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.

Complete Removal of Residual Particles and Realization of Mechanical Properties to Improve Osseointegration in Additively Manufactured Ti6Al4 V Scaffolds through Flowing Acid Etching.

Massive unmelted Ti6Al4 V (Ti64) particles presented across all surfaces of additively manufactured Ti64 scaffolds significantly impacted the designed surface topography, mechanical properties, and permeability, reducing the osseointegration of the scaffolds. In this study, the proposed flowing acid etching (FAE) method presented high efficiency in eliminating Ti64 particles and enhancing the surface modification capacity across all surfaces of Ti64 scaffolds. The Ti64 particles across all surfaces of the scaffolds were completely removed effectively and evenly. The surface topography of the scaffolds closely resembled the design after the 75 s FAE treatment. The actual elastic modulus of the treated scaffolds (3.206 ± 0.040 GPa) was closer to the designed value (3.110 GPa), and a micrometer-scale structure was constructed on the inner and outer surfaces of the scaffolds after the 90 s FAE treatment. However, the yield strength of scaffolds was reduced to 89.743 ± 0.893 MPa from 118.251 ± 0.982 MPa after the 90 s FAE treatment. The FAE method also showed higher efficiency in decreasing the roughness and enhancing the hydrophilicity and surface energy of all of the surfaces. The FAE treatment improved the permeability of scaffolds efficiently, and the permeability of scaffolds increased to 11.93 ± 0.21 × 10-10 mm2 from 8.57 ± 0.021 × 10-10 mm2 after the 90 s FAE treatment. The treated Ti64 scaffolds after the 90 s FAE treatment exhibited optimized osseointegration effects in vitro and in vivo. In conclusion, the FAE method was an efficient way to eliminate unmelted Ti64 particles and obtain ideal surface topography, mechanical properties, and permeability to promote osseointegration in additively manufactured Ti64 scaffolds.

Improving the corrosion resistance of micro-arc oxidation coated Mg-Zn-Ca alloy.

Four additives (Na2WO4, nano-hydroxyapatite, K2TiF6 and NaF) were added into the Na5P3O10 + NaOH + C3H8O3 base electrolyte according to the orthogonal design of four factors three levels (L9 (34)). Nine different micro-arc oxidation (MAO) coatings were fabricated on Mg-2Zn-0.5Ca alloys through orthogonal experiments. The effects of four additives on the microstructure, mechanical properties, corrosion resistance and biocompatibility of MAO coatings were investigated through X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), electrochemical corrosion test and in vitro degradation test. The addition of nano-hydroxyapatite and K2TiF6 showed self-sealing effects and contributed to the corrosion resistance of the samples significantly. The addition of 0.5 g L-1 Na2WO4 markedly elevated the bonding strength of the coatings with the substrate. The optimal combination of factors and levels considering both mechanical properties and corrosion resistance was: 0.5 g L-1 Na2WO4, 0 g L-1 NaF, 5 g L-1 n-HAp, 5 g L-1 K2TiF6. The growth mechanism of MAO coatings combining with the visual phenomenon was discussed as well.

Degradable magnesium-based alloys for biomedical applications: The role of critical alloying elements.

Magnesium-based alloys exhibit biodegradable, biocompatible and excellent mechanical properties which enable them to serve as ideal candidate biomedical materials. In particular, their biodegradable ability helps patients to avoid a second surgery. The corrosion rate, however, is too rapid to sustain the healing process. Alloying is an effective method to slow down the corrosion rate. However, currently magnesium alloys used as biomaterials are mostly commercial alloys without considering cytotoxicity from the perspective of biosafety. This article comprehensively reviews the status of various existing and newly developed degradable magnesium-based alloys specially designed for biomedical application. The effects of critical alloying elements, compositions, heat treatment and processing technology on the microstructure, mechanical properties and corrosion resistance of magnesium alloys are discussed in detail. This article covers Mg-Ca based, Mg-Zn based, Mg-Sr based, Mg-RE based and Mg-Cu-based alloy systems. The novel methods of fabricating Mg-based biomaterials and surface treatment on Mg based alloys for potential biomedical applications are summarized.

Mixed-valent MnSiO3/C nanocomposite for high-performance asymmetric supercapacitor.

In this work, carbon-coated manganese silicate (MnSiO3/C) nanocomposite with excellent cycling stability was fabricated via a cost-effective process. The carbon coating followed with a CO2 heat treatment process on the manganese silicate results in mixed-valent hierarchically-porous nanoparticles, which tightly connects with an ultrathin (∼1.5 nm) and ordered carbon coating layer. This composite features rectangular-like cyclic voltammetry curve with two couples of redox peaks, suppressing the irreversible reactions and thus providing a broad and stable working voltage. By fitting the CV curves, the MnSiO3/C demonstrates a capacitive energy-storage behavior. The as-assembled activated carbon//MnSiO3/C asymmetric supercapacitor in 1 M Na2SO4 aqueous electrolyte is found to have excellent cycling stability, with 95.5% retention of initial after 10,000 cycles. This device could deliver 25.8 W h kg-1 energy density at the power density of 1 kW kg-1 with ∼10 mg cm-2 high mass loading, suggesting a bright prospect in practical application.

Effect of the second-step voltages on the structural and corrosion properties of silicon-calcium-phosphate (Si-CaP) coatings on Mg-Zn-Ca alloy.

The applications of magnesium (Mg) alloys as biodegradable orthopedic implants are mainly restricted due to their rapid degradation rate in the physiological environment. In this study, Si-CaP micro-arc oxidation (MAO) coatings were prepared on a Mg-Zn-Ca alloy by a second-step MAO process at different voltages in order to decrease the degradation rate and increase the bioactivity of the alloy. The microstructure and morphology of the samples were characterized using XRD, FT-IR SEM and EDS. The degradation behaviours of samples were evaluated using electrochemical techniques, and immersion tests in simulated body fluid (SBF). The results indicate that the morphology of the Si-CaP coatings changed significantly with the increase in Ca/P ratio as the second-step voltage increased. The Si-CaP containing coating produced at 450 V could significantly decrease the degradation rate of Mg and caused a slow increase in pH of the SBF solution. The haemolysis test concluded that the coating C3 did not cause a haemolytic reaction. The corrosion resistance of Mg alloy was greatly improved with the Si-CaP coatings, and the Mg alloy with Si-CaP coating prepared at 450 V had the best corrosion resistance, which indicates that the Si-CaP coatings are promising for improving the biodegradation properties of Mg-based orthopedic implants. Haemolysis tests indicated that the Si-CaP coating prepared at 450 V conforms to the given standard (YY/T0127.1-93).

Corrosion behaviour of micro-arc oxidation coatings on Mg-2Sr prepared in poly(ethylene glycol)-incorporated electrolytes.

Microarc oxidized calcium phosphate (CaP) ceramic coatings were fabricated on Mg-2Sr alloy from silicate electrolytes with different concentration gradient poly(ethylene glycol) (PEG1000). The microstructure, phase and degradability of the ceramic coatings were evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and simulation body fluid (SBF) immersion tests respectively. An electrochemical workstation was used to investigate the electrochemical corrosion properties of the coatings. It is found that microstructure, thickness, adhesive strength and degradation rate are influenced by PEG1000 incorporation through adjusting the electrolyte activity and then altering the coating growth mechanism. Similar thicknesses (39.0-42.2 µm) are observed in PEG1000-containing coatings while their PEG1000-free counterparts possess the maximum value (51.5 µm). The weight gain in the first two days of SBF immersion suggests that a new layer containing CaP apatites is generated. Results show that ceramic coatings prepared in the electrolyte containing 8 g L-1 PEG1000 exhibits the highest corrosion resistance and lowest degradation rate.

Effect of phosphate additives on the microstructure, bioactivity, and degradability of microarc oxidation coatings on Mg-Zn-Ca-Mn alloy.

Calcium phosphate coatings were prepared on the surface of self-designed Mg-Zn-Ca-Mn alloy using microarc oxidization technology. To characterize the microstructures, cross-section morphologies, and compositions of the coatings, the authors used scanning electron microscopy equipped with an energy-disperse spectrometer, x-ray diffraction, and Fourier transform infrared spectroscopy. Potentiodynamic polarization in the simulated body fluid (SBF) was used to evaluate the corrosion behaviors of the samples. An SBF immersion test was used to evaluate the coating bioactivity and degradability. After the immersion tests, some bonelike apatite formed on the coating surfaces indicate that bioactivity of the coatings is excellent. The coating prepared in electrolyte containing (NaPO3)6 had slower degradation rate after immersion test for 21 days.