Surface Characterization of an Amorphous Magnesium–Calcium-Based Coating in Cell Culture Medium Using X-Ray Photoelectron Spectroscopy

The Ca-containing amorphous metallic Mg layer (MC30) is an attractive surface coating for achieving bone reconstruction and infection prevention on Ti surfaces due to its functional changes from antibacterial to pro-osteogenic activities <i>in vitro</i>. In this study, surface changes of MC30 in a cell culture medium were thoroughly characterized in terms of ion release, surface morphology, crystal phase, and chemical state to elucidate the mechanism underlying its <i>in vitro</i> functional changes. The surface of MC30 initially possessed a thin passive layer composed of calcite and Mg²⁺ compounds before immersion in the cell culture medium. Furthermore, MC30 rapidly dissolved in the cell culture medium, leading to subsequent protein adsorption and the formation of calcite on its surface. X-ray photoelectron spectroscopy (XPS) analysis revealed that the binding energies of Mg 1s on the MC30 surface remained almost unchanged, whereas those of Mg 2p shifted to lower values after immersion in the cell culture medium. This shift in the binding energy of Mg 2p indicates that a chemical reaction associated with an increase in electron density around Mg 2p, such as the transformation of MgO to Mg(OH)₂, occurred on the MC30 surface after immersion in the cell culture medium. Therefore, calcite and Mg(OH)₂ were formed on the MC30 surface following its rapid dissolution in the cell culture medium within 6 h, which may govern its <i>in vitro</i> functional transformation. Our findings provide fundamental insights into the design of Mg-based thin layers as well as their deterioration and degradation mechanisms <i>in vivo</i>.