## 2 Materials and Methods
### Design and Prototyping of Porous Compound Cage  
Porous composite fusion cages have several design requirements. The instrument should induce bone fusion after surgery and should be able to reduce subsidence compared to conventional titanium alloy commercial products, and the fused complex should not be separated. Also, special equipment dedicated to surgery is required, and the connection must be possible between two instruments and separation also must be possible after surgery. Ti6 Al4V-ELI (per ASTM F136) material, which has been widely used and already verified [11; 12] as a medical device material for orthopedics and neurosurgery to improve bone union rate, was selected, and the porous structure and size confirmed through animal experiments were applied. The porous structure of the spinal fusion cage is advantageous for increasing the bone union rate, but due to the unique rough surface, there is a risk of damage to the nerves and blood vessels during the insertion of the cage in surgery. Therefore, the side part has a solid structure that was treated with polishing, and only the center of the vertebral end plate is designed as a porous structure. To reduce subsidence, medical PEEK (polyetheretherketone per ASTM F2026), which is a typical biomaterial, was selected. PEEK is known to have similar elastic moduli to human cancellous bone. Since two different materials are were being combined, they were not to be separated after surgery. Also, because it is difficult to use adhesives to combine the two components, because the medical device is inserted in the living body, a precision machining method was used to combine the two materials. Thus, it completely restrained the three directions and minimized the possibility of separation into a dove-tail structure. The PEEK was constructed with a pocket inside and a slanted side, so that it could not come out again when the metal part was forcibly pushed in from above. In order to use the cage, a device called a cage holder was necessary. It has a structure in which the cage and the holder are connected and can be separated after the operation. Also, since excessive compression and rotational force are delivered during the operation, a sufficient fixing force of the portion connected to the cage was required. Therefore, a hole was drilled in the back of the cage and a  
Figure 1: The 3D printing porosity size samples for SEM. Figure 2: The 3D printing design of animal experiment implant specimen: From the left—Additive Manufacturing Type 1, Additive Manufacturing Type 2, Additive Manufacturing type 3, Additive Manufacturing Solid and Machined Solid. 3 mm tap (M3 \(\times\) Pitch 0.5 mm) was machined, and a rectangular pocket was used to prevent rotation. All designs and simulation verification were done using Solid works 2013 Premium (Dassault System, Velizy-villacoublay, France), a commercial 3D CAD software. (Figure 3). The final prototype of the cover part was printed using EBM (Electron Beam Melting) within an ARCAM A1 (ARCAM AB, Molnlycke, Sweden). The ARCAM EBM A1 machines use EBM. During the EBM process, the electron beam melts metal powder layer-by-layer to build the implant. The vacuum environment in the EBM machine maintains the chemical composition of the material and provides an excellent environment for building parts in reactive materials such as titanium alloys. The high power of the electron beam ensures a high rate of deposit and an even temperature distribution within the part. The results trigger the full melting of the metal powder and high strength properties of the material. The EBM machine titanium Ti6 Al4V ELI (Grade 23) is a gas-atomized powder with a particle size between 45 and 100 microns. This limit on the minimum particle size ensures safe handling of the powder. Ti6 Al4V-ELI has numerous applications in the medical industry. The biocompatibility of Ti6 Al4V-ELI is excellent, especially when direct contact with tissue or bone is required.