Mechanism of Biomineralization of Apatite on a Sodium Silicate Glass: TEM−EDX Study In Vitro

Abstract

The mechanism of biomineralization of apatite on a Na2O−SiO2 glass was investigated in vitro, in which the glass surface was surveyed by transmission electron microscopy and energy-dispersive X-ray spectrometry as a function of soaking time in simulated body fluid (SBF), complemented with Fourier transform infrared reflection spectroscopy of the glass surface and atomic emission spectroscopy of the fluid. The glass was found to exchange Na+ ions with H3O+ ions in the SBF to form silanol (Si−OH) groups on its surface at an early stage of soaking. Immediately after they were formed, the Si−OH groups incorporated calcium ions in the fluid to form an amorphous calcium silicate. After a long soaking time, the calcium silicate incorporated phosphate ions and further calcium ions in the fluid to form an amorphous calcium phosphate with a low Ca/P atomic ratio of 1.43. The amorphous calcium phosphate was eventually converted into crystalline apatite, which contained small amounts of Na, Mg, and Cl and had a Ca/P ratio of 1.65, similar to bone mineral. The bonelike apatite grew spontaneously by consuming the remaining calcium and phosphate ions in the fluid. We propose that the initial formation of the calcium silicate is a consequence of an electrostatic interaction of negatively charged ≡Si−O- units, formed by dissociation of the Si−OH groups, with the positively charged calcium ions in the fluid. The calcium silicate is postulated to gain a positive charge and interact with the negatively charged phosphate ions in the fluid to form an amorphous calcium phosphate, which then stabilizes into the crystalline apatite.