Direct growth of oriented Mg–Fe layered double hydroxide (LDH) on pure Mg substrates and in vitro corrosion and cell adhesion testing of LDH-coated Mg samples

Abstract

This work presents a novel method for directly forming highly-oriented Mg–Fe–CO₃ LDH coating on pure Mg samples by treating the sample in pH 5.6 aqueous Fe³⁺/HCO₃⁻/CO₃²⁻ at 50 °C, and then immersing it in pH 9.5 aqueous HCO₃⁻/CO₃²⁻ at 50 °C. The former step was performed to yield Mg²⁺ in aqueous solution at pH 5.6 by corroding the Mg sample. A two-layered thin film was thus formed on the Mg substrate, of which the outer layer (∼1 μm-thick) comprised fine platelet-like Mg–Fe–CO₃ LDH. The latter treatment in pH 9.5 aqueous HCO₃⁻/CO₃²⁻ at 50 °C resulted in the growth of the fine LDH platelets into a strongly-oriented Mg–Fe–CO₃ LDH. The chemical formula of the Mg–Fe–CO₃ LDH is Mg_0.74Fe_0.26(OH)₂(CO₃)_0.13·mH₂O. The method used herein involves a metal salt-free system, which requires no addition of Mg(NO₃)₂ and Fe(NO₃)₃. Several in vitro tests of the Mg–Fe–CO₃ LDH coating on a Mg sample were performed. Based on the measured contact angle between the sample surface and human whole blood, the Mg–Fe–CO₃ LDH coating can improve the hydrophilicity of a pure Mg surface. According to the results of an in vitro corrosion test in revised simulated body fluid (R-SBF), the Mg–Fe–CO₃ LDH coated sample had a much higher corrosion resistance than the pure Mg substrate. The results of in vitro human mesenchymal stem cell adhesion tests showed that the Mg–Fe–CO₃ LDH coated sample had a better cell spreading and cell–cell interaction behavior than the pure Mg substrate.