45S5 bioactive glass surface charge variations and the formation of a surface calcium phosphate layer in a solution containing fibronectin.

This study investigated the effect of fibronectin adsorption on surface charge variations and calcium phosphate (Ca-P) layer formation kinetics on the surface of 45S5 bioactive glass (BG). We hypothesize that the adsorption of fibronectin on BG changes the surface charge and alters the kinetics of Ca-P layer formation on the glass surface. The charge at a material's surface modulates surface chemistry, protein adsorption, and interactions with bone cells. The zeta potential of BG in a solution containing human plasma fibronectin (TE-FN) was measured as a function of time by particle electrophoresis, and Ca-P layer formation was characterized using SEM, EDXA, and FTIR. Si, Ca, and P solution concentrations also were determined. It was found that the adsorption of fibronectin reduced the initial electronegativity of the BG surface and delayed the formation of both the amorphous and the crystalline Ca-P layers. The delayed formation of these surface layers may be attributed to the competitive binding of Ca2+ ions by the fibronectin molecule. In addition, the formation of an amorphous Ca-P layer correlated with the reversal from a negatively to a positively charged surface, independent of the presence of fibronectin. The addition of a single protein (in this case fibronectin) can significantly alter material surface parameters, such as charge, and subsequently affect the formation of a surface Ca-P layer. Furthermore, the formation of an amorphous Ca-P layer is an important event in the reactions leading to bioactive behavior, and proteins such as FN are actively involved in the transformation of the surface into a Ca-P layer.