Bioactive HA/TiO2 coating on magnesium alloy for biomedical applications

Magnesium alloys are new class of biodegradable alloys having many favourable properties to overcome the limitations of currently used biomedical alloys. Recently, several coatings have been developed to overcome their higher degradation rate. In this regard, a new attempt has been made to develop Hydroxyapatite and Hydroxyapatite/TiO₂ coatings on magnesium alloys to increase the biocompatibility and reduce the corrosion rate. The coated surfaces were characterized by Fourier-Transform Infrared Spectroscopy (FT-IR), X-ray Diffraction Analysis (XRD), Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray (EDX) Spectroscopy, Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). Contact angle measurements proved higher hydrophilic nature of HA/TiO₂ coating compared to HA coating. In-vitro studies showed that HA–TiO₂ coated alloy exhibited higher osteoinduction compared to HA coated alloy. Hydrogen evolution studies and corrosion studies confirmed greater reduction in degradation rate of HA/TiO₂ coated alloy. Vickers microhardness test also showed enhancement in mechanical strength of the composite coated alloy compared to HA coated alloy. Three point bend test depicted better adherence of the HA/TiO₂ coating compared to HA coating on the substrate. Cell culture studies proved higher cell attachment and proliferation on composite coated alloy by controlling the release of magnesium ions into the surrounding body tissue.     

Long-term corrosion resistance and fast mineralization behavior of micro-nano hydroxyapatite coated magnesium alloy in vitro

To improve the long-term corrosion resistance of biodegradable AZ31 magnesium alloy, the micro-nano structural hydroxyapatite (HA) coating was fabricated on AZ31 substrate by hydrothermal treatment. The compact and high crystallinity HA coating prepared at 120 °C had excellent electrochemical properties. Moreover, the cell viability experiment revealed that the micro-nano structure coating was conducive to the viability and proliferation of MC3T3-E1 osteoblasts. The immersion experiment in simulated body fluid (SBF) solution showed that the micro-nano structural HA coatings could quickly induce the production of HA mineralization, and then the mineralization evolved into a compact mineralized layer on the surface of coated sample, which provided a long-term protection for the specimen. Even after 147 days of immersion, the coated samples remained the relatively complete macroscopic shape, the corrosion rates were lower than 0.500 mm/y and the pH values of the SBF solution maintained in the range of 7.10–7.80, suggesting when these coated AZ31 magnesium alloys were used as degradable biomaterial implants, they could provide a long-term mechanical support during the healing of damaged bones.     

Corrosion Resistance of MgZn Alloy Covered by Chitosan-Based Coatings

Chitosan coatings are deposited on the surface of Mg20Zn magnesium alloy by means of the spin coating technique. Their structure was investigated using Fourier Transform Infrared Spectroscopy (FTIR) an X-ray photoelectron spectroscopy (XPS). The surface morphology of the magnesium alloy substrate and chitosan coatings was determined using Scanning Electron Microscope (FE-SEM) analysis. Corrosion tests (linear sweep voltamperometry and chronoamperometry) were performed on uncoated and coated magnesium alloy in the Hank’s solution. In both cases, the hydrogen evolution method was used to calculate the corrosion rate after 7-days immersion in the Hank’s solution at 37 °C. It was found that the corrosion rate is 3.2 mm/year and 1.2 mm/year for uncoated and coated substrates, respectively. High corrosion resistance of Mg20Zn alloy covered by multilayer coating (CaP coating + chitosan water glass) is caused by formation of CaSiO₃ and Ca₃(PO₄)₂ compounds on its surface.     

Self-healing hybrid coating of phytic acid/silane for improving the corrosion resistance of magnesium alloy

In order to improve the corrosion resistance of a biodegradable magnesium alloy, a series of phytic acid/3-aminopropyltrimethoxysilane (γ-APS) hybrid coatings was prepared on AZ31 magnesium alloys by dipping the magnesium alloy into the mixing solution of phytic acid and γ-APS. During the preparation of hybrid coatings, the pH values of the mixing solutions greatly affected the uniformity of the coatings and subsequently influenced their corrosion resistance. Electrochemical tests indicated that the hybrid coating prepared in the solution of pH = 8.0 could highly improve corrosion resistance of AZ31 magnesium alloys. Meanwhile, corrosion current density of the hybrid coating coated sample was significantly decreased from the uncoated sample of 138.1 ± 11.9 to 8.5 ± 0.8 μA cm^?2. Immersion test in simulated body fluid revealed that the cracks on the surface of the hybrid coating gradually healed up during the lengthy immersion.     

Development of HA-CNTs composite coating on AZ31 Magnesium alloy by cathodic electrodeposition. Part 2: Electrochemical and in-vitro behavior

The carbon nano-tubes (CNTs) reinforced hydroxyapatite (HA), with various functionalized CNTs concentration ranging from 0 to 1.5?wt%, were deposited on AZ31 magnesium alloy by direct and pulse cathodic electrodeposition methods. The corrosion resistance of the coatings was tested in simulated body fluid (SBF) using different electrochemical methods such as open circuit potential, polarization and electrochemical impedance spectroscopy. The in-vitro behavior, changes in solution pH as well as the amount of evolved hydrogen of these coatings were also evaluated during five days immersion in SBF. The results indicated that the pulse deposited HA having 1% CNTs coating was the optimum condition which decreased the corrosion current density of AZ31 magnesium alloy from 44.25?µA/cm² to 0.72?µA/cm². Moreover, it stabilized the alkalization behavior of AZ31 alloy and caused a tenfold decrease in the amount of hydrogen generation in SBF. Additionally, the formation of new hydroxyapatite layer on the surface of the pre-exist coatings after five days immersion in SBF was confirmed by SEM characterization.     

Influence of chloride ion concentration on the electrochemical corrosion behaviour of plasma electrolytic oxidation coated AM50 magnesium alloy

The electrochemical degradation of a silicate- and a phosphate-based plasma electrolytic oxidation (PEO) coated AM50 magnesium alloy obtained using a pulsed DC power supply was investigated using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) in NaCl solutions of different chloride ion concentrations viz., 0.01 M, 0.1 M, 0.5 M and 1 M. The surface of the PEO coated specimens after 50 h of immersion/EIS testing was examined by optical microscopy and scanning electron microscopy. The results showed that the corrosion deterioration of PEO coated magnesium alloy in NaCl solutions was significantly influenced by chloride ion concentration. The silicate-based coating was found to offer a superior corrosion resistance to the magnesium substrate than the phosphate-based coatings in lower chloride ion concentration NaCl solutions (0.01 M and 0.1 M NaCl). On the other hand both these PEO coatings were found to be highly susceptible to localized damage, and could not provide an effective corrosion protection to Mg alloy substrate in solutions containing higher chloride concentrations (0.5 M and 1 M). The extent of localized damage was observed to be more with increase in chloride concentration in both the cases.     

Composite anticorrosion coatings for AZ91D magnesium alloy with molybdate conversion coating and silicon sol–gel coatings

This study evaluated the corrosion resistance of AZ91D magnesium alloy coated by composite coatings which consisted of a molybdate conversion coating and three layers of silicon sol–gel coatings. For molybdate conversion treatment, various conditions including the pH of the molybdate baths, immersion time and bath temperature were investigated using electrochemical measurements. The corrosion resistance of the AZ91D magnesium alloy was improved to some extent by the conversion coating with the optimal conversion parameters (7.3 g/L (NH₄)₆Mo₇O₂₄·6H₂O solution with pH 5 for 30 min at 30 °C).     

Preparation and characterization of hydroxyapatite coating by magnetron sputtering on Mg–Zn–Ag alloys for orthopaedic trauma implants

The aim of the present paper was to evaluate the effect of hydroxyapatite coatings on the two types of Mg–Zn–Ag alloys as a possible solution to control magnesium alloy degradation. The coatings were prepared by the radio frequency magnetron sputtering method at a deposition temperature of 300 °C. To perform this evaluation, the coated alloys were immersed in a simulated body fluid solution at body temperature (37 ± 0.5 °C) to determine the corrosion resistance through electrochemical and immersion tests. Moreover, the investigation also consisted of the evaluation of microchemical, mechanical, and morphological properties. The deposition temperature of 300 °C was enough to obtain a crystalline hydroxyapatite structure with a Ca/P ratio close to the stochiometric one. The adhesion of coatings was not influenced by the nature of Mg–Zn–Ag alloys, so similar values for both coated alloys were found. The results showed that the coating was homogonous deposited on the Mg–Zn–Ag alloys and the corrosion resistance of uncoated magnesium alloys was improved.     

Novel bi-layered nanostructured SiO2/Ag-FHAp coating on biodegradable magnesium alloy for biomedical applications

In this study, a novel bi-layered nanostructured silica (SiO₂)/ silver-doped fluorohydroxyapatite (Ag-FHAp) coating was deposited on biodegradable Mg-1.2Ca-4.5Zn alloy via physical vapor deposition (PVD) combined with electrodeposition (ED). The nano-SiO₂ underlayer had a compact columnar microstructure with thickness of around 1 µm while the Ag-FHAp overlayer presented large plate-like crystals accompanied with small rounded particles with thickness about 10 µm. Potentiodynamic polarization test exhibited that the double layer SiO₂/Ag-FHAp coated Mg alloy has superior corrosion resistance compared to uncoated and single layer SiO₂ coated samples. Contact angle measurement showed that Ag-FHAp coating over nano-SiO₂ layers significantly increased surface wettability which is favorable for the attachment of cells. Cytotoxicity tests indicated that the nanostructured SiO₂/Ag-FHAp coating enabled higher cell viability compared to nano-SiO₂ coating and uncoated samples. In addition, bi-layer and single-layer coatings considerably improved the ability of cell attachment than that of the uncoated samples. The cell viability of coated and uncoated samples increased with increasing incubation time. The double layer SiO₂/Ag-FHAp coated biodegradable Mg alloy possessed high corrosion resistance and cytocompatibility and can be considered as a promising material for implant applications.     

Fabrication and characterization of hybrid coating on Mg–Zn–Ca Mg alloy for enhanced corrosion and degradation resistance as medical implant

Magnesium (Mg)-based alloys have appealing properties as promising implants for medical applications. However, their clinical applications are hindered due to the rapid corrosion and degradation rate in the physiological environment. In this investigation, we reported a novel interfacial engineering approach for the fabrication of polymer/ceramic hybrid coating on Mg–Zn–Ca Mg alloy. Firstly, hydroxyapatite (HA) coating was fabricated on the Mg–Zn–Ca sample followed by an alkali treatment that was performed in 1 M NaOH solution at 60 °C. Finally, polycaprolactone (PCL) coating was synthesized using a dip-coating approach on the top of the HA-coated Mg–Zn–Ca specimen. Microhardness test and adhesion test revealed that PCL/HA hybrid coating significantly improved mechanical properties and enhanced biointerface property between the substrate and coating. The immersion tests showed that the hybrid coating considerably slowed down the degradation in the simulated body fluid (SBF) solution. In addition, in vitro electrochemical investigations confirmed that PCL/HA coating significantly improved corrosion resistance and greatly reduced corrosion rate by about 10 times compared to HA coating and about 900 times to untreated Mg–Zn–Ca sample. Moreover, cytotoxicity assessment exhibited PCL/HA hybrid coating enhanced biocompatibility and bioactivity due to adopting a suitable interfacial engineering approach.     

Controlling the degradation rate of bioactive magnesium implants by electrophoretic deposition of akermanite coating

In order to improve the corrosion resistance and the surface bioactivity of biodegradable magnesium alloys, a nanostructured akermanite (Ca₂MgSi₂O₇) coating was grown on AZ91 magnesium alloy through electrophoretic deposition (EPD) assisted with micro arc oxidation (MAO) method. The crystalline structures, morphologies and compositions of samples were characterized by X–ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The in vitro bio–corrosion (biodegradability) and bioactivity behaviors of samples were investigated by electrochemical and immersion tests. The experimental results indicated that the nanostructured akermanite coating could slow down the corrosion rate and improve the in vitro bioactivity of biodegradable magnesium alloy. Thus, magnesium alloy coated with nanostructured akermanite may be a promising candidate to be used as biodegradable bone implants.     

In-vitro corrosion inhibition mechanism of fluorine-doped hydroxyapatite and brushite coated Mg–Ca alloys for biomedical applications

Magnesium alloys have received great attention as a new kind of biodegradable metallic biomaterials. However, they suffer from poor corrosion resistance. In this study, Mg–Ca alloy was coated with nano-fluorine-doped hydroxyapatite (FHA), and brushite (DCPD); via electrochemical deposition (ED). Coatings were characterized by X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The results revealed that nano-fluorine-doped hydroxyapatite coating produced more dense and uniform coating layer, compared to the brushite coating. The compression tests of the ED-coated Mg alloy samples immersed in simulated body fluid for different time periods showed higher yield strength (YS) and ultimate tensile strength (UTS), compared to those of the uncoated samples. The degradation behavior and corrosion properties of the ED-coated Mg alloy samples were examined via electrochemical measurements and immersion tests. The results showed that FHA coating could effectively induce the precipitation of more Ca²⁺ and PO₄³⁻ ions than DCPD coating, because the nanophase can provide higher specific surface area. It was also found that FHA and DCPD coatings can significantly decline the initial degradation rate of the alloy. A corrosion mechanism of the ED-coated alloy is proposed and discussed in this paper.     

Hydroxyapatite-carboxymethyl cellulose-graphene composite coating development on AZ31 magnesium alloy: Corrosion behavior and mechanical properties

In this study, hydroxyapatite (HA) coatings containing carboxymethyl cellulose (CMC) and graphene (Gr) were developed on AZ31 magnesium alloy through two-step electrophoretic deposition method. The morphology and chemical bonding of coatings were characterized and also the phase identification was done using scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction, respectively. Moreover, the corrosion behavior of the applied coatings was compared with the bare AZ31 Mg alloy substrate in the simulated body fluid by the means of potentiodynamic polarization test and electrochemical impedance spectroscopy. Obtained results revealed that the novel HA-CMC-Gr coating possesses the highest corrosion resistance compared to the HA, HA-CMC, and HA-Gr coatings due to its uniform and compact structure. To investigate the mechanical properties and to elucidate the effect of CMC on the adhesion of coating-alloy interface, pull-off test was employed, where results demonstrated that the addition of CMC increases the adhesion force from 1.06 MPa to 1.62 MPa. Besides, the modulus of elasticity and the hardness of HA and HA-Gr composite coatings were compared by applying nanoindentation test. Interestingly, it is detected that the presence of Gr has considerably increased the elastic modulus of the coating by approximately 30% in comparison to the pure HA coating.     

High corrosion protection of a polyaniline/organophilic montmorillonite coating for magnesium alloys

Epoxy coatings containing polyaniline (PANI) and polyaniline/organophilic montmorillonite (PANI/OMMT) powders were prepared on the surface of AZ91D magnesium alloy. The corrosion performance of the coatings was evaluated by electrochemical impedance spectroscopy (EIS) and open-circuit potential analysis in 3.5% NaCl. The results indicate that the PANI/OMMT coating retained its high corrosion protection for AZ91D magnesium alloy after 6000 h of immersion. The protective mechanism conferred by the PANI/OMMT coating was also discussed. The effects of oxygen on the protective mechanism of PANI were evaluated by EIS measurements in a 3.5% deaerated NaCl solution.     

Influence of substrate composition on corrosion protection of sol–gel thin films on magnesium alloys in 0.6 M NaCl aqueous solution

The corrosion protection behaviour of organic–inorganic hybrid thin films on AZ31 and AZ61 magnesium alloy substrates has been studied. These films were prepared by a sol–gel dip-coating method. The organopolysiloxane precursors were γ-methacryloxypropyltrimethoxysilane (MAPTMS) and tetramethoxysilane (TMOS). An attempt was made to determine the possible relationships between the degradation of the sol–gel film and composition of the metal substrate during the exposure of the metal/coating system to 0.6 M NaCl aqueous solutions. For this purpose electrochemical impedance spectroscopy (EIS) and hydrogen evolution measurements were applied. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) analyses revealed that the sol–gel films formed on the surface of AZ61 alloy were far more perfect and uniform than those formed on the AZ31 alloy. This behaviour was attributed to the effect of the native oxide film initially present on the surface of the AZ61 alloy, which inhibited the attack of magnesium. Results indicated that the sol–gel coated AZ61 substrate tended to develop corrosion products slower than the sol–gel coated AZ31 substrate, trend that could change by prolonging exposure time. After 6 days of immersion, a clear inhibitive effect of the corrosion products formed during the test was observed in the case of the sol–gel coated AZ31, but not with the coated AZ61 alloy substrate, a phenomenon explained by the carbonate enrichment observed by XPS.     

Structure and Properties of ZrO2 Ceramic Coatings on AZ91D Mg Alloy by Plasma Electrolytic Oxidation

ZrO₂ ceramic coatings were prepared in situ on an AZ91D Mg alloy by plasma electrolytic oxidation in a K₂ZrF₆ solution. The phase composition and the surface morphology of the coatings were examined with X-ray diffraction and scanning electron microscopy. The thermal shock resistance of the coatings was evaluated by a thermal shock test. The corrosion resistance of the coated samples was examined by the polarizing curve method in a 3.5% NaCl solution. The prepared coating was composed of t -ZrO₂ and a small amount of c -ZrO₂. There were many residual discharging channels on the coating surface. The coated samples showed excellent thermal shock resistance under 500°C, which improved with increasing frequency or decreasing current density or PEO time. Besides, the coating improved the corrosion resistance of AZ91D Mg alloy considerably. In the experiments, the corrosion current density of the coated samples prepared under 1000 Hz was the least, which also decreased with the current density during the PEO process.     

Electrophoretic deposition of bioactive glass nanopowders on magnesium based alloy for biomedical applications

To slow down the initial biodegradation rate of magnesium (Mg) alloy, crystalline nano-sized bioactive glass coating was used to deposit on micro-arc oxidized AZ91 samples via electrophoretic deposition (EPD). Zeta potential and conductivity of the bioactive glass suspension were characterized at various pH values to identify the most stable dispersion conditions. The bone-bonding properties of bioactive glass coated samples were evaluated in terms of apatite-forming ability during the immersion in simulated body fluid (SBF) solution. Results revealed that the ability to form a bioactive glass coating via EPD was influenced by the degree of its crystalline phase composition. Moreover, the potentiodynamic polarization tests recorded significant drops in corrosion current density and corrosion rate of the coated samples which implies a good level of corrosion protective behavior. These preliminary results show that this process will enable the development of Mg implants in the later stage of bone healing.     

Corrosion behaviors of Zn/Al-Mn alloy composite coatings deposited on magnesium alloy AZ31B (Mg-Al-Zn)

After being pre-plated a zinc layer, an amorphous Al-Mn alloy coating was applied onto the surface of AZ31B magnesium alloy with a bath of molten salts. Then the corrosion performance of the coated magnesium alloy was examined in 3.5% NaCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results showed that the single Zn layer was active in the test solution with a high corrosion rate while the Al-Mn alloy coating could effectively protect AZ31B magnesium alloy from corrosion in the solution. The high corrosion resistance of Al-Mn alloy coating was ascribed to an intact and stable passive film formed on the coating. The performances of the passive film on Al-Mn alloy were further investigated by Mott-Schottky curve and X-ray photoelectron spectroscopy (XPS) analysis. It was confirmed that the passive film exhibited n-type semiconducting behavior in 3.5% NaCl solution with a carrier density two orders of magnitude less than that formed on pure aluminum electrode. The XPS analysis indicated that the passive film was mainly composed of AlO(OH) after immersion for long time and the content of Mn was negligible in the outer part of the passive film. Based on the EIS measurement, electronic structure and composition analysis of the passive film, a double-layer structure, with a compact inner oxide and a porous outer layer, of the film was proposed for understanding the corrosion process of passive film, with which the experimental observations might be satisfactorily interpreted.     

Low-Cost Biobased Coatings for AM60 Magnesium Alloys for Food Contact and Harsh Environment Applications

Low-cost, environmentally friendly and easily applicable coating for Mg alloys, able to resist in real world conditions, are studied. Coatings already used for other metals (aluminum, steel) and never tested on Mg alloy for its different surface and reactivity were deposited on AM60 magnesium alloys to facilitate their technological applications, also in presence of chemically aggressive conditions. A biobased PA11 powder coating was compared to synthetic silicon-based and polyester coatings, producing lab scale samples, probed by drop deposition tests and dipping in increasingly aggressive, salty, basic and acid solutions, at RT and at higher temperatures. Coatings were analyzed by SEM/EDX to assess their morphology and compositions, by optical and IR-ATR microscopy analyses, before and after the drop tests. Migration analyses from the samples were performed by immersion tests using food simulants followed by ICP-OES analysis of the recovered simulant to explore applications also in the food contact field. A 30 μm thick white lacquer and a 120 μm PA11 coating resulted the best solutions. The thinner siliconic and lacquer coatings, appearing brittle and thin in the SEM analysis, failed some drop and/or dipping test, with damages especially at the edges. The larger thickness is thus the unique solution for edgy or pointy samples. Finally, coffee cups in AM60 alloy were produced, as real word prototypes, with the best performing coatings and tested for both migration by dipping, simulating also real world aging (2 h in acetic acid at 70° and 24 h in hot coffee at 60 °C): PA11 resulted stable in all the tests and no migration of toxic metals was observed, resulting a promising candidate for many real world application in chemically aggressive environments and also food and beverage related applications.     

AZ31镁合金稀土转化成膜工艺研究

本文对AZ31镁合金表面稀土转化成膜工艺进行了研究。分析了不同的成膜工艺参数丽土盐溶液组成、转化成膜时间）对稀土转化膜的形貌及耐蚀性能的影响。扫描电镜分析了不同成膜工艺形成的稀土转化膜的表面形貌;极化曲线研究了转化膜的电化学腐蚀行为。结果表明：当转化液中硝酸铈浓度为4．3423g·L^-1和硝酸镧浓度为4．3302g·L^-1时,转化膜的耐蚀性能最好;成膜时间对膜的耐蚀性也有不同程度的影响。