Guangyin Yuan

Microstructure refinement of Mg–Al–Zn–Si alloys

Magnesium alloys containing Mg2Si particles, as a promising cheap heat-resistant magnesium alloy for automobile engine applications, are attracting more and more attention of both material scientists and design engineers. Refinement of the Chinese script Mg2Si particle is a key for using this alloy in sand casting or permanent mould casting. In the present work, the refinement effect of antimony(Sb) or calcium(Ca) on the microstructure was investigated. The study shows that Sb or Ca addition promotes the formation of fine polygonal type Mg2Si particles by providing the nucleation sites. Moreover, the grain sizes of Sb or Ca modified Mg–Al–Zn–Si alloys are much finer than that of base alloy. The effect of modification and refinement with Sb addition is more effective than that with Ca addition. Such improved microstructure of the modified alloys results in large improvement in tensile properties and toughness as compared to the base alloy.

Microstructure, mechanical properties, biocorrosion behavior, and cytotoxicity of as-extruded Mg–Nd–Zn–Zr alloy with different extrusion ratios

Recently, commercial magnesium (Mg) alloys containing Al (such as AZ31 and AZ91) or Y (such as WE43) have been studied extensively for biomedical applications. However, these Mg alloys were developed as structural materials, not as biomaterials. In this study, a patented Mg-Nd-Zn-Zr (denoted as JDBM) alloy was investigated as a biomedical material. The microstructure, mechanical properties, biocorrosion behavior, and cytotoxicity of the alloy extruded at 320 °C with extrusion ratios of 8 and 25 were studied. The results show that the lower extrusion ratio results in finer grains and higher strength, but lower elongation, while the higher extrusion ratio results in coarser grains and lower strength, but higher elongation. The biocorrosion behavior of the alloy was investigated by hydrogen evolution and mass loss tests in simulated body fluid (SBF). The results show that the alloy extruded with lower extrusion ratio exhibits better corrosion resistance. The corrosion mode of the alloy is uniform corrosion, which is favorable for biomedical applications. Aging treatment on the as-extruded alloy improves the strength and decreases the elongation at room temperature, and has a small positive influence on the corrosion resistance in SBF. The cytotoxicity test indicates that the as-extruded JDBM alloy meets the requirement of cell toxicity.

In vitro degradation behavior and biocompatibility of Mg–Nd–Zn–Zr alloy by hydrofluoric acid treatment

In this paper, Mg-Nd-Zn-Zr alloy (denoted as JDBM) coated with hydrofluoric acid (HF) chemical conversion film (MgF2) was researched as a potential biodegradable cardiovascular stent material. The microstructures, in vitro degradation and biocompatibility were investigated. The field emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) showed that a compact MgF2 film was formed on the surface of JDBM. The corrosion rate decreased in artificial plasma from 0.337 to 0.253 mm·y(-1) and the electrochemical measurement demonstrated that the corrosion resistance of JDBM alloy could be obviously improved due to the protective MgF2 film on the surface of the substrate. Meanwhile, the hemolysis ratio of JDBM decreased from 52.0% to 10.1% and the cytotoxicity met the requirement of cellular application after HF treatment. In addition, JDBM and MgF2 film showed good anti-platelet adhesion, which is a very favorable property for implant material in contact with blood directly.

Effects of magnesium alloys extracts on adult human bone marrow-derived stromal cell viability and osteogenic differentiation.

In this study, adult human bone marrow-derived stromal cells (hBMSCs) were cultured in extracts of magnesium (Mg) and the Mg alloys AZ91D and NZ30K for 12 days. We studied the indirect effects of Mg alloys on hBMSC viability. Alkaline phosphatase activity and the expression of osteogenic differentiation marker genes were used to evaluate the effects of the Mg alloys on the osteogenic differentiation of hBMSCs. The results indicate that <or=10 mM concentration of Mg in the extracts did not inhibit the viability and osteogenic differentiation of hBMSCs. However, the results suggest that the high pH of the extracts, which is a result of the rapid corrosion of Mg and the Mg alloys, is unfavorable to the viability and osteogenic differentiation of hBMSCs.

Enhanced biocorrosion resistance and biocompatibility of degradable Mg-Nd-Zn-Zr alloy by brushite coating.

To further improve the corrosion resistance and biocompatibility of Mg-Nd-Zn-Zr alloy (JDBM), a biodegradable calcium phosphate coating (Ca-P coating) with high bonding strength was developed using a novel chemical deposition method. The main composition of the Ca-P coating was brushite (CaHPO4·2H2O). The bonding strength between the coating and the JDBM substrate was measured to be over 10 MPa, and the thickness of the coating layer was about 10-30 μm. The in vitro corrosion tests indicated that the Ca-P treatment improved the corrosion resistance of JDBM alloy in Hanks solution. Ca-P treatment significantly reduced the hemolysis rate of JDBM alloy from 48% to 0.68%, and induced no toxicity to MC3T3-E1 cells. The in vivo implantation experiment in New Zealands rabbit tibia showed that the degradation rate was reduced obviously by the Ca-P treatment and less gas was produced from Ca-P treated JDBM bone plates and screws in early stage of the implantation, and at least 10weeks degradation time can be prolonged by the present coating techniques. Both Ca-P treated and untreated JDBM Mg alloy induced bone growth. The primary results indicate that the present Ca-P treatment is a promising technique for the degradable Mg-based biomaterials for orthopedic applications.

Enhanced Bioactivity of Mg–Nd–Zn–Zr Alloy Achieved with Nanoscale MgF2 Surface for Vascular Stent Application

Magnesium (Mg) alloys have revolutionized the application of temporary load-bearing implants as they meet both engineering and medical requirements. However, rapid degradation of Mg alloys under physiological conditions remains the major obstacle hindering the wider use of Mg-based implants. Here we developed a simple method of preparing a nanoscale MgF2 film on Mg-Nd-Zn-Zr (denoted as JDBM) alloy, aiming to reduce the corrosion rate as well as improve the biological response. The corrosion rate of JDBM alloy exposed to artificial plasma is reduced by ∼20% from 0.337 ± 0.021 to 0.269 ± 0.043 mm·y(-1) due to the protective effect of the MgF2 film with a uniform and dense physical structure. The in vitro cytocompatibility test of MgF2-coated JDBM using human umbilical vein endothelial cells indicates enhanced viability, growth, and proliferation as compared to the naked substrate, and the MgF2 film with a nanoscale flakelike feature of ∼200-300 nm presents a much more favorable environment for endothelial cell adhesion, proliferation, and alignment. Furthermore, the animal experiment via implantation of MgF2-coated JDBM stent to rabbit abdominal aorta confirms excellent tissue compatibility of the well re-endothelialized stent with no sign of thrombogenesis and restenosis in the stented vessel.

Enhancement of Osteogenesis and Biodegradation Control by Brushite Coating on Mg–Nd–Zn–Zr Alloy for Mandibular Bone Repair

To diminish incongruity between bone regeneration and biodegradation of implant magnesium alloy applied for mandibular bone repair, a brushite coating was deposited on a matrix of a Mg-Nd-Zn-Zr (hereafter, denoted as JDBM) alloy to control the degradation rate of the implant and enhance osteogenesis of the mandible bone. Both in vitro and in vivo evaluations were carried out in the present work. Viability and adhesion assays of rabbit bone marrow mesenchyal stem cells (rBM-MSCs) were applied to determine the biocompatibility of a brushite-coated JDBM alloy. Osteogenic gene expression was characterized by quantitative real-time polymerase chain reaction (RT-PCR). Brushite-coated JDBM screws were implanted into mandible bones of rabbits for 1, 4, and 7 months, respectively, using 316L stainless steel screws as a control group. In vivo biodegradation rate was determined by synchrotron radiation X-ray microtomography, and osteogenesis was observed and evaluated using Van Giesons picric acid-fuchsin. Both the naked JDBM and brushite-coated JDBM samples revealed adequate biosafety and biocompatibility as bone repair substitutes. In vitro results showed that brushite-coated JDBM considerably induced osteogenic differentiation of rBM-MSCs. And in vivo experiments indicated that brushite-coated JDBM screws presented advantages in osteoconductivity and osteogenesis of mandible bone of rabbits. Degradation rate was suppressed at a lower level at the initial stage of implantation when new bone tissue formed. Brushite, which can enhance oeteogenesis and partly control the degradation rate of an implant, is an appropriate coating for JDBM alloys used for mandibular repair. The Mg-Nd-Zn-Zr alloy with brushite coating possesses great potential for clinical applications for mandibular repair.

Research on a Zn-Cu alloy as a biodegradable material for potential vascular stents application

Zn-based alloys have been viewed as new potential materials for biodegradable implants, such as cardiovascular stents, mainly in consideration of their lower corrosion rate when compared with that of Mg alloys. In this study we developed a new Zinc-4wt.%Copper (Zn-4Cu) alloy as a biodegradable material. Hot extrusion was applied to Zn-4Cu to refine the microstructure and consequently improve its mechanical properties and corrosion resistance. After extrusion, dendritic CuZn5 phases were broken and distributed along the extrusion direction. The grains were refined obviously due to dynamical recrystallization. The yield strength (YS), ultimate tensile strength (UTS) and elongation of the as-extruded alloy are 250±10MPa, 270±10MPa and 51±2%, respectively. The corrosion rate of the as-extruded alloy in Hanks solution is about 9.41(±1.34)μmyear(-1). In vitro evaluation shows that Zn-4Cu presents acceptable toxicity to human endothelial cells, and could effectively inhibit bacteria adhesion and biofilm formation. The present study indicates that the as-extruded Zn-4Cu alloy exhibits excellent strength and ductility, uniform and slow degradation, good biocompatibility and significant antibacterial effect, which make it an excellent candidate material for biodegradable implants, especially for cardiovascular stents application.

In vivo degradation behavior and biocompatibility of Mg-Nd-Zn-Zr alloy at early stage

The neotype magnesium alloy, Mg-Nd-Zn-Zr (NZK) alloy, was implanted into the rabbit femur to investigate its in vivo degradation behavior and biocompatibility. Seventy-two New Zealand white rabbits were randomly divided into the NZK alloy group, titanium alloy group and sham-operated group. Then NZK alloy rods were embedded in the rabbit femur in the NZK alloy group, titanium alloy rods were embedded in the titanium alloy group, and only bone tunnel was established in the sham-operated group. Prior to surgery and at 1, 7, 14, 28 and 56 days after operation, the serum alanine transaminase, creatinine, creatine kinase and magnesium ion concentration were examined in each group. An X-ray of the implanted region was taken at 7, 14, 28 and 56 days after implantation. The pathological changes in heart, liver, kidney and bone from the implant region were examined at 28 and 56 days postoperatively. The degradation behavior of the NZK alloy was observed using scanning electron microscope with an energy dispersive spectroscopy system. There were no significant differences in serum alanine transaminase, creatinine, creatine kinase and magnesium ion concentrations among each group at the same time point (P>0.05). The histology of heart, liver, kidney and bone from implant region was altered. The results demonstrate that the NZK alloy implanted into the rabbit femur could be absorbed gradually, and that the NZK alloy has excellent biocompatibility in vivo.

The processing of Mg alloy micro-tubes for biodegradable vascular stents

In this study, through a combination of hot extrusion, cold rolling and drawing, three Mg alloys, Mg-Nd-Zn-Zr (abbr. JDBM), AZ31 and WE43, were successfully fabricated into the high-quality micro-tubes with 3.00mm outer diameter and 180μm thickness for biodegradable stents. This processing method overcame the shortcoming of the poor workability of Mg alloys and could be applied to fabricate sufficiently long tubes with low dimensional errors within 2.8%. Microstructure observation demonstrated that the as-annealed JDBM, AZ31 and WE43 micro-tubes had more uniformly distributed grains with an average size of 10.9μm, 12.9μm and 15.0μm, respectively. Tensile mechanical test results showed that the as-annealed JDBM, AZ31 and WE43 micro-tubes respectively exhibited the yield strength of 123MPa, 172MPa and 113MPa, and significantly different breaking elongation of 26%, 16% and 10%. The following SEM observation showed microvoid coalescence, quasi-cleavage and cleavage fracture, respectively. In addition, EBSD analyses revealed that the as-annealed AZ31 tubes had a strong texture component 21¯1¯0 with a low Schmid factor for basal slip, while JDBM and WE43 tubes respectively exhibited weak textures 101¯0 and 101¯0+202¯1 with a similarly high Schmid factor for basal slip.