Fig. 10. Formation stages of apatite on diverse substrates, including: (a) stage 1: fast ion exchange Na+ and Ca2+ ions with H+ from solution, (b) stage 2: breaking Si–O–Si bonds Si(OH)4 to solution, (c) stage 3: re-polymerization of a SiO2-rich layer, (d) stage 4: migration Ca2+ and PO43− to surface SiO2-rich layer forming amorphous CaO–P2O5 rich (ACP) film, and (e) crystallization ACP film by incorporation OH− and CO32− from solution forming hydroxycarbonate–apatite.18,58,138.
