This study was approved by the Institutional Review Board of the University of Hong Kong/Hospital Authority Hong Kong West Cluster (IRB UW13-555). Extracted sound human third molars were collected with patient consent. They were stored in distilled water at 4 °C and were used within one month of extraction. Thirty dentine blocks of 2 × 2 × 4 mm3 were prepared from a tooth root modified from our previous protocol . The blocks were treated with 1% acetic acid for 5 s and ultrasonically washed with deionized water to remove the smear layer . In addition, 36 hydroxyapatite disks were prepared by compressing hydroxyapatite powder (Sigma-Aldrich Co., St. Louis, MO, USA) into a circular mold (12 mm in diameter, 1 mm in thickness). They were sintered at 900 °C for 2 h. The surfaces of the hydroxyapatite disks were polished using micro-fine 4000 grid sanding paper. Commercially available methyl-PEG-amine (mPEG-NH2 Mw = 2000) (Shanghai Science Peptide Biological Technology Co., Ltd., Shanghai, China) and polydopamine (Sigma-Aldrich, St. Louis, MO, USA) were used in this study.