A novel two-step sintering for nano-hydroxyapatite scaffolds for bone tissue engineering

Abstract

In this study, nano-hydroxyapatite scaffolds with high mechanical strength and an interconnected porous structure were prepared using NTSS for the first time. The first step was performed using a laser characterized by the rapid heating to skip the surface diffusion and to obtain the driving force for grain boundary diffusion. Additionally, the interconnected porous structure was achieved by SLS. The second step consisted of isothermal heating in a furnace at a lower temperature (T2) than that of the laser beam to further increase the density and to suppress grain growth by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration. The results indicated that the mechanical properties first increased and then decreased as T2 was increased from 1050 to 1250°C. The optimal fracture toughness, compressive strength and stiffness were 1.69 MPam(1/2), 18.68 MPa and 245.79 MPa, respectively. At the optimal point, the T2 was 1100°C, the grain size was 60 nm and the relative density was 97.6%. The decrease in mechanical properties was due to the growth of grains and the decomposition of HAP. The cytocompatibility test results indicated that cells adhered and spread well on the scaffolds. A bone-like apatite layer formed, indicating good bioactivity.

Figure 1. SEM images and optical photographs.

(a) SEM image of the nano-HAP powder. Optical photographs (b) and SEM images (c–e) of the scaffold.

Figure 2. XRD patterns.

(a) The nano-HAP powder. (b) The SLS scaffold without the second sintering step. (c–g) The scaffolds sintered at different T₂.

Figure 3. FTIR spectra.

(a) The nano-HAP powder. (b) The SLS scaffold without the second step sintering. (c–g) The scaffolds sintered at different T₂.

Figure 4. Relative densities of the struts sintered using NTSS at different T₂.

Figure 5. The etched surfaces of the scaffolds without the second sintering step (a); sintered at T₂ of 1100 (b) °C, 1150°C (c), 1200°C (d), and 1250°C (e); and the average grain sizes of the scaffolds prepared using SLS without the second sintering step and of those using NTSS at different T₂ (f).

Figure 6. Vickers hardness and fracture toughness (a) and compressive strength and stiffness (b) of the SLS scaffold without the second sintering step and of scaffolds sintered at different T₂ (1050, 1100, 1150, 1200 and 1250°C).

Difference (*P < 0.05 and **P < 0.001) between the scaffolds prepared using NTSS compared to those without the second sintering step.

Figure 7. SEM micrographs of the scaffolds soaked in SBF for 2 (a, b), 7 (c, d), 14 (e, f) and 21 (g, h) days; (b), (d), (f) and (h) are higher magnifications images of those presented in (a), (c), (e) and (g), respectively.

Figure 8. EDS analysis for the scaffolds soaked in SBF for 0 (a), 2 (b), 7 (c), 14 (d) and 21 (e) days; changing trend of Ca/P ratios (f) on the surface of scaffolds after soaking in SBF.

Figure 9. SEM images of MG-63 cells cultured on the scaffolds for 4 hours (a) and for 1 (b), 3 (c) and 5 (d) days.

Figure 10. Sintering profiles for NTSS and TSS.