Bioactive materials such as 4SSS bioactive glass (BG) bond to bone by forming a calcium phosphate (Ca-P) layer in vivo. Currently, the precise mechanisms associated with bioactivity have not been elucidated. We hypothesize that bioactive surface reactions will result in a surface charge variation, and this variation will in turn affect the kinetics of these reactions and consequently, formation of a surface Ca-P layer. We combined electrophoresis measurements with surface and solution analysis techniques to examine the relationship between bioactive surface zeta potential (ZP) variations and the formation of a surface Ca-P layer in electrolyte solutions with and without protein. We also quantified cell attachment to the changing bioactive surface as a surface Ca-P layer was formed.

We found the initial BG surface to be negatively charged, and temporal surface ZP variations directly corresponded to the formation of a surface Ca-P layer. We measured two sign reversals in ZP of BG within 3 days of immersion in an electrolyte solution. The first sign reversal from negative to positive corresponded to the formation of an amorphous Ca-P layer. The second sign reversal from positive to negative was due to precipitation of phosphate ions from solution, and it corresponded to the formation of a crystalline Ca-P layer. Addition of fibronectin significantly reduced BG surface electronegativity and delayed the formation of the amorphous and crystalline Ca-P layers. A sign reversal from negative to positive correlated with amorphous Ca-P layer formation. Competitive binding of Ca2+ by fibronectin may delay the formation of the surface Ca-P layer. Pre-adsorption of fibronectin on BG significantly increased cell attachment. Primary osteoblasts responded at higher percentage to the adhesion substrates than cells from an osteosarcoma line. There is evidence that charge and the presence of a Ca-P layer on BG can affect osteoblastic attachment.

In summary, the initial BG surface was negatively charged in electrolyte solutions and temporal ZP variations corresponded to the formation of a Ca-P layer. Fibronectin adsorption reduced surface electronegativity and delayed Ca-P layer formation. Cell attachment to BG was dependent on fibronectin adsorption, cell type and the presence of a surface Ca-P layer.