## 4 Discussion  
The main objective of this study is to identify a combination of PCU material hardness and molding method that exhibits comprehensive properties more suitable for replacing the natural intervertebral disc. To this end, four PCU materials with different hardness were molded using the commonly employed molding methods of IM, CM, and 3D. Various material performance studies were then conducted to assess the impact of different molding methods and material hardness on the overall properties of the PCUs. This study found that the three molding methods did not affect the material hardness. The results convincingly demonstrated that none of the aforementioned molding methods significantly impacted the hardness characteristics of the PCU material, thereby validating the rationality of these molding processes in terms of temperature settings and processing procedures. No degradation, resynthesis, or other reactions that could alter the original properties of the material were triggered by improper processing conditions. Furthermore, an interesting trend emerged from the hardness data: IM > CM > 3D. Although slight differences existed among the three, these differences were not statistically significant, indicating comparable hardness properties of the materials produced by these three molding methods. This finding aligned with previous research results, and the work by [PERSON] et al. [27] also emphasized the importance of selecting molding techniques in maintaining material performance, yet similarly found no significant differences in hardness within their specific material system. In summary, our findings further support the notion that appropriate molding techniques can reliably produce materials with consistent hardness properties, regardless of the specific method employed. Although the molding method itself has little influence on the hardness of PCU, when pursuing high-precision applications, it is still necessary to consider the minor hardness variations that may arise from different molding processes, in order to make the optimal choice based on specific requirements. The PCU materials selected for this study are intended for future use as nucleus pulposus materials in artificial intervertebral discs. Consequently, there are specific requirements for the microstructure of PCUs to minimize failure factors such as stress concentration, fatigue cracks, and wear debris in subsequent practical applications due to unsatisfactory surface morphology, thereby enhancing the service life of artificial intervertebral discs. [PERSON] et al. [24] observed during the processing of PCU specimens using the CM method  
Figure 8: WCA distribution histogram of gradient-hardness PCUs via different molding methods (* represents statistically significant differences (\(p<0.05\)); I represents the value of standard deviation of the data). that an inappropriate melting temperature during the compression process likely caused the surface of the obtained PCU specimens to appear rough under SEM examination, with the presence of incompletely melted particles. The SEM results of this study differed from theirs; through appropriate temperature control, a uniform distribution of the material on the specimen surface was achieved without the presence of incompletely melted particles. However, possibly due to excessively high demolding speed or insufficient cooling time, minor flocculent marks were present on the surface of the CM specimens. The observations from this study revealed that although all three molding methods could produce dense and essentially defect-free PCU samples, there were still subtle differences in surface morphology. The surface smoothness of IM and 3D samples was superior to that of CM samples. These subtle differences might arise from the unique processing conditions and mechanisms associated with each molding method. The smoother surface of IM samples might be attributed to the higher processing pressure and more controlled cooling rate employed in this technique.