Microstructural investigation of hydroxyapatite formation in bioactive borosilicate glass

Abstract

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.