On the evaluation of ALD TiO₂, ZrO₂ and HfO₂ coatings on corrosion and cytotoxicity performances

Magnesium alloys have been widely studied as materials for temporary implants, but their use has been limited by their corrosion rate. Recently, coatings have been proven to provide an effective barrier. Though only little explored in the field, Atomic Layer Deposition (ALD) stands out as a coating technology due to the outstanding film conformality and density achievable. Here, we provide first insights into the corrosion behavior and the induced biological response of 100 nm thick ALD TiO₂, HfO₂ and ZrO₂ coatings on AZ31 alloy by means of potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), hydrogen evolution and MTS colorimetric assay with L929 cells. All three coatings improve the corrosion behavior and cytotoxicity of the alloy. Particularly, HfO₂ coatings were characterized by the highest corrosion resistance and cell viability, slightly higher than those of ZrO₂ coatings. TiO₂ was characterized by the lowest corrosion improvements and, though generally considered a biocompatible coating, was found to not meet the demands for cellular applications (it was characterized by grade 3 cytotoxicity after 5 days of culture). These results reveal a strong link between biocompatibility and corrosion resistance and entail the need of taking the latter into consideration in the choice of a biocompatible coating to protect degradable Mg-based alloys.