Crystal structure of calcium carbonate coating modulates degradation and bone formation of biodegradable magnesium

Surface coatings on biodegradable Mg alloys have been extensively explored to inhibit the corrosion of Mg substrate and to promote the bone formation. However, the self-degradation of the coating is rarely seriously considered when it is used to modulate the corrosion of Mg substrate and the osteogenic activity. Based on our previously developed robust CaCO₃ coating, considering the stability of different CaCO₃ crystalline, the current study evaluates how the crystal structure of CaCO₃ coating affects its protection ability to Mg substrate and the corresponding biological response of osteoblasts. To this end, different CaCO₃ coatings, calcite coating (C-coating), vaterite coating (V-coating), aragonite coating (A-coating) and mixture coating (C+V+A-coating), are fabricated on pure Mg respectively, for in vitro corrosion performance and osteoblast response as well as in vivo self-degradation of the coatings and their bio-safety. It revealed the most prominent protection ability of plate-like aggregated V-coating to Mg substrate, decreasing in the order of V-coating > C+V+A-coating > C-coating > A-coating. The anti-corrosion ability of the coatings seemly correlates with the compactness and the self-degradation ability of the coating, surprisingly differing from the thermodynamic stability of CaCO₃ polymorphs. Correspondingly, V-coating, C-coating and C+V+A-coating exhibited higher cytocompatibility and fortified osteogenic ability for MC3T3-E1 cells due to the hospitable surrounding ion environment and the desired surface features. Noteworthily, the structural reorganization of the coatings into Ca-P products on the surface also contributes to their exceptional osteogenic performance, which is more significant for V-coating, followed by C-coating. The findings of this work suggest that the coating structure could be one of the vital indicators for its corrosion protection ability and the bone regeneration promotion ability of Mg implants, strengthening the underlying functional mechanisms for CaCO₃-based coating on biomaterials. STATEMENT OF SIGNIFICANCE: The present study emphasizes the self-degradation behaviour of CaCO₃ coating composed of single crystalline phase, i.e. calcite, vaterite or aragonite, in terms of their corrosion protection ability for biodegradable Mg substrate and the corresponding osteogenic activity. Differring from the thermodynamic stability (calcite > aragonite > vaterite), vaterite coating with plate-like packed structure exhibits the slowest in vitro and in vivo self-degradation, and the corresponding highest corrosion resistance. Meaningwhile, it possessed higher osteogenic activity due to the suitable surrounding environment and the enhanced structural reorganzation. It highlights the importance of the structural stability of CaCO₃ coating, which is beneficial to optimize and design coating on biodegradable Mg with promising degradation performance and osteogenic bioactivity.