Effect of B₂O₃ Substitution for SiO₂ on the Network Structure and Degradation Behavior of Borosilicate Glasses

The effects of B₂O₃ substitution for SiO₂ on the network structure and in vitro degradation behavior of borosilicate glasses were investigated using a combined approach of molecular dynamics (MD) simulation and experimental methods. The results show that an increasing B₂O₃/SiO₂ molar ratio shifts the main network former from [SiO₄] to [BO₃] and [BO₄] units. Notably, [BO₃] units exhibit a tendency for spatial aggregation, which reduces network stability, whereas [SiO₄] tetrahedra increase their bridging oxygen connectivity to compensate and stabilize the network. These structural changes critically determine the degradation mechanism: For silicate-rich glasses (e.g., 0B, 1B), a protective silica-rich layer forms, leading to a fluctuating, layer-by-layer ion release of Ca, B, and Si. In contrast, for borate-rich glasses (e.g., 2B, 3B), the network undergoes rapid, bulk disruption due to the vulnerability of clustered [BO₃] domains, resulting in the near-synchronous and rapid release of B and Ca. Furthermore, the applicability of F_net for predicting the ion release behaviors of borosilicate glasses is demonstrated. This study validates the integrated MD-experimental approach for revealing the atomic-scale origins of degradation behavior and demonstrates the potential of structural descriptors like F_net for guiding the composition design of bioactive glasses with tailored ion release profiles.