The role of calcium nitrate in early crystallization of hydroxyapatite and its effects on the bioactivity and protein adsorption of a 58S glass

Early crystallization of hydroxyapatite during the syntheses of sol-gel-derived bioactive glasses has been frequently reported in the literature. However, there is no consensus on how this early crystallization occurs or how it affects the subsequent biological behavior of the glass. In this research, we explored how the time at which the calcium precursor was added to the sol-gel synthesis affected this early crystallization process, the consequent bioactivity and the protein adsorption capacity of 58S glass (58SiO ₂–33CaO-9P ₂O ₅, wt.%). The addition of calcium nitrate after alkoxide hydrolysis produced a heterogeneous microstructure in which Ca ²⁺ -rich regions led to a devitrification-like process with the formation of nanocrystalline hydroxyapatite domains within the amorphous silicate network. Larger mesopores and higher amounts of Q ¹ and Q ² silicate units were produced. This heterogeneous microstructure resulted in an increased capacity for albumin adsorption. However, the glass bioactivity was not significantly affected by this devitrification-like process.