## 3 Results and Discussion
### The Effect of SBF Immersion on the Microstructure of an HA/WS Skeleton
Due to the proven biomineralization process, the growth of apatite nanocrystals on the surface and internal micropores structures in weeks is critical for the success of artificial skeleton implantation. On the other hand, the results in Figure 4a show that the apatite nanocrystals formed during the immersion process with SBF can cover the holes and gaps, thus proving that the apatite nanocrystals formed by the biomineralization process of the HA/WS skeleton and SBF can repair micro-cracks in the natural HA system such as in the teeth and bones. Figure 3: The complex microstructure of the three-dimensional porous HA/WS skeleton. (**a**) Completely closed and half-closed holes. (**b**) Enlarged view of the mid-section in (**a**), showing different interior sail-like nano-platelet clusters. (**c**) Enlarged view of (**b**), showing a small entry has been sealed by the nano-platelet crystal formations. (**d**) Enlarged view of an enclosed cavity at the bottom-right in (**a**). On the relatively uneven surface of the three-dimensional porous HA/WS skeleton (left of Figure 4a), the vertices of the HA/WS skeleton grains are surrounded by clusters of calcium-rich apatite nanocrystals and calcium-poor sail-like apatite nanocrystals. The combination of the two types of apatite nanocrystals effectively doubles the total thickness of the coating, which is advantageous if bone integration is a major consideration for a particular implant. Figure 4b,c show the interface between the HA/WS skeleton and the apatite nanocrystals; this may be the first time that the interface between the artificial skeleton and the apatite nanocrystals has been clearly observed using scanning electron microscopy, and this interface is perfect even at the nanoscale. In addition, the interface between the HA/WS skeleton and apatite nanocrystals shows no signs of cracking, indicating strong interface bonding. It can be seen from Figure 4c that the apatite nanocrystals at the interface are of the smallest size, which is only about 50 nm. As apatite nanocrystals grow outward gradually, the sizes and shapes of the apatite nanocrystals increase gradually. The disparity between Figures 4 and 5 illustrates the generation of distinct apatite nanocrystals on the external and internal surfaces of the exposed three-dimensional porous HA/WS skeleton. Figure 4: A crystal-clear view of the surface at nano-scale. (**a**) An HA/WS grain apex on the uneven skeleton surface. (**b**) The interface between HA/WS and apatite nanocrystals. (**c**) Enlarged view of (**b**). The extension of the interface in (**b**), showing a perfect interface at the nano-scale. Figure 5: Nanocrystals close to the interface inside the scaffold. (**a**) Apatite nano-crystals formed on the HA. (**b**) Top view of sail-like nano-platelet clusters. Figure 6 shows a natural cross-sectional view of the subtle changes in the microstructures of apatite nanocrystals in a half-full cavity. The relatively diluted SBF and limited calcium ions within this internal cavity facilitate the formation of apatite nanocrystals exclusively on the bottom surface of the groove, which is distinct from their absence on the three-dimensional fracture plane, as depicted in Figure 4 or Figure 5. Another obvious difference is that the apatite nanocrystals on the outer surface of the HA/WS skeleton are tightly arranged while the apatite nanocrystals in the inner half-full cavity are loosely arranged, as shown in Figure 6c. Therefore, the observations in Figures 5 and 6 are consistent, and they both reflect the influence of calcium ion concentration inside the HA/WS skeleton on the formation of apatite nanocrystals. The two distinct internal apatite nanocrystal structures depicted in Figure 7a,b were utilized in the illustration presented in Figure 2 to emphasize the objective of this study, which was to investigate the microstructural diversity of apatite nanocrystals within a three-dimensional porous HA/WS skeleton. In Figure 7a, a portion of the pure wall can be observed on the inner surface of the cavity.