In vitro degradation and biocompatibility of vitamin C loaded Ca-P coating on a magnesium alloy for bioimplant applications

Molecular recognition was utilized to fabricate bioinspired calcium phosphate (Ca-P) coating on bioabsorbable magnesium alloys through small biomolecules such as Vitamin C (VC). Ca-P and VC hybrid coating (Ca-P_VC) was successfully fabricated on AZ31 Mg alloy. The surface morphology and chemical composition of the coatings were investigated using SEM, XRD, and FTIR together with XPS. The results showed that the Ca-P_VC coating was composed of bamboo leaf-like Ca-P particles with a thickness of about three times that of the Ca-P coating. The surface roughness of the Ca-P_VC coating (1.12 ± 0.12 μm) was lower than that (3.14 ± 1.93 μm) of Ca-P coating, suggesting the formation of refined Ca-P particles resulting from the VC addition. The corrosion resistance of the coated samples was characterized via electrochemical polarization, impedance spectroscopy, and immersion hydrogen evolution tests. The cell toxicity of the coated samples was evaluated utilizing mouse MC3T3-E1 pre-osteoblasts. The charge transfer resistance (R_ct) of the Ca-P_VC coated alloy increased as compared to the bare and Ca-P coated alloy samples. The Ca-P_VC coated alloy exhibited minimal corrosion current density (1.36 × 10⁻⁶ A cm⁻²), which is one order of magnitude lower in comparison to that of the Ca-P coated alloy. These results confirm that VC addition greatly enhanced the coating resistance on AZ31 Mg alloy. It was also noticed that the Ca-P_VC coated samples rapidly induced the formation of apatite after immersion in Hank's solution. VC was mainly transformed to L-Threonic acid, which facilitated the nucleation process of the Ca-P_VC coating and significantly increased the thickness, density, and bonding strength of the coating. With enhanced corrosion resistance property and excellent biocompatibility, Ca-P_VC coating has great potential for application in biodegradable Mg-based alloys.