## 3 Results and Discussion
### The Effect of SBF Immersion on the Microstructure of an HA/WS Skeleton
It can be seen that there are relatively dense apatite nanocrystal layers and sail-like apatite nanocrystals on the surface. According to the overall formation trend of calcium-rich apatite nanocrystals and calcium-poor apatite nanocrystals under quantitative SBF conditions, the relatively dense apatite crystal layer comprises the first calcium-rich apatite nanocrystals formed at the initial crystallization stage while the sail-like apatite nanocrystals are the calcium-poor apatite nanocrystals formed at the later crystallization stage. As shown in Figure 3a-c, the SBF permeates the inner micropores, micro-channels, and micro-holes of the three-dimensional porous HA/WS skeleton surface and forms calcium-rich apatite nanocrystals. Due to the relatively abundant supply of calcium ions in the external body-fluid environment at the initial crystallization stage, calcium-rich apatite nanocrystals are formed. Moreover, these calcium-rich apatite nanocrystals gradually form a dense coating of apatite nanocrystals on the outer surface of the three-dimensional porous HA/WS skeleton. Holes with closed surfaces or small openings are less infiltrated by the SBF than holes with completely open or large openings, and so, the supply of calcium ions is limited, resulting in the formation of calcium-poor apatite nanocrystals. As shown in Figure 3a-c, since the outer surface of the three-dimensional porous HA/WS skeleton is in full contact with the SBF solution, more calcium ions are available for the formation of apatite nanocrystals, and so, a thicker and dense coating of calcium-rich apatite nanocrystals is formed on the outer surface of the three-dimensional porous HA/WS skeleton. As can be seen from Figure 3d, the dividing line between the HA/WS skeleton  
Figure 2: Microstructure of a nanocrystal. **(a)** Tilted landscape and near-surface cross-section view of the porous HA/WS skeleton. Red and blue insets stand for two different nanocrystal structures. **(b)** Three typical internal environments inside HA/WS skeleton: **(i)** fully filled by SBFs, **(ii)** half-full, **(iii)** wall-only (SBF is diffused through and attached only to the cavity wall). and apatite nanocrystals is relatively obvious. Because of the different SBF environments, the different calcium ion concentrations, and the different micropore structures, different forms of apatite nanocrystals are formed inside and outside the three-dimensional porous HA/WS skeleton. It must be noted that the formation of the tunnel wall (about 8 \(\upmu\)m thick) shown in Figure 3d has similar characteristics to the apatite film (about 500 nm) observed under TEM [33]. The results show that the concentration of calcium ions has a great influence on the type and thickness of apatite nanocrystal coating. Figure 4a is the microstructure picture of the outer surface of the three-dimensional porous HA/WS skeleton. It can be observed that the boundary between the mirror-like surface and the rough area is easily distinguished. The completely open micro-holes and micro-channels in the three-dimensional porous HA/WS skeleton and the pores near the surface are completely filled with apatite nanocrystals, and a dense apatite nanocrystal coating is formed on the surface of the HA/WS skeleton. This may have a significant impact on the role of micro-holes, micro-channels, and micropores in bone regeneration. In actual bone implantation surgery, whether to promote or restrict the growth of apatite nanocrystals depends on the reaction of the skeleton with bone cells. In part, this may explain why bone marrow cells and regenerative stem cells should be transplanted into the skeleton fold before actually being implanted [34; 35; 36]. Since the total time of SBF immersion was only three weeks, the experimental results show that the dense apatite nanocrystals coating of about 2 \(\upmu\)m as shown in Figure 4b can be formed within 21 days and the deposition rate of apatite nanocrystals exceeds 100 nm per day.