The influence of HF treatment on corrosion resistance and in vitro biocompatibility of Mg-Zn-Zr alloy

The samples made of a Mg-2.5wt.%Zn-0.5wt.%Zr alloy were immersed in the 20% hydrofluoric acid (HF) solution at room temperature for different time, with the aim of improving the properties of magnesium (Mg) alloy in applications as biomaterials. The corrosion resistance and in vitro biocompatibility of untreated and fluoride-coated samples were investigated. The results show that the optimum process is to immerse Mg alloys in the 20% HF solution for 6 h. After the immersion, a dense magnesium fluoride (MgF₂) coating of 0.5 μm was synthesized on the surface of Mg-Zn-Zr alloy. Polarization tests recorded a reduction in the corrosion current density from 2.10 to 0.05 μA/cm² due to the MgF₂ protective coating. Immersion tests in the simulated body fluid (SBF) also reveal a much milder corrosion on the fluoride-coated samples, and its corrosion rate was calculated to be 0.05 mm/yr. Hemolysis test suggests that the conversion coated Mg alloy has no obvious hemolysis reaction. The hemolysis ratio (HR) of the samples decreases from 11.34% to 1.86% with the HF treatment, which meets the requirements of biomaterials (HR < 5%). The coculture of 3T3 fibroblasts with Mg alloy results in the adhesion and proliferation of cells on the surface of fluoride-coated samples. All the results show that the MgF₂ conversion coating would markedly improve the corrosion resistance and in vitro biocompatibility of Mg-Zn-Zr alloy.     

In vitro degradation,hemolysis and MC3T3-E1 cell adhesion of biodegradable Mg–Zn alloy

In this study a kind of patent binary Mg–6 wt.%Zn magnesium alloy was investigated as degradable biomedical material. The results of in vitro degradation including electrochemical measurements and immersion tests in simulated body fluid (SBF) revealed that zinc could elevate both the corrosion potential and Faraday charge transfer resistance of magnesium and thus improve the corrosion resistance. XRD and EDS analysis proved that the corrosion products on the surface of Mg–Zn contained hydroxyapatite (HA), Mg(OH)₂ and other Mg/Ca phosphates, which could reduce the degradation rate. The degradation process of magnesium alloy and the mechanism of corrosion layer formation were also discussed in this work, i.e. the byproducts of degradation of magnesium, Mg²⁺ and OH^?, reacted with the phosphate and Ca²⁺ in the SBF, thus the corrosion layer containing HA, Mg(OH)₂ and other magnesium-substituted apatite precipitated in corrosion pits and covered the surface of magnesium alloy.The hemolysis test found that the hemolysis rate of Mg–Zn was 3.4%, which is lower than the safe value of 5% according to ISO 10993-4. For the cell culture experiments, after 2 h incubation the pre-osteoblastic cell MC3T3-E1 was able to adhere and spread on the corrosion layer of Mg–Zn alloy, indicating that despite the fluctuation of pH value of DMEM culture solution, Mg–Zn alloy could still support the earlier adhesion of pre-osteoblastic cells on the surface. Hemolysis and adhesion of cells display good biocompatibility of Mg–Zn alloy in vitro.     

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.     

Hydroxyapatite coating on magnesium with MgF<Subscript>2</Subscript> interlayer for enhanced corrosion resistance and biocompatibility

Hydroxyapatite (HA) was coated onto pure magnesium (Mg) with an MgF₂ interlayer in order to reduce the surface corrosion rate and enhance the biocompatibility. Both MgF₂ and HA were successfully coated in sequence with good adhesion properties using the fluoride conversion coating and aerosol deposition techniques, respectively. In a simulated body fluid (SBF), the double layer coating remarkably enhanced the corrosion resistance of the coated Mg specimen. The in vitro cellular responses of the MC3T3-E1 pre-osteoblasts were examined using a cell proliferation assay and an alkaline phosphatase (ALP) assay, and these results demonstrated that the double coating layer also enhanced cell proliferation and differentiation levels. In the in vivo study, the HA/MgF₂ coated Mg corroded less than the bare Mg and had a higher bone-to-implant contact (BIC) ratio in the cortical bone area of the rabbit femora 4 weeks after implantation. These in vitro and in vivo results suggested that the HA coated Mg with the MgF₂ interlayer could be used as a potential candidate for biodegradable implant materials.     

Effect of fluoride coating on in vitro dynamic degradation of Mg–Zn alloy

A compact and flat fluoride coating with some pores was prepared on a Mg–Zn alloy in order to control its degradation behavior. The electrochemical tests demonstrated that the real impedance (Z_re) of the fluoride-coated Mg–Zn was approximately 10 times as large as that of the untreated alloy. The free corrosion potential (E_corr), compared to that of the uncoated Mg–Zn alloy, increased 646 mV for the coated metal. The free corrosion current (I_corr) of the Mg–Zn specimen with the fluoride film was about one tenth of that of the uncoated one. The in vitro dynamic degradation tests showed that the average weight loss of the fluoride-coated Mg–Zn was lower than that of the untreated alloy in the initial 4 h of the tests, indicating the film could function as a barrier coating on Mg–Zn matrix. However, the coating cracked and peeled severely after 4 h dynamic tests, which implied that the fluoride coating could not endure the sustaining washing of the modified simulated body fluid.     

Blood compatibility of zinc–calcium phosphate conversion coating on Mg–1.33Li–0.6Ca alloy

Magnesium alloys as a new class of biomaterials possess biodegradability and biocompatibility in comparison with currently used metal implants. However, their rapid corrosion rates are necessary to be manipulated by appropriate coatings. In this paper, a new attempt was used to develop a zinc-calcium phosphate (Zn-Ca-P) conversion coating on Mg-1.33Li-0.6Ca alloys to increase the biocompatibility and improve the corrosion resistance. In vitro blood biocompatibility of the alloy with and without the Zn-Ca-P coating was investigated to determine its suitability as a degradable medical biomaterial. Blood biocompatibility was assessed from the hemolysis test, the dynamic cruor time test, blood cell count and SEM observation of the platelet adhesion to membrane surface. The results showed that the Zn-Ca-P coating on Mg-1.33Li-0.6Ca alloys had good blood compatibility, which is in accordance with the requirements for medical biomaterials.     

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 Hank's 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 Zealand's 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 10 weeks 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.     

Effects of Ca on microstructure, mechanical and corrosion properties and biocompatibility of Mg–Zn–Ca alloys

Zn and Ca were selected as alloying elements to develop an Mg–Zn–Ca alloy system for biomedical application due to their good biocompatibility. The effects of Ca on the microstructure, mechanical and corrosion properties as well as the biocompatibility of the as-cast Mg–Zn–Ca alloys were studied. Results indicate that the microstructure of Mg–Zn–Ca alloys typically consists of primary α-Mg matrix and Ca₂Mg₆Zn₃/Mg₂Ca intermetallic phase mainly distributed along grain boundary. The yield strength of Mg–Zn–Ca alloy increased slightly with the increase of Ca content, whilst its tensile strength increased at first and then decreased. Corrosion tests in the simulated body fluid revealed that the addition of Ca is detrimental to corrosion resistance due to the micro-galvanic corrosion acceleration. In vitro hemolysis and cytotoxicity assessment disclose that Mg–5Zn–1.0Ca alloy has suitable biocompatibility.     

Anti-corrosion mechanism of epoxy-resin and different content Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> coatings on magnesium alloy

In this study, anti-corrosion coatings were prepared and coated successfully on magnesium alloy substrates by mixing nanopowders, solvent, curing agent with epoxy resin. The effect of the amount of iron trioxide (Fe₂O₃) on the adhesion strength and corrosion resistance on magnesium alloy was investigated with standard protocols, and electrochemical measurements were also made in 3.5 wt.% NaCl solutions. The surface morphology and corrosion mechanism after corrosion tests was characterized using FESEM analysis. Nanoparticles in matrix acted as filler, and interstitial cross-linked spaces and other coating artifacts regions (micro cracks and voids) would all affect the anti-corrosion properties of coating. The results showed the proper powder content not only provided adhesion strength to these coatings but also improved obviously their anticorrosion. Hydrogen bound to the amine nitrogen (¹N) could take part in the curing process rather than hydrogen of the amide site due to the smaller ΔG and the more stable configuration.     

Effect of Mg<Superscript>2+</Superscript> concentration on biocompatibility of pure magnesium

The aim of the present study is to find the correlation between the Mg²⁺ concentration degraded from pure magnesium material and the biocompatibility of the material. Hemolysis ratio (HR) of the extracts of pure magnesium with different Mg²⁺ concentration were measured according to ISO 10993.4 standard. The cytotoxicity tests were carried out by both indirect contact with fibroblast L929 and preosteoblasts MC3T3-E1, and MTT tests were used. Cytotoxicity of the pure magnesium with and without surface modification was further evaluated by direct contact method. Samples were cultured with Osteoblast MC3T3-E1 and the effects of the material on viability and activity of cells were discussed. The results showed that the hemolysis rate and cytotoxicity of the modified Mg could meet the requirement for biomaterials. In our test, the hemolysis rate of the extracts was qualified when the concentration of Mg²⁺ ? 42 mg/L; the extracts with 202 mg/L Mg²⁺ met the cytotoxicity requirement, and the extracts with 156 mg/L Mg²⁺ promoted cell proliferation. Therefore, the biocompatibility of magnesium-based materials can be improved by suitable surface modification.     

Preparation and Performance of Electroless Nickel on AZ91D Magnesium Alloy

The corrosion of magnesium alloy in different plating solutions was researched. The results demonstrated that corrosive condition of the alloy immersed in nickel chloride solution and nickel sulfate solution is serious and in nickel acetate solution and nickel nitrate solution is less. Magnesium alloy was handled with four acid pickling formulas and two activation formulas and the effects of different pickling formulas and activation formulas were researched through comparative experiment. The experimental results indicated that after handed with pickling formula about 500 mL L^?1 H₃PO₄ (85%), 110 mL L^?1 HNO₃ (68%), room temperature for 30 s and activation formula about 375 mL L^?1 HF (40%), room temperature for 10 min, magnesium alloy could realize electroless nickel plating directly and the performance of the prepared plating was much better. The properties of the nickel-plating coating were researched by electrochemical workstation, scanning electron microscope, and X-ray diffraction. The results demonstrated that this Ni–P coating was very uniform and meticulous; the structure of Ni–P coating was amorphous; and comparing with magnesium alloy, the corrosion potential of this plating increased about 799 V and the corrosion current density declined obviously. The nickel-plating coating effectively improved the anticorrosion performance of magnesium alloy.     

Fluoride treatment and in vitro corrosion behavior of an AZ31B magnesium alloy

As a new class of biodegradable material, magnesium alloys have attracted much attention in recent years. In order to improve the corrosion resistance, a fluoride coating was prepared on the surface of AZ31B magnesium alloy. The surface characterization analysis showed a dense coating with some irregular pores was formed. The TF-XRD analysis indicated that the coating was mainly composed of MgO and MgF₂. Electrochemical and immersion tests proved that the fluoride conversion coating significantly improved the corrosion resistance of AZ31B. Three-point bending test revealed that the degradation behavior of the fluoride treated AZ31B could meet the requirement as a biodegradable material.     

Corrosion resistance of AZ91D magnesium alloy with electroless plating pretreatment and Ni–TiO2 composite coating

In this paper, a protective multilayer coating, with electroless Ni coating as bottom layer and electrodeposited Ni–TiO₂ composite coating as top layer, was successfully prepared on AZ91D magnesium alloy by a combination of electroless and electrodeposition techniques. Scanning electron microscopy and X-ray diffraction were employed to investigate the surface, cross-section morphologies and phase structure of coatings, respectively. The electrochemical corrosion behaviors of coatings in 3.5 wt.% NaCl solutions were evaluated by electrochemical impedance spectroscopy, open circuit potential and potentiodynamic polarization techniques. The results showed that the corrosion process of Ni–TiO₂ composite coating was mainly composed of three stages in the long-term immersion test in the aggressive media, and could afford better corrosion and mechanical protection for the AZ91D magnesium alloy compared with single electroless Ni coating. The micro-hardness of the Ni–TiO₂ composite coating improved more than 5 times than that of the AZ91D magnesium alloy.     

Effect of applied potential on the microstructure,composition and corrosion resistance evolution of fluoride conversion film on AZ31 magnesium alloy

AZ31 magnesium (Mg) alloy was potentiostatic polarized in 0.1?M deaerated KF solution with pH 7.5 from ?0.4?V to ?1.4?V with an interval of ?0.2?V. The polarization process was described by the potentiostatic current decay. The resultant film was analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The results demonstrated that the deposited film included a Mg(OH)₂/MgF₂ containing inner layer and a Mg(OH)₂/MgF₂/KMgF₃ comprising outer layer. The high polarized potential produced high content of MgF₂ but low content of KMgF₃ and thin film. Conversely, the low polarized potential produced small content of MgF₂ but high content of KMgF₃ and thick film. The optimal corrosion resistance of the deposited film was obtained at ?1.4?V, which was closely related with the content of MgF₂ and KMgF₃ and the film thickness.     

Microstructure,mechanical and corrosion properties and biocompatibility of Mg–Zn–Mn alloys for biomedical application

Mn and Zn were selected to develop a Mg–Zn–Mn magnesium alloy for biomedical application due to the good biocompatibility of Zn and Mn elements. Microstructure, mechanical properties, corrosion properties and biocompatibility of the Mg–Zn–Mn alloys have been investigated by use of optical microscope, scanning electron microscope, tensile testing, and blood hemolysis and cell toxicity. Microstructure observation has shown that the addition of Zn and the extrusion significantly refined the grain size of both the as-cast and the extruded magnesium alloys, which mainly contributes to the high tensile strength and good elongation. Polarization test has shown Zn could accelerate the formation of a passivation film, which provides good protection to the magnesium alloy against simulate body fluid. Cell culture and hemolysis tests have shown that the magnesium alloy did not have cell toxicity, showing good cytocompatibility, but the alloy caused hemolysis to blood system. It was suggested that surface modification have to be adopted to improve the blood compatibility of the magnesium alloy for the application in blood environment.     

Influence of fluoride treatment on surface properties,biodegradation and cytocompatibility of Mg–Nd–Zn–Zr alloy

Fluoride treatment is a commonly used technique or pre-treatment to optimize the degradation kinetic and improve the biocompatibility of magnesium-based implant. The influence of changed surface properties and degradation kinetics on subsequent protein adsorption and cytocompatibility is critical to understand the biocompatibility of the implant. In this study, a patent magnesium alloy Mg–Nd–Zn–Zr alloy (JDBM) designed for cardiovascular stent application was treated by immersion in hydrofluoric acid. A 1.5 μm thick MgF₂ layer was prepared. The surface roughness was increased slightly while the surface zeta potential was changed to a much more negative value after the treatment. Static contact angle test was performed, showing an increase in hydrophilicity and surface energy after the treatment. The MgF₂ layer slowed down in vitro degradation rate, but lost the protection effect after 10 days. The treatment enhanced human albumin adsorption while no difference of human fibrinogen adsorption amount was observed. Direct cell adhesion test showed many more live HUVECs retained than bare magnesium alloy. Both treated and untreated JDBM showed no adverse effect on HUVEC viability and spreading morphology. The relationship between changed surface characteristics, degradation rate and protein adsorption, cytocompatibility was also discussed.     

Influence of dicalcium phosphate dihydrate coating on the <Emphasis Type="Italic">in vitro</Emphasis> degradation of Mg-Zn alloy

To reduce the degradation rate and further to improve the biocompatibility of magnesium alloy, dicalcium phosphate dihydrate (CaHPO₄·2H₂O, DCPD) has been fabricated on a kind of magnesium-zinc alloy by way of electrodeposition method. The experimental results of XRD, SEM and EDS showed that the electrodeposited coating on the Mg-Zn alloy mainly contains the flake-like DCPD, along with some octacalcium phosphate (Ca₈(HPO₄)₂(PO₄)₄·4H₂O, OCP). After the in vitro degradation of the coated alloy in modified-simulated body fluid (m-SBF), it was proved that the coating could reduce the degradation rate effectively, and the samples were covered by calcium phosphate salts with a higher Ca/P ratio. Therefore, it indicates that compared with the bare alloy, the DCPD coating rendered a more biocompatible surface, and is a promising coating candidate for biomedical magnesium materials.     

Corrosion resistance of in-situ growth of nano-sized Mg(OH)2 on micro-arc oxidized magnesium alloy AZ31—Influence of EDTA

One of the major obstacles for the clinical use of biodegradable magnesium (Mg)-based materials is their high corrosion rate. Micro-arc oxidation (MAO) coatings on Mg alloys provide mild corrosion protection owing to their porous structure. Hence, in this study a dense Mg(OH)₂ film was fabricated on MAO-coated Mg alloy AZ31 in an alkaline electrolyte containing ethylenediamine tetraacetic acid disodium (EDTA-2Na) to reinforce the protection. Surface morphology, chemical composition and growth process of the MAO/Mg(OH)₂ hybrid coating were examined using field-emission scanning electron microscopy, energy dispersive X-ray spectrometer, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared spectrophotometer. Corrosion resistance of the coatings was evaluated via potentiodynamic polarization curves and hydrogen evolution tests. Results manifested that the Mg(OH)₂ coating possesses a porous nano-sized structure and completely seals the micro-pores and micro-cracks of the MAO coating. The intermetallic compound of AlMn phase in the substrate plays a key role in the growth of Mg(OH)₂ film. The current density of Mg(OH)₂-MAO composite coating decreases three orders of magnitude in comparison with that of bare substrate, indicating excellent corrosion resistance. The Mg(OH)₂-MAO composite coating is beneficial to the formation of calcium phosphate corrosion products on the surface of Mg alloy AZ31, demonstrating a great promise for orthopaedic applications.     

Micromorphological effect of calcium phosphate coating on compatibility of magnesium alloy with osteoblast

Abstract

Octacalcium phosphate (OCP) and hydroxyapatite (HAp) coatings were developed to control the degradation speed and to improve the biocompatibility of biodegradable magnesium alloys. Osteoblast MG-63 was cultured directly on OCP- and HAp-coated Mg-3Al-1Zn (wt%, AZ31) alloy (OCP- and HAp-AZ31) to evaluate cell compatibility. Cell proliferation was remarkably improved with OCP and HAp coatings which reduced the corrosion and prevented the H₂O₂ generation on Mg alloy substrate. OCP-AZ31 showed sparse distribution of living cell colonies and dead cells. HAp-AZ31 showed dense and homogeneous distribution of living cells, with dead cells localized over and around corrosion pits, some of which were formed underneath the coating. These results demonstrated that cells were dead due to changes in the local environment, and it is necessary to evaluate the local biocompatibility of magnesium alloys. Cell density on HAp-AZ31 was higher than that on OCP-AZ31 although there was not a significant difference in the amount of Mg ions released in medium between OCP- and HAp-AZ31. The outer layer of OCP and HAp coatings consisted of plate-like crystal with a thickness of around 0.1 μm and rod-like crystals with a diameter of around 0.1 μm, respectively, which grew from a continuous inner layer. Osteoblasts formed focal contacts on the tips of plate-like OCP and rod-like HAp crystals, with heights of 2–5 μm. The spacing between OCP tips of 0.8–1.1 μm was wider than that between HAp tips of 0.2–0.3 μm. These results demonstrated that cell proliferation depended on the micromorphology of the coatings which governed spacing of focal contacts. Consequently, HAp coating is suitable for improving cell compatibility and bone-forming ability of the Mg alloy.     

Mechanical and bio-corrosion properties of quaternary Mg–Ca–Mn–Zn alloys compared with binary Mg–Ca alloys

Binary Mg–xCa alloys and the quaternary Mg–Ca–Mn–xZn were studied to investigate their bio-corrosion and mechanical properties. The surface morphology of specimens was characterized by X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The results of mechanical properties show that the yield strength (YS), ultimate tensile strength (UTS) and elongation of quaternary alloy increased significantly with the addition of zinc (Zn) up to 4 wt.%. However, further addition of Zn content beyond 4 wt.% did not improve yield strength and ultimate tensile strength. In contrast, increasing calcium (Ca) content has a deleterious effect on binary Mg–Ca alloys. Compression tests of the magnesium (Mg) alloys revealed that the compression strength of quaternary alloy was higher than that of binary alloy. However, binary Mg–Ca alloy showed higher reduction in compression strength after immersion in simulated body fluid. The bio-corrosion behaviour of the binary and quaternary Mg alloys were investigated using immersion tests and electrochemical tests. Electrochemical tests shows that the corrosion potential (E_corr) of binary Mg–2Ca significantly shifted toward nobeler direction from −1996.8 to −1616.6 mV_SCE with the addition of 0.5 wt.% manganese (Mn) and 2 wt.% Zn content. However, further addition of Zn to 7 wt.% into quaternary alloy has the reverse effect. Immersion tests show that the quaternary alloy accompanied by two secondary phases presented higher corrosion resistance compared to binary alloys with single secondary phase. The degradation behaviour demonstrates that Mg–2Ca–0.5Mn–2Zn alloy had the lowest degradation rate among quaternary alloys. In contrast, the binary Mg–2Ca alloy demonstrated higher corrosion rates, with Mg–4Ca alloy having the highest rating. Our analysis showed the Mg–2Ca–0.5Mn–2Zn alloy with suitable mechanical properties and excellent corrosion resistance can be used as biodegradable implants.