Effects of grain refinement on the biocorrosion and in vitro bioactivity of magnesium

Development of a Bioreactor-Coupled Flow-Cell Setup for 3D In Situ Nanotomography of Mg Alloy Biodegradation

Functional materials feature hierarchical microstructures that define their unique set of properties. The prediction and tailoring of these require a multiscale knowledge of the mechanistic interaction of microstructure and property. An important material in this respect is biodegradable magnesium alloys used for implant applications. To correlate the relationship between the microstructure and the nonlinear degradation process, high-resolution in situ three-dimensional (3D) imaging experiments must be performed. For this purpose, a novel experimental flow cell is presented which allows for the in situ 3D-nano imaging of the biodegradation process of materials with nominal resolutions below 100 nm using nanofocused hard X-ray radiation from a synchrotron source. The flow cell setup can operate under adjustable physiological and hydrodynamic conditions. As a model material, the biodegradation of thin Mg-4Ag wires in simulated body fluid under physiological conditions and a flow rate of 1 mL/min is studied. The use of two full-field nanotomographic imaging techniques, namely transmission X-ray microscopy and near-field holotomography, is compared, revealing holotomography as the superior imaging technique for this purpose. Additionally, the importance of maintaining physiological conditions is highlighted by the preliminary results. Supporting measurements using electron microscopy to investigate the chemical composition of the samples after degradation are performed.

Antibacterial PLA/Mg composite with enhanced mechanical and biological performance for biodegradable orthopedic implants

Biodegradability, bone-healing rate, and prevention of bacterial infection are critical factors for orthopedic implants. Polylactic acid (PLA) is a good candidate biodegradable material; however, it has insufficient mechanical strength and bioactivity for orthopedic implants. Magnesium (Mg), has good bioactivity, biodegradability, and sufficient mechanical properties, similar to that of bone. Moreover, Mg has an inherent antibacterial property via a photothermal effect, which generates localized heat, thus preventing bacterial infection. Therefore, Mg is a good candidate material for PLA composites, to improve their mechanical and biological performance and add an antibacterial property. Herein, we fabricated an antibacterial PLA/Mg composite for enhanced mechanical and biological performance with an antibacterial property for application as biodegradable orthopedic implants. The composite was fabricated with 15 and 30 vol% of Mg homogeneously dispersed in PLA without the generation of a defect using a high-shear mixer. The composites exhibited an enhanced compressive strength of 107.3 and 93.2 MPa, and stiffness of 2.3 and 2.5 GPa, respectively, compared with those of pure PLA which were 68.8 MPa and 1.6 GPa, respectively. Moreover, the PLA/Mg composite at 15 vol% Mg exhibited significant improvement of biological performance in terms of enhanced initial cell attachment and cell proliferation, whereas the composite at 30 vol% Mg showed deteriorated cell proliferation and differentiation because of the rapid degradation of the Mg particles. In turn, the PLA/Mg composites exerted an antibacterial effect based on the inherent antibacterial property of Mg as well as the photothermal effect induced by near-infrared (NIR) treatment, which can minimize infection after implantation surgery. Therefore, antibacterial PLA/Mg composites with enhanced mechanical and biological performance may be a candidate material with great potential for biodegradable orthopedic implants.

Microstructural Assessment, Mechanical and Corrosion Properties of a Mg-Sr Alloy Processed by Combined Severe Plastic Deformation

The development of high-performance biodegradable alloys with controllable corrosion rates to be used for manufacturing advanced implants is a hot topic of modern materials science and biomedicine. This work features the changes in microstructure, corrosion behavior and mechanical properties of the Mg-2 wt.%Sr alloy progressively induced by equal-channel angular pressing, high-pressure torsion and annealing. We show that such processing leads to significant microstructure refinement including diminishing grain size, defect accumulation and fragmentation of the initial eutectics. We demonstrate that the application of severe plastic deformation and heat treatment is capable of considerably enhancing the mechanical and corrosion performance of a biodegradable alloy of the Mg-Sr system. The best trade-off between strength, plasticity and the corrosion resistance has been achieved by annealing of the Mg-Sr alloy subjected to combined severe plastic deformation processing.

Novel Bioactive Magnesium-Hopeite composite by friction stir processing for orthopedic implant applications

Magnesium (Mg) shows excellent potential for orthopedic implant applications owing to its equivalent mechanical properties compared to cortical bone and its biocompatibility. However, the rapid degradation rate of magnesium and its alloys in the physiological environment results in losing their mechanical integrity before complete bone healing. In light of this, friction stir processing (FSP), a solid-state process, is used to fabricate Hopeite (Zn(PO₄)₂.4H₂O) reinforced novel magnesium composite. As a result of the novel composite fabricated by FSP, grain refinement of the matrix phase occurs significantly. The samples were immersed in simulated body fluid (SBF) for in-vitro bioactivity and biodegradability tests. The corrosion behavior of pure Mg, FSP Mg, and FSP Mg-Hopeite composite samples was compared using electrochemical and immersion tests in SBF. It found that Mg-Hopeite composite has better corrosion resistance than FSP Mg and pure Mg. Because of grain refinement and the presence of secondary phase Hopeite in the composite, the mechanical properties and corrosion resistance improved. The bioactivity test was performed in the SBF environment, and a rapid apatite layer was formed on the surface of Mg-Hopeite composite samples during the test. Osteoblast-like MG63 cells were exposed to samples, and the MTT assay confirmed the non-toxicity of the FSP Mg-Hopeite composite. The wettability of the Mg-Hopeite composite was improved than pure Mg. The present research findings showed that the novel Mg-Hopeite composite fabricated by FSP is a promising candidate for orthopedic implant applications, unreported in the literature.

In Vitro Degradation and In Vivo Biocompatibility of Strontium-Doped Magnesium Phosphate-Reinforced Magnesium Composites

Magnesium is projected for use as a degradable orthopedic biomaterial. However, its fast degradation in physiological media is considered as a significant challenge for its successful clinical applications. Bioactive reinforcements containing Mg-based composites constitute one of the promising approaches for developing degradable metallic implants because of their adjustable mechanical behaviors, corrosion resistance, and biological response. Strontium is a trace element known for its role in enhancing osteoblast activity. In this study, bioactive SrO-doped magnesium phosphate (MgP)-reinforced Mg composites containing 1, 3, and 5 wt % MgP were developed through the casting route. The influence of the SrO-doped MgP reinforcement on degradation behaviors of the composites along with its cell-material interactions and in vivo biocompatibility was investigated. The wt % and distribution of MgP particles significantly improved the mechanical properties of the composite. HBSS immersion study indicated the least corrosion rate (0.56 ± 0.038 mmpy) for the Mg-3MgP composite. The higher corrosion resistance of Mg-3MgP leads to a controlled release of Sr-containing bioactive reinforcement, which eventually enhanced the cytotoxicity as measured using MG-63 cell-material interactions. The in vivo biocompatibility of the composite was evaluated using the rabbit femur defect model. Micro-computed tomography (μ-CT) and histological analysis supported the fact that Mg-3MgP maintained its structural integrity and enhanced osteogenesis (50.36 ± 2.03%) after 2 months of implantation. The results indicated that the Mg-MgP composite could be used as a degradable internal fracture fixation device material.

Biological Applications of Severely Plastically Deformed Nano-Grained Medical Devices: A Review

Metallic materials are widely used for fabricating medical implants due to their high specific strength, biocompatibility, good corrosion properties, and fatigue resistance. Recently, titanium (Ti) and its alloys, as well as stainless steel (SS), have attracted attention from researchers because of their biocompatibility properties within the human body; however, improvements in mechanical properties while keeping other beneficial properties unchanged are still required. Severe plastic deformation (SPD) is a unique process for fabricating an ultra-fine-grained (UFG) metal with micrometer- to nanometer-level grain structures. SPD methods can substantially refine grain size and represent a promising strategy for improving biological functionality and mechanical properties. This present review paper provides an overview of different SPD techniques developed to create nano-/ultra-fine-grain-structured Ti and stainless steel for improved biomedical implant applications. Furthermore, studies will be covered that have used SPD techniques to improve bone cell proliferation and function while decreasing bacterial colonization when cultured on such nano-grained metals (without resorting to antibiotic use).

Additively manufactured biodegradable porous metals

Partially due to the unavailability of ideal bone substitutes, the treatment of large bony defects remains one of the most important challenges of orthopedic surgery. Additively manufactured (AM) biodegradable porous metals that have emerged since 2018 provide unprecedented opportunities for fulfilling the requirements of an ideal bone implant. First, the multi-scale geometry of these implants can be customized to mimic the human bone in terms of both micro-architecture and mechanical properties. Second, a porous structure with interconnected pores possesses a large surface area, which is favorable for the adhesion and proliferation of cells and, thus, bony ingrowth. Finally, the freeform geometrical design of such biomaterials could be exploited to adjust their biodegradation behavior so as to maintain the structural integrity of the implant during the healing process while ensuring that the implant disappears afterwards, paving the way for full bone regeneration. While the AM biodegradable porous metals that have been studied so far have shown many unique properties as compared to their solid counterparts, the unprecedented degree of flexibility in their geometrical design has not yet been fully exploited to optimize their properties and performance. In order to develop the ideal bone implants, it is important to take advantage of the full potential of AM biodegradable porous metals through detailed and systematic study on their biodegradation behavior, mechanical properties, biocompatibility, and bone regeneration performance. This review paper presents the state of the art in AM biodegradable porous metals and is focused on the effects of material type, processing, geometrical design, and post-AM treatments on the mechanical properties, biodegradation behavior, in vitro biocompatibility, and in vivo bone regeneration performance of AM porous Mg, Fe, and Zn as well as their alloys. We also identify a number of knowledge gaps and the challenges encountered in adopting AM biodegradable porous metals for orthopedic applications and suggest some promising areas for future research.

Development and properties of dental Ti-Zr binary alloys

In this study, two medium Zr-containing Ti-based alloys with commercially pure titanium as control were systematically investigated to assess their potential biomedical application. After samples subjected to TMP and CR, it was found that the Zr addition significantly affected the microstructure, phase constitutions, mechanical properties and cytocompatibility. The microstructural results showed that increasing Zr concentrations resulted in more refined grains. Furthermore, Zr changed the phase constitution: CR Ti-20Zr was formed by the single α-phase while CR Ti-30Zr alloy was formed by the coexistence of α and deformation-induced FCC phases. The P-type FCC phase was dominant and more prone to occur than the B-type one. The mechanical tests demonstrated that the increasing Zr content led to a simultaneous increase in micro-hardness, strength and plasticity of CR samples due to the combined effects of solution strengthening, work hardening and the FCC phase. The SEM fractography indicated that the brittle fracture of CR Ti-20Zr due to deformation twins and ductile fracture of CR Ti-30Zr because of FCC phase. Furthermore, Ti-Zr alloys presented comparable cytocompatibility to the CP-Ti control based on cell viability, proliferation and intracellular O^2- content of MSCs. Specifically, alkaline phosphatase activity in BMSCs were significantly higher for grain refined CR Ti-30Zr. Considering all these results, CR Ti-30Zr alloy exhibited the optimal comprehensive performance to be potential dental materials.

Recent Developments in Magnesium Metal-Matrix Composites for Biomedical Applications: A Review

Recently, there is a growing interest in developing magnesium (Mg) based degradable biomaterial. Although corrosion is a concern for Mg, other physical properties, such as low density and Young's modulus, combined with good biocompatibility, lead to significant research and development in this area. To address the issues of corrosion and low yield strength of pure Mg, several approaches have been adopted, such as, composite preparation with suitable bioactive reinforcements, alloying, or surface modifications. This review specifically focuses on recent developments in Mg-based metal matrix composites (MMCs) for biomedical applications. Much effort has gone into finding suitable bioactive, bioresorbable reinforcements and processing techniques that can improve upon existing materials. In summary, this review provides a comprehensive overview of existing Mg-based composite preparation and their mechanical and corrosion properties and biological responses and future perspectives on the development of Mg-based composite biomaterials.

Biological evaluation of bone substitute

Critical-sized defects (CSDs) caused by trauma, tumor resection, or skeletal abnormalities create a high demand for bone repair materials (BRMs). Over the years, scientists have been trying to develop BRMs and evaluate their efficacy using numerous developed methods. BRMs are characterized by osteogenesis and angiogenesis promoting properties, the latter of which has rarely been studied in vitro and in vivo. While blood vessels are required to provide nutrients. Bone mass maintains a dynamic balance under the joint action of osteolytic and osteogenic activity in which monocytes differentiate into osteolytic cells, and osteoprogenitor cells differentiate into osteogenic cells. This review would be helpful for inexperienced researchers as well as present a comprehensive overview of methods used to investigate the effect of BRMs on osteogenic cells, osteolytic cells, and blood vessels, as well as their biocompatibility and biological performance. This review is expected to facilitate further research and development of new BRMs.

In vitro degradation of pure magnesium-the synergetic influences of glucose and albumin

The biocorrosion of magnesium in the external physiological environment is still difficult to accurately evaluate the degradation behavior in vivo, particularly, in the microenvironment of the patients with hyperglycemia or diabetes. Thus, we explored the synergistic effects of glucose and protein on the biodegradation of pure magnesium, so as to have a deeper understanding the mechanism of the degradation in vivo. The surface morphology and corrosion product composition of pure magnesium were investigated using SEM, EDS, FTIR, XRD and XPS. The effect of glucose and albumin on the degradation rate of pure magnesium was investigated via electrochemical and immersion tests. The adsorption of glucose and albumin on the sample surface was observed using fluorescence microscopy. The results showed that the presence of 2 g/L glucose changed the micromorphology of corrosion products on the magnesium surface by reacting with metal cations, thus inhibiting the corrosion of pure magnesium. Protein formed a barrier layer to protect the magnesium at early stage of immersion. The chelation reaction between protein and magnesium surface might accelerate the degradation at later stage. There may be a critical glucose (albumin) content. Biodegradation of pure magnesium was inhibited at low concentrations and promoted at high concentrations. The synergistic effect of glucose and protein restrained the adsorption of aggressive chloride ions to a certain extent, and thus inhibited the degradation of pure magnesium considerably. Moreover, XPS results indicated that glucose promoted the adsorption of protein on the sample surface.

Enhancing the Corrosion Resistance Performance of Mg-1.8Zn-1.74Gd-0.5Y-0.4Zr Biomaterial via Solution Treatment Process

: Microstructure and corrosion behavior of the solution-treated Mg-1.8Zn-1.74Gd-0.5Y-0.4Zr (wt%) alloy were studied. The results of microstructure indicated that the second phases of as-cast alloy was mainly comprised of Mg12Zn(Gd,Y) phase, Mg3Zn3(Gd,Y)2 phase and (Mg,Zn)3(Gd,Y) phase. After solution treatment process, the second phase gradually dissolved into the matrix, and the grain size increased. The effect of microgalvanic corrosion between α-Mg matrix and second phase was also improved. At the range of 470~510 °C solution treatment temperature, the corrosion resistance of the samples increases at first and then decreases slightly at 510 °C. All the solution-treated Mg-Zn-Gd-Y-Zr samples exhibit better corrosion resistance in comparison with as-cast sample. The existence form of the remaining phase affects the morphology of the corroded surface that relatively complete dissolution with homogeneous microstructure makes the sample more effective to obtain uniform corrosion form. The optimum temperature for solution treatment is 490 °C, which shows a much better corrosion resistance and uniform corrosion form after soaking for a long time.

In vitro corrosion of pure Mg in phosphate buffer solution-Influences of isoelectric point and molecular structure of amino acids

Influences of proteins on degradation of magnesium alloys are of great significance but not well understood. In particular the roles of amino acids, the basic unit of proteins in regulating the progress of biodegradation of magnesium based materials remain unclear. This study aims to investigate the impacts of alanine, glutamic acid and lysine on degradation of pure magnesium in phosphate buffer solution through SEM, XPS, FTIR, potentiodynamic polarisation curves, electrochemical impedance spectroscopy and immersion tests. The changed contents of amino acids in solutions were detected by UV-vis spectrophotometer. Results demonstrate that the charges of the selected amino acids imposed significant contribution to suppressing the degradation of pure magnesium in phosphate buffer solution. The presence of amino acids led to the formation of phosphate-based corrosion products, increasing free corrosion potential, and reduction in corrosion current density and solution pH depending on their isoelectric points and molecular structures. A plausible corrosion mechanism organised by amino acids on pure magnesium was proposed.

Manipulating the degradation behavior and biocompatibility of Mg alloy through a two-step treatment combining sliding friction treatment and micro-arc oxidation

Manipulating the degradation rate of biomedical Mg alloys has always been a challenge. In this study, a two-step treatment including sliding friction treatment (SFT) and micro-arc oxidation (MAO) was adopted to acquire a unique Mg-based architecture containing three typical layers comprising a MAO coating/nanocrystalline (NC) layer/coarse-grained (CG) matrix. It was found that the modified topmost MAO coating possessed enhanced corrosion resistance, cytocompatibility and hemocompatibility. The intermediate NC layer sandwiched between the coating and CG matrix was an ideal transition layer capable of avoiding degradation rate upsurge caused by coating breakdown; meanwhile, it provided an effective reinforcing effect on the overall mechanical strength. More importantly, the corrosion resistance of these layers was ranked in the order: MAO coating > NC layer > CG matrix. This kind of gradually increasing corrosion rate of the three layers with depth renders the two-step treatment a promising approach to design Mg-based implants possessing controllable degradation rates.

Mechanical and in vitro degradation behavior of magnesium-bioactive glass composites prepared by SPS for biomedical applications

In order to make magnesium (Mg) a successful candidate for fracture fixation devices, it is imperative to control the corrosion rate and enhance its elastic modulus. In the present work, we have prepared bioactive glass (BG) reinforced magnesium composite using spark plasma sintering (SPS). Simultaneous application of heat and pressure during SPS decreased the softening point of BG (600°C), allowing it to coat the Mg particles partially. As a result, BG was found along the Mg particle boundaries, which was confirmed by elemental mapping. Addition of BG improved microhardness and elastic modulus of Mg-BG composites. Corrosion behavior was studied by hydrogen evolution and immersion corrosion in phosphate buffered saline (PBS). After 64 h of immersion, Mg-10 wt % BG composite showed highest corrosion resistance. Quantitative micro-computed tomography (micro-CT) results indicated porosity increase in Mg-BG composites during immersion. The maximum increase in porosity (1.66%) was noticed for pure Mg while the minimum for Mg-10 wt % BG composite. MG63 cell-material interactions, using extract method, showed good cytocompatibility for Mg-10 wt % BG composite. The concentration of Mg ion in cell culture media was measured using atomic absorption spectroscopy after 24 h immersion of Mg/BG composites. The results indicated that using BG as reinforcement and SPS as sintering method; we can prepare corrosion resistant and high modulus Mg-BG composites that can be used for fabricating bone fracture fixation plates. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 107B: 352-365, 2019.

Investigation on the in vitro cytocompatibility of Mg-Zn-Y-Nd-Zr alloys as degradable orthopaedic implant materials

Mg-Zn-Y-Nd-Zr alloy has been developed as a new type of biodegradable orthopaedic implant material by the authors' research group with its excellent mechanical properties and controllable degradation rate. In this study, the cytocompatibility of Mg-Zn-Y-Nd-Zr alloy was systematically evaluated through in vitro cell culture method. MTT assay was applied to evaluate the cytotoxicity of Mg-Zn-Y-Nd-Zr alloy and no toxic effect was observed on L929 and MC3T3-E1 cells followed the protocol of ISO 10993 standard. Considering the potential ion accumulation in the bony environment, this study further investigated the cytotoxic effect of accumulated metallic ions during the alloy degradation by extending the extract preparation time. When the extract preparation time was prolonged to 1440 h, the accumulated metallic ions leaded to severe cell apoptosis, of which the combined ion concentration was determined as 39.5-65.8 µM of Mg²⁺, 3.5-5.9 µM of Zn²⁺, 0.44-0.74 µM of Y³⁺, 0.3-0.52 µM of Nd³⁺ and 0.11-0.18 µM of Zr⁴⁺ for L929, and 65.8-92.2 µM of Mg²⁺, 5.9-8.3 µM of Zn²⁺, 0.74-1.04 µM of Y³⁺, 0.52-0.73 µM of Nd³⁺ and 0.18-0.25 µM of Zr⁴⁺ for MC3T3-E1 cells. Besides the cell viability assessment, high expression of ALP activity and calcified nodules implied that metal elements in Mg-Zn-Y-Nd-Zr alloys can promote the osteogenic differentiation. Hence, excellent cytocompatibility has equipped Mg-Zn-Y-Nd-Zr alloy as a promising candidate for orthopaedic implant application, which can remarkably guide the magnesium-based alloy design and provide scientific evidence for clinical practice in future.

A study of degradation resistance and cytocompatibility of super-hydrophobic coating on magnesium

Calcium stearate based super-hydrophobic coating was deposited on plasma electrolytic oxidation (PEO) pre-treated magnesium substrate. The pre-treated magnesium and super-hydrophobic coating covered sample were characterized by scanning electron microscopy, X-ray diffraction and electrochemical corrosion measurements. The cytocompatibility and degradation resistance of magnesium, pre-treated magnesium and super-hydrophobic coating were analysed in terms of cell adhesion and osteoblast differentiation. The results indicate that the calcium stearate top coating shows super-hydrophobicity and that the surface is composed of micro/nanostructure. The super-hydrophobic coating covered sample shows higher barrier properties compared with the PEO pre-treated magnesium and bare magnesium. Human osteoblast proliferation, but not differentiation is enhanced by the PEO coating. Contrary, the super-hydrophobic coating reduces proliferation, but enhances differentiation of osteoblast, observable by the formation of hydroxyapatite. The combination of corrosion protection and cell reaction indicates that this system could be interesting for biomedical applications.

Stimulatory effects of the degradation products from Mg-Ca-Sr alloy on the osteogenesis through regulating ERK signaling pathway

The influence of Mg-1Ca-xwt.% Sr (x = 0.2, 0.5, 1.0, 2.0) alloys on the osteogenic differentiation and mineralization of pre-osteoblast MC3T3-E1 were studied through typical differentiation markers, such as intracellular alkaline phosphatase (ALP) activity, extracellular collagen secretion and calcium nodule formation. It was shown that Mg-1Ca alloys with different content of Sr promoted cell viability and enhanced the differentiation and mineralization levels of osteoblasts, and Mg-1Ca-2.0Sr alloy had the most remarkable and significant effect among all. To further investigate the underlying mechanisms, RT-PCR and Western Blotting assays were taken to analyze the mRNA expression level of osteogenesis-related genes and intracellular signaling pathways involved in osteogenesis, respectively. RT-PCR results showed that Mg-1Ca-2.0Sr alloy significantly up-regulated the expressions of the transcription factors of Runt-related transcription factor 2 (RUNX2) and Osterix (OSX), Integrin subunits, as well as alkaline phosphatase (ALP), Bone sialoprotein (BSP), Collagen I (COL I), Osteocalcin (OCN) and Osteopontin (OPN). Western Blotting results suggested that Mg-1Ca-2.0Sr alloy rapidly induced extracellular signal-regulated kinase (ERK) activation but showed no obvious effects on c-Jun N terminal kinase (JNK) and p38 kinase of MAPK. Taken together, our results demonstrated that Mg-1Ca-2.0Sr alloy had excellent biocompatibility and osteogenesis via the ERK pathway and is expected to be promising as orthopedic implants and bone repair materials.