Biocompatibility and in vitro degradation behavior of magnesium–calcium alloy coated with calcium phosphate using an unconventional electrolyte

Calcium phosphate (CaP) was electrochemically coated on a magnesium–calcium (Mg–Ca) alloy using an unconventional electrolyte and a pulse-potential method. The CaP particles of the coating were relatively large, flat, and irregularly oriented; however, they covered the entire alloy surface with a coating thickness of 5 μm. Cytocompatibility tests using L929 cells inoculated in Eagle minimum essential medium supplemented with 10% (v/v) fetal bovine serum (E-MEM+FBS) revealed that CaP coating improved the cytocompatibility of the alloy. It also showed effective suppression of Mg²⁺ ion release from the substrate of the coated alloy and consequently reduced the pH increase of the medium. In vitro degradation experiments using electrochemical techniques in simulated body fluid (SBF) also suggested significant enhancement of the alloy degradation resistance by CaP coating. Potentiodynamic polarization results showed that the corrosion current density of the coated alloy was ∼95% lower than that of the bare metal. Electrochemical impedance spectroscopy results revealed that the polarization resistance (Rₚ) of the coated alloy was more than an order of magnitude higher than that of the bare metal after 2 h of immersion in SBF. Interestingly, after 72 h of immersion, the measured Rₚ had decreased by ∼82%, and the coating appeared cracked and damaged. The results suggest that SBF is more aggressive than E-MEM+FBS cell culture medium