Microstructural investigation of hydroxyapatite formation in bioactive borosilicate glass

Two types of glass particles were immersed in simulated body fluid (SBF) at 37 °C. Silicate glass 45S5 with a trademark name “Bioglass” only formed hydroxyapatite (HAp) near the particle surface, leaving a large amount of silica gel in the interior. The second glass type was borosilicate glass with a composition of the 45S5 glass in which a major part of SiO2 was replaced with B2O3; in the samples from it, glass was completely replaced with HAp inside the particles. We used Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy-energy dispersive spectroscopy, field-emission scanning electron microscopy, and transmission electron microscopy (TEM) to characterize the microscopic structure of samples. When the 45S5 glass was immersed in SBF, the PO43−(Q0) in glass quickly formed amorphous calcium phosphate (ACP), a precursor of HAp. Ca2+ preferentially coordinated with PO43−(Q0) forming ACP [Ca2+–PO43−(Q0)], which is considered stable, where the notation Qn represents the number (n) of the bridging oxygen atoms per PO4 tetrahedron. The TEM results showed that the columnar crystals of HAp grew along the c-axis. In the borosilicate glass, the reaction is thought to progress to the inside through ion transport into the space where B2O3 was eluted.