Although the initial in vitro dissolution of bioactive glasses (BAG) is well characterized, the long-term behaviour of crystallized BAG scaffolds in a continuous fluid flow is incompletely understood. A detailed understanding of the long-term dissolution of scaffolds is vital for predicting their behaviour in clinical applications. Here, we explored the dissolution and reaction mechanisms of partly crystalline and glass–ceramic scaffolds based on the bioactive glasses S53P4 and 45S5 using a continuous flow-through method in Tris-buffer (Tris) and simulated body fluid (SBF) for up to 21 days. Granules of the parent glasses were used as references. The main crystalline phase in both scaffolds was sodium-calcium-silicate. The scaffolds’ dissolution suggested that the sodium-calcium-silicate crystals dissolved incongruently to yield hydrous silica. The silica phase then provided abundant nucleation sites for hydroxyapatite precipitation, resulting in fine-grained crystalline structures. When exposed to Tris, the scaffolds almost completely dissolved within the test period, leaving only highly porous remnant phases. For the 45S5 scaffolds, the calcium phosphate reaction layers that formed on their surfaces effectively slowed the dissolution in SBF. In contrast, this effect was less apparent for the S53P4 specimens.

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Bioglass 45S5 is the most widely used bioactive glass composition in orthopedics to regenerate bone tissue. Although its reactions in vitro and in vivo are well established, the impact of the local environment on the dissolution behavior in the dynamic environment is not fully understood. Here, we show that the ion concentrations released from 45S5 into Tris-buffer solution and simulated body fluid (SBF) in a cascade system of three reactors in a series significantly differed depending on the ion concentrations in the inflow solutions to each reactor. The ion concentrations and pH of the solutions were measured after each reactor for up to 7 days. Also, the reaction layers at the glass particles were analyzed.The release of Si species into Tris decreased in the consecutive reactors from 92% to 26% and 24% at 168 h. Correspondingly, the release of Si species into SBF decreased from 35% to 20% and 19%. The share of elements in the remaining glass particles markedly varied between the reactors throughout the dissolution. The results imply that gradients of local ion concentrations have an essential effect on the dissolution of 45S5. The results provide guidelines for utilizing Bioglass 45S5 in different bone and soft tissue applications.

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