In vitro and in vivo studies of biodegradable fine grained AZ31 magnesium alloy produced by equal channel angular pressing

Magnesium-Based Temporary Implants: Potential, Current Status, Applications, and Challenges

Biomedical implants are important devices used for the repair or replacement of damaged or diseased tissues or organs. The success of implantation depends on various factors, such as mechanical properties, biocompatibility, and biodegradability of the materials used. Recently, magnesium (Mg)-based materials have emerged as a promising class of temporary implants due to their remarkable properties, such as strength, biocompatibility, biodegradability, and bioactivity. This review article aims to provide a comprehensive overview of current research works summarizing the above-mentioned properties of Mg-based materials for use as temporary implants. The key findings from in-vitro, in-vivo, and clinical trials are also discussed. Further, the potential applications of Mg-based implants and the applicable fabrication methods are also reviewed.

An Overview on the Effect of Severe Plastic Deformation on the Performance of Magnesium for Biomedical Applications

There has been a great interest in evaluating the potential of severe plastic deformation (SPD) to improve the performance of magnesium for biological applications. However, different properties and trends, including some contradictions, have been reported. The present study critically reviews the structural features, mechanical properties, corrosion behavior and biological response of magnesium and its alloys processed by SPD, with an emphasis on equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). The unique mechanism of grain refinement in magnesium processed via ECAP causes a large scatter in the final structure, and these microstructural differences can affect the properties and produce difficulties in establishing trends. However, the recent advances in ECAP processing and the increased availability of data from samples produced via HPT clarify that grain refinement can indeed improve the mechanical properties and corrosion resistance without compromising the biological response. It is shown that processing via SPD has great potential for improving the performance of magnesium for biological applications.

A Superior Corrosion Protection of Mg Alloy via Smart Nontoxic Hybrid Inhibitor-Containing Coatings

The increase of corrosion resistance of magnesium and its alloys by forming the smart self-healing hybrid coatings was achieved in this work in two steps. In the first step, using the plasma electrolytic oxidation (PEO) treatment, a ceramic-like bioactive coating was synthesized on the surface of biodegradable MA8 magnesium alloy. During the second step, the formed porous PEO layer was impregnated with a corrosion inhibitor 8-hydroxyquinoline (8-HQ) and bioresorbable polymer polycaprolactone (PCL) in different variations to enhance the protective properties of the coating. The composition, anticorrosion, and antifriction properties of the formed coatings were studied. 8-HQ allows controlling the rate of material degradation due to the self-healing effect of the smart coating. PCL treatment of the inhibitor-containing layer significantly improves the corrosion and wear resistance and retains an inhibitor in the pores of the PEO layer. It was revealed that the corrosion inhibitor incorporation method (including the number of steps, impregnation, and the type of solvent) significantly matters to the self-healing mechanism. The hybrid coatings obtained by a 1-step treatment in a dichloromethane solution containing 6 wt.% polycaprolactone and 15 g/L of 8-HQ are characterized by the best corrosion resistance. This coating demonstrates the lowest value of corrosion current density (3.02 × 10−7 A cm−2). The formation of the hybrid coating results in the corrosion rate decrease by 18 times (0.007 mm year−1) as compared to the blank PEO layer (0.128 mm year−1). An inhibitor efficiency was established to be 83.9%. The mechanism of corrosion protection of Mg alloy via smart hybrid coating was revealed.

Characterization of AZ31/HA Biodegradable Metal Matrix Composites Manufactured by Rapid Microwave Sintering

This study reports the development of magnesium alloy/hydroxyapatite-based biodegradable metal matrix composites (BMMCs) through rapid microwave sintering. Magnesium alloy (AZ31) and hydroxyapatite powder were used in four compositions 0, 10, 15 and 20% by weight. Developed BMMCs were characterized to evaluate physical, microstructural, mechanical and biodegradation characteristics. XRD results show Mg and HA as major phases and MgO as a minor phase. SEM results correlate with the XRD findings by identifying the presence of Mg, HA and MgO. The addition of HA powder particles reduced density and increased the microhardness of BMMCs. The compressive strength and Young's modulus increased with increasing HA up to 15 wt.%. AZ31-15HA exhibited the highest corrosion resistance and lowest relative weight loss in the immersion test for 24 h and weight gain after 72 and 168 h due to the deposition of Mg(OH)₂ and Ca(OH)₂ layers at the sample surface. XRD analysis of the AZ31-15HA sintered sample after an immersion test was carried out and these results revealed the presence of new phases Mg(OH)₂ and Ca(OH)₂ that could be the reason for enhancing the corrosion resistance. SEM elemental mapping result also confirmed the formation of Mg(OH)₂ and Ca(OH)₂ at the sample surface, which acted as protective layers and prevented the sample from further corrosion. It showed that the elements were uniformly distributed over the sample surface. In addition, these microwave-sintered BMMCs showed similar properties to the human cortical bone and help bone growth by depositing apatite layers at the surface of the sample. Furthermore, this apatite layer can enhance osteoblast formation due to the porous structure type, which was observed in the BMMCs. Therefore, it is indicative that developed BMMCs can be an artificial biodegradable composite for orthopedic applications.

Effect of ECAP Route Type on the Microstructural Evolution, Crystallographic Texture, Electrochemical Behavior and Mechanical Properties of ZK30 Biodegradable Magnesium Alloy

In this study, billets of the ZK30 (Mg-3Zn-0.6 Zr-0.4 Mn, wt%) alloy were Equal Channel Angle Pressing (ECAP) processed for up to four passes of routes Bc (with rotating the sample 90° in the same direction between the subsequent passes), A (without sample rotation), and C (with sample rotating 180°) after each pass at a temperature of 250 °C and a ram speed of 10 mm/min using a die with an internal channel angle of 90°. The microstructural evolution and the crystallographic texture were investigated using a Scanning Electron Microscope (SEM) equipped with the Electron Back-Scatter Diffraction (EBSD) technique. Corrosion measurements were conducted in ringer lactate which is a simulated body fluid. The Vickers microhardness test and tensile tests were conducted for the alloy before and after processing. The as-annealed billets exhibited a bimodal structure as fine grains (more than 3.39 µm) coexisted with almost-equiaxed coarse grains (less than 76.73 µm); the average grain size was 26.69 µm. Further processing until four passes resulted in enhanced grain refinement and full Dynamic Recrystallization (DRX). ECAP processing through 4-Bc, 4-A, and 4-C exhibited significant reductions in grain size until they reached 1.94 µm, 2.89 µm, and 2.25 µm, respectively. Four-pass processing also resulted in the transformation of low-angle grain boundaries into high-angle grain boundaries. The previous conclusion was drawn from observing the simultaneous decrease in the fraction of low-angle grain boundaries and an increase in the fraction of high-angle grain boundaries. The pole figures revealed that 4-Bc, 4-A, and 4-C reduced the maximum texture intensity of the as-annealed billets. The potentiodynamic polarization findings revealed that route Bc is the most effective route in improving the corrosion rate, whereas the Electrochemical Impedance Spectroscopy (EIS) revealed that routes A and Bc improved the corrosion resistance with nearly identical values. Finally, 4-Bc resulted in the highest increase in Vickers hardness, yield stress, and ultimate tensile strength with values of 80.8%, 19.3%, and 44.5%, alongside a 31% improvement in ductility, all compared to the AA condition.

Influence of Ultrafine-Grained Microstructure and Texture Evolution of ECAPed ZK30 Magnesium Alloy on the Corrosion Behavior in Different Corrosive Agents

Magnesium-Zinc-Zirconium (Mg-Zn-Zr) alloys have caught considerable attention in medical applications where biodegradability is critical. The combination of their good biocompatibility, improved strength, and low cytotoxicity makes them great candidates for medical implants. This research investigation is focused on providing further insight into the effects of equal channel angular processing (ECAP) on the corrosion behavior, microstructure evolution, and mechanical properties of a biodegradable ZK30 alloy. Billets of Mg-3Zn-0.6 Zr (ZK30) alloy were processed through ECAP up to 4 passes of route Bc (rotating the billets 90° in the same direction between the subsequent passes) at 250 °C. Electron back-scatter diffraction (EBSD) was utilized to investigate the microstructural evolution as well as the crystallographic texture. Several electrochemical measurements were carried out on both a simulated body fluid and a 3.5% sodium chloride (NaCl) solution. Mechanical properties such as Vicker's hardness and tensile properties were also assessed. The as-annealed (AA) microstructure was dominated by equiaxed coarse recrystallized grains with an average grain size of 26.69 µm. After processing, a geometric grain subdivision took place due to the severe plastic deformation. Processed samples were characterized by grain refinement and high density of substructures. The 4-passes sample experienced a reduction in the grain size by 92.8% compared with its AA counterpart. The fraction of high-angle grain boundaries increased significantly after 4-passes compared to the 1-pass processed sample. With regards to the crystallographic texture, the AA condition had its {0001} basal planes mostly oriented parallel to the transversal direction. On the other hand, ECAP processing resulted in crystallographic texture changes, such as the shifting of the ZK30 shear plane to be aligned at 45° relative to the extrusion direction (ED). Furthermore, the maximum texture intensity was reduced from 14 times random (AA billets) to 8 times random after ECAP processing through 4-passes. The corrosion rate of the 4-passes sample was tremendously reduced by 99% and 45.25% compared with its AA counterpart in the simulated body fluid and the NaCl solution, respectively. The pitting corrosion resistance of ZK30 showed notable improvements in the simulated body fluid by 471.66% and 352% during processing through 1-pass and 4-passes, respectively, compared with the 3.5% NaCl findings. Finally, significant improvements in the tensile strength, hardness, and ductility were also achieved.

In vitro and in vivo degradation assessment and preventive measures of biodegradable Mg alloys for biomedical applications

Magnesium (Mg) and its alloys have been widely explored as a potential biodegradable implant material. However, the fast degradation of Mg-based alloys under physiological environment has hindered their widespread use for implant applications till date. The present review focuses on in vitro and in vivo degradation of biodegradable Mg alloys, and preventive measures for biomedical applications. Initially, the corrosion assessment approaches to predict the degradation behavior of Mg alloys are discussed along with the measures to control rapid corrosion. Furthermore, this review attempts to explore the correlation between in vitro and in vivo corrosion behavior of different Mg alloys. It was found that the corrosion depends on experimental conditions, materials and the results of different assessment procedures hardly matches with each other. It has been demonstrated the corrosion rate of magnesium can be tailored by alloying elements, surface treatments and heat treatments. Various researches also studied different biocompatible coatings such as dicalcium phosphate dihydrate (DCPD), β-tricalcium phosphate (β-TCP), hydroxyapatite (HA), polycaprolactone (PCL), polylactic acid (PLA), and so on, on Mg alloys to suppress rapid degradation and examine their influence on new bone regeneration as well. This review shows the need for a standard method of corrosion assessment to predict the in vivo corrosion rate based on in vitro data, and thus reducing the in vivo experimentation.

Bio-corrosion impacts on mechanical integrity of ZM21 Mg for orthopaedic implant application processed by equal channel angular pressing

The mechanical integrity of rolled ZM21 Mg was improved by equal channel angular pressing (ECAP) to function as a potential biodegradable bone screw implant. Electron backscattered diffraction (EBSD) revealed deformed grains of 45 µm observed in rolled ZM21 Mg. They were transformed to equiaxed fine grains of 5.4 µm after 4^th pass ECAP. The yield strength of rolled and ECAPed ZM21 Mg alloys were comparable. In contrast, 4^th pass ZM21 Mg exhibited relatively higher elongation when compared to rolled sample. The mechanical properties of rolled and ECAPed ZM21 Mg were dependant on both grain refinement and crystallographic texture. The rolled and 4^th pass ECAPed tensile samples exhibited nonlinear deterioration of mechanical properties when tested after 7, 14, 21 and 28 days immersion in Hank's solution. The evaluation signifies that regardless their processing condition, ZM21 Mg alloys are suitable for surgical areas that requires high mechanical strength. In addition, the 4^th pass ECAP samples were viable to MG-63 cells proving themselves to be promising candidates for future in vivo studies.

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.

Improved Corrosion Behavior of AZ31 Alloy through ECAP Processing

This study aims to establish the effects of equal channel angular pressing (ECAP) processing on the corrosion behavior and hardness values of the AZ31 Mg alloy. The AZ31 billets were processed through ECAP successfully at 250 °C and their microstructural evolution was studied using optical and field emission scanning electron microscopy. The corrosion resistance of the AZ31 alloy was studied before and after processing through ECAP. The homogeneity of the hardness distribution was studied using both sections cut parallel and perpendicular to the extrusion direction. ECAP processing resulted in highly deformed central regions with elongated grains aligned parallel to the extrusion direction, whereas the peripheral regions showed an ultra-fine-grain recrystallized structure. After processing, small ultra-fine secondary particles were found to be homogeneously dispersed alongside the grain boundaries of the α-Mg matrix. Regarding the corrosion properties, measurements showed that ECAP processing through 1-P and 2-Bc resulted in decreasing their corrosion rate to 67.7% and 78.3%, respectively, of their as-annealed counterpart’s. The corrosion resistance of the ECAPed Mg alloy increased with the number of processing passes. This was due to the refinement of the grain size of the α-Mg matrix and secondary phases till ultra-fine size, caused by the accumulation of strain during processing. On the other hand, ECAP processing through 2-Bc resulted in increasing the Vickers hardness values by 132% and 71.8% at the peripheral and central areas, respectively, compared to the as-annealed counterpart.

In-vitro biomineralization and biocompatibility of friction stir additively manufactured AZ31B magnesium alloy-hydroxyapatite composites

The present study aims to evaluate effect of hydroxyapatite (HA, Ca₁₀(PO₄)₆OH₂), a ceramic similar to natural bone, into AZ31B Mg alloy matrix on biomineralization and biocompatibility. The novel friction stir processing additive manufacturing route was employed to fabricate Mg-HA composites. Various HA contents (5, 10, 20 wt%) were incorporated into Mg matrix.

Microstructural observation and chemical composition analysis revealed that refined Mg grains and dispersion of HA particles at micro/nanoscales were achieved in Mg-HA composites after the friction stir processing. The biomineralization evaluation were carried out using immersion experiments in simulated body fluid followed by mineral morphology observation and chemical composition analysis. The wettability measurements were conducted to correlate the biomineralization behavior. The results showed improvement in wettability and bone-like Ca/P ratio in apatite deposit on the composites compared to as-received Mg. In addition, the increase of blood compatibility, cell viability and spreading were found in the higher HA content composites, indicating the improved biocompatibility. Therefore, friction stir processed Mg-20 wt%HA composite exhibited the highest wettability and better cell adhesion among other composites due to the effect of increased HA content within Mg matrix.

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Friction stir additive manufacturing technique was employed to fabricate AZ31B magnesium-hydroxyapatite composite.

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Hydroxyapatite incorporated in Mg matrix varied from micro-to nano-length scales.

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Refined microstructure and increase in hydroxyapatite content enhanced wettability, biomineralization, and biocompatibility.

Bilayer nanostructure coated AZ31 magnesium alloy implants: in vivo reconstruction of critical-sized rabbit femoral segmental bone defect

Healing or reconstruction of critical-sized bone defects is still challenging in orthopaedic practice. In this study, we developed a new approach to control the degradation and improve the bone regeneration of the AZ31 magnesium substrate, fabricated as mesh cage implants. Subsequently, bilayer nanocomposite coating was carried out using polycaprolactone (PCL) and nano-hydroxyapatite (nHA) by dip-coating and electrospinning. Lastly, the healing capacity of the implants was studied in New Zealand White (NZW) rabbit critical-sized femur bone defects. X-ray analysis showed the coated implant group bridged and healed the critical defects 100% during four weeks of post-implantation. Micro-computed tomography (Micro-CT) study showed higher total bone volume (21.10%), trabecular thickness (0.73), and total porosity (85.71%) with bilayer coated implants than uncoated. Our results showed that nanocomposite coated implants controlled the in vivo degradation and improved bioactivity. Hence, the coated implants can be used as a promising bioresorbable implant for critical segmental bone defect repair applications.

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.

The Functional Properties of Mg-Zn-X Biodegradable Magnesium Alloys

The implantation of metallic devices in orthopaedic surgical procedures and coronary angioplasty is associated with the risk of various adverse events: (i) mechanical (premature failure), (ii) chemo-mechanical (corrosion and corrosion-fatigue degradation) and (iii) biomedical (chronic local inflammatory reactions, tissue necrosis, etc.). In this regard, the development of biodegradable implants/stents, which provide the necessary mechanical support for the healing period of the bone or the vessel wall and then are completely resorbed, has bright prospects. Magnesium alloys are the most suitable candidates for that purpose due to their superior mechanical performance, bioresorbability and biocompatibility. This article presents the results of the comparative research on several wrought biodegradable alloys, assessing their potential for biomedical applications. The Mg-Zn-X alloys with different chemical compositions and microstructures were produced using severe plastic deformation techniques. Functional properties pivotal for biomedical applications-mechanical strength, in vitro corrosion resistance and cytotoxic activity-were included in the focus of the study. Excellent mechanical performance and low cytotoxic effects are documented for all alloys with a notable exception for one of two Mg-Zn-Zr alloys. The in vitro corrosion resistance is, however, below expectations due to critical impurities, and this property has yet to be drastically improved through the cleaner materials fabrication processing before they can be considered for biomedical applications.

Investigation of a cyanine dye assay for the evaluation of the biocompatibility of magnesium alloys by direct and indirect methods

Magnesium and its alloys are promising candidates for a new generation of biodegradable metals in orthopaedic applications due to their excellent biocompatibility, biodegradability, and mechanical properties that are similar to natural bone. However, direct in vitro assessment of these materials in the presence of cells is complicated by degradation products from the alloy that lead to a false positive for the most commonly used cell adhesion and cell proliferation assays. In this paper, a cyanine dye was used to quantitatively evaluate the in vitro biocompatibility of a Mg AZ31 alloy by both direct and indirect methods. The cytotoxicity of the corrosion products was evaluated via an indirect method; a 25% decrease in cell viability compared to control samples was observed. Moreover, direct assessment of cell adhesion and proliferation showed a statistically significant increase in cell number at the surface after 72 h. In addition, the degradation rate and surface characteristics of the Mg AZ31 alloy were evaluated for both direct and indirect tests. The degradation rate was unaffected by the presence of cells while evidence of an increase in calcium phosphate deposition on the magnesium alloy surface in the presence of cells was observed. This study demonstrates that a cyanine dye based assay provides a more accurate assessment of the overall in vitro biocompatibility of biodegradable metals than the more commonly used assays reported in the literature to date.

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Quantitative analysis of cell numbers on the surface of magnesium has been performed.

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No false positive was observed.

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Cyanine dye assays are an excellent alternative to tetrazolium salts for in vitro evaluation of bioabsorbable implants.

Enhancement of stress corrosion cracking of AZ31 magnesium alloy in simulated body fluid thanks to cryogenic machining

Magnesium and its alloys have recently attracted great attention as potential materials for the manufacture of biodegradable implants. Unfortunately, their inadequate resistance to the simultaneous action of corrosion and mechanical stresses in the human body have hampered their use as implant materials. This work aims at evaluating the Stress Corrosion Cracking (SCC) susceptibility of the AZ31 Mg alloy after being machined under cryogenic cooling. The SCC behaviour was evaluated by means of Slow Strain Rate Tests (SSRTs) in Simulated Body Fluid (SBF) at 37 °C. Prior to testing, a full characterization of the machined surface integrity, including microstructural observations, residual stress, nano-hardness measurements and surface texture analysis was carried out together with the assessment of the corrosion properties through potentiodynamic polarization curves. In addition, the morphology of the fracture surfaces after SSRTs was analysed by means of 3D optical profiler and Scanning Electron Microscopy (SEM). The improved corrosion resistance due to the increased extension of the nano-surface layer and to the compressive residual stresses represents the reason of the reduced SCC susceptibility of cryogenically machined AZ31 samples as compared to dry machined ones.

Endovascular Metal Devices for the Treatment of Cerebrovascular Diseases

Cerebrovascular disease involves various medical disorders that obstruct brain blood vessels or deteriorate cerebral circulation, resulting in ischemic or hemorrhagic stroke. Nowadays, platinum coils with or without biological modification have become routine embolization devices to reduce the risk of cerebral aneurysm bleeding. Additionally, many intracranial stents, flow diverters, and stent retrievers have been invented with uniquely designed structures. To accelerate the translation of these devices into clinical usage, an in-depth understanding of the mechanical and material performance of these metal-based devices is critical. However, considering the more distal location and tortuous anatomic characteristics of cerebral arteries, present devices still risk failing to arrive at target lesions. Consequently, more flexible endovascular devices and novel designs are under urgent demand to overcome the deficiencies of existing devices. Herein, the pros and cons of the current structural designs are discussed when these devices are applied to the treatment of diseases ranging broadly from hemorrhages to ischemic strokes, in order to encourage further development of such kind of devices and investigation of their use in the clinic. Moreover, novel biodegradable materials and drug elution techniques, and the design, safety, and efficacy of personalized devices for further clinical applications in cerebral vasculature are discussed.

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.

Updates on the research and development of absorbable metals for biomedical applications

Absorbable metals, metals that corrode in physiological environment, constitute a new class of biomaterials intended for temporary medical implant applications. The introduction of these metals has shifted the established paradigm of metal implants from preventing corrosion to its direct application. Interest toward absorbable metals has been growing in the past decade. This is proved by the rapid increase in scientific publication, progressive development of standards, and launching the first commercial products. Iron, magnesium, zinc, and their alloys are the current three absorbable metals families. Magnesium-based metals are the most progressing family with a large data set obtained from both basic and translational research. Iron-based metals are still facing a major challenge of low in vivo corrosion rate despite the significant efforts that have been put to overcome its weakness. Zinc-based metals are the new alternative absorbable metals with moderate corrosion rates that fall between those of iron and magnesium. This manuscript provides a brief review on the latest progress in the research and development of absorbable metals, the most important findings, the remaining challenges, and the perspective on the future direction.

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.

Fabrication of biodegradable Zn-Al-Mg alloy: Mechanical properties, corrosion behavior, cytotoxicity and antibacterial activities

In this work, binary Zn-0.5Al and ternary Zn-0.5Al-xMg alloys with various Mg contents were investigated as biodegradable materials for implant applications. Compared with Zn-0.5Al (single phase), Zn-0.5Al-xMg alloys consisted of the α-Zn and Mg₂(Zn, Al)₁₁ with a fine lamellar structure. The results also revealed that ternary Zn-Al-Mg alloys presented higher micro-hardness value, tensile strength and corrosion resistance compared to the binary Zn-Al alloy. In addition, the tensile strength and corrosion resistance increased with increasing the Mg content in ternary alloys. The immersion tests also indicated that the corrosion rates in the following order Zn-0.5Al-0.5Mg<Zn-0.5Al-0.3Mg<Zn-0.5Al-0.1Mg<Zn-0.5Al. The cytotoxicity tests exhibited that the Zn-0.5Al-0.5Mg alloy presents higher viability of MC3T3-E1 cell compared to the Zn-0.5Al alloy, which suggested good biocompatibility. The antibacterial activity result of both Zn-0.5Al and Zn-0.5Al-Mg alloys against Escherichia coli presented some antibacterial activity, while the Zn-0.5Al-0.5Mg significantly prohibited the growth of Escherichia coli. Thus, Zn-0.5Al-0.5Mg alloy with appropriate mechanical properties, low corrosion rate, good biocompatibility and antibacterial activities was believed to be a good candidate as a biodegradable implant material.