## 3 Results  
The purpose of the experiments was to determine the most stable, reliable and stretchable combination of the materials used. Thus, the experimental work was carried out with 33 different sample sets (Table 4), which included 66 samples in total. The Design-of-Experiments that was performed compared the influence of four parameters--meander shape, TPU-stack, fabric type and whether or not the meander is reinforced. In the first analyses, given in Figure 7, the percentage of surviving tracks is plotted as a function of the number of washing cycles, without taking into account the TPU stack or the fabric type. A track of which the resistance stayed under 50 Ohm, due to washing or other treatment, is hereafter called a \"surviving track\". From this graph, it can be seen that the meander design 2 was less resilient to the high forces induced by the washing tests, both in terms of first occurring failure (1 washing cycle) as in terms of the total number of losses at the end of the experiment. Meander style 1 shows an earlier failure compared to meander style 3. While Type 1 and 3 seem statistically evenly reliable, the isolated data showed that type 1 survived more cycles in more cases, when comparing similar conditions, e.g., sample 16 (type 1) survived longer than sample 18 (type 3), resulting in a higher number of survivors after 25 washing cycles. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Nr** & **TPU** & **Meander Style** & **Fabric with Weight** \\ \hline  
\hline \hline \end{tabular}
\end{table} 
Table 4: Overview of experimental work set-up. The reinforced meander types seem to outperform the designs 1, 2, 3, although it can not be concluded whether this is a result of the reinforced design or a consequence of the novel stack as shown in Figure 5b. Although the failure of design 2-2 seems to confirm the earlier conclusion on the worse robustness to washing cycles of this meander design, a closer inspection of this failed sample revealed a failure at the interconnection between the (flexible) track and the (rigid) interposer PCBs: a small crack was found next to the interposer transition to the tracks. One of the reasons for that specific crack could be the failure of encapsulation layering and meander design together. Although the number of samples used in this experiment was too small to make sound statistical conclusions, Figure 7 illustrates the importance of a well-balanced meander design, tuned to the specific mechanical load applied in the use-case (such as machine washing in this paper). As mentioned in the previous paragraph, all measurements of track resistances were averaged per sample, and the overall resistance of the track per bridged distance was calculated (mOhms/cm). The bridged distance is the distance in between two islands of components that needs to be bridged by the tracks, while the resistance measured will depend on the shape of the meander. Based on the geometric parameters given in Table 2, the track length per unit cell (cfr. Figure 2) and the bridged distance can be calculated, resulting in a geometrical form factor:  
FF = Track length per unit cell/bridged distance per unit cell  
As all copper tracks are designed with the same width (w = 0.15 mm) and have the same thickness t = 18 \(\upmu\)m, the resistance per bridged distance can be calculated as:  
\[\mathrm{R}=\frac{\mathrm{r}\mathrm{o}\times\mathrm{FF}}{(\mathrm{t}\times \mathrm{w})}\]  
With _ro_ being the bulk resistivity of the copper layer. In Figure 8, the average values of the measured copper tracks are given for all designs--as a function of the washing cycles, and the maximum and minimum values that were measured.