## 3 Results and Discussion
### Cohesive Characteristics of the HAp/BC Bilayer Model  
To evaluate the dependence of the BC/HAp cohesion on the crosslinking density, two types of the pull-out tests were conducted in this study: (1) a test in the opening mode and (2) a test in the sliding mode. For the opening-mode test, the upper part of the BC sample was fixed and the HAp sample was moved in the -_z_ direction at a speed of 0.5 m/s, using the NVT dynamics at room temperature. During the pull-out process, the traction stress was obtained by tracking the force applied to the HAp substrate and dividing it by the surface area. The sliding-mode test was performed by moving the HAp substrate in the \(x\) or \(y\) directions, under the same BC constraint and under the same substrate velocity condition, while the traction was derived by averaging the responses for the different directions. First, the traction-separation response was calculated, while 40% of the top of the BC was fixed to derive the cohesive properties throughout the entire system. As shown in Figure 6a, when the separations were applied in the direction perpendicular to the surface, a linear increase in the traction stress was observed. In the corresponding elastic region, many uniform-size nanovoids were created in the polymer network, whereas the nanovoids did not grow [35]. Especially, the magnitude of the traction stress was proportional to the occurrence of crosslinking, indicating improved cohesive properties. In the subsequent damage region resulting from the continuous descent of HAp, these nanovoids gradually integrated over separation, decreasing the traction stress exponentially. Note that because only the partial region of the BCs was fixed, the current cohesive response is a comprehensive result that includes not only the BC/HAp interfacial characteristics, but also the mechanical response of the BC. To separately examine the contribution of the interfacial region, the same pull-out test was performed with the top 85% of the BC fixed, as shown in Figure 6b. Because the 85% constraint on the BC excludes the variation of the mechanical properties by the curing of the BC material, only the interfacial response can be identified under this condition. Still, stronger traction was clearly observed during the interface opening, for high crosslinking ratios. In conclusion, the crosslinking reaction of  
Figure 5: Density distributions for the HAp/BC bilayer models. The density profiles of the HAp substrate and the BC were described independently. BC provides both the benefits of the rigid microstructure and improved interface response with the bone scaffold from the mechanical point of view. In the traction-separation relationship, the \(\delta_{0}\) and \(\delta_{f}\) parameters represent the elastic distance and the failure distance, respectively. In addition, \(K_{eff}\) is the cohesive stiffness in the loading direction, and \(G_{c}\) is the critical energy required for fracture, which can be calculated from the area under the curve. The characterized cohesive parameters are listed in Tables 2 and 3. In all of the considered cases, \(K_{eff}\) and \(G_{c}\) significantly improved with a progress in the crosslinking ratio (\(K_{eff}\) improved by 32%; \(G_{c}\) improved by 98% for the system separation. In addition, for the interface separation, \(K_{eff}\) improved by 14% while \(G_{c}\) improved by 9%). In the case of the entire system separation, an increase in \(\delta_{f}\) was also observed because the effective length of the polymer network increased owing to the high crosslinking ratio. The relationship between HAp and BC mainly originated from the weak non-bonded interactions rather than physical bonds such as covalent anchoring. For this reason, the critical problems within the body such as loosening of the resulting product due to the slippery interface can occur if the interaction strength is not sufficiently high.