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.

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

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

Systems, Properties, Surface Modification and Applications of Biodegradable Magnesium-Based Alloys: A Review

In recent years, biodegradable magnesium (Mg) alloys have attracted the attention of many researchers due to their mechanical properties, excellent biocompatibility and unique biodegradability. Many Mg alloy implants have been successfully applied in clinical medicine, and they are considered to be promising biological materials. In this article, we review the latest research progress in biodegradable Mg alloys, including research on high-performance Mg alloys, bioactive coatings and actual or potential clinical applications of Mg alloys. Finally, we review the research and development direction of biodegradable Mg alloys. This article has a guiding significance for future development and application of high-performance biodegradable Mg alloys, promoting the future advancement of the magnesium alloy research field, especially in biomedicine.

Potential bioactive coating system for high-performance absorbable magnesium bone implants

Magnesium alloys are considered the most suitable absorbable metals for bone fracture fixation implants. The main challenge in absorbable magnesium alloys is their high corrosion/degradation rate that needs to be controlled. Various coatings have been applied to magnesium alloys to slow down their corrosion rates to match their corrosion rate to the regeneration rate of the bone fracture. In this review, a bioactive coating is proposed to slow down the corrosion rate of magnesium alloys and accelerate the bone fracture healing process. The main aim of the bioactive coatings is to enhance the direct attachment of living tissues and thereby facilitate osteoconduction. Hydroxyapatite, collagen type I, recombinant human bone morphogenetic proteins 2, simvastatin, zoledronate, and strontium are six bioactive agents that show high potential for developing a bioactive coating system for high-performance absorbable magnesium bone implants. In addition to coating, the substrate itself can be made bioactive by alloying magnesium with calcium, zinc, copper, and manganese that were found to promote bone regeneration.

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Bioactive-coated magnesium implant could accelerate bone fracture healing time to match with magnesium degradation.

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Hydroxyapatite, collagen type I, recombinant human bone morphogenetic proteins 2, simvastatin, zoledronate, and strontium are high potential bioactive coating materials.

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The incorporation of Ca, Zn, Cu, Sr, and Mn in Mg base-metal could further enhance bone formation.

Mechanical Behavior and In Vitro Corrosion of Cubic Scaffolds of Pure Magnesium Processed by Severe Plastic Deformation

Reports in the literature show that severe plastic deformation can improve mechanical strength, ductility, and corrosion resistance of pure magnesium, which suggests good performance for biodegradable applications. However, the reported results were based on testing of small samples on limited directions. The present study reports compression testing of larger samples, at different directions, in pure magnesium processed by hot rolling, equal channel angular pressing (ECAP), and high pressure torsion (HPT). The results show that severe plastic deformation through ECAP and HPT reduces anisotropy and increases strength and strain rate sensitivity. Also, scaffolds were fabricated from the material with different processing histories and immersed in Hank’s solution for up to 14 days. The as-cast material displays higher corrosion rate and localized corrosion and it is reported that severe plastic deformation induces uniform corrosion and reduces the corrosion rate.

Sequential activation of heterogeneous macrophage phenotypes is essential for biomaterials-induced bone regeneration

Macrophage has been gradually recognized as a central regulator in tissue regeneration, and the study of how macrophage mediates biomaterials-induced bone regeneration through immunomodulatory pathway becomes popular. However, the current understanding on the roles of different macrophage phenotypes in regulating bone tissue regeneration remains controversial. In this study, we demonstrate that sequential infiltration of heterogeneous phenotypes of macrophages triggered by bio-metal ions effectively facilitates bone healing in bone defect. Indeed, M1 macrophages promote the recruitment and early commitment of osteogenic and angiogenic progenitors, while M2 macrophages and osteoclasts support the deposition and mineralization of the bone matrix, as well as the maturation of blood vessels. Moreover, we have identified a group of bone biomaterial-related multinucleated cells that behave similarly to M2 macrophages with wound-healing features rather than participate in the bone resorption cascade similarly to osteoclasts. Our study shows how sequential activation of macrophage-osteoclast lineage contribute to a highly orchestrated immune response in the bone tissue microenvironment around biomaterials to regulate the complex biological process of bone healing. Therefore, we believe that the temporal activation pattern of heterogeneous macrophage phenotypes should be considered when the next generation of biomaterials for bone regeneration is engineered.

Zn0.8Li0.1Sr-a biodegradable metal with high mechanical strength comparable to pure Ti for the treatment of osteoporotic bone fractures: In vitro and in vivo studies

The first in vivo investigation of Zn-based biodegradable metal aiming to treat osteoporotic bone fractures, a soaring threat to human health, is reported in this paper. Among the newly developed biodegradable metal system (ZnLiSr), Zn0.8Li0.1Sr exhibits excellent comprehensive mechanical properties, with an ultimate tensile strength (524.33 ± 18.01 MPa) comparable to pure Ti (the gold standard for orthopaedic implants), and a strength-ductility balance over 10 GPa%. The in vitro degradation tests using simulated body fluid (SBF) shows that Zn0.8Li0.1Sr manifests a uniform degradation morphology and smaller corrosion pits, with a degradation rate of 10.13 ± 1.52 μm year^-1. Real-time PCR and western blotting illustrated that Zn0.8Li0.1Sr successfully stimulated the expression of critical osteogenesis-related genes (ALP, COL-1, OCN and Runx-2) and proteins. Twenty-four weeks' in vivo implantations within ovariectomized (OVX) rats were conducted to evaluate the osteoporotic-bone-fracture-treating effects of Zn0.8Li0.1Sr, with pure Ti as control group. Micro-CT, histological and immunohistochemical evaluations all revealed that Zn0.8Li0.1Sr possesses a similar biosafety level to, while significantly superior osteogenesis-inducing and osteoporotic-bone-fracture-treating effects than pure Ti. ZnLiSr biodegradable alloys manifest excellent comprehensive mechanical properties, good biosafety and osteoporotic-bone-fracture-treating effects, which would provide preferable choices for future medical applications, especially in load-bearing positions.

Tailoring grain sizes of the biodegradable iron-based alloys by pre-additive manufacturing microalloying

We demonstrated the design of pre-additive manufacturing microalloying elements in tuning the microstructure of iron (Fe)-based alloys for their tunable mechanical properties. We tailored the microalloying stoichiometry of the feedstock to control the grain sizes of the metallic alloy systems. Two specific microalloying stoichiometries were reported, namely biodegradable iron powder with 99.5% purity (BDFe) and that with 98.5% (BDFe-Mo). Compared with the BDFe, the BDFe-Mo powder was found to have lower coefficient of thermal expansion (CTE) value and better oxidation resistance during consecutive heating and cooling cycles. The selective laser melting (SLM)-built BDFe-Mo exhibited high ultimate tensile strength (UTS) of 1200 MPa and fair elongation of 13.5%, while the SLM-built BDFe alloy revealed a much lower UTS of 495 MPa and a relatively better elongation of 17.5%, indicating the strength enhancement compared with the other biodegradable systems. Such an enhanced mechanical behavior in the BDFe-Mo was assigned to the dominant mechanism of ferrite grain refinement coupled with precipitate strengthening. Our findings suggest the tunability of outstanding strength-ductility combination by tailoring the pre-additive manufacturing microalloying elements with their proper concentrations.

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.