. Using 30% in-situ recycled chips material allows manufacturers to keep the total GWP of semifinished bar produced by the tested process route to about 76.2 kgC02eq per kg of bar.

Investigation and Analytical Description of Acoustic Production by Magneto-Acoustic Mixing Technology

Magneto-Acoustic Mixing Technology is a novel manufacturing method that combines two magnetic fields to produce high-intensity sonication for liquid-state materials processing. This method may be adapted to the manufacture of various materials that benefit from a combination of high temperature, magnetic fields, and acoustic energy. In this work, acoustic generation mechanisms are described in detail and found to be dependent on the skin depth of the induction currents. Analytical models of acoustic pressure are derived, based on two mutually exclusive vibration mechanisms, crucible and melt vibration. Additionally, grain size evidence of acoustic pressure distribution is presented as preliminary model validation.

Near Surface Stoichiometry in UO2: A Density Functional Theory Study

The mechanisms of oxygen stoichiometry variation in UO2 at different temperature and oxygen partial pressure are important for understanding the dynamics of microstructure in these crystals. However, very limited experimental studies have been performed to understand the atomic structure of UO2 near surface and defect effects of near surface on stoichiometry in which the system can exchange atoms with the external reservoir. In this study, the near (110) surface relaxation and stoichiometry in UO2 have been studied with density functional theory (DFT) calculations. On the basis of the point-defect model (PDM), a general expression for the near surface stoichiometric variation is derived by using DFT total-energy calculations and atomistic thermodynamics, in an attempt to pin down the mechanisms of oxygen exchange between the gas environment and defected UO2. By using the derived expression, it is observed that, under poor oxygen conditions, the stoichiometry of near surface is switched from hyperstoichiometric at 300 K with a depth around 3 nm to near-stoichiometric at 1000 K and hypostoichiometric at 2000 K. Furthermore, at very poor oxygen concentrations and high temperatures, our results also suggest that the bulk of the UO2 prefers to be hypostoichiometric, although the surface is near-stoichiometric.