Design of multilayer hybrid sol-gel coatings with bifunctional barrier-bioactive response on the Elektron 21 magnesium alloy for biomedical applications

The present study aims to develop multilayer barrier-bioactive hybrid sol-gel coatings from a mixture of the silane precursors tetraethylorthosilicate (TEOS) and glycidoxypropyltriethoxysilane (GPTMS) deposited on the Elektron 21 magnesium alloy. The purpose of the inner layer (barrier coating) was to provide corrosion protection to the magnesium alloy, whereas the outer layer (bioactive coating) was doped with different Ca and Mg contents to produce a bioactive material. The coatings were characterised using scanning electron microscopy (SEM) and their corrosion behaviour was evaluated by anodic polarisation and electrochemical impedance spectroscopy after immersion in simulated body fluid (SBF) at 37 ± 0.5 °C. The experimental results showed that the multilayer coatings increased the corrosion resistance of the alloy up to three orders of magnitude during immersion in SBF solution. On the other hand, the presence of Ca and Mg in the bioactive coating promoted the growth of apatite-like phases. However, an increment of salt content favoured the formation of porous coatings, which allowed the access of the electrolyte to the substrate leading to their rapid deterioration. Despite the latter, this research endorses the premise that the TEOS-GPTMS hybrid system represents a promising alternative to produce bifunctional barrier-bioactive coatings.