Mechanical properties of magnesium alloys for medical application: A review

Functionalized magnesium alloys obtained by superplastic forming process retain osteoinductive and antibacterial properties: An in-vitro study

OBJECTIVES

This study aimed to investigate the biocompatibility, osteogenic and antibacterial activity of biomedical devices based on Magnesium (Mg) Alloys manufactured by Superplastic Forming process (SPF) and subjected to Hydrothermal (HT) and Sol-Gel Treatment (Sol-Gel).

METHODS

Mg-SPF devices subjected to Hydrothermal (Mg-SPF+HT) and Sol-Gel Treatment (Mg-SPF+Sol-Gel) were investigated. The biocompatibility of Mg-SPF+Sol-Gel and Mg-SPF+HT devices was observed by indirect and direct cytotoxicity assays, whereas the colonization of sample surfaces was assessed by confocal microscopy. qRT-PCR analysis and microbial growth curve analyses were employed to evaluate the osteogenic and antibacterial activity of both SPF-Mg treated devices, respectively.

RESULTS

Mg-SPF+HT and Mg-SPF+Sol-Gel showed a high degree of biocompatibility. Analysis of mRNA expression of osteogenic genes in cells cultured on Mg-treated devices revealed a significant upregulation of the expression levels of BMP2 and Runx-2. Furthermore, the bacterial growth in strains developed in contact with both the Mg-SPF+HT and Mg-SPF+Sol-Gel devices was lower than that observed in the control.

SIGNIFICANCE

Hydrothermal and Sol-Gel Treatments of Mg alloys obtained through the SPF process demonstrated bioactive, osteogenic and antibacterial activity, offering a promising alternative to conventional Mg-based devices. The obtained Mg-based materials may have the potential to enhance the tunability of temporary devices in maxillary reconstruction, eliminating the need for second surgeries, and ensuring a good bone reconstruction and a reduced implant failure rate due to bacterial infections.

Coupling biomechanical models of implants with biodegradation models: A case study for biodegradable mandibular bone fixation plates

In fracture fixation, biodegradable implant materials are an interesting alternative to conventional non-biodegradable materials as the latter often require a second implant removal surgery to avoid long-term complications. In this study, we present an in silico strategy to design/study biodegradable metal implants focusing on mandibular fracture fixation plates of WE43 (Mg alloy). The in silico strategy is composed of an orchestrated interaction between three separate computational models. The first model simulates the mass loss of the degradable implant based on the chemistry of Mg biodegradation. A second model estimates the loading on the jaw plate in the physiological environment, incorporating a phenomenological dynamic bone regeneration process. The third model characterizes the mechanical behavior of the jaw plate and the influence of material degradation on the mechanical behavior. A sensitivity analysis was performed on parameters related to choices regarding numerical implementation and parameter dependencies were implemented to guarantee robust and correct results. Different clinical scenarios were tested, related to the amount of screws used to fix the plate. The results showed a lower initial strength when more screw holes were left open, as well as a faster decrease over time in strength due to the increased area available for surface degradation. The obtained degradation results were found to be in accordance with previously reported data of in vivo studies with biodegradable plates. The combination of these three models allows for the design of patient-specific biodegradable fixation implants able to deliver the desired mechanical behavior tuned to the bone regeneration process.

Strengthening of Mg Alloy with Multiple RE Elements with Ag and Zn Doping via Heat Treatment

Strengthening Mg alloys with rare earth elements has been a research focus for several decades. To minimize the usage of rare earth elements while enhancing mechanical properties, we adopted the strategy of alloying with multiple rare earth elements, namely Gd, Y, Nd, and Sm. Additionally, to promote the precipitation of basal precipitate, Ag and Zn doping was also induced. Thus, we designed a new cast Mg-2Gd-2Y-2Nd-2Sm-1Ag-1Zn-0.5Zr (wt.%) alloy. The microstructure of the alloy and its relevance to mechanical properties in various heat treatment conditions were investigated. After undergoing a heat treatment process, the alloy demonstrated exceptional mechanical properties, with a yield strength of 228 MPa and an ultimate tensile strength of 330 MPa achieved through peak-aging for 72 h at 200 °C. The excellent tensile properties are due to the synergistic effect of basal γ″ precipitate and prismatic β' precipitate. In its as-cast state, its primary mode of fracture is inter-granular, whereas in the solid-solution and peak-aging conditions, the predominant mode of fracture is a mixture of trans-granular and inter-granular fractures.

Porous Biocoatings Based on Diatomite with Incorporated ZrO2 Particles for Biodegradable Magnesium Implants

In the present work, the surface of a biodegradable Mg alloy was modified to create porous diatomite biocoatings using the method of micro-arc oxidation. The coatings were applied at process voltages in the range of 350-500 V. We have studied the influence of the addition of ZrO₂ microparticles on the structure and properties of diatomite-based protective coatings for Mg implants. The structure and properties of the resulting coatings were examined using a number of research methods. It was found that the coatings have a porous structure and contain ZrO₂ particles. The coatings were mostly characterized by pores less than 1 μm in size. However, as the voltage of the MAO process increases, the number of larger pores (5-10 μm in size) also increases. However, the porosity of the coatings varied insignificantly and amounted to 5 ± 1%. It has been revealed that the incorporation of ZrO₂ particles substantially affects the properties of diatomite-based coatings. The adhesive strength of the coatings has increased by approximately 30%, and the corrosion resistance has increased by two orders of magnitude compared to the coatings without zirconia particles.

Bredigite-CNTs Reinforced Mg-Zn Bio-Composites to Enhance the Mechanical and Biological Properties for Biomedical Applications

Magnesium (Mg) and its compounds have been investigated as biodegradable metals for bone implants. However, high corrosion rates and low bioactivity that cause loss of mechanical properties are factors that have limited their biomedical applications. The purpose of this work is to remedy the weaknesses of the Mg-Zn (MZ) alloy matrix. For this purpose, we have synthesized Mg-based composites with different concentrations of bredigite (Br; Ca₇MgSi₄O₁₆)-carbon nanotubes (CNTs) using mechanical alloying and semi-powder metallurgy processes with spark plasma sintering. Then, we studied the effect of the simultaneous addition of Br-CNTs on in vitro degradation, as well as its effect on the composites' mechanical and antibacterial properties. Increases of 57% and 72% respectively were observed in the microhardness and compressive strength of the MZ/Br-CNTs composite in comparison to the MZ alloy. In addition, the rate of degradation of Mg-based composites in simulated body fluids (SBF) was almost 2 times lower. An assessment of antibacterial behavior disclosed that the simultaneous adding of Br-CNTs to Mg can meaningfully prevent the growth and invasion of E. coli and S. aureus. These research findings demonstrate the potential application of MZ/Br-CNTs composites to implants and the treatment of bone infections.

Microstructures, mechanical and corrosion properties of graphene nanoplatelet-reinforced zinc matrix composites for implant applications

Zinc (Zn)-based alloys and composites are gaining increasing interest as promising biodegradable implant materials due to their appropriate biodegradation rates and biological functionalities. However, the inadequate mechanical strength and ductility of pure Zn have restricted its application. In this study, Zn matrix composites (ZMCs) reinforced with 0.1-0.4 wt.% graphene nanoplatelets (GNP) fabricated via powder metallurgy were investigated as potential biodegradable implant materials. The microstructures, mechanical properties, and corrosion behaviors of the GNP-reinforced ZMCs were characterized using optical microscopy, scanning electron microscopy combined with energy-dispersive X-ray spectroscopy, Raman spectroscopy, compression testing, and electrochemical and immersion testing in Hanks' balanced salt solution (HBSS). The microstructural study revealed that the GNP was uniformly dispersed in the ZMCs after ball milling and sintering at 420°C for 6 h. The microhardness, compressive yield strength, ultimate compressive strength, and compressive strain of the ZMC-0.2GNP were 69 HV, 123 MPa, 247 MPa, and 23 %, respectively, improvements of ∼ 18 %, 50%, ∼ 28%, and ∼ 15% compared to pure Zn. The corrosion rate of the ZMCs were lower than that of the pure Zn in HBSS, and the ZMC-0.2GNP composite exhibited the lowest corrosion rate of 0.09 mm/y as measured by electrochemical testing. Biocompatibility assessment indicated that the diluted extracts of pure Zn and GNP-reinforced ZMCs with concentrations of 12.5% and 6.25% exhibited no cytotoxicity after cell culturing for up to 5 days, and the diluted extracts of ZMC-0.2 GNP composite revealed more than 90% cell viability after cell culturing of 3 days, showing the satisfying cytocompatibility. STATEMENT OF SIGNIFICANCE: Biodegradable Zn is a promising candidate material for orthopedic implant applications. Nonetheless, the inadequate mechanical strength and ductility of pure Zn limited its clinical application. In this study, Zn matrix composites (ZMCs) reinforced with 0.1-0.4 wt.% graphene nanoplatelets (GNP) were developed via powder metallurgy, and the reinforcing efficacy of GNP on their mechanical properties was investigated. The addition of GNP significantly improved the compressive properties of ZMCs, with the Zn-0.2GNP composite exhibiting the best compressive properties, including 123 MPa compressive yield strength, 247 MPa ultimate compressive strength, and 22.9% compressive strain. Further, the 12.5% concentration extract of the ZMCs exhibited no cytotoxicity after cell culturing for 5 d toward SaOS2 cells.

Assessment of the Mechanical and Corrosion Properties of Mg-1Zn-0.6Ca/Diamond Nanocomposites for Biomedical Applications

In this work, the microstructure, mechanical properties, and corrosion behavior of the Mg-1Zn-0.6Ca matrix alloy (ZX10), reinforced by adding various amounts of nanodiamond particles (0.5, 1, and 2 wt.%), prepared by the ultrasound-assisted stir-casting method, were investigated as they are deemed as potential implant materials in biomedical applications. Microstructure, nanoindentation, mechanical tensile, immersion, and potentiodynamic polarization tests were performed for evaluating the influence of the addition of nanodiamond particles on the alloy's mechanical and biocorrosion properties. The results revealed that the addition of nanodiamond particles causes a reduction in the alloy's grain size. The alloy's nanohardness and elastic modulus values increased when the amount of added nanodiamond particles were increased. The nanocomposite with an addition of 0.5% ND showed the best composition with regard to an acceptable corrosion rate as the corrosion rates are too high with higher additions of 1 or 2% NDs. At the same time, the yield strength, tensile strength, and elongation improved slightly compared to the matrix alloy.

Recent advances on the mechanical behavior of zinc based biodegradable metals focusing on the strain softening phenomenon

Zinc based biodegradable metals (BMs) show great potential to be used in various biomedical applications, owing to their superior biodegradability and biocompatibility. Some high-strength (ultimate tensile strength > 600 MPa) Zn based BMs have already been developed through alloying and plastic working, making their use in load-bearing environments becomes a reality. However, different from Mg and Fe based BMs, Zn based BMs exhibit significant "strain-softening" effect that leads to limited uniform deformation. Non-uniform deformation is detrimental to Zn based devices or implants, which will possibly lead to unexpected failure. People might be misled by the considerable fracture elongation of Zn based BMs. Thus, it is important to specify uniform elongation as a term of mechanical requirements for Zn based BMs. In this review, recent advances on the mechanical properties of Zn based BMs have been comprehensively summarized, especially focusing on the strain softening phenomenon. At first, the origin and evaluation criteria of strain softening were introduced. Secondly, the effects of alloying elements (including element type, single or multiple addition, and alloying content) and microstructural characteristics (grain size, constituent phase, phase distribution, etc.) on mechanical properties (especially for uniform elongation) of Zn based BMs were summarized. Finally, how to get a good balance between strength and uniform elongation was generally discussed based on the service environment. In addition, possible ways to minimize or eliminate the strain softening effect were also proposed, such as controlling of twins, solute clusters, and grain boundary characteristics. All these items above would be helpful to understand the mechanical instability of Zn based BMs, and to make the full usage of them in the future medical device design. STATEMENT OF SIGNIFICANCE: Biodegradable metals (BMs) is a hotspot in the field of metallic biomaterials. Fracture elongation is normally adopted to quantify the deformability of Mg and Fe based BMs owing to their negligible necking strain, yet the strain softening would occur in Zn based BMs, which is extremely detrimental to performance of their medical device. In this review paper, a better understanding the mechanical performance of Zn-based BMs with the term "uniform elongation" instead of "fracture elongation" was depicted, and possible ways to minimize or eliminate the strain softening effect were also proposed, such as twins, solute clusters, self-stable dislocation network, and grain boundary characteristics. It would be helpful to understand the mechanical instability of Zn based BMs and making full usage of it in the future medical device design.

Effect and mechanism of a novel Mg-Nd-Gd-Sr alloy on osteogenic differentiation of bone marrow mesenchymal stem cells

We investigated the effect and mechanism of a novel Mg-3Nd-1Gd-0.3Sr-0.2Zn-0.4Zr (abbreviated to Mg-Nd-Gd-Sr) alloy on the osteogenic differentiation of bone marrow mesenchymal stem cells extracted from Sprague-Dawley rats. Cultured cells were divided into five groups: a control group cultured in osteogenic induction medium alone without Mg-Nd-Gd-Sr alloy extract, and four experimental groups cultured in the same medium with 25%, 50%, 75%, and 100% Mg-Nd-Gd-Sr alloy extracts, respectively. After 14 days of culture, ALP activity was determined and expressions of osteogenesis-related factors Runx2, OCN, and OPN at the mRNA level and Runx2, OCN, and OPN at the protein level were detected by RT-PCR and western blot, respectively. After 21 days of culture, mineralized nodules were detected by alizarin red staining. The results showed that bone marrow mesenchymal stem cells from Sprague-Dawley rats were successfully isolated in vitro using the whole bone marrow adherence method. Flow cytometry revealed that the cells expressed high levels of CD44 and CD90, but low levels of CD31 and CD45. Alizarin red staining indicated the formation of mineralized nodules in all five groups. Compared with the control group, the number of mineralized nodules was increased significantly in the four experimental groups (p < 0.05). The ALP activity in each group was significantly higher on day 14 than on day 7, and was significantly higher in the four experimental groups compared with the control group (p < 0.05). Moreover, the ALP activity was highest when the concentration of Mg-Nd-Gd-Sr alloy extract was 75% (p < 0.05). RT-PCR results showed that, compared with the control group, the mRNA expression of Runx2, OPN, and OCN was significantly higher in the four experimental groups (p < 0.05), and the highest mRNA expression of Runx2, OPN, and OCN was observed in the 75% experimental group (p < 0.05). Western blotting showed that Mg-Nd-Gd-Sr alloy extract significantly increased the protein expression of Runx2, OCN, and OPN compared with the control group (p < 0.05). Our data indicate that the novel Mg-Nd-Gd-Sr alloy can promotes the osteogenic differentiation of bone marrow mesenchymal stem cells isolated from Sprague-Dawley rats. During this process, there is an increase in the expressions of Runx2, OPN, and OCN mRNAs and Runx2, OCN, and OPN proteins.

A review on magnesium alloys for biomedical applications

Magnesium (Mg) and Mg alloys are considered as potential candidates for biomedical applications because of their high specific strength, low density, and elastic modulus, degradability, good biocompatibility and biomechanical compatibility. However, the rapid corrosion rate of Mg alloys results in premature loss of mechanical integrity, limiting their clinical application in load-bearing parts. Besides, the low strength of Mg alloys restricts their further application. Thus, it is essential to understand the characteristics and influencing factors of mechanical and corrosion behavior, as well as the methods to improve the mechanical performances and corrosion resistance of Mg alloys. This paper reviews the recent progress in elucidating the corrosion mechanism, optimizing the composition, and microstructure, enhancing the mechanical performances, and controlling the degradation rate of Mg alloys. In particular, the research progress of surface modification technology of Mg alloys is emphasized. Finally, the development direction of biomedical Mg alloys in the future is prospected.

Mapping knowledge structure and themes trends of biodegradable Mg-based alloy for orthopedic application: A comprehensive bibliometric analysis

Background: Since Lambotte and Payr first studied Mg-based alloys for orthopedics in 1900, the research of this field has finally ushered in vigorous development in the 21st century. From the perspective of quantitative analysis, this paper clearly demonstrated the global research trend from 2005 to 2021 by using bibliometrics and scientometric analysis.

Methods: We obtained the publications from the Web of Science Core Collection (WoSCC) database. The bibliometric and scientometric analysis was conducted by using R software, CiteSpace software, VOSviewer software, Pajek software and Microsoft Excel program.

Results: In total, 1921 publications were retrieved. It can be found that the number of publications is gradually increasing year by year. We can find that the most prolific countrie, institution and researcher are China, Chinese Academy of Sciences and Zheng Yufeng, respectively. The most influential journals in this field are Acta Biomaterialia and Biomaterials, with 16,511 and 12,314 total citations, respectively. By conducting the co-cited documents-based clustering analysis, 16 research hotspots and their representative studies have been identified. Besides, by conducting analysis of keywords, we divided the keyword citation bursts representing the development of the field into three stages.

Conclusion: The number of researches on the biodegradable Mg-based alloys increased sharply all over the world in the 21st century. China has made significant progress in biodegradable Mg-based alloy research. More focus will be placed on osteogenic differentiation, fabrication, graphene oxide, antibacterial property, bioactive glass and nanocomposite, which may be the next popular topics in the field.

The Role of the Sol-Gel Synthesis Process in the Biomedical Field and Its Use to Enhance the Performance of Bioabsorbable Magnesium Implants

In the present day, the increment in life expectancy has led to the necessity of developing new biomaterials for the restoration or substitution of damaged organs that have lost their functionalities. Among all the research about biomaterials, this review paper aimed to expose the main possibilities that the sol-gel synthesis method can provide for the fabrication of materials with interest in the biomedical field, more specifically, when this synthesis method is used to improve the biological properties of different magnesium alloys used as biomaterials. The sol-gel method has been widely studied and used to generate ceramic materials for a wide range of purposes during the last fifty years. Focused on biomedical research, the sol-gel synthesis method allows the generation of different kinds of biomaterials with diverse morphologies and a high potential for the biocompatibility improvement of a wide range of materials commonly used in the biomedical field such as metallic implants, as well as for the generation of drug delivery systems or interesting biomaterials for new tissue engineering therapies.

Mg2+ -mediated autophagy-dependent polarization of macrophages mediates the osteogenesis of bone marrow stromal stem cells by interfering with macrophage-derived exosomes containing miR-381

Magnesium ion (Mg²⁺ ) has received increased attention due to the roles it plays in promoting osteogenesis and preventing inflammation. This study was designed to investigate the mechanism by which Mg²⁺ influences the osteoblastic differentiation of bone marrow stromal stem cells (BMSCs). The polarization of Mø (macrophages) was measured after treatment with Mg²⁺ . Meanwhile, autophagy in Mø was measured by detecting LC3B expression. Mø-derived exosomes were isolated and cocultured with BMSCs; after which, osteogenic differentiation was evaluated by Alizarin Red staining and detection of alkaline phosphatase (ALP). Our results showed that Mg²⁺ could induce autophagy in macrophages and modulate the M1/M2 polarization of macrophages. Mg²⁺ -mediated macrophages could facilitate the osteogenic differentiation of BMSCs by regulating autophagy, and this facilitation by Mg²⁺ -mediated macrophages was closely related to macrophage-derived exosomes, and especially exosomes containing miR-381. However, miR-381 in macrophages did not influence autophagy or the polarization of Mg²⁺ -mediated macrophages. Furthermore, macrophage-derived exosomes containing miR-381 mainly determined the osteogenic differentiation of BMSCs. Mg²⁺ -mediated macrophages were shown to promote the osteogenic differentiation of BMSCs via autophagy through reducing miR-381 in macrophage-derived exosomes. In conclusion, our results suggest Mg²⁺ -mediated macrophage-derived exosomes containing miR-381 as novel vehicles for promoting the osteogenic differentiation of BMSCs.

Simulating In Vitro the Bone Healing Potential of a Degradable and Tailored Multifunctional Mg-Based Alloy Platform

This work intended to elucidate, in an in vitro approach, the cellular and molecular mechanisms occurring during the bone healing process, upon implantation of a tailored degradable multifunctional Mg-based alloy. This was prepared by a conjoining anodization of the bare alloy (AZ31) followed by the deposition of a polymeric coating functionalized with hydroxyapatite. Human endothelial cells and osteoblastic and osteoclastic differentiating cells were exposed to the extracts from the multifunctional platform (having a low degradation rate), as well as the underlying anodized and original AZ31 alloy (with higher degradation rates). Extracts from the multifunctional coated alloy did not affect cellular behavior, although a small inductive effect was observed in the proliferation and gene expression of endothelial and osteoblastic cells. Extracts from the higher degradable anodized and original alloys induced the expression of some endothelial genes and, also, ALP and TRAP activities, further increasing the expression of some early differentiation osteoblastic and osteoclastic genes. The integration of these results in a translational approach suggests that, following the implantation of a tailored degradable Mg-based material, the absence of initial deleterious effects would favor the early stages of bone repair and, subsequently, the on-going degradation of the coating and the subjacent alloy would increase bone metabolism dynamics favoring a faster bone formation and remodeling process and enhancing bone healing.

The Microstructure Evolution of Mg-RE Alloy Produced by Reciprocating Upsetting Extrusion during Hot Compression

Mg-13Gd-4Y-2Zn-0.4Zr (wt. %) alloy bar produced by three passes reciprocating upsetting extrusion (named as RUE-ed bar) exhibited fine grain with the average grain size of 3.02 μm. Hot compression tests of the RUE-ed bar were carried out on Gleeble-3800 compression unit at different deformation temperatures (653, 683, 713, and 743 K) and strain rates (0.001–1 s, 0.01–1 s, 0.1–1 s, and 0.5–1 s). This alloy showed work hardening and softening stages in hot compression, the thermal activation energy of the RUE-ed bar was 150 ± 1 kJ/mol and the constitutive equation was: ε˙=1.80×109[sinh(0.0174σ)]2.47exp[−150×1038.314×T]. Numerous Mg5 (Gd, Y, Zn) phase re-dissolved in α-Mg matrix appeared in the RUE-ed samples during hot compression deformation. The movement of the dislocation stimulated the re-dissolution of the Mg5 (Gd, Y, Zn) phase. The re-dissolution of Mg5 (Gd, Y, Zn) phase promoted texture strengthening and DRX grains growth in this experiment. In addition, the transformation and kinking of LPSO phase played an important coordinating role in the process of hot compression; 18R-LPSO was changed to 14H-LPSO phase at low strain rate while the LPSO phase kinked dominant to coordinated deformation at high strain rate.

A Systematic Review and Network Meta-Analysis of Biomedical Mg Alloy and Surface Coatings in Orthopedic Application

Magnesium alloys have great application prospects as ideal bone implant materials. However, their poor corrosion resistance limits their clinical orthopedic application. Surface modification promotes the corrosion resistance of magnesium. Conversion coatings, such as calcium phosphate (Ca-P) coating, microarc oxidation (MAO) treatment, and fluoride (FLU) treatment, have been extensively investigated in in vivo studies. This systematic review and network meta-analysis compared the influence of different conversion coatings on bone repair, material properties, and systemic host response in orthopedic applications. Using the PICOS model, the inclusion criteria for biodegradable magnesium and its alloys were determined for in vivo studies. Four databases were used. The standard and weight mean differences with 95% confidence intervals were used to analyze new bone formation and degradation rate. Network structure and forest plots were created, and ranking probabilities were estimated. The risk of bias and quality of evidence were assessed using SYRCLE, CERQual, and GRADE tools. In the qualitative analysis, 43 studies were selected, and the evaluation of each outcome indicator was not entirely consistent from article to article. In the quantitative analysis, 21 articles were subjected to network meta-analysis, with 16 articles on implant degradation and 8 articles for new bone formation. Additionally, SUCRA indicated that Ca-P coating exhibited the highest corrosion resistance, followed by FLU treatment. MAO demonstrated the best capability for new bone formation, followed by Ca-P coating. Ca-P coating exhibited the highest overall performance. To conclude, coated Mg can promote better new bone formation than bare Mg and has considerable biocompatibility. Ca-P-coated Mg and MAO-coated Mg have the greatest potential to significantly promote corrosion resistance and bone regeneration, respectively. The findings of this study will provide a theoretical basis for the investigation of composite coatings and guidance for the orthopedic application of Mg bone implants.

In Vitro Corrosion Performance of As-Extruded Mg–Gd–Dy–Zr Alloys for Potential Orthopedic Applications

In this study, different contents of rare earth elements with high solid solubility (Gd and Dy) were added into Mg and fabricated through homogenization and hot extrusion processes that enable few second phase formation to efficaciously inhibit the galvanic corrosion. The microstructure and phase characterization of the as-extruded Mg–Gd–Dy–Zr alloys were analyzed by scanning electron microscopy, electron backscattered diffraction, and X-ray diffraction. The in vitro biodegradation behavior of the as-extruded Mg–Gd–Dy–Zr alloys was investigated via the electrochemical measurement and immersion test. The results revealed that all the as-extruded alloys with different RE additions exerted fully recrystallized microstructures. The average grain size was appropriately 20 μm to 30 μm for all alloys and gradually increased by adding more RE. Only a few tiny second-phase particles less than 5 μm dispersed for all the samples and the volume fraction of particles increased slightly with the increase in RE content. The as-extruded Mg–Gd–Dy–Zr alloys with low RE content (GD0.6) allowed for a satisfactory corrosion resistance in Hank’s solution with a controlled corrosion rate less than 0.5 mm/year, which is considered as the tolerance limit for the corrosion rate of orthopedic implants. This study provides a cost-effective choice for promoting biodegradable magnesium alloys for potential orthopedic applications with low rare earth content in Mg alloys.

Making Hardware Removal Unnecessary by Using Resorbable Implants for Osteosynthesis in Children

Introduction: Following osteosynthesis, children generally require a second surgery to remove the hardware. This becomes unnecessary, by using resorbable implants. Limiting the number of required surgeries and their associated risks, this technique provides critical aspects of minimally invasive surgery. This review focuses on resorbable implants for osteosynthesis for the treatment of fractures in children and discusses their clinical features. Method: We provide an overview of the two most common technologies used in resorbable osteosynthesis materials: polymer- and magnesium-based alloys. Clinical examples of osteosynthesis are presented using polymer-based ActivaTM products and magnesium-based Magnezix^® products. Results: Polymer-based implants demonstrate surgical safety and efficacy. Due to their elasticity, initial placement of polymer-based products may demonstrate technical challenges. However, stability is maintained over the course of healing. While maintaining good biocompatibility, the rate of polymer-resorption may be controlled by varying the composition of polyesters and copolymers. Similarly, magnesium-based implants demonstrate good mechanical stability and resorption rates, while these characteristics may be controlled by varying alloy components. One of the significant shortcomings of magnesium is that metabolism results in the production of hydrogen gas. Both technologies provide equally good results clinically and radiographically, when compared to non-resorbable implants. Conclusion: Resorbable osteosynthesis materials demonstrate similar therapeutic results as conventional materials for osteosynthesis. Resorbable implants may have the potential to improve patient outcomes, by sparing children a second surgery for hardware removal.

Bone Union Quality after Fracture Fixation of Mandibular Head with Compression Magnesium Screws

For some years now, fixation devices created with resorbable magnesium alloys for the mandibular head have been clinically available and are beginning to be used. It is thus valuable to evaluate the quality of unions in these cases. The aim of this study was radiological comparison of magnesium versus titanium open reduction and rigid fixations in the mandible condylar head. Thirty-one patients were treated for fractures of the mandibular head with magnesium WE43 alloy headless compression screws (diameter 2.3 mm) and, as a reference group, 29 patients were included with similar construction titanium screws (diameter 1.8 mm). The 12-month results of the treatment were evaluated by the texture analysis of CT. Near similar treatment results were found with magnesium screws in traditional titanium fixation. Magnesium screws result in a higher density of the bone structure in the mandibular head. Conclusions: The quantitative evaluation of bone union after surgical treatment of mandibular head fracture with magnesium compression headless screws indicates that stable consolidation was achieved. Undoubtedly, the resorption process of the screws was found to be incomplete after 12 months, evidenced by a marked densification of the bone structure at the fracture site.

Biodegradable Mg-Zn-Ca-Based Metallic Glasses

Biodegradable Mg-Zn-Ca-based metallic glasses (MGs) present improved strength and superior corrosion resistance, compared to crystalline Mg. In particular, in vivo and in vitro attempts reveal that biodegradable Mg-Zn-Ca-based MGs possess excellent biocompatibility, suggesting that they are ideal candidates for temporary implant materials. However, the limited size and severe brittleness prevent their widespread commercialization. In this review, we firstly summarize the microstructure characteristic and mechanical properties of Mg-Zn-Ca-based MGs. Then, we provide a comprehensive and systematic understanding of the recent progress of the biocorrosion and biocompatibility of Mg-Zn-Ca-based MGs. Last, but not least, the outlook towards the fabrication routes, composition design, structure design, and reinforcement approaches of Mg-Zn-Ca-based MGs are briefly proposed.

Tailoring of Biodegradable Magnesium Alloy Surface with Schiff Base Coating via Electrostatic Spraying for Better Corrosion Resistance

In this study, three new Schiff bases were synthesized from paeonol and amino acids to prepare a compound Schiff base coating on the Mg-Zn-Y-Nd alloy (ZE21B alloy) surface by electrostatic spraying, and these three single Schiff base coatings were prepared on the ZE21B alloy as control. The results of SEM and XPS confirmed the successful preparation of the coating. Immersion tests and electrochemical tests showed that both the single coating and the compound coating significantly improved the corrosion resistance of ZE21B alloy, and the compound coating could play a synergistic corrosion inhibition effect, thus showing the best corrosion resistance.

A review of current challenges and prospects of magnesium and its alloy for bone implant applications

Medical application materials must meet multiple requirements, and the designed implant must mimic the bone structure in shape and support the formation of bone tissue (osteogenesis). Magnesium (Mg) alloys, as a "smart" biodegradable material and as "the green engineering material in the twenty-first century", have become an outstanding bone implant material due to their natural degradability, smart biocompatibility, and desirable mechanical properties. Magnesium is recognised as the next generation of orthopaedic appliances and bioresorbable scaffolds. At the same time, improving the mechanical properties and corrosion resistance of magnesium alloys is an urgent challenge to promote the application of magnesium alloys. Nevertheless, the excessively quick deterioration rate generally results in premature mechanical integrity disintegration and local hydrogen build-up, resulting in restricted clinical bone restoration applicability. The condition of Mg bone implants is thoroughly examined in this study. The relevant approaches to boost the corrosion resistance, including purification, alloying treatment, surface coating, and Mg-based metal matrix composite, are comprehensively revealed. These characteristics are reviewed to assess the progress of contemporary Mg-based biocomposites and alloys for biomedical applications. The fabricating techniques for Mg bone implants also are thoroughly investigated. Notably, laser-based additive manufacturing fabricates customised forms and complicated porous structures based on its distinctive additive manufacturing conception. Because of its high laser energy density and strong controllability, it is capable of fast heating and cooling, allowing it to modify the microstructure and performance. This review paper aims to provide more insight on the present challenges and continued research on Mg bone implants, highlighting some of the most important characteristics, challenges, and strategies for improving Mg bone implants.

Can Hardware Removal be Avoided Using Bioresorbable Mg-Zn-Ca Screws After Medial Malleolar Fracture Fixation? Mid-Term Results of a First-In-Human Study

Ankle is the most common site of hardware removal, mainly performed within 12 months of the primary surgery. The prominence of the metallic hardware is a frequent cause of pain after fracture fixation. Over the last decade, the development of bioresorbable materials based on magnesium (Mg) has increased. Bioresorbable metals aim to avoid a second surgery for hardware removal.

METHODS

Twenty patients with isolated, bimalleolar, or trimalleolar ankle fractures were treated with bioresorbable screws made of Mg, 0.45wt% calcium (Ca) and 0.45wt% zinc (Zn) (ZX00). Patient-reported outcome measures (PROMs) including visual analogue scale (VAS) for pain, the presence of complications 6 and 12 months after surgery and the AOFAS scale after 12 months were reported. The functional outcomes were analysed through the range of motion (ROM) of the ankle joint with a standard goniometer. Degradation products and the bioresorbability of the screws were evaluated using plane radiographs.

RESULTS

One patient was lost to follow-up. All patients were free of pain, no complications, shoe conflict or misalignement were reported after 12 months of follow-up. No Mg screws were surgically removed. An additional fixation of the distal fibula or the dorsal tibial fragment with conventional titanium implants (Ti) was performed in 17 patients. Within 12 months after primary refixation, 12 of these patients (71%) underwent a second surgery for Ti hardware removal. The mean AOFAS score was 89.8±7.1 and the difference between the treated and the non-treated site in the ROM of the talocrural joint was 2°±11° after 12 months. Radiolucent areas around the screws were attributed to degradation and did not affect clinical or functional outcomes. After one year, the Mg screw heads could not be detected in the plane radiographs of 17 patients which suggests that the majority of the screw head is degraded without introducing adverse reactions.

CONCLUSIONS

At 6 and 12 months, the bioresorbable Mg screws show excellent PROMs without complications or need for screw removal. The resorbability of the screw heads in most of the patients after one year could also provide an advantage over conventional bio-inert implants by avoiding related skin irritation due for instance to shoe conflict.

Effect of Vacancies on Dynamic Response and Spallation in Single-Crystal Magnesium by Molecular Dynamic Simulation

The effect of vacancies on dynamic response and spallation in single-crystal magnesium (Mg) is investigated by nonequilibrium molecular dynamics simulations. The initial vacancy concentration (Cv) ranges from 0% to 2.0%, and the shock loading is applied along [0001] and [10–10] directions. The simulation results show that the effects of vacancy defects are strongly dependent on the shock directions. For shock along the [0001] direction, vacancy defects have a negligible effect on compression-induced plasticity, but play a role in increasing spall damage. In contrast, for shock along the [10–10] orientation, vacancy defects not only provide the nucleation sites for compression-induced plasticity, which mainly involves crystallographic reorientation, phase transition, and stacking faults, but also significantly reduce spall damage. The degree of spall damage is probably determined by a competitive mechanism between energy absorption and stress attenuation induced by plastic deformation. Void evolution during spallation is mainly based on the emission mechanism of dislocations. The {11–22} <11–23> pyramidal dislocation facilitates the nucleation of void in the [0001] shock, as well as the {1–100} <11–20> prismatic dislocation in the [10–10] shock. We also investigated the variation of spall strength between perfect and defective Mg at different shock velocities. The relevant results can provide a reference for future investigations on spall damage.

Corrosion Resistance and Cytocompatibility of Magnesium-Calcium Alloys Modified with Zinc- or Gallium-Doped Calcium Phosphate Coatings

In orthopedic surgery, metals are preferred to support or treat damaged bones due to their high mechanical strength. However, the necessity for a second surgery for implant removal after healing creates problems. Therefore, biodegradable metals, especially magnesium (Mg), gained importance, although their extreme susceptibility to galvanic corrosion limits their applications. The focus of this study was to control the corrosion of Mg and enhance its biocompatibility. For this purpose, surfaces of magnesium-calcium (MgCa1) alloys were modified with calcium phosphate (CaP) or CaP doped with zinc (Zn) or gallium (Ga) via microarc oxidation. The effects of surface modifications on physical, chemical, and mechanical properties and corrosion resistance of the alloys were studied using surface profilometry, goniometry, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), nanoindentation, and electrochemical impedance spectroscopy (EIS). The coating thickness was about 5-8 μm, with grain sizes of 43.1 nm for CaP coating and 28.2 and 58.1 nm for Zn- and Ga-doped coatings, respectively. According to EIS measurements, the capacitive response (Y_c) decreased from 11.29 to 8.72 and 0.15 Ω^-1 cm^-2 sⁿ upon doping with Zn and Ga, respectively. The E_corr value, which was -1933 mV for CaP-coated samples, was found significantly electropositive at -275 mV for Ga-doped ones. All samples were cytocompatible according to indirect tests. In vitro culture with Saos-2 cells led to changes in the surface compositions of the alloys. The numbers of cells attached to the Zn-doped (2.6 × 10⁴ cells/cm²) and Ga-doped (6.3 × 10⁴ cells/cm²) coatings were higher than that on the surface of the undoped coating (1.0 × 10³ cells/cm²). Decreased corrosivity and enhanced cell affinity of the modified MgCa alloys (CaP coated and Zn and Ga doped, with Ga-doped ones having the greatest positive effect) make them novel and promising candidates as biodegradable metallic implant materials for the treatment of bone damages and other orthopedic applications.

Fluoride Treatment and In Vitro Corrosion Behavior of Mg-Nd-Y-Zn-Zr Alloys Type

Fluoride conversion coatings on Mg present many advantages, among which one can find the reduction of the corrosion rate under “in vivo” or “in vitro” conditions and the promotion of the calcium phosphate deposition. Moreover, the fluoride ions released from MgF₂ do not present cytotoxic effects and inhibit the biofilm formation, and thus these treated alloys are very suitable for cardiovascular stents and biodegradable orthopedic implants. In this paper, the biodegradation behavior of four new magnesium biodegradable alloys that have been developed in the laboratory conditions, before and after surface modifications by fluoride conversion (and sandblasting) coatings, are analyzed. We performed structural and surface analysis (XRD, SEM, contact angle) before and after applying different surface treatments. Furthermore, we studied the electrochemical behavior and biodegradation of all experimental samples after immersion test performed in NaCl solution. For a better evaluation, we also used LM and SEM for evaluation of the corroded samples after immersion test. The results showed an improved corrosion resistance for HF treated alloy in the NaCl solution. The chemical composition, uniformity, thickness and stability of the layers generated on the surface of the alloys significantly influence their corrosion behavior. Our study reveals that HF treatment is a beneficial way to improve the biofunctional properties required for the studied magnesium alloys to be used as biomaterials for manufacturing the orthopedic implants.

A comprehensive review on metallic implant biomaterials and their subtractive manufacturing

There is a tremendous increase in the demand for converting biomaterials into high-quality industrially manufactured human body parts, also known as medical implants. Drug delivery systems, bone plates, screws, cranial, and dental devices are the popular examples of these implants - the potential alternatives for human life survival. However, the processing techniques of an engineered implant largely determine its preciseness, surface characteristics, and interactive ability with the adjacent tissue(s) in a particular biological environment. Moreover, the high cost-effective manufacturing of an implant under tight tolerances remains a challenge. In this regard, several subtractive or additive manufacturing techniques are employed to manufacture patient-specific implants, depending primarily on the required biocompatibility, bioactivity, surface integrity, and fatigue strength. The present paper reviews numerous non-degradable and degradable metallic implant biomaterials such as stainless steel (SS), titanium (Ti)-based, cobalt (Co)-based, nickel-titanium (NiTi), and magnesium (Mg)-based alloys, followed by their processing via traditional turning, drilling, and milling including the high-speed multi-axis CNC machining, and non-traditional  abrasive water jet machining (AWJM), laser beam machining (LBM), ultrasonic machining (USM), and electric discharge machining (EDM) types of subtractive manufacturing techniques. However, the review further funnels down its primary focus on Mg, NiTi, and Ti-based alloys on the basis of the increasing trend of their implant applications in the last decade due to some of their outstanding properties. In the recent years, the incorporation of cryogenic coolant-assisted traditional subtraction of biomaterials has gained researchers' attention due to its sustainability, environment-friendly nature, performance, and superior biocompatible and functional outcomes fitting for medical applications. However, some of the latest studies reported that the medical implant manufacturing requirements could be more remarkably met using the non-traditional subtractive manufacturing approaches. Altogether, cryogenic  machining among the traditional routes and EDM among the non-traditional means along with their variants, were identified as some of the most effective subtractive manufacturing techniques for achieving the dimensionally accurate and biocompatible metallic medical implants with significantly modified surfaces.

Effects of Mg-Nd-Gd-Sr alloy on bone marrow mesenchymal stem cell function derived from SD rats

Introduction

The aim of this study was to determine the effect of a new type of Mg-3Nd-1Gd-0.3Sr-0.2Zn-0.4Zr (abbreviated to Mg-Nd-Gd-Sr) alloy on bone marrow mesenchymal stem cell (BMSCs) function derived from SD rats.

Methods

BMSCs were first isolated and cultured in vitro using the whole bone marrow adherence method, and identified by BMSC surface biomarkers and osteogenic induction. The in vitro biological safety of the Mg-Nd-Gd-Sr alloy was studied by cytotoxicity and apoptosis experiments, and the in vitro biological functions were studied by cell adhesion and cell proliferation experiments.

Results

The results showed that high-purity BMSCs were isolated using the whole bone marrow adherence method. Flow cytometry showed high expression of CD44 and CD90, and low expression of CD31 and CD45 in the BMSCs. Osteogenic induction showed that the BMSCs differentiated into osteoblasts, and mineralized nodules were observed. The cytotoxicity of the Mg-Nd-Gd-Sr alloy to SD rat BMSCs was 0–1 grade, suggesting that the Mg-Nd-Gd-Sr alloy had no significant cytotoxic effect on SD rat BMSCs; compared with the control group, there was no significant cell apoptosis in any of the experimental groups ( p > 0.05). Cell adhesion experiments showed that the number of adherent cells increased with the duration of culture with the exception of the 100% concentration group; compared with the control group, the 75% concentration group had the highest number of adherent cells at the 1st, 3rd, 5th, and 7th hours ( p < 0.05). Cell proliferation experiments showed that the number of cells in all experiment groups was higher than the control group ( p < 0.05) on the 1st, 3rd, 5th, and 7th days, with the highest number of cells in the 75% concentration group ( p < 0.05).

Conclusion

Our data indicate that the extracts of new type of Mg-Nd-Gd-Sr alloy has no apparent cytotoxicity to BMSCs, does not affect cell apoptosis, and has good biocompatibility. Different concentrations of Mg-Nd-Gd-Sr alloy extracts promoted the adhesion and proliferation of BMSCs. The alloy had good biological functions, and is thus a promising bone repair material.

Nutrient alloying elements in biodegradable metals: a review

As a new generation of biomedical metallic materials, biodegradable metals have become a hot research topic in recent years because they can completely degrade in the human body, thus preventing secondary surgery, and reducing the pain and economic burden for patients. Clinical applications require biodegradable metals with adequate mechanical properties, corrosion resistance, and biocompatibility. Alloying is an important method to create biodegradable metals with required and comprehensive performances. Since nutrient elements already have important effects on various physiological functions of the human body, the alloying of nutrient elements with biodegradable metals has attracted much attention. The present review summarizes and discusses the effects of nutrient alloying elements on the mechanical properties, biodegradation behavior, and biocompatibility of biodegradable metals. Moreover, future research directions of biodegradable metals with nutrient alloying elements are suggested.

Clinical translation and challenges of biodegradable magnesium-based interference screws in ACL reconstruction

As one of the most promising fixators developed for anterior cruciate ligament (ACL) reconstruction, biodegradable magnesium (Mg)-based interference screws have gained increasing attention attributed to their appropriate modulus and favorable biological properties during degradation after surgical insertion. However, its fast degradation and insufficient mechanical strength have also been recognized as one of the major causes to limit their further application clinically. This review focused on the following four parts. Firstly, the advantages of Mg or its alloys over their counterparts as orthopaedic implants in the fixation of tendon grafts in ACL reconstruction were discussed. Subsequently, the underlying mechanisms behind the contributions of Mg ions to the tendon-bone healing were introduced. Thirdly, the technical challenges of Mg-based interference screws towards clinical trials were discussed, which was followed by the introduction of currently used modification methods for gaining improved corrosion resistance and mechanical properties. Finally, novel strategies including development of Mg/Titanium (Ti) hybrid fixators and Mg-based screws with innovative structure for achieving clinically customized therapies were proposed. Collectively, the advancements in the basic and translational research on the Mg-based interference screws may lay the foundation for exploring a new era in the treatment of the tendon-bone insertion (TBI) and related disorders.

Electrochemical Pourbaix diagrams of Mg-Zn alloys from first-principles calculations and experimental thermodynamic data

Mg-Zn alloys have attracted much attention as biodegradable alloys owing to their superior mechanical properties and excellent biocompatibility. However, their corrosion/degradation behaviour has become a major issue for various biomedical applications. To understand their corrosion behaviours in aqueous environments, the first-principles informed Pourbaix diagrams, that is, electrochemical phase diagrams with respect to electrode potential and solution pH, were constructed for Mg-Zn alloys and compared with experimental observations. It was found that for Mg-rich alloys, the MgZn phase has a higher potential than the Mg matrix and may act as a cathode, resulting in galvanic corrosion, while for Zn-rich alloys, the phase Mg₂Zn₁₁ corrodes first. In Zn-rich alloys, Mg(OH)₂ preferably precipitates under alkaline conditions, thus hindering the increase in pH and preventing the release of dissolved ZnO₂^2- ions. In a Cl-containing solution, the soluble ZnCl₂ eases the corrosion of the Zn matrix by decreasing the corrosion potential. These results are supported by various experimental observations; thus, they provide an in-depth understanding of the degradation behaviour of various Mg-Zn alloys as well as a feasible pathway in the design of biocompatible Mg-Zn alloys with first-principles informed Pourbaix diagrams.

Longitudinally centered embossed structure in the locking compression plate for biodegradable bone implant plate: a finite element analysis

In the current revolution of internal fixation implant in orthopaedics, a biodegradable implant is the most awaited and exceptional medical device where biodegradable material has paid more attention to the success of a biodegradable implant than the design of a biodegradable bone implant plate. By far, LCP is the most traditionally used implant plate (using non-biodegradable material) because of its experimental success, but not with qualified biodegradable material (Mg-alloy). This lack of mechanical performance is a major drawback that can be rectified by better structural design. This will help avoid few other problems as well. Therefore, with proper consideration, the LCP has been added to a semicircular filleted longitudinally centered embossed (LCE) structure to enhance overall mechanical performance that can help emphasize mechanical support even after continuous degradation when applied in a physiological environment. For mechanical verification of this advanced design of biodegradable bone implant plate, four-point bending test (4PBT) and axial compression test (ACT) have been performed using FEM on LCELCP, LCP, continuously degraded (CD)-LCELCP, and CD-LCP. LCELCP showed reduced stress of about 22% and 10% in 4PBT and ACT, respectively, compared to LCP. CD-LCELCP is safe during ACT over 6 months of continuous degradation when the degradation rate is assumed to be 4 mm/year. These results also ensured accuracy using mesh convergence and also mesh checked for quality assurance. Overall, LCELCP can be considered as a biodegradable bone implant plate because of its superior performance, if its ultimate validation is carried out through animal/human trials as future work.

Osteogenesis and degradation behavior of magnesium alloy plate in vivo

Introduction: The magnesium alloy was fabricated into orthopedic plates, and used to repair tibial fractures of New Zealand white rabbits. The osteogenesis and degradation behavior of magnesium alloy plates were investigated in vivo. Methods: 38 rabbits were randomly divided into an experimental group using the magnesium alloy plate and control group using a titanium alloy plate. Tibial fractures in the experimental group and control group were fixed with magnesium alloy plates and titanium alloy plates, respectively. An X-ray of the fracture site was taken at 1, 2, 4, 8, and 16 weeks after surgery. The formation of callus and expression of bone morphogenetic protein (BMP-2) in each group were examined at 4, 8, and 16 weeks postoperatively. The degradation behavior of the magnesium alloy plate was observed using a scanning electron microscope with an energy dispersive spectroscopy system. Results: The results of X-ray showed that the fracture healed gradually and there was significant callus around the plate in the magnesium alloy plate group than that in the titanium alloy plate groups. The formation of callus and the expression of BMP-2 in the magnesium alloy plate group were more significant than that in the titanium plate group. The degradation behavior of the magnesium alloy plates deepened in vivo with the implantation time. Conclusion: The results demonstrated that the magnesium alloy plate implanted into the rabbit tibia could promote the formation of callus and result in osteogenesis in vivo. Meanwhile, the magnesium alloy plate was absorbed gradually in vivo.

Applications of Magnesium and Its Alloys: A Review

Magnesium is a promising material. It has a remarkable mix of mechanical and biomedical properties that has made it suitable for a vast range of applications. Moreover, with alloying, many of these inherent properties can be further improved. Today, it is primarily used in the automotive, aerospace, and medical industries. However, magnesium has its own set of drawbacks that the industry and research communities are actively addressing. Magnesium’s rapid corrosion is its most significant drawback, and it dramatically impeded magnesium’s growth and expansion into other applications. This article reviews both the engineering and biomedical aspects and applications for magnesium and its alloys. It will also elaborate on the challenges that the material faces and how they can be overcome and discuss its outlook.

Design approaches and challenges for biodegradable bone implants: a review

Introduction: Biodegradable materials have been at the forefront of cutting-edge research and offer a truly viable option in the designing and manufacturing of bone implants in biomedical engineering. Most research regarding these materials has focused on their biological characteristics and mechanical behavior vis-à-vis nonbiodegradable (NB) materials; but the design aspects and parametric configurations of biodegradable bone implant have somehow not received as much attention as they deserved.Area covered: This review aims to develop insight into the parametrically conceptualized design of biodegradable bone implant and takes into due consideration the characteristics of bone-biodegradable implant interface (BBII), design techniques employed for conventionally used bone implants to optimize parameters using standard test methods, traditional design, and finite element analysis approaches for implant and healing behavior, manufacturing techniques, real-time surgical simulations, and so on.Expert opinion: Some successful and conventionally used NB bone implants do not dissolve or degrade with time and require removal through a complicated surgery after fulfilling the intended objectives. These bone implants should be reconceptualized and designed with an appropriate biodegradable material while paying due attention to all factors/parameters involved and striking a balance between these factors with the ultimate objective of fulfilling all desired orthopedic requirements.

Biodegradable magnesium-based biomaterials: An overview of challenges and opportunities

As promising biodegradable materials with nontoxic degradation products, magnesium (Mg) and its alloys have received more and more attention in the biomedical field very recently. Having excellent biocompatibility and unique mechanical properties, magnesium-based alloys currently cover a broad range of applications in the biomedical field. The use of Mg-based biomedical devices eliminates the need for biomaterial removal surgery after the healing process and reduces adverse effects induced by the implantation of permanent biomaterials. However, the high corrosion rate of Mg-based implants leads to unexpected degradation, structural failure, hydrogen evolution, alkalization, and cytotoxicity. To overcome these limitations, alloying Mg with suitable alloying elements and surface treatment come highly recommended. In this area, open questions remain on the behavior of Mg-based biomaterials in the human body and the effects of different factors that have resulted in these challenges. In addition to that, many techniques are yet to be verified to turn these challenges into opportunities. Accordingly, this article aims to review major challenges and opportunities for Mg-based biomaterials to minimize the challenges for the development of novel biomaterials made of Mg and its alloys.

Microalloying Design of Biodegradable Mg-2Zn-0.05Ca Promises Improved Bone-Implant Applications

Mg and its alloys have been comprehensively studied and show huge potential for clinical orthopedic applications. However, balancing the mechanical strength and corrosion resistance of alloys is still a challenge. In light of this, micro-level contents of Zn and Ca were added to pure Mg to fabricate a Mg-2Zn-0.05Ca microalloy to expectedly enhance the mechanical strength and concurrently improve the corrosion resistance. The characteristics of the rolled Mg-2Zn-0.05Ca microalloy were explored using optical microscopy, X-ray diffraction, and tensile tests. The corrosion behavior and mechanical strength loss were explored using electrochemical and immersion tests. The effects of the microalloy extract on the proliferation, adhesion, and osteogenic differentiation of MC3T3-E1 cells were systematically studied. Moreover, implantations were done in femoral condyles of rabbits to study the degradation properties, osteogenic effect, mechanical strength loss, and biosafety of the microalloy. The ultimate tensile strength and yield strength of the rolled microalloy were found to be significantly elevated to 257 ± 2.74 and 237.6 ± 8.29 MPa, respectively. The microalloy showed a stable and gradual strength loss during degradation, both in vivo and in vitro. Concurrently, the microalloy exhibited improved corrosion resistance ability and especially, in vivo, the rolled microalloy exhibited a comparable degradation rate to that of rolled pure Mg within the initial 12 weeks of implantation. Additionally, the microalloy promoted osteogenesis, both in vitro and in vivo, and no short- and long-term toxicities of the microalloy were observed in rabbits. This study suggested that the rolled Mg-2Zn-0.05Ca microalloy effectively balanced the mechanical strength and corrosion resistance and showed potential application as bone implants.

Bioresorbable Magnesium-Based Alloys as Novel Biomaterials in Oral Bone Regeneration: General Review and Clinical Perspectives

Bone preservation and primary regeneration is a daily challenge in the field of dental medicine. In recent years, bioresorbable metals based on magnesium (Mg) have been widely investigated due to their bone-like modulus of elasticity, their high biocompatibility, antimicrobial, and osteoconductive properties. Synthetic Mg-based biomaterials are promising candidates for bone regeneration in comparison with other currently available pure synthetic materials. Different alloys based on Mg were developed to fit clinical requirements. In parallel, advances in additive manufacturing offer the possibility to fabricate experimentally bioresorbable metallic porous scaffolds. This review describes the promising clinical results of resorbable Mg-based biomaterials for bone repair in osteosynthetic application and discusses the perspectives of use in oral bone regeneration.

Effect of sterilization on 3-point dynamic response to in vitro bending of an Mg implant

Background

The aim of the study is to characterize a biomedical magnesium alloy and highlighting the loss of mechanical integrity due to the sterilization method. Ideally, when using these alloys is to delay the onset of degradation so that the implant can support body loads and avoid toxicological effects due to the release of metal ions into the body.

Methods

Standardized procedures according to ASTM F-1264 and ISO-10993-5 were used, respecting detailed methodological controls to ensure accuracy and reproducibility of the results, this testing methodology is carried out in accordance with the monographs of the Pharmacopoeia for the approval of medical devices and obtaining a health registration. An intramedullary implant (IIM) manufactured in magnesium (Mg) WE43 can support loads of the body in the initial period of bone consolidation without compromising the integrity of the fractured area. A system with these characteristics would improve morbidity and health costs by avoiding secondary surgical interventions.

Results

As a property, the fatigue resistance of Mg in aggressive environments such as the body environment undergoes progressive degradation, however, the autoclave sterilization method drastically affects fatigue resistance, as demonstrated in tests carried out under in vitro conditions. Coupled with this phenomenon, the relatively poor biocompatibility of Mg WE43 alloys has limited applications where they can be used due to low acceptance rates from agencies such as the FDA. However, Mg alloy with elements such as yttrium and rare earth elements (REEs) have been shown to delay biodegradation depending on the method of sterilization and the physiological solution used. With different sterilization techniques, it may be possible to keep toxicological effects to a minimum while still ensuring a balance between the integrity of fractured bone and implant degradation time. Therefore, the evaluation of fatigue resistance of WE43 specimens sterilized and tested in immersion conditions (enriched Hank’s solution) and according to ASTM F-1264, along with the morphological, crystallinity, and biocompatibility characterization of the WE43 alloy allows for a comprehensive evaluation of the mechanical and biological properties of WE43.

Conclusions

These results will support decision-making to generate a change in the current perspective of biomaterials utilized in medical devices (MDs), to be considered by manufacturers and health regulatory agencies. An implant manufactured in WE43 alloy can be used as an intramedullary implant, considering keeping elements such as yttrium-REEs below as specified in its designation and with the help of a coating that allows increasing the life of the implant in vivo.

Recent research and progress of biodegradable zinc alloys and composites for biomedical applications: Biomechanical and biocorrosion perspectives

Biodegradable metals (BMs) gradually degrade in vivo by releasing corrosion products once exposed to the physiological environment in the body. Complete dissolution of biodegradable implants assists tissue healing, with no implant residues in the surrounding tissues. In recent years, three classes of BMs have been extensively investigated, including magnesium (Mg)-based, iron (Fe)-based, and zinc (Zn)-based BMs. Among these three BMs, Mg-based materials have undergone the most clinical trials. However, Mg-based BMs generally exhibit faster degradation rates, which may not match the healing periods for bone tissue, whereas Fe-based BMs exhibit slower and less complete in vivo degradation. Zn-based BMs are now considered a new class of BMs due to their intermediate degradation rates, which fall between those of Mg-based BMs and Fe-based BMs, thus requiring extensive research to validate their suitability for biomedical applications. In the present study, recent research and development on Zn-based BMs are reviewed in conjunction with discussion of their advantages and limitations in relation to existing BMs. The underlying roles of alloy composition, microstructure, and processing technique on the mechanical and corrosion properties of Zn-based BMs are also discussed.

Rare-earth- and aluminum-free, high strength dilute magnesium alloy for Biomedical Applications

Lightweight, recyclable, and plentiful Mg alloys are receiving increased attention due to an exceptional combination of strength and ductility not possible from pure Mg. Yet, due to their alloying elements, such as rare-earths or aluminum, they are either not economical or biocompatible. Here we present a new rare-earth and aluminum-free magnesium-based alloy, with trace amounts of Zn, Ca, and Mn (≈ 2% by wt.). We show that the dilute alloy exhibits outstanding high strength and high ductility compared to other dilute Mg alloys. By direct comparison with annealed material of the same chemistry and using transmission electron microscopy (TEM), high-resolution TEM (HR-TEM) and atom probe tomography analyses, we show that the high strength can be attributed to a number of very fine, Zn/Ca-containing nanoscale precipitates, along with ultra-fine grains. These findings show that forming a hierarchy of nanometer precipitates from just miniscule amounts of solute can invoke simultaneous high strength and ductility, producing an affordable, biocompatible Mg alloy.

Recent Advances in the Control of the Degradation Rate of PEO Treated Magnesium and Its Alloys for Biomedical Applications

Mg and Mg alloys have been studied for almost two centuries; nevertheless, commercial biomedical devices are still not available. The main issue that limits their use in the biomedical field is the rapid degradation rate combined with suitable surface properties. Novel approaches need to be designed for the development of biodegradable Mg-based devices, which could include the use of multifunctional coatings and/or new alloys designed “ad hoc”. The present article reviews on various properties, parameters and improvement methods concerning plasma electrolytic oxidation (PEO) coatings on Mg alloys substrates for biomedical applications. In this regard, (i) optimizing the PEO parameters, (ii) using additives and nanoparticles, (iii) creating combined layers of hard and/or soft particles, (iv) coating the PEO layer with a biodegradable polymer, could be the way to control their degradation rate. The review of recent scientific articles highlights that none of the techniques proposed may be preferred over the others and the need to deepen the studies for allowing the use of Mg-based devices in the biomedical field.

Mg-Based Metallic Glass-Polymer Composites: Investigation of Structure, Thermal Properties, and Biocompatibility

In this work, the biomedical applicability and physical properties of magnesium-based metallic glass/polycaprolactone (PCL) composites are explored. The composites were fabricated via mechanical alloying and subsequent coextrusion. The coextrusion process was carried out at a temperature near to the supercooled liquid region of the metallic glass and the viscous region of the polymer. The structures, as well as thermal and mechanical properties of the obtained samples were characterized, and in vivo investigations were undertaken. The composite samples possess acceptable thermal and mechanical properties. Tensile tests indicate the ability of the composites to withstand more than 100% deformation. In vivo studies reveal that the composites are biologically compatible and could be promising for biomedical applications.

Are Magnesium Screws Proper for Mandibular Condyle Head Osteosynthesis?

Recently, magnesium alloys have gained a significant amount of recognition as potential biomaterials for degradable implants for craniofacial bone screws. Purpose: The aim of this work was to compare screws made specifically for mandibular head osteosynthesis from different materials. Materials and Methods: Screws measuring 14 mm made by one manufacturer specifically for mandibular head osteosynthesis out of the following materials were selected: magnesium (MgYREZr), titanium (Ti6Al7Nb), and polymer (PLGA). The axial pull-out strength and torsional properties were investigated. Results: Each type of screw presented different pull-out forces (Kruskal–Wallis test, p < 0.001). The magnesium screw had the highest pull-out force of 399 N (cracked without the screw out being pulled out), followed by the titanium screw, with a force of 340 N, and the PLGA screw, with a force of 138 N (always cracked at the base of the screw head without the screw being pulled out). ANOVA was performed for the maximal torques before damage to the screw (torsional properties), revealing that the maximal torque of the magnesium screw was 16 N·cm, while that of the titanium screw was 19 N·cm. The magnesium screw was significantly weaker than the titanium screw (p < 0.05). The measured torque and pull-out force were not related to each other (p > 0.05). Conclusion: Among the screws compared, the metal biodegradable magnesium screw seems to be the most suitable material for multiscrew mandibular head osteosynthesis, considering the condition of the fragile screwdriver socket.

Corrosion Resistance and Biocompatibility Assessment of a Biodegradable Hydrothermal-Coated Mg-Zn-Ca Alloy: An in Vitro and in Vivo Study

A hydrothermal (HT) coating was applied to the biomedical Mg-Zn-Ca alloy surface by microarc oxidation (MAO) and heat treatment. Then, the corrosion resistance and biocompatibility of the coated alloy was evaluated in vitro and in vivo. The corrosion rate (CR) of HT-coated implants was significantly lower in experiment. In addition, this CR increased over time in vivo but was stable, albeit higher, in vitro. The proliferation, adhesion, and live activity of bone marrow stem cells (BMSCs) were significantly greater on the surface of the HT-coated Mg alloy in vitro. Serum Mg²⁺ was always within the normal range in rabbits with implants, although Ca²⁺ was higher than normal for both uncoated and coated scaffolds. There were no significant pathological effects on the main organs of alloy-implanted rabbits compared with healthy animals. Thus, the HT coating significantly improved the corrosion resistance and biocompatibility of the Mg-Zn-Ca alloy.

Effect of solid-solution and aging treatment on corrosion behavior of orthogonal designed and vacuum melted Mg-Zn-Ca-Mn alloys

Fast degradation rate and inhomogeneous corrosion are obstacles for magnesium alloy bio-corrosion properties. In this paper, a quaternary Mg-Zn-Ca-Mn alloy was designed by an orthogonal method and prepared by vacuum induction melting to investigate its bio-corrosion. Microstructure, corrosion morphology, and bio-corrosion properties of as-cast alloys 1 to 5 with good corrosion resistance were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction with immersion and electrochemical tests in simulated body fluid (SBF), respectively. Both the orthogonal method and in vitro degradation experiments demonstrated that alloy 3 exhibited the lowest degradation rate among the tested quaternary Mg-Zn-Ca-Mn alloys. Then, as-cast alloy 3 was treated by solid-solution and solid-solution aging. In vitro experimental results indicated that as-cast alloy 3 showed better corrosion resistance than heat-treated specimens and the average corrosion rate was approximately 0.15 mm/y. Heat-treated alloy 3 exhibited more uniform corrosion than as-cast alloy specimens. These results suggest that alloy 3 has the potential to become a biodegradable candidate material.