Influence of Magnesium Alloy Degradation on Undifferentiated Human Cells

Challenges and Pitfalls of Research Designs Involving Magnesium-Based Biomaterials: An Overview

Magnesium-based biomaterials hold remarkable promise for various clinical applications, offering advantages such as reduced stress-shielding and enhanced bone strengthening and vascular remodeling compared to traditional materials. However, ensuring the quality of preclinical research is crucial for the development of these implants. To achieve implant success, an understanding of the cellular responses post-implantation, proper model selection, and good study design are crucial. There are several challenges to reaching a safe and effective translation of laboratory findings into clinical practice. The utilization of Mg-based biomedical devices eliminates the need for biomaterial removal surgery post-healing and mitigates adverse effects associated with permanent biomaterial implantation. However, the high corrosion rate of Mg-based implants poses challenges such as unexpected degradation, structural failure, hydrogen evolution, alkalization, and cytotoxicity. The biocompatibility and degradability of materials based on magnesium have been studied by many researchers in vitro; however, evaluations addressing the impact of the material in vivo still need to be improved. Several animal models, including rats, rabbits, dogs, and pigs, have been explored to assess the potential of magnesium-based materials. Moreover, strategies such as alloying and coating have been identified to enhance the degradation rate of magnesium-based materials in vivo to transform these challenges into opportunities. This review aims to explore the utilization of Mg implants across various biomedical applications within cellular (in vitro) and animal (in vivo) models.

Multiscale morphological analysis of bone microarchitecture around Mg-10Gd implants

The utilization of biodegradable magnesium (Mg)-based implants for restoration of bone function following trauma represents a transformative approach in orthopaedic application. One such alloy, magnesium-10 weight percent gadolinium (Mg-10Gd), has been specifically developed to address the rapid degradation of Mg while enhancing its mechanical properties to promote bone healing. Previous studies have demonstrated that Mg-10Gd exhibits favorable osseointegration; however, it exhibits distinct ultrastructural adaptation in comparison to conventional implants like titanium (Ti). A crucial aspect that remains unexplored is the impact of Mg-10Gd degradation on the bone microarchitecture. To address this, we employed hierarchical three-dimensional imaging using synchrotron radiation in conjunction with image-based finite element modelling. By using the methods outlined, the vascular porosity, lacunar porosity and the lacunar-canaliculi network (LCN) morphology of bone around Mg-10Gd in comparison to Ti in a rat model from 4 weeks to 20 weeks post-implantation was investigated. Our investigation revealed that within our observation period, the degradation of Mg-10Gd implants was associated with significantly lower (p < 0.05) lacunar density in the surrounding bone, compared to Ti. Remarkably, the LCN morphology and the fluid flow analysis did not significantly differ for both implant types. In summary, a more pronounced lower lacunae distribution rather than their morphological changes was detected in the surrounding bone upon the degradation of Mg-10Gd implants. This implies potential disparities in bone remodelling rates when compared to Ti implants. Our findings shed light on the intricate relationship between Mg-10Gd degradation and bone microarchitecture, contributing to a deeper understanding of the implications for successful osseointegration.

Exploring the Usability of α-MSH-SM-Liposome as an Imaging Agent to Study Biodegradable Bone Implants In Vivo

Novel biodegradable metal alloys are increasingly used as implant materials. The implantation can be accompanied by an inflammatory response to a foreign object. For studying inflammation in the implantation area, non-invasive imaging methods are needed. In vivo imaging for the implanted area and its surroundings will provide beneficiary information to understand implant-related inflammation and help to monitor it. Therefore, inflammation-sensitive fluorescent liposomes in rats were tested in the presence of an implant to evaluate their usability in studying inflammation. The sphingomyelin-containing liposomes carrying alpha-melanocyte-stimulating hormone (α-MSH)-peptide were tested in a rat bone implant model. The liposome interaction with implant material (Mg-10Gd) was analyzed with Mg-based implant material (Mg-10Gd) in vitro. The liposome uptake process was studied in the bone-marrow-derived macrophages in vitro. Finally, this liposomal tracer was tested in vivo. It was found that α-MSH coupled sphingomyelin-containing liposomes and the Mg-10Gd implant did not have any disturbing influence on each other. The clearance of liposomes was observed in the presence of an inert and biodegradable implant. The degradable Mg-10Gd was used as an alloy example; however, the presented imaging system offers a new possible use of α-MSH-SM-liposomes as tools for investigating implant responses.

The osteogenetic activities of mesenchymal stem cells in response to Mg2+ ions and inflammatory cytokines: a numerical approach using fuzzy logic controllers

Magnesium (Mg²⁺) ions are frequently reported to regulate osteogenic activities of mesenchymal stem cells (MSCs). In this study, we propose a numerical model to study the regulatory importance of Mg²⁺ ions on MSCs osteoblastic differentiation in the presence of an inflammatory response. A fuzzy logic controller was formulated to receive the concentrations of Mg²⁺ ions and the inflammatory cytokines of TNF-α, IL-10, IL-1β, and IL-8 as cellular inputs and predict the cells’ early and late differentiation rates. Five sets of empirical data obtained from published cell culture experiments were used to calibrate the model. The model successfully reproduced the empirical data regarding the concentration- and phase-dependent effect of Mg²⁺ ions on the differentiation process. In agreement with the experiments, the model showed the stimulatory role of Mg²⁺ ions on the early differentiation phase, once administered at low concentration, and their inhibitory role on the late differentiation phase. The numerical approach used in this study suggested 6–8 mM as the most effective concentration of Mg²⁺ ions in promoting the early differentiation process. Also, the proposed model sheds light on the fundamental differences in the behavioral properties of cells cultured in different experiments, e.g. differentiation rate and the sensitivity of the cultured cells to stimulatory signals such as Mg²⁺ ions. Thus, it can be used to interpret and compare different empirical findings. Moreover, the model successfully reproduced the nonlinearities in the concentration-dependent role of the inflammatory cytokines in early and late differentiation rates. Overall, the proposed model can be employed in studying the osteogenic properties of Mg-based implants in the presence of an inflammatory response.

Magnesium (Mg) is an attractive material for bone implants as it fully degrades after implantation, saving pain and cost of the second surgery for implant removal. To advance its application in the orthopedic industry, it is paramount to fully understand the biological impact of the degradation products, in particular Mg²⁺ ions. Here, we propose a computer model to study the effects of Mg²⁺ ions on bone regeneration. The model focuses on stem cells and includes both the direct stimulation effects of Mg²⁺ ions on cells and the indirect stimulus through the inflammatory system. The proposed model successfully reproduced the experimental data of five different studies. The model additionally highlighted differences amongst different experiments in terms of the cellular response to Mg²⁺ ions. The proposed system therefore provides an important addition to the field of Mg implant research.

High-resolution ex vivo analysis of the degradation and osseointegration of Mg-xGd implant screws in 3D

Biodegradable magnesium (Mg) alloys can revolutionize osteosynthesis, because they have mechanical properties similar to those of the bone, and degrade over time, avoiding the need of removal surgery. However, they are not yet routinely applied because their degradation behavior is not fully understood.

In this study we have investigated and quantified the degradation and osseointegration behavior of two biodegradable Mg alloys based on gadolinium (Gd) at high resolution.

Mg-5Gd and Mg-10Gd screws were inserted in rat tibia for 4, 8 and 12 weeks. Afterward, the degradation rate and degradation homogeneity, as well as bone-to-implant interface, were studied with synchrotron radiation micro computed tomography and histology. Titanium (Ti) and polyether ether ketone (PEEK) were used as controls material to evaluate osseointegration.

Our results showed that Mg-5Gd degraded faster and less homogeneously than Mg-10Gd. Both alloys gradually form a stable degradation layer at the interface and were surrounded by new bone tissue. The results were correlated to in vitro data obtained from the same material and shape. The average bone-to-implant contact of the Mg-xGd implants was comparable to that of Ti and higher than for PEEK. The results suggest that both Mg-xGd alloys are suitable as materials for bone implants.

•

High-resolution non-destructive synchrotron micro computed tomography of biodegradable Mg alloys ex vivo.

•

Mg-xGd implants exhibit a high bone-to-implant contact, similar to titanium implants.

•

Initially Mg-xGd implants are surrounded by a lesser bone volume fraction but reach similar levels as reference materials.

•

Mg-xGd implants degrade at rates lower than 1 mm a⁻¹ in vivo, decreasing to less than 0.4 mm a⁻¹ after 12 weeks.

Comprehensive Review on the Degradation Chemistry and Toxicity Studies of Functional Materials

In recent decades there has been growing interest of material chemists in the successful development of functional materials for drug delivery, tissue engineering, imaging, diagnosis, theranostic, and other biomedical applications with advanced nanotechnology tools. The efficacy and safety of functional materials are determined by their pharmacological, toxicological, and immunogenic effects. It is essential to consider all degradation pathways of functional materials and to assess plausible intermediates and final products for quality control. This review provides a brief insight into chemical degradation mechanisms of functional materials like oxidation, photodegradation, and physical and enzymatic degradation. The intermediates and products of degradation were confirmed with analytical methods such as proton nuclear magnetic resonance (¹H NMR), gel permeation chromatography (GPC), UV-vis spectroscopy (UV-vis), infrared spectroscopy (IR), differential scanning calorimetry (DSC), mass spectroscopy, and other sophisticated analytical methods. These analytical methods are also used for regulatory, quality control, and stability purposes in industry. The assessment of degradation is important to predetermine the behavior of functional materials in specific storage conditions and can be relevant to their behavior during in vivo applications. Another important aspect is the evaluation of the toxicity of functional materials. Toxicity can be accessed with various methods using in vitro, in vivo, ex vivo, and in silico models. In vitro cell culture methods are used to determine mitochondrial damage, reactive oxygen species, stress responses, and cellular toxicity. In vitro cellular toxicity can be measured by MTT assay, LDH leakage assay, and hemolysis. In vivo studies are performed using various animal models involving zebrafish, rodents (mice and rats), and nonhuman primates. Ex vivo studies are also used for efficacy and toxicity determinations of functional materials like ex vivo potency assay and precision-cut liver slice (PCLS) models. The in silico tools with computational simulations like quantitative structure-activity relationships (QSAR), pharmacokinetics (PK) and pharmacodynamics (PD), dose and time response, and quantitative cationic-activity relationships ((Q)CARs) are used for prediction of the toxicity of functional materials. In this review, we studied the principle methods used for degradation studies, different degradation pathways, and mechanisms of functional material degradation with prototype examples. We discuss toxicity assessments with different toxicity approaches used for estimation of the safety and efficacy of functional materials.

Anti-tumour activity of Mg-6%Ag and Mg-10%Gd alloys in mice with inoculated melanoma

Cytotoxicity in vitro and anti-tumour activity in vivo of magnesium alloys Mg-6%Ag and Mg-10%Gd was studied. It was shown that specifically designed and thermomechanically processed Mg alloys produced a sizeable cytotoxic effect on PC-3 line tumour cells in vitro through inhibition of their proliferation. A pronounced grain refinement in combination with the formation of second phases and precipitates attained by means of equal-channel angular pressing (ECAP) accellerated the degradation and gave rise to enhanced anti-tumour activity of both alloys. The gadolinium-containing alloy outperformed the silver-containing one substantially. In an in vivo assay, intratumoural implantation of pins made from both alloys in the homogenised and the ECAP states in mice with inoculated B16 melanoma gave rise to an indubitable anti-tumour effect. It was documented in a decrease of Ki-67(+) cells and the occurrence of regions of destruction of tumoural tissue that were filled with gas evolving in the course of biodegradation of the implants. The data obtained suggest that intratumoural implantation of gadolinium-containing magnesium alloys can be considered for therapy of inoperable or chemoresistant forms of cancer.

Tissue responses after implantation of biodegradable Mg alloys evaluated by multimodality 3D micro-bioimaging in vivo

The local response of tissue triggered by implantation of degradable magnesium-based implant materials was investigated in vivo in a murine model. Pins (5.0 mm length by 0.5 mm diameter) made of Mg, Mg-10Gd, and Ti were implanted in the leg muscle tissue of C57Bl/6N mice (n = 6). Implantation was generally well tolerated as documented by only a mild short term increase in a multidimensional scoring index. Lack of difference between the groups indicated that the response was systemic and surgery related rather than material dependent. Longitudinal in vivo monitoring utilizing micro-computed tomography over 42 days demonstrated the highest and most heterogeneous degradation for Mg-10Gd. Elemental imaging of the explants by micro X-ray fluorescence spectrometry showed a dense calcium-phosphate-containing degradation layer. In order to monitor resulting surgery induced and/or implant material associated local cell stress, sphingomyelin based liposomes containing indocyanine green were administered. An initial increase in fluorescent signals (3-7 days after implantation) indicating cell stress at the site of the implantation was measured by in vivo fluorescent molecular tomography. The signal decreased until the 42nd day for all materials. These findings demonstrate that Mg based implants are well tolerated causing only mild and short term adverse reactions.

Magnesium ions regulate mesenchymal stem cells population and osteogenic differentiation: A fuzzy agent-based modeling approach

Mesenchymal stem cells (MSCs) are proliferative and multipotent cells that play a key role in the bone regeneration process. Empirical data have repeatedly shown the bioregulatory importance of magnesium (Mg) ions in MSC growth and osteogenesis. In this study, we propose an agent-based model to predict the spatiotemporal dynamics of the MSC population and osteogenic differentiation in response to Mg2+ ions. A fuzzy-logic controller was designed to govern the decision-making process of cells by predicting four cellular processes of proliferation, differentiation, migration, and mortality in response to several important bioregulatory factors such as Mg2+ ions, pH, BMP2, and TGF-β1. The model was calibrated using the empirical data obtained from three sets of cell culture experiments. The model successfully reproduced the empirical observations regarding live cell count, viability, DNA content, and the differentiation-related markers of alkaline phosphate (ALP) and osteocalcin (OC). The simulation results, in agreement with the empirical data, showed that Mg2+ ions within 3-6 mM concentration have the highest stimulation effect on cell population growth. The model also correctly reproduced the stimulatory effect of Mg2+ ions on ALP and its inhibitory effect on OC as the early and late differentiation markers, respectively. Besides, the numerical simulation shed light on the innate cellular differences of the cells cultured in different experiments in terms of the proliferative capacity as well as sensitivity to Mg2+ ions. The proposed model can be adopted in the study of the osteogenesis around Mg-based implants where ions released due to degradation interact with local cells and regulate bone regeneration.

Zinc and cerium synergistically enhance the mechanical properties, corrosion resistance, and osteogenic activity of magnesium as resorbable biomaterials

Magnesium and its alloys have the potential to serve as a revolutionary class of biodegradable materials, specifically in the field of degradable implants for orthopedics. However, the corrosion rate of commercially pure magnesium is high and does not match the rate of regeneration of bone tissues. In this work, magnesium alloys containing zinc and cerium, either alone or in combination, were investigated and compared with commercially-pure magnesium as biomaterials. The microstructure, mechanical properties, corrosion resistance, and response of osteoblastsin vitrowere systematically assessed. Results reveal that alloying with Ce results in grain refinement and weakening of texture. The tensile test revealed that the ternary alloy offered the best combination of elastic modulus (41.1 ± 0.5 GPa), tensile strength (234.5 ± 4.5 MPa), and elongation to break (17.1 ± 0.4%). The ternary alloy was also the most resistant to corrosion (current of 0.85 ± 0.05 × 10^-4A cm^-2) in simulated body fluid than the other alloys. The response of MC3T3-E1 cellsin vitrorevealed that the ternary alloy imparts minimal cytotoxicity. Interestingly, the ternary alloy was highly efficient in supporting osteogenic differentiation, as revealed by the expression of alkaline phosphatase and calcium deposition. In summary, the extruded Mg alloy containing both Zn and Ce exhibits a combination of mechanical properties, corrosion resistance, and cell response that is highly attractive for engineering biodegradable orthopedic implants.

Fibrinogen and magnesium combination biomaterials modulate macrophage phenotype, NF-kB signaling and crosstalk with mesenchymal stem/stromal cells

Macrophage behavior upon biomaterial implantation conditions the inflammatory response and subsequent tissue repair. The hypothesis behind this work was that fibrinogen (Fg) and magnesium (Mg) biomaterials, used in combination (FgMg) could act synergistically to modulate macrophage activation, promoting a pro-regenerative phenotype. Materials were characterized by scanning electron microscopy, Fg and Mg degradation products were quantified by atomic absorption spectroscopy and ELISA. Whole blood immune cells and primary human monocyte-derived macrophages were exposed to the biomaterials extracts in unstimulated (M0) or pro-inflammatory LPS or LPS-IFNγ (M1) conditions. Macrophage phenotype was evaluated by flow cytometry, cytokines secreted by whole blood cells and macrophages were measured by ELISA, and signaling pathways were probed by Western blotting. The secretomes of macrophages preconditioned with biomaterials extracts were incubated with human mesenchymal stem/stromal cells (MSC) and their effect on osteogenic differentiation was evaluated via Alkaline Phosphatase (ALP) activity and alizarin red staining. Scaffolds of Fg, alone or in the FgMg combination, presented similar 3D porous architectures. Extracts from FgMg materials reduced LPS-induced TNF-α secretion by innate immune cells, and macrophage M1 polarization upon LPS-IFNγ stimulation, resulting in lower cell surface CD86 expression, lower NFκB p65 phosphorylation and reduced TNF-α secretion. Moreover, while biomaterial extracts per se did not enhance MSC osteogenic differentiation, macrophage secretome, particularly from cells exposed to FgMg extracts, increased MSC ALP activity and alizarin red staining, compared with extracts alone. These findings suggest that the combination of Fg and Mg synergistically influences macrophage pro-inflammatory activation and crosstalk with MSC. STATEMENT OF SIGNIFICANCE: Modulating macrophage phenotype by degradable and bioactive biomaterials is an increasingly explored strategy to promote tissue repair/regeneration. Fibrinogen (Fg) and magnesium (Mg)-based materials have been explored in this context. Previous work from our group showed that monocytes interact with fibrinogen adsorbed onto chitosan surfaces through TLR4 and that fibrinogen scaffolds promote in vivo bone regeneration. Also, magnesium ions have been reported to modulate macrophage pro-inflammatory M1 stimulation and to promote bone repair. Here we report, for the first time, the combination of Fg and Mg materials, hypothesizing that it could act synergistically on macrophages, directing them towards a pro-regenerative phenotype. As a first step towards proving/disproving our hypothesis we used extracts obtained from Fg, Mg and FgMg multilayer constructs. We observed that FgMg extracts led to a reduction in the polarization of macrophages towards a pro-inflammatory phenotype. Also, the secretome of macrophages exposed to extracts of the combination material promoted the expression of osteogenic markers by MSCs.

Optimizing an Osteosarcoma-Fibroblast Coculture Model to Study Antitumoral Activity of Magnesium-Based Biomaterials

Osteosarcoma is among the most common cancers in young patients and is responsible for one-tenth of all cancer-related deaths in children. Surgery often leads to bone defects in excised tissue, while residual cancer cells may remain. Degradable magnesium alloys get increasing attention as orthopedic implants, and some studies have reported potential antitumor activity. However, most of the studies do not take the complex interaction between malignant cells and their surrounding stroma into account. Here, we applied a coculture model consisting of green fluorescent osteosarcoma cells and red fluorescent fibroblasts on extruded Mg and Mg-6Ag with a tailored degradation rate. In contrast to non-degrading Ti-based material, both Mg-based materials reduced relative tumor cell numbers. Comparing the influence of the material on a sparse and dense coculture, relative cell numbers were found to be statistically different, thus relevant, while magnesium alloy degradations were observed as cell density-independent. We concluded that the sparse coculture model is a suitable mechanistic system to further study the antitumor effects of Mg-based material.

Bio-inspired biomaterial Mg-Zn-Ca: a review of the main mechanical and biological properties of Mg-based alloys

The toxicity of alloying elements in magnesium alloys used for biomedical purposes is an interesting and innovative subject, due to the great technological advances that would result from their application in medical devices (MDs) in traumatology. Recently promising results have been published regarding the rates of degradation and mechanical integrity that can support Mg alloys; this has led to an interest in understanding the toxicological features of these emerging biomaterials. The growing interest of different segments of the MD market has increased the determination of different research groups to clarify the behavior of alloying elements in vivo. This review covers the influence of the alloying elements on the body, the toxicity of the elements in Mg-Zn-Ca, as well as the mechanical properties, degradation, processes of obtaining the alloy, medical approaches and future perspectives on the use of the Mg in the manufacture of MDs for various medical applications.

Analysis of the bone ultrastructure around biodegradable Mg-xGd implants using small angle X-ray scattering and X-ray diffraction

Magnesium alloys are increasingly researched as temporary biodegradable metal implants in bone applications due to their mechanical properties which are more similar to bone than conventional implant metals and the fact that Magnesium occurs naturally within the body. However, the degradation processes in vivo and in particular the interaction of the bone with the degrading material need to be further investigated. In this study we are presenting the first quantitative comparison of the bone ultrastructure formed at the interface of biodegradable Mg-5Gd and Mg-10Gd implants and titanium and PEEK implants after 4, 8 and 12 weeks healing time using two-dimensional small angle X-ray scattering and X-ray diffraction. Differences in mineralization, orientation and thickness of the hydroxyapatite are assessed. We find statistically significant (p < 0.05) differences for the lattice spacing of the (310)-reflex of hydroxyapatite between titanium and Mg-xGd materials, as well as for the (310) crystal size between titanium and Mg-5Gd, indicating a possible deposition of Mg within the bone matrix. The (310) lattice spacing and crystallite size further differ significantly between implant degradation layer and surrounding bone (p < 0.001 for Mg-10Gd), suggesting apatite formation with significant amounts of Gd and Mg within the degradation layer. STATEMENT OF SIGNIFICANCE: Biodegradable Magnesium-based alloys are emerging as a viable alternative for temporary bone implant applications. However, in order to understand if the degradation of the implant material influences the bone ultrastructure, it is necessary to study the bone structure using high-resolution techniques. We have therefore employed 2D small angle X-ray scattering and X-ray diffraction to study the bone ultrastructure surrounding Magnesium-Gadolinium alloys as well as Titanium and PEEK alloys at three different healing times. This is the first time, that the bone ultrastructure around these materials is directly compared and that a statistical evaluation is performed. We found differences indicating a possible deposition of Mg within the bone matrix as well as a local deposition of Mg and/or Gd at the implant site. DATA AVAILABILITY STATEMENT: The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Improved In Vitro Test Procedure for Full Assessment of the Cytocompatibility of Degradable Magnesium Based on ISO 10993-5/-12

Magnesium (Mg)-based biomaterials are promising candidates for bone and tissue regeneration. Alloying and surface modifications provide effective strategies for optimizing and tailoring their degradation kinetics. Nevertheless, biocompatibility analyses of Mg-based materials are challenging due to its special degradation mechanism with continuous hydrogen release. In this context, the hydrogen release and the related (micro-) milieu conditions pretend to strictly follow in vitro standards based on ISO 10993-5/-12. Thus, special adaptions for the testing of Mg materials are necessary, which have been described in a previous study from our group. Based on these adaptions, further developments of a test procedure allowing rapid and effective in vitro cytocompatibility analyses of Mg-based materials based on ISO 10993-5/-12 are necessary. The following study introduces a new two-step test scheme for rapid and effective testing of Mg. Specimens with different surface characteristics were produced by means of plasma electrolytic oxidation (PEO) using silicate-based and phosphate-based electrolytes. The test samples were evaluated for corrosion behavior, cytocompatibility and their mechanical and osteogenic properties. Thereby, two PEO ceramics could be identified for further in vivo evaluations.

Magnesium ion leachables induce a conversion of contractile vascular smooth muscle cells to an inflammatory phenotype

Phenotype switching is a characteristic response of vascular smooth muscle cells (vSMCs) to the dynamic microenvironment and contributes to all stages of atherosclerotic plaque. Here, we immersed pure magnesium and AZ31 alloy in the completed medium under cell culture condition, applied the resultant leaching extracts to the isolated contractile rat aortic vSMCs and investigated how vSMCs phenotypically responded to the degradation of the magnesium-based stent materials. vSMCs became more proliferative and migratory but underwent more apoptosis when exposed to the degradation products of pure magnesium; while the AZ31 extracts caused less cell division but more apoptosis, thus slowing cell moving and growing. Noticeably, both leaching extracts dramatically downregulated the contractile phenotypic genes at mRNA and protein levels while significantly induced the inflammatory adhesive molecules and cytokines. Exogenously added Mg ions excited similar transformations of vSMCs. With the liberation or supplementation of Mg²⁺ , the expression patterns of the pro-contractile transactivator myocardin and the pro-inflammatory transcriptional factor kruppel-like factor 4 (KLF4) were reversed. Overall, the degradation of the Mg-based materials would evoke a shift of the contractile vSMCs to an inflammatory phenotype via releasing Mg ions to induce a transition from the phenotypic control of vSMCs by the myocardin to that by the KLF4. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 988-1001, 2019.

Numerical Modelling of Effects of Biphasic Layers of Corrosion Products to the Degradation of Magnesium Metal In Vitro

Magnesium (Mg) is becoming increasingly popular for orthopaedic implant materials. Its mechanical properties are closer to bone than other implant materials, allowing for more natural healing under stresses experienced during recovery. Being biodegradable, it also eliminates the requirement of further surgery to remove the hardware. However, Mg rapidly corrodes in clinically relevant aqueous environments, compromising its use. This problem can be addressed by alloying the Mg, but challenges remain at optimising the properties of the material for clinical use. In this paper, we present a mathematical model to provide a systematic means of quantitatively predicting Mg corrosion in aqueous environments, providing a means of informing standardisation of in vitro investigation of Mg alloy corrosion to determine implant design parameters. The model describes corrosion through reactions with water, to produce magnesium hydroxide Mg(OH) 2 , and subsequently with carbon dioxide to form magnesium carbonate MgCO 3 . The corrosion products produce distinct protective layers around the magnesium block that are modelled as porous media. The resulting model of advection-diffusion equations with multiple moving boundaries was solved numerically using asymptotic expansions to deal with singular cases. The model has few free parameters, and it is shown that these can be tuned to predict a full range of corrosion rates, reflecting differences between pure magnesium or magnesium alloys. Data from practicable in vitro experiments can be used to calibrate the model's free parameters, from which model simulations using in vivo relevant geometries provide a cheap first step in optimising Mg-based implant materials.

Gadolinium accumulation in organs of Sprague-Dawley® rats after implantation of a biodegradable magnesium-gadolinium alloy

Biodegradable magnesium implants are under investigation because of their promising properties as medical devices. For enhancing the mechanical properties and the degradation resistance, rare earth elements are often used as alloying elements. In this study Mg10Gd pins were implanted into Sprague-Dawley® rats. The pin volume loss and a possible accumulation of magnesium and gadolinium in the rats' organs and blood were investigated in a long-term study over 36weeks. The results showed that Mg10Gd is a fast disintegrating material. Already 12weeks after implantation the alloy is fragmented to smaller particles, which can be found within the intramedullary cavity and the cortical bones. They disturbed the bone remodeling until the end of the study. The results concerning the elements' distribution in the animals' bodies were even more striking, since an accumulation of gadolinium could be observed in the investigated organs over the whole time span. The most affected tissue was the spleen, with up to 3240μgGd/kg wet mass, followed by the lung, liver and kidney (up to 1040, 685 and 207μgGd/kg). In the brain, muscle and heart, the gadolinium concentrations were much smaller (less than 20μg/kg), but an accumulation could still be detected. Interestingly, blood serum samples showed no accumulation of magnesium and gadolinium. This is the first time that an accumulation of gadolinium in animal organs was observed after the application of a gadolinium-containing degradable magnesium implant. These findings demonstrate the importance of future investigations concerning the distribution of the constituents of new biodegradable materials in the body, to ensure the patients' safety.

STATEMENT OF SIGNIFICANCE

In the last years, biodegradable Mg alloys are under investigation due to their promising properties as orthopaedic devices used for bone fracture stabilization. Gadolinium as Rare Earth Element enhances the mechanical properties of Mg-Gd alloys but its toxicity in humans is still questionable. Up to now, there is no study investigating the elements' metabolism of a REE-containing Magnesium alloy in an animal model. In this study, we examined the gadolinium distribution and accumulation in rat organs during the degradation of Mg10Gd. Our findings showed that Gd is accumulating in the animal organs, especially in spleen, liver and kidney. This study is of crucial benefit regarding a safe application of REE-containing Magnesium alloys in humans.

Microhardness and In Vitro Corrosion of Heat-Treated Mg-Y-Ag Biodegradable Alloy

Magnesium alloys are promising candidates for biodegradable medical implants which reduce the necessity of second surgery to remove the implants. Yttrium in solid solution is an attractive alloying element because it improves mechanical properties and exhibits suitable corrosion properties. Silver was shown to have an antibacterial effect and can also enhance the mechanical properties of magnesium alloys. Measurements of microhardness and electrical resistivity were used to study the response of Mg-4Y and Mg-4Y-1Ag alloys to isochronal or isothermal heat treatments. Hardening response and electrical resistivity annealing curves in these alloys were compared in order to investigate the effect of silver addition. Procedures for solid solution annealing and artificial aging of the Mg-4Y-1Ag alloy were developed. The corrosion rate of the as-cast and heat-treated Mg-4Y-1Ag alloy was measured by the mass loss method. It was found out that solid solution heat treatment, as well artificial aging to peak hardness, lead to substantial improvement in the corrosion properties of the Mg-4Y-1Ag alloy.

Effects of Corroded and Non-Corroded Biodegradable Mg and Mg Alloys on Viability, Morphology and Differentiation of MC3T3-E1 Cells Elicited by Direct Cell/Material Interaction

This study investigated the effect of biodegradable Mg and Mg alloys on selected properties of MC3T3-E1 cells elicited by direct cell/material interaction. The chemical composition and morphology of the surface of Mg and Mg based alloys (Mg2Ag and Mg10Gd) were analysed by scanning electron microscopy (SEM) and EDX, following corrosion in cell culture medium for 1, 2, 3 and 8 days. The most pronounced difference in surface morphology, namely crystal formation, was observed when Pure Mg and Mg2Ag were immersed in cell medium for 8 days, and was associated with an increase in atomic % of oxygen and a decrease of surface calcium and phosphorous. Crystal formation on the surface of Mg10Gd was, in contrast, negligible at all time points. Time-dependent changes in oxygen, calcium and phosphorous surface content were furthermore not observed for Mg10Gd. MC3T3-E1 cell viability was reduced by culture on the surfaces of corroded Mg, Mg2Ag and Mg10Gd in a corrosion time-independent manner. Cells did not survive when cultured on 3 day pre-corroded Pure Mg and Mg2Ag, indicating crystal formation to be particular detrimental in this regard. Cell viability was not affected when cells were cultured on non-corroded Mg and Mg alloys for up to 12 days. These results suggest that corrosion associated changes in surface morphology and chemical composition significantly hamper cell viability and, thus, that non-corroded surfaces are more conducive to cell survival. An analysis of the differentiation potential of MC3T3-E1 cells cultured on non-corroded samples based on measurement of Collagen I and Runx2 expression, revealed a down-regulation of these markers within the first 6 days following cell seeding on all samples, despite persistent survival and proliferation. Cells cultured on Mg10Gd, however, exhibited a pronounced upregulation of collagen I and Runx2 between days 8 and 12, indicating an enhancement of osteointegration by this alloy that could be valuable for in vivo orthopedic applications.

In vivo degradation of binary magnesium alloys – a long-term study

Bioresorbable magnesium materials are widely investigated because of their promising properties as orthopedic devices. Pure magnesium (99.99%) and two binary magnesium alloys (Mg2Ag and Mg10Gd) were used to investigate the degradation behavior, the bone adherence and bone-implant interface mechanics of these materials in growing Sprague-Dawley