# Evaluating the Degradation of WE43 for Implant Applications: Optical and Mechanical Insights
## 4 Results and Discussion
### Results from the Degradation Experiments  
By using HBSS as body replacement fluid without another buffer introduced, the pH value changed over the time of the experiment (Figure 9). Before starting the experiment, the pH was set to a physiological value of about 7.5. After the start of the experiment (placing the sample in the liquid), the pH value increases sharply within a short time. As a result, the value was between 8.4 and 8.8 after 20 h. In the further course, an almost constant value of 8.5 to 8.9 was reached (after 80 h). This exemplary course was observed in an approximate manner for all samples and in all experimental chambers of sample geometry 1. During all tests, the set temperature remained constant at 37 \({}^{\circ}\)C (\(\pm\)0.2). The behaviour of the pH value in this study is improved compared to the findings of [PERSON] et al. (V/S ratio of 40 mL/cm\({}^{2}\)) [24] and [PERSON] et al. (V/S ratio not mentioned) [27]. In both studies, WE43 samples were immersed in HBSS liquid for a defined duration, during which the pH value was observed to increase rapidly initially (pH value between 9.0 and 10.5 after 120 h). [PERSON] et al. [25] reported a pH range of 10.0 to 10.5 after five days (V/S ratio not mentioned), whereas [PERSON] et al. [16] measured a value of approximately 11.0 (V/S ratio of 30 mL/cm\({}^{2}\)). Comparing with the existing literature, it becomes clear that an appropriate V/S ratio must be selected. The significantly higher increase in pH value within the first five days in the studies by [PERSON] et al. [24] and [PERSON] et al. [16] can be explained by their lower V/S ratio. Another influence could be minimal differences in the HBSS composition. Overall, it can be stated that in the experimental chamber presented here, a pH value increase of 0.7 to 1.2 was observed within the first 24 h. This leads to the conclusion that an additional coating might be recommended for future applications to slow down degradation and, consequently, mitigate its impact on the pH value. This can reduce the body's need to compensate for pH changes. The results for the positioning of sample geometry 1 and 2 in the experimental chamber are shown in Figure 10. Before the start of the experiments, the roughness depth R\({}_{z}\) was 2.55 \(\upmu\)m for the samples (Figures 7 and 10a). After five days in the HBSS, an increase in the roughness depth is measured. Depending on the positioning, the value is between 10.0 \(\upmu\)m (X\({}_{2}\) sample geometry 2) and 20.5 \(\upmu\)m (X\({}_{1}\) sample geometry 2). Overall, position X\({}_{1}\) exhibits  
Figure 9: Development of the pH value during the experiment time of sample geometry 1 and the comparison with literature values [24; 27]. the highest resulting roughness for both sample geometries. The spread of the values is highest for position X\({}_{1}\) compared to the other two positions. This greater spread can be attributed to its location in the experimental chamber. In general, both geometries lead to very similar results. An exemplary view of the surface roughness can be seen in Figure 10b. A cloud-like layer structure can be observed in the topmost layer of the image. These \"clods\" are the degradation layer that lies loosely on top of the base material. In the bottom-most layer of the image, the base material is visible, and the initial signs of degradation can be identified. To contextualise the results of the roughness analysis, Figure 11 presents the findings from the analysis of flow conditions. A non-directional flow is present at position X\({}_{3}\) (left position, largest distance to the pump). This is due to the position in the installation space, which is located directly in the corner. Position X\({}_{2}\) (middle position) experiences a directional flow, characterised by a constant flow that exerts uniform loads on the degradation layer due to fluid movement. Position X\({}_{1}\) (the smallest distance to the pump, not visible in the figure) undergoes the direct flow of the pump due to its positioning close to the pump outlet.