After implantation of S53P4 bioactive glass, surface reactions ensure deposition of a calcium phosphate layer when exposed to (body) fluid. Sodium, silica, calcium, and phosphate ions are released from the surface and increase the local pH and osmotic pressure. Thereafter, a silica gel layer is formed on the glass surface, and amorphous calcium phosphates precipitate on this layer.These amorphous structures then crystalize to natural hydroxyapatite, which starts the activation of osteoblasts for the formation of new bone [7–9]. This mechanism of action is illustrated in Figure 1. Because of the continuous reactions and layer formation, the glass will finally be absorbed [7, 8]. The surface reactions not only are beneficial for the formation of new bone but also ensure that bioactive glass contains antibacterial properties and potentially promotes angiogenesis [10–17].

Figure 1: An illustration of the surface reactions of bioactive glass after implantation. When bioactive glass is implanted in a septic bone defect it will exchange alkali from the glass surface with the hydronium in the surrounding microenvironment, which will increase the local pH.The release of ions of the glass surfacewill also increase the osmotic pressure locally. A silica gel layer will be formed near the glass surface to which amorphous calcium phosphate precipitates and subsequently will crystallise into natural hydroxyapatite. The hydroxyapatite will induce the osteostimulative effect by activating osteogenic cells. This figure was kindly provided by BonAlive Biomaterials Ltd.