Zn-Li alloy after extrusion and drawing: Structural, mechanical characterization, and biodegradation in abdominal aorta of rat

Zinc shows great promise as a bio-degradable metal. Our early in vivo investigations implanting pure zinc wires into the abdominal aorta of Sprague-Dawley rats revealed that metallic zinc does not promote restenotic responses and may suppress the activities of inflammatory and smooth muscle cells. However, the low tensile strength of zinc remains a major concern. A cast billet of the Zn-Li alloy was produced in a vacuum induction caster under argon atmosphere, followed by a wire drawing process. Two phases of the binary alloy identified by x-ray diffraction include the zinc phase and intermetallic LiZn4 phase. Mechanical testing proved that incorporating 0.1wt% of Li into Zn increased its ultimate tensile strength from 116±13MPa (pure Zn) to 274±61MPa while the ductility was held at 17±7%. Implantation of 10mm Zn-Li wire segments into abdominal aorta of rats revealed an excellent biocompatibility of this material in the arterial environment. The biodegradation rate for Zn-Li was found to be about 0.008mm/yr and 0.045mm/yr at 2 and 12months, respectively.

Preparation and characterization of a calcium-phosphate-silicon coating on a Mg-Zn-Ca alloy via two-step micro-arc oxidation

Magnesium alloys are the most promising implant materials due to their excellent biodegradability. However, their high degradation rate limits their practical application. In this study, we produced a calcium-phosphate (Ca-P) coating and a calcium-phosphate-silicon (Ca-P-Si) coating via one-step and two-step micro-arc oxidation processes, respectively. The microstructure and chemical composition of the MAO coatings were characterized using SEM, XRD and EDS. The degradation behaviors of the MAO coatings and the substrate were investigated using electrochemical techniques and immersion tests in simulated body fluid (SBF). The results show that the silicate was successfully incorporated into the Ca-P coating in the second MAO step, and this also increased the thickness of the coating. The Ca-P-Si coatings remarkably reduced the corrosion rate of the Mg alloy and Ca-P coating during 18 days of immersion in SBF. In addition, the bone-like apatite layer on the sample surface demonstrated the good biomineralization ability of the Ca-P-Si coating. Potentiodynamic polarization results showed that the MAO coating could clearly enhance the corrosion resistance of the Mg alloy. Moreover, we propose the growth mechanism of the MAO coating in the second step.

In situ incorporation of heparin/bivalirudin into a phytic acid coating on biodegradable magnesium with improved anticorrosion and biocompatible properties

Drugs were incorporated into a phytic acid coating on Mg by an in situ chemical route for corrosion control and biocompatibility.

Effect of heat treatment on mechanical and biodegradable properties of an extruded ZK60 alloy

ZK60 magnesium alloy possess good mechanical properties and is a potential biodegradable material. But its high degradation rate is not desirable. In this study the effect of heat treatment on the biodegradable property of ZK60 alloy was investigated. T5 treated, T6 treated, as-cast and as-extruded ZK60 alloys were studied. Microstructure characterization, electrochemical measurement and immersion test were carried out. The results showed that both the mechanical properties and degradation behavior were improved after T5 treatment due to the formation of small and uniformly distributed MgZn phases. The as-cast alloys also exhibited good corrosion resistance. However, the as-extruded and T6 treated samples were severely corroded due to the formation of large amounts of second phases accelerating the corrosion rate owing to the galvanic corrosion. The corrosion resistance of ZK60 alloy was as following: T5 treated > as-cast > T6 treated > as-extruded.

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The strength and corrosion resistance of ZK60 alloys are improved by T5 treatment due to the formation of small and uniformly distributed MgZn phases.

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After T6 treatment, certain amount of MgZn₂ phase is precipitated along the grain boundary which deteriorates the corrosion resistance.

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The as-extruded ZK60 exhibits the worst corrosion resistance. The content of ZnZr phase is high in the second phase.

Mechanical properties, in vitro corrosion and biocompatibility of newly developed biodegradable Mg-Zr-Sr-Ho alloys for biomedical applications

Our previous studies have demonstrated that Mg-Zr-Sr alloys can be anticipated as excellent biodegradable implant materials for load-bearing applications. In general, rare earth elements (REEs) are widely used in magnesium (Mg) alloys with the aim of enhancing the mechanical properties of Mg-based alloys. In this study, the REE holmium (Ho) was added to an Mg-1Zr-2Sr alloy at different concentrations of Mg1Zr2SrxHo alloys (x = 0, 1, 3, 5 wt. %) and the microstructure, mechanical properties, degradation behaviour and biocompatibility of the alloys were systematically investigated. The results indicate that the addition of Ho to Mg1Zr2Sr led to the formation of the intermetallic phases MgHo3, Mg2Ho and Mg17Sr2 which resulted in enhanced mechanical strength and decreased degradation rates of the Mg-Zr-Sr-Ho alloys. Furthermore, Ho addition (≤5 wt. %) to Mg-Zr-Sr alloys led to enhancement of cell adhesion and proliferation of osteoblast cells on the Mg-Zr-Sr-Ho alloys. The in vitro biodegradation and the biocompatibility of the Mg-Zr-Sr-Ho alloys were both influenced by the Ho concentration in the Mg alloys; Mg1Zr2Sr3Ho exhibited lower degradation rates than Mg1Zr2Sr and displayed the best biocompatibility compared with the other alloys.

In Vitro Corrosion Study of Friction Stir Processed WE43 Magnesium Alloy in a Simulated Body Fluid

Corrosion behavior of friction stir processing (FSP) WE43 alloy in a simulated body fluid (SBF) was investigated. Micro-galvanic corrosion was the dominated corrosion behavior, and the corrosion resistance of FSP WE43 alloy was improved compared to the cast counterpart. Furthermore, due to the fine-grained and homogeneous microstructure, uniform corrosion morphology was observed on FSP WE43 alloy. According to the tensile properties of specimens with different immersion time intervals, FSP WE43 alloy shows better performance to maintain the mechanical integrity in SBF as compared to the as-cast alloy.

In Vitro Corrosion and Cytocompatibility of a Microarc Oxidation Coating and Poly(L-lactic acid) Composite Coating on Mg-1Li-1Ca Alloy for Orthopedic Implants

Manipulating the degradation rate of biomedical magnesium alloys poses a challenge. The characteristics of a microarc oxidation (MAO), prepared in phytic acid, and poly(L-lactic acid) (PLLA) composite coating, fabricated on a novel Mg-1Li-1Ca alloy, were studied through field emission scanning electron microscopy (FE-SEM), electron probe X-ray microanalysis (EPMA), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The corrosion behaviors of the samples were evaluated via hydrogen evolution, potentiodynamic polarization and electrochemical impedance spectroscopy in Hanks' solution. The results indicated that the MAO/PLLA composite coatings significantly enhanced the corrosion resistance of the Mg-1Li-1Ca alloy. MTT and ALP assays using MC3T3 osteoblasts indicated that the MAO/PLLA coatings greatly improved the cytocompatibility, and the morphology of the cells cultured on different samples exhibited good adhesion. Hemolysis tests showed that the composite coatings endowed the Mg-1Li-1Ca alloys with a low hemolysis ratio. The increased solution pH resulting from the corrosion of magnesium could be tailored by the degradation of PLLA. The degradation mechanism of the composite coatings was discussed. The MAO/PLLA composite coating may be appropriate for applications on degradable Mg-based orthopedic implants.

Influence of bicarbonate concentration on the conversion layer formation onto AZ31 magnesium alloy and its electrochemical corrosion behaviour in simulated body fluid

The electrochemical corrosion behaviour of a magnesium carbonate conversion layer-coated AZ31 magnesium alloy was evaluated in simulated body fluid (SBF) solution.

Ultra low water adhesive metal surface for enhanced corrosion protection

A superhydrophobic surface with ultra low water adhesive force is fabricated on various metals for enhanced corrosion protection.

In vivo evaluation of a magnesium-based degradable intramedullary nailing system in a sheep model

The biocompatibility and the degradation behavior of the LAE442 magnesium-based intramedullary interlocked nailing system (IM-NS) was assessed in vivo in a comparative study (stainless austenitic steel 1.4441LA) for the first time. IM-NS was implanted into the right tibia (24-week investigation period; nails/screws diameter: 9 mm/3.5 mm, length: 130 mm/15-40 mm) of 10 adult sheep (LAE442, stainless steel, n=5 each group). Clinical and radiographic examinations, in vivo computed tomography (CT), ex vivo micro-computed tomography (μCT), mechanical and histological examinations and element analyses of alloying elements in inner organs were performed. The mechanical examinations (four-point bending) revealed a significant decrease of LAE442 implant stiffness, force at 0.2% offset yield point and maximum force. Periosteal (new bone formation) and endosteal (bone decline) located bone alterations occurred in both groups (LAE442 alloy more pronounced). Moderate gas formation was observed within the LAE442 alloy group. The CT-measured implant volume decreased slightly (not significant). Histologically a predominantly direct bone-to-implant interface existed within the LAE442 alloy group. Formation of a fibrous tissue capsule around the nail occurred in the steel group. Minor inflammatory infiltration was observed in the LAE442 alloy group. Significantly increased quantities of rare earth elements were detected in the LAE442 alloy group. μCT examination showed the beginning of corrosion in dependence of the surrounding tissue. After 24 weeks the local biocompatibility of LAE442 can be considered as suitable for a degradable implant material.

STATEMENT OF SIGNIFICANCE

An application oriented interlocked intramedullary nailing system in a comparative study (degradable magnesium-based LAE442 alloy vs. steel alloy) was examined in a sheep model for the first time. We focused in particular on the examination of implant degradation by means of (μ-)CT, mechanical properties (four-point bending), clinical compatibility, local bone reactions (X-ray and histology) and possible systemic toxicity (histology and element analyses of inner organs). A significant decrease of magnesium (LAE442 alloy) implant stiffness and maximum force occurred. Moderate not clinically relevant gas accumulation was determined. A predominantly direct bone-to-implant contact existed within the magnesium (LAE442 alloy) group compared to an indirect contact in the steel group. Rare earth element accumulation could be observed in inner organs but H&E staining was inconspicuous.

New horizon for high performance Mg-based biomaterial with uniform degradation behavior: Formation of stacking faults

Designing the new microstructure is an effective way to accelerate the biomedical application of magnesium (Mg) alloys. In this study, a novel Mg-8Er-1Zn alloy with profuse nano-spaced basal plane stacking faults (SFs) was prepared by combined processes of direct-chill semi-continuous casting, heat-treatment and hot-extrusion. The formation of SFs made the alloy possess outstanding comprehensive performance as the biodegradable implant material. The ultimate tensile strength (UTS: 318 MPa), tensile yield strength (TYS: 207 MPa) and elongation (21%) of the alloy with SFs were superior to those of most reported degradable Mg-based alloys. This new alloy showed acceptable biotoxicity and degradation rate (0.34 mm/year), and the latter could be further slowed down through optimizing the microstructure. Most amazing of all, the uniquely uniform in vitro/vivo corrosion behavior was obtained due to the formation of SFs. Accordingly we proposed an original corrosion mechanism for the novel Mg alloy with SFs. The present study opens a new horizon for developing new Mg-based biomaterials with highly desirable performances.

Effects of zirconium and strontium on the biocorrosion of Mg-Zr-Sr alloys for biodegradable implant applications

The successful applications of magnesium (Mg) alloys as biodegradable orthopedic implants are mainly restricted due to their rapid degradation rate in the physiological environment, leading to a loss of mechanical integrity. This study systematically investigated the degradation behaviors of novel Mg-Zr-Sr alloys using electrochemical techniques, hydrogen evolution, and weight loss in simulated body fluid (SBF). The microstructure and degradation behaviors of the alloys were characterized using optical microscopy, XRD, SEM, and EDX. The results indicate that Zr and Sr concentrations in Mg alloys strongly affected the degradation rate of the alloys in SBF. A high concentration of 5 wt% Zr led to acceleration of anodic dissolution, which significantly decreased the biocorrosion resistance of the alloys and their biocompatibility. A high volume fraction of Mg₁₇Sr₂ phases due to the addition of excessive Sr (over 5 wt%) resulted in enhanced galvanic effects between the Mg matrix and Mg₁₇Sr₂ phases, which reduced the biocorrosion resistance. The average Sr release rate is approximately 0.15 mg L^-1 day^-1, which is much lower than the body burden and proves its good biocompatibility. A new biocorrosion model has been established to illustrate the degradation of alloys and the formation of degradation products on the surface of the alloys. It can be concluded that the optimal concentration of Zr and Sr is less than 2 wt% for as-cast Mg-Zr-Sr alloys used as biodegradable orthopedic implants.

Evaluation of a new Mg-Zn-Ca-Y alloy for biomedical application

In this study, a new Mg-Zn-Ca-Y alloy was evaluated for blood compatibility and in vivo biocompatibility in rabbits after implantation in the sacral crest muscle. Blood test and HE staining was performed to examine the host response, and scanning electron microscope was used to observe the fibrous membrane and corrosion of the magnesium alloy. The results showed that hemolysis rate decreased with the Mg(2+) concentration, in particularly, the hemolysis rate was 47.24 % for the magnesium alloy 100 % mixture solution, while was 0.1372 % for the 1 % extract solution. After implantation, the rabbits showed generally good condition, without swelling and wound secretions. One week after implantation, in the experimental group, a few lymphocytes and macrophages could be observed around the local muscle tissue, and fiber membrane structure had not yet formed; after 2 weeks, loose fiber membranes formed, while the number of inflammatory cells decreased; the fiber membrane became thinner at 4 and 12 weeks,. The fiber membrane thickness at 24 weeks were measured by scanning electron microscopy, at about 15-25 μm, which accord with the U.S. ASTM-F4 implant requirements (<30 μm). Acceptable degradation and corrosion were observed after implantation into rabbits. Through the in vivo study, the new magnesium alloy exhibited good biocompatibility and non-toxic in the experimental animals. Addition of Zn, Ca and Y can slow the degradation rate, and have acceptable side effects in vivo, resulting in improved corrosion properties and desirable biocompatibility at the same time.

Effect of the addition of low rare earth elements (lanthanum, neodymium, cerium) on the biodegradation and biocompatibility of magnesium

Rare earth elements are promising alloying element candidates for magnesium alloys used as biodegradable devices in biomedical applications. Rare earth elements have significant effects on the high temperature strength as well as the creep resistance of alloys and they improve magnesium corrosion resistance. We focused on lanthanum, neodymium and cerium to produce magnesium alloys with commonly used rare earth element concentrations. We showed that low concentrations of rare earth elements do not promote bone growth inside a 750 μm broad area around the implant. However, increased bone growth was observed at a greater distance from the degrading alloys. Clinically and histologically, the alloys and their corrosion products caused no systematic or local cytotoxicological effects. Using microtomography and in vitro experiments, we could show that the magnesium-rare earth element alloys showed low corrosion rates, both in in vitro and in vivo. The lanthanum- and cerium-containing alloys degraded at comparable rates, whereas the neodymium-containing alloy showed the lowest corrosion rates.

Applications of dynamic electrochemical impedance spectroscopy (DEIS) to evaluate protective coatings formed on AZ31 magnesium alloy

Increase in magnitude (IZI) and phase angle maximum values with applied potential reveal the protective nature of the coating.

In vitro evaluation of biodegradable magnesium alloys containing micro-alloying additions of strontium, with and without zinc

(a) Alloying of microlevels of Sr do not promote thein vitrocathodic reaction, whilst it slightly moderates the anodic kinetics of Mg in MEM and (b) adopted microadditions of Sr around its solid solubility in a Mg implant favourable for bone growth.

Recent advances on the development of magnesium alloys for biodegradable implants

In recent years, much progress has been made on the development of biodegradable magnesium alloys as "smart" implants in cardiovascular and orthopedic applications. Mg-based alloys as biodegradable implants have outstanding advantages over Fe-based and Zn-based ones. However, the extensive applications of Mg-based alloys are still inhibited mainly by their high degradation rates and consequent loss in mechanical integrity. Consequently, extensive studies have been conducted to develop Mg-based alloys with superior mechanical and corrosion performance. This review focuses on the following topics: (i) the design criteria of biodegradable materials; (ii) alloy development strategy; (iii) in vitro performances of currently developed Mg-based alloys; and (iv) in vivo performances of currently developed Mg-based implants, especially Mg-based alloys under clinical trials.