# Evaluating the Degradation of WE43 for Implant Applications: Optical and Mechanical Insights
## 3 Materials and Methods
### Preparing and Conducting the Experimental Tests  
The degradation was analysed depending on the positioning in the experimental setup of the three different experimental chambers. This was used to identify which position is suitable for determining a degradation similar to that in the human body or as reported in the literature [18; 20] and what influence arises from the distance to the heating rod or the pump. For this purpose, three different positions were investigated (Figure 5). These experiments were carried out in the same way in all chambers to verify their independence. For this purpose, the samples were placed in the HBSS at these positions for five days. Only one sample was in each chamber at a time. For better comparison with the initial state and to identify uniform surface corrosion, one half of the sample was always taped with parafilm. It was not recommended to use adhesive tape or similar adhesive-based utensils, as these could have interacted with the samples or the HBSS. Parafilm is stable in the environment and leaves no residue on the samples [26]. The investigation also involved the analysis of the degradation rate as a function of the sample geometry. Since the focus here was on the later use as an osteosynthesis implant, such as screws or plates, the geometric conditions occurring there were used as a basis for abstraction. Therefore, two different sample shapes were designed, manufactured, and analysed (Figure 6). Sample geometry 1 was the reference sample and at the same time corresponds to the shank area of a screw with a partial thread (Figure 6a). Sample geometry 2 features two equally sized through-holes (Figure 6b), each inserted axially from the front side of the sample. These holes are centrally positioned within each half of the sample. This configuration, which simplifies to a plate used in the osteosynthesis treatment of bone fractures, was primarily analysed to assess the impact of the holes on the structural integrity and performance of the implant. Both geometries were constructed in such a way that they are symmetrical and thus make it possible to cover half of the sample. Covering part of the sample allows for the examination of both the initial and final states during subsequent characterisation. This comparison enables the determination of  
Figure 4: CAD model of the sample holder with and without a sample. Figure 5: Different positions of the samples during some tests (top view of a single experimental chamber). height reduction due to uniform surface corrosion. In total, each geometry was placed in the experimental setup with HBSS three times for five days each. The size of the samples was chosen so that there was a large ratio of HBSS volume (1.5 L) to sample surface area (V/S ratio). Sample geometry 1 had a V/S ratio of 139.54 mL/cm\({}^{2}\), and sample geometry 2 had 126.26 mL/cm\({}^{2}\), as listed in the table in Figure 6c. The respective surface area is listed there for the subsequent calculation of the degradation rate. If the V/S ratio is chosen too small, the degradation rate will accelerate depending on the immersion time, which means that the desired abstraction of the application site is not achieved. This effect is prevented with a ratio of over 80 mL/cm\({}^{2}\)[25; 35]. In addition, a large ratio corresponds to the abstraction for application in the human body. All samples were ground with 1400-grit silicon carbide paper. Cleaning sample geometry 2 with the grit silicon carbide paper required special care. They were subsequently cleaned with ethanol to remove any surface residue and then rinsed with distilled water. Finally, the samples were air-dried for one day. Then, all samples were weighed (Kern & Sohn GmbH, Balingen-Frommern, Germany) three times to measure the initial weight. For sample geometry 1, the initial weight was 4.67 g (\(\pm\) 0.01), and for sample geometry 2, 4.55 g (\(\pm\)0.007). To outline the exact initial surface topology, an optical measurement was carried out with the VR-3200 3D profilometer from Keyence.