## 3 Results and Discussion
### The Effect of SBF Immersion on the Microstructure of an HA/WS Skeleton
The occurrence of the pure wall phenomenon on the inner hole surface is attributed to the high concentration of wollastonite's glass phase, which is formed during the sintering process at elevated temperatures. The microstructure of the tiny caverns inside, as shown in Figure 7b, exhibits a more intricate composition, suggesting that the distinct microstructures of the apatite nanocrystals reflect variations in the SBF contact environment. The internal micropore structure of the bone implant skeleton may therefore require redesigning in accordance with the preferences of bone cells and bone regeneration. The internal micropore structure of an SBF needs to undergo in vitro cell tests after different periods of infiltration, highlighting the necessity of the careful tailoring of biomaterials with appropriate chemical composition, microstructure, pore size, and porosity to optimize conditions for stimulating bone tissue regeneration [37; 38; 39; 40]. Engineered cells and gene-activated materials [41; 42; 43; 44] will open up new avenues for research on bone growth and regeneration by incorporating bioabsorbable apatite nanocrystals with diverse micro morphologies into the three-dimensional micropore structure of the material. Figure 6: A microscopic view of the V-bottom interior of HA/WS skeleton. (**a**) Micrographs of the interior environment of the half-full cavity. (**b**) Enlarged view of the flat surface and the V-bottom section, showing seaweed-like nanocrystals. (**c**) Further enlarged view at the V-bottom. Figure 7c illustrates the presence of a narrow wall, measuring approximately 1 \(\upmu\)m, separating the two fully occupied internal micro-caves. The micropores in the cavity wall were filled with calcium-rich apatite nanocrystals, which subsequently formed a compact coating of approximately 1 \(\upmu\)m thick apatite nanocrystals on both sides. The presence of clusters consisting of calcium-deficient apatite nanocrystals on both sides indicates the closure of pre-existing micro-holes in the thin wall by these nanocrystals, suggesting a modification in the opening system within the skeleton. Consequently, further research is required to optimize skeleton design considering fluid infiltration and apatite nanocrystal formation, enabling the efficient utilization of cells and genes for activating apatite crystal formation while minimizing potential adverse side effects on the open-hole skeleton system. The internal environment of the sintered three-dimensional porous HA/WS skeleton is significantly more intricate compared to the flat and fully exposed surface utilized in current biomineralization studies [8; 28; 29; 45]. In addition to the large open micropores ranging from 50 \(\upmu\)m to 500 \(\upmu\)m in size, there exist numerous micropores and nanopores on the inner wall, which are interconnected through various means. The formation conditions of apatite nanocrystals in three distinct SBF contact environments, as depicted in Figure 2b, further contribute to the intricate nature of the structure. The observations demonstrate that the inherent variations in internal conditions inevitably give rise to the heterogeneity of apatite nanocrystals. The crystallization process, even when constrained by a specific and well-defined condition such as the internal angle or nanoscale surface roughness, continues to present novel discoveries and challenges. The key functions of the apatite nanocrystal structure in the three-dimensional porous skeleton need to be investigated from various perspectives including crystal engineering [46; 47; 48] and tissue engineering [49; 50; 51; 52; 53; 54]. A three-dimensional structural image of the skeleton was acquired using a micro-computed tomography (Micro-CT) system, specifically the Skyscan 1176 Micro-CT, provided by the Harry Perkins Medical Center at Xihua University.