2.3.2. X-ray Photoelectron Spectroscopy (XPS)

The surface chemical compositions of Ti, Hepa/Ti, PDGF/Hepa/Ti, BMP/Hepa/Ti, and PDGF/BMP/Hepa/Ti disc were investigated by X-ray photoelectron spectroscopy (XPS; K-Alpha spectrometer; Thermo Electron, Rockford, IL, USA). Amounts of heparin immobilized onto Ti were measuredusing toluidine blue. Hepa/Ti disc was immersed in 1 mL PBS buffer (pH 7.4) containing 1 mL 0.005% toluidine blue, gently shaken for 30 min, and 2 mL of hexane was added. After removing the disc, absorbance of the aqueous phase was measured by a Flash Multimode Reader (Varioskan™, Thermo Scientific, Waltham, MA, USA) at 620 nm. The amount of heparin immobilized onto disc were calculated using a calibration curve prepare using different concentrations of heparin.
