# Design and 3D Printing of Interbody Fusion Cage Based on TPMS Porous Structure
## 3 Experiment and Discussion  
### Experiment  
To verify the feasibility of the porous structure proposed in this paper, in the application of the interbody fusion cage, the printing and pressure analysis of SLM porous structure were carried out. The EP-M250 introduced earlier was selected as the printing equipment, and the material was titanium alloy powder. The different porosity structures of TPMS-G and TMPS-P were printed and the mechanical properties were tested. The microstructure of the printed hole structure were detected by SEM. Considering the actual processing process, due to the different process parameters of SLM printing, such as laser power, laser spot diameter, powder layer thickness, scanning speed, and whether the substrate was preheated or not, will lead to some specific phenomena of bone tissue  
Figure 9: Diagram of finite element analysis results (the figure on the left shows equivalent stress and the figure on the right shows total deformation). (**(a)** Standing still; (**b**) left bend; (**c**) right bend; (**d**) forward flexion; (**e**) backward extension; (**f**) left rotation; (**g**) right rotation).  
engineering scaffolds. The SLM printing and SEM analysis process of bone tissue engineering scaffold in this paper are shown in Figure 10, where the a diagram shows the L3-L4 vertebral body model; the b diagram shows the interbody fusion cage in the L3-L4 verteb model printed by SLM; the c diagram shows the pore structure inside the interbody fusion cage, and d and e are SEM analysis diagrams of the porous structure.  
The printed results were processed by scanning electron microscope (SEM). It can be seen that obvious defects and cracks occurred in the printing process, and all appear in the central area of the structure. The main reason for this phenomenon is that the Gaussian distributed laser energy caused a large temperature gradient in the material in the laser scanning area, and the uneven deformation of the material in each area during the cooling process caused residual stress. Moreover, during the SLM printing process, the heat conduction between the powder layers in the z direction was poor, resulting in a temperature gradient, resulting in different residual stress distributions in the upper and lower parts of the print. The rapid heating and cooling during the SLM process will produce huge temperature gradients. This temperature gradient will cause the material to expand or contract, causing residual stress. If the maximum tensile strength of the alloy is exceeded, cracks will form. Based on the analysis of its generation mechanism, this paper proposes two methods of substrate heating and material heat treatment to suppress cracks and pores. When the substrate is used for preheating, it indirectly increases the input of external energy, reduces the laser energy, and the temperature distribution of the entire substrate tends to be consistent, to reduce the effect of residual stress. From the nature of the material itself, increasing the temperature can increase the material toughness and reduce brittleness; when the same deformation occurs, it can reduce the generation of cracks.  
Figure 10: SLM printing and SEM analysis process of bone tissue engineering scaffolds. (**(a)** L3–L4 vertebral body model. (b) Interbody fusion cage in the L3–L4 verteb model printed by SLM. (c) Pore structure inside the interbody fusion cage; (d) and (e) SEM analysis diagrams of the porous structure).