Cryo‐X‐ray analysis—A novel tool to better understand the physicochemical reactions at the bioglass/biological fluid interface

The present study deals with the short-term physicochemical reactions at the interface between bioactive glass particles [55SiO2-20CaO-9P2O5-12Na2O-4MgO. mol%] and biological fluid (Dulbecco Modified Eagle's Medium (DMEM)). The physicochemical reactions within the interface are characterized by scanning transmission electron microscopy (TEM) (STEM) associated with Energy-dispersive X-ray spectroscopy (EDXS). Microanalysis of diffusible ions such as sodium, potassium, or oxygen requires a special care. In the present investigation the cryo-technique was adopted as a suitable tool for the specimen preparation and characterization. Cryosectioning is essential for preserving the native distribution of ions so that meaningful information about the local concentrations can be obtained by elemental microanalysis. The bioglass particles immersed in biological fluid for 24 h revealed five reaction stages: (i) dealkalization of the surface by cationic exchange (Na+, Ca2+ with H+ or H3O+); (ii) loss of soluble silica in the form of Si(OH)4 to the solution resulting from the breakdown of SiOSi bonds (iii); repolymerization of Si(OH)4 leading to condensation of SiO2); (iv) migration of Ca2+ and PO43− to the surface through the SiO2-rich layer to form CaO-P2O5 film; (v) crystallization of the amorphous CaO-P2O5 by incorporating OH or CO32− anions with the formation of three different surface layers on the bioactive glass periphery. The thickness of each layer is ≈300 nm and from the inner part to the periphery they consist of SiOH, which permits the diffusion of Ca2+ and PO43− ions and the formation of the middle CaP layer, and finally the outer layer composed of NaO, which acts as an ion exchange layer between Na+ ions and H+ or H3O+ from the solution. Microsc. Res. Tech., 2008. © 2008 Wiley-Liss, Inc.