## 3 Results and Discussion  
### The Effect of SBF Immersion on the Microstructure of an HA/WS Skeleton  
The microstructures of apatite nanocrystals \"hidden\" on the inner surface of the three-dimensional porous HA/WS skeleton during the inward growth of bone cells were investigated by using a high-intensity field emission scanning electron microscope (SEM). It has been shown that two different types of apatite nanocrystals are formed during the process of biomineralization and the inward growth of bone cells [15; 28; 29]. In this study, we observed two different types of apatite nanocrystals: one was a circular apatite nanocrystal, located on the surface of the sintered three-dimensional porous HA/WS skeleton, and the other was a sail-like nanocrystal cluster located on the top of the circular nanocrystal. The two main types of apatite nanocrystals can be observed from the oblique surface microstructure and in a cross-sectional view of the three-dimensional porous HA/WS skeleton, as shown in Figure 2a. The red and blue illustrations show two different types of apatite nanocrystals inside the three-dimensional porous HA/WS skeleton. The purpose of this study was to study the interior of the three-dimensional porous HA/WS skeleton to explore the microscopic diversity of apatite nanocrystals \"hidden\" in it and their influence on the adhesion of biological cells. In this study, a three-dimensional porous HA/WS skeleton with a random micropore structure was prepared using the freeze-drying method [30; 31; 32]. In addition to the changes in the internal pore structure of the three-dimensional porous HA/WS skeleton, it is also necessary to consider three representative structures in the three-dimensional porous HA/WS skeleton. When the three-dimensional porous HA/WS skeleton and SBF come into contact, three different SBF contact environments are formed. The three SBF contact environments are shown in Figure 2b: (i) micropores fully filled with SBFs, (ii) half-full-with-SBF contact environments that exist due to the presence of partial air pressure in the pores or limited SBF infiltrate, and (iii) completely closed micropores, with only a wall or where just a limited SBF quantity infiltrates through micropore channels and adheres to the inner walls of the micropores. Detailed studies of apatite nanocrystals formed in these unique SBF contact environments will enable us to understand the internal microstructure changes resulting from the formation of apatite nanocrystals to assess their potential impact on bone cell growth and bone tissue regeneration in future studies. [PERSON] et al. [15] explained the formation process of apatite nanocrystals distributed on the surface after SBF contact with sintered HA. First, calcium ions of positive valence contact and react with the exposed HA surface to form calcium-rich apatite nanocrystals. Then, the calcium-rich nanocrystals contact and react with negative phosphate ions to form calcium-poor apatite nanocrystals. The alternating process and dynamics of crystal formation are affected by the total amount of positive calcium and the number of negative phosphate ions available for crystallization. If the total concentration of positive calcium ions and negative phosphate ions changes, different types of apatite nanocrystals will be formed. Therefore, according to the results of [PERSON] et al., it can be concluded that in a settled concentration of SBFs, the general trend of forming apatite nanocrystals is that the calcium-rich apatite nanocrystals are located at or near the surface of sintered HA while the calcium-poor apatite nanocrystals are located away from the surface of sintered HA. Figure 3a is the original electron microscope photograph with a scale of the three-dimensional porous HA/WS skeleton, showing the surface microstructure of the micro-holes inside the HA/WS skeleton. It can be seen from Figure 3a that there are completely closed holes and half-closed holes in the HA/WS skeleton. Figure 3b-d show a detailed enlarged image of Figure 3a.