The dentine blocks (2 × 2 × 4 mm3) and hydroxyapatite disks (diameter: 10 mm) were autoclaved for sterilization. They were randomly and equally divided into 4 experimental groups. In Group 1, dentine blocks and hydroxyapatite disks were immersed in 2 mg/mL of freshly prepared polydopamine solution (in 10 mM Tris buffer, pH = 8.5) at 23 °C for 12 h in the dark . After rinsing for 10 min with Tris buffer to remove non-attached polydopamine and drying under nitrogen, the blocks and the disks were then immersed in a freshly prepared solution of 1 mg/mL of mPEG-NH2 (in 10 mM Tris buffer, pH = 8.5) at 23 °C for 4 h. They were then rinsed with Tris buffer for another 10 min to remove any extra solution . In Group 2, dentine blocks and hydroxyapatite disks were immersed in a freshly prepared solution of 1 mg/mL of mPEG-NH2 at 23 °C temperature for 4 h. They were rinsed with Tris buffer for 10 min to remove residual solution. In Group 3, dentine blocks and hydroxyapatite disks were immersed in 2 mg/mL of freshly prepared polydopamine solution at 23 °C for 12 h in the dark. They were rinsed with Tris buffer for 10 min to remove nonattached polydopamine. In Group 4, dentine blocks and hydroxyapatite disks were immersed in distilled water.