To evaluate the biocompatibility of composite hydrogel, hMSCs were purchased from the American Type Culture Collection (ATCC, Manassas, VA, USA). The cells were cultured in α-MEM (Gibco-BRL, Gaithersbug, MD, USA) containing 10% fetal bovine serum (FBS) and 1% antibiotics at 37 °C under 5% CO2 and 100% humidity. Osteoblast differentiation was induced using osteoblast differentiation reagents (10 mM β-glycerophosphate, 50 μg/mL ascorbic acid, and 100 nM dexamethasone (Sigma-Aldrich, St. Louis, MO, USA). The number of viable cells was determined using the CellTiter96® aqueous one solution kit (Promega, Madison, WI, USA). Briefly, cells were seeded to the hydrogel. At a predetermined time point (6 days), 200 μL of MTS reagent was mixed with 500 μL of culture media and added to each well. After incubation for 2 h, absorbance of the supernatant was measured at 490 nm using an ELISA reader (SpectraMAX M3; Molecular Devices, Sunnyvale, CA, USA). After 6 days of cultivation, cell-loaded hydrogels were rinsed with PBS to remove the phenol red, and were with PBS. In addition, the Live/Dead® Viability/Cytotoxicity staining kit (Molecular Probe, Eugene, OR, USA) reagent solution was added. After incubation for 30 min in a CO2 incubator, the samples were observed using an inverted fluorescence microscope (DM IL LED Fluo; Leica Microsystems, Wetzlar, Germany). SEM was used to observe cell adhesion to the hydrogels. After 6 days of culture, the cell-loaded hydrogels were fixed with 2.5% glutaraldehyde, and additional-fixation was performed with 0.1% osmium tetroxide (Sigma-Aldrich, St. Louis, MO, USA). After dehydration with a graded ethanol series (50%, 75%, 95% and 100%), the samples were sputter-coated with gold, and observed by SEM (EM-30; Coxem, Daejeon, Korea) .