Heterogeneous amorphous structure and bioactivity in silver-doped bioactive glasses synthesized by spray drying methods

Abstract

In this study, Ag-doped SiO₂-CaO-P₂O₅ bioactive glasses (BGs) were synthesized by a spray drying method, and their atomistic structures were investigated by transmission electron microscopy, X-ray diffraction, and nuclear magnetic resonance. The constituent elements (Si, Ca, P, and O) were homogeneously distributed in the pure (non-doped) BG, but a small Ca- and P-rich layer was present on the surface. The segregation of Ca and P on the surface became more pronounced with increasing Ag concentration. The atomic pair-distribution functions extracted by nanobeam electron diffraction revealed that the Ag-doped BG particles had a heterogeneous amorphous structure: the interior and surface of the BG particles were similar to those of SiO₂ and phosphate-based glasses, respectively. The addition of Ag resulted in less non-bridging oxygen, which may be due to the removal of the network modifier ion (Ca) from the SiO₂ network. In vitro bioactive tests of the pure BG and Ag-doped BG particles revealed that the formation of hydroxyapatite was enhanced by Ag doping, although the amount of non-bridging oxygen decreased. This is attributed to the P- and Ca-rich surface layer, which triggers the nucleation and growth of hydroxyapatite.