## 2 Materials and Methods  
### Material Preparations and Processing  
The raw materials used in this study were all medical-grade PCU pellets purchased from Lubrizol, belonging to the Carbothane(tm) PC Series (Lubrizol, Wickliffe, OH, USA). We ultimately selected four grades of pellets with different hardness: PC-357A, PC-3585A, PC-3595A, and PC-3555D. PC-3575A was classified as a low-hardness material, while PC-3585A and PC-3595A were medium-hardness materials, and PC-3555D was a high-hardness material. The difference in hardness among these four PCU grades essentially stemmed from varying contents of hard segments. As the material's hardness increased, so did its rigidity, tensile strength, and compressive strength, but its elasticity might decrease. Conversely, lower-hardness materials were softer and more elastic, typically exhibiting better elongation but possibly lacking in rigidity. Natural intervertebral discs required both sufficient stiffness to support the weight of the head and maintain cervical spine height, as well as elasticity to cushion and absorb shock, thereby protecting the cervical spine. Therefore, to explore the potential application of PCU in artificial cervical discs, we conducted a comprehensive study encompassing materials from low to high hardness, with a focus on the mechanical properties of PCU, such as tensile and compressive performance, in order to identify a PCU material with properties close to those of natural intervertebral discs. For better differentiation, we classified the four materials based on their hardness and grade: 75A for PC-3575A, 85A for PC-3585A, 95A for PC-3595A, and 55D for PC-3555D. The processing methods employed in the study encompassed three of the most used molding techniques at present [18]: injection molding (IM), compression molding (CM), and fused deposition modeling in three-dimensional printing (3D). Thermoplastic polyurethane products are susceptible to the detrimental effects of moisture when in a molten state. To achieve optimal results, all pellets were dried in a vacuum oven at 65 \({}^{\circ}\)C (150 \({}^{\circ}\)F) for at least four hours prior to processing, ensuring a moisture content of 0.05% by weight or less. Subsequently, the dried PCU particles were introduced into the machinery for further processing. For the injection-molded specimens, the dried PCU pellets were loaded into the hopper of the injection molding machine (MA 900 III; HaiTian Plastics Machinery, Ningbo, Zhejiang, China). The pellets were then melted and injected at a speed of 40 mm/s. Referring to the parameter ranges provided in the Lubrizol technical sheet and considering the actual operating environment, the final temperatures for the mold, rear, middle, front, and nozzle are shown in Table 1 below. For the compression-molded specimens, a large-scale hot press machine (KSH R100; Dongguan Kesheng Industry, Dongguan, Guangdong, China) was employed. During the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Temperature (\({}^{\circ}\)C)** & **75A** & **85A** & **95A** & **55D** \\ \hline Mold & 25 & 25 & 25 & 25 \\ Rear & 185 & 185 & 190 & 190 \\ Middle & 190 & 190 & 196 & 196 \\ Front & 196 & 196 & 202 & 202 \\ Nozzle & 196 & 202 & 210 & 210 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of actual operating temperatures for injection molding of PCU with gradient hardness. hot-press molding process, the temperatures of the upper and lower platens were both set to 200 \({}^{\circ}\)C for 75A and 85A materials, while 210 \({}^{\circ}\)C was applied for 95A and 55D. The pressure was uniformly maintained at 20 MPa. The preheating phase lasted for 8 min, followed by a hot-pressing duration of 10 min, and subsequently, air cooling for 10 min. The dried pellets were placed into standard molds and pressed into thin sheets or cylinders of specified thickness for subsequent experimental research and applications.