Analysis and modelling of in vitro bioactivity for bioactive glass microspheres and granules in continuous fluid flow conditions

In vitro reactions of granules and microspheres (45–90 µm) of the bioactive glass S53P4 and an experimental composition with 5 mol.% K₂O substituted for Na₂O in S53P4 were studied in continuous fluid flow conditions. The dissolution experiments were performed with a 0.2 mL/min flow rate in TRIS buffer up to 24 h. Granules provided higher initial release and pH changes, while at the longest time point, 24 h, the release of Na+K, Ca, and Si was similar for granules and microspheres. The reaction layers (Si-rich and CaP) were evaluated via SEM-EDX analysis. Granules exhibited a thicker Si-rich layer than the microspheres of both glasses. At the same time, a more developed CaP surface layer was detected for S53P4 microspheres than granules. Microspheres, with their uniform shape and size distribution, provided a controlled porosity in the reactor, allowing a more uniform solution flow through the particle bed. A shrinking core model based on the external mass transport coefficient, diffusion through the Si-rich layer, and dissolution rate coefficient was used to predict the initial dissolution kinetics of the glass microspheres in the TRIS buffer. The calculated model parameters provided an appropriate fit with the experimental data for the calcium and alkalis release from the microspheres. The results imply that microspheres offer a good platform for calculating the model parameters for predicting the dissolution mechanism of silicate-based bioactive glasses.