The isolation and characterization of DFCs and dNC-PCs were described in previous studies [4,7,12]. DFCs were routinely cultivated in DMEM (Sigma-Aldrich, St. Louis, MO, USA) supplemented with 10% fetal bovine serum (Sigma-Aldrich, St. Louis, MO, USA) and 100 μg/ml penicillin/streptomycin (standard cell culture medium). dNC-PCs were cultivated in DMEM (Sigma-Aldrich) supplemented with 15% fetal bovine serum (Sigma-Aldrich) and 100 μg/ml penicillin/streptomycin (standard cell culture medium). For experiments, both cell types were used after passage 6. DFCs and dNC-PCs expressed typical markers for dental stem cells such as CD105, Nestin, and STRO-1 (Additional file 1: Figure S1).

Five milliliter of PA gel solution with the concentration of 8% acrylamide and 0.06% bis-acrylamide (Bio-Rad, Hercules, CA, USA) were mixed and degas under vacuum for at least 20 min to remove oxygen. Then, 30 μl of 0.1 mg/mL ammonium persulfate (Sigma-Aldrich, St. Louis, MO, USA) and 20 μl TEMED (Applichem, Omaha, NE, USA) were added and placed into the mini protean casting strand and frame (Bio-Rad) to form 1-mm thickness of substrate. After letting the gel to polymerize for 30 to 45 min, gently remove and rinse gel with 50-mM HEPES, pH 8.5 (Applichem, Omaha, NE, USA). PA gel was then cut into circular shape with 14 mm diameters and placed in 24 well plates for the experiment. Sulfo-SANPAH (Pierce Biotechnologies, Rockford, IL USA) 0.5 mg/mL in 50-mM HEPES, pH 8.5 was pipetted onto the surface and exposed to the UV light for photoactivation procedure. After photoactivation, the substrate was washed several times in 50-mM HEPES. A 0.2 mg/mL of type I collagen (Sigma-Aldrich, St. Louis, MO, USA) was then layered onto the surface of gel and incubated 4 h at room temperature or overnight at 4°C on a shaker. After washing with PBS, the gels were stored in PBS at 4°C. Before platting the cells, the gel was exposed to UV for 15 min for the sterilization and replace PBS with complete culture medium for 1 h at 37°C.