In vitro transformation of bioactive glass granules into Ca-P shells

Bioactive glass (BG) granules of narrow size are excavated when implanted in mandibular bone of beagles. Bone tissue forms within these internally hollowed particles without a connection to the bone at the margins of the defect. In this study the internal excavation of BG granules was simulated by in vitro immersion experiments. Postimmersion solutions were analyzed for changes in Si, Ca, and P concentrations. Using scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and Fourier Transform Infrared (FTIR) spectroscopy, granules were analyzed for compositional, morphologic, and structural changes resulting from immersion. Only when the solution was continuously replenished and only if this solution was composed of electrolyte- and protein-containing serum was excavation achieved. Without solution replenishment, that is, under so-called integral immersion conditions, the solution quickly became saturated in silicon, and the silicon no longer dissolved. When the glass was immersed in a solution with serum, a porous surface structure with fine precipitates was formed, in contrast to a dense surface reaction layer with closely packed globular precipitates that was formed in a solution without serum. The combined effect of continuous solution replenishment and the use of a solution containing serum proteins led to the formation of a surface reaction layer that did not impede continued corrosion. As such, all Si was released, and eventually a hollow Ca-P shell was formed. Thus this study supports the hypothesis that there is a physico-chemical mechanism of Si transport through the Ca-P-rich layer followed by Si dissolution. This mechanism may be operative in vivo and thereby may contribute to the observed in vivo excavation.