Osteoclast and osteoblast responsive carbonate apatite coatings for biodegradable magnesium alloys

Corrosion-control coatings which can enhance bone formation and be completely replaced by bone are attractive for biodegradable Mg alloys. Carbonate apatite (CAp) and hydroxyapatite (HAp) coatings were formed on Mg-4 wt% Y-3 wt% rare earth (WE43) alloy as a corrosion-control and bioabsorbable coating in the coating solution with various concentrations of NaHCO₃. The incorporation of carbonate group in apatite structure was examined using X-ray diffraction and Fourier transform infrared spectroscopy. Rat osteoclast precursor and MC3T3-E1 osteoblast cells were cultured on the CAp- and HAp-coated WE43 to examine the osteoclastic resorption and the alkaline phosphatase (ALP) activity, respectively. Mg ions in the used medium were quantified to examine the corrosion-control ability. The NaHCO₃ addition in the solution resulted in the formation of B-type CAp in which the phosphate group of apatite structure was substituted with the carbonate group. The osteoclastic resorption was observed only for the CAp coatings as the cracking of the coatings and the corrosion of substrate WE43 strongly localized under osteoclast cell bodies. The CAp and HAp coatings significantly enhanced the ALP activity of osteoblasts. The CAp-coated WE43 specimens showed 1/5 smaller amount of Mg ion release than the uncoated WE43 on the first day of culturing osteoblasts. For the subsequent 22 days, the Mg ion release was reduced to 1/2 by the CAp coatings. In the presence of osteoclasts, the CAp coatings showed slightly lower corrosion protectiveness than the HAp coating. It was demonstrated that the CAp coatings can be a bioabsorbable and corrosion-control coating for biodegradable Mg alloys.