*2.1. Materials*

Titanium discs (diameter 1.2 cm; height 0.3 cm) were supplied by Cowellmedi (Busan, Korea). Recombinant human platelet-derived growth factor-BB (rhPDGF-BB), recombinant human bone morphogenic protein-2 (rhBMP-2), rhPDGF-BB, and rhBMP-2 ELISA kits were purchased from PeproTech Inc. (Rocky Hill, NJ, USA). Ascorbic acid, dexamethasone, β-glycerophosphate, and dopamine were from Sigma-Aldrich (St. Louis, MO, USA), and heparin sodium (molecular weight: 12,000–15,000 g/moL) was from Acrose Organics (Belgium, NJ, USA). Dulbecco's modified Eagle's medium (DMEM), fetal bovine serum (FBS), phosphate-buffered saline (PBS), and penicillin-streptomycin (PS) were from Gibco BRL (Rockville, MD, USA).
