Insights into Calcium Phosphate Formation Induced by the Dissolution of 45S5 Bioactive Glass

Although models have been proposed to explain the mechanisms of bioglass (BG) dissolution and subsequent calcium phosphate (CaP) mineralization, open questions remain. The processes in which phase transition occurs in aqueous solutions and their dynamics remain underexplored partly because traditional instruments/techniques do not allow for direct observations at the adequate time and length scales at which such phase transformations occur. For instance, given the crucial role of the silica gel in CaP formation during BG dissolution, uncertainty exists about how such a silica gel forms on the BG surface. In the case of CaP formation driven by BG dissolution, questions can also be added, i.e., how CaP develops into an apatitic-like structure, how many transient phases there are, and, in general, phenomena occurring in the solid-liquid interface during BG dissolution. Several approaches were taken to study CaP mineralization driven by BG dissolution, mainly examining the solid-liquid interface and the BG after-reaction surface. This paper focuses on gaining insight into silica gel formation on the BG's surface during dissolution. Electron microscopy techniques were used, including scanning electron microscopy and focused ion beam cross sections. Other analysis techniques, such as time-of-flight secondary ion mass spectrometry, were utilized. Cross sections of reacted BG-blocks gave essential insights into the BG dissolution, particularly its strong dependency on experimental conditions, and tentative evidence has shown that soluble silica from BG dissolution may not reprecipitate/repolymerize on BG blocks' surface; thus, we wonder where it precipitates. Additionally, complementary analysis techniques determined that CaP, during BG dissolution, transitions from amorphous calcium phosphate to a calcium-deficient nanocrystalline apatitic structure with minimal contents of Si⁴⁺ and Na⁺ ions that may be molecularly part of CaP. The Hench model has been the core guide for BG dissolution and subsequent CaP formation for many years. However, this study shows tentative evidence that contributes to and somewhat differs from it.