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Abstract
The biomedical applications of Mg-based alloys are limited by their rapid corrosion rate in the body fluid. In this study, the hydrothermal synthesis is employed to produce protective bioactive hydroxyapatite coating (HAC) and strontium-substituted hydroxyapatite coating (Sr-HAC) to further enhance the corrosion resistance and in vitro biocompatibility of biodegradable AZ91D Mg alloy in physiological environments. For comparison, the brucite Mg(OH)2 prepared by the alkaline pre-treatment is designated as a control group. Experimental evidences of XRD and XPS analysis confirm that Sr2+ ions can be incorporated into HA crystal structure. It is noted that the hydrothermally synthesized Sr-HAC conversion coating composed of a specific surface topography with the nanoscaled flake-like fine crystallites is constructed on the AZ91D Mg alloy. The hydrophilicity of Mg substrate is effectively enhanced with the decrease in static contact angles after performing alkaline and hydrothermal treatments. Potentiodynamic polarization measurements reveal that the nanostructured Sr-HAC-coated specimens exhibit superior corrosion resistance than HAC and alkaline pre-treated Mg(OH)2. Moreover, immersion tests demonstrate that Sr-HAC provides favorable long-term stability for the Mg alloy with decreasing concentration of released Mg2+ ions in the SBF and the reduced corrosion rate during the immersion length of 30 days. The cells cultured on Sr-HAC specimens exhibit higher viability than those on the alkaline-pre-treated Mg(OH)2 and HAC specimens. The Sr-substituted HA coating with a nanostructured surface topography can help to stimulate the cell viability of osteoblastic cells.
Highlights
Magnesium (Mg) and its alloys are the candidates of the state-of-the-art new generation biodegradable metallic materials for the applications of cardiovascular devices, hard tissue replacements, regeneration therapies and bone graft scaffolds of tissue engineering [1,2,3,4,5,6]
It represents that a brucite Mg(OH)2 conversion surface layer is deposited on the Mg alloy after the alkaline pre-treatment
Surface treatments of AZ91D Mg alloy including alkaline pre-treated Mg(OH)2, hydrothermal synthesized hydroxyapatite coating (HAC) and Sr-HAC conversion coatings are analyzed in terms of anti-corrosion performances and cell responses for biomedical applications
Summary
Magnesium (Mg) and its alloys are the candidates of the state-of-the-art new generation biodegradable metallic materials for the applications of cardiovascular devices, hard tissue replacements, regeneration therapies and bone graft scaffolds of tissue engineering [1,2,3,4,5,6]. Mg2+ ion is a basic and beneficial cation for the human body to induce bone cell adhesion and the growth of new bone tissues with a firm bone-implant fixation for the implantation of Mg alloys [11,12]. The mismatch between the degradation rate of Mg implants and the growth of new bone tissues may result in a prolonged healing process after implantation. Since the appropriate concentration of Mg2+ ions play an essential role in bone remodeling [14], a controllable and protective coating is needed for regulating the degradation rate of Mg alloys [15,16]. Some surface modification methods, such as chemical conversion coatings, chemical modification with organic compounds, electrochemical deposition, micro-arc oxidation and hydrothermal crystallization methods, are adopted to deposit a surface coating for improving the corrosion resistance and reducing the degradation rate of Mg alloys [17,18,19,20,21]
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