## 2 Materials and Methods
#### Methodology and Layering  
In general, the method of the study was set up in 7 steps, as applied to samples 1-24:  
1. Preparation of the meander design through the etching process based on the standard lithography technique;
2. Laminating the designs with ShengYi SF305C overlay;
3. Laser-cutting the designs in the shape of the circuits;
4. Assembly of the dummy PCB (printed circuit board) to centre interposer on the meander. Soldering was done in a vapor phase oven using SAC305 solder;
5. Lamination of fabric swatches with base TPU film;
6. Lamination of meander designs between these base fabric + TPU film (step 3) and the top TPU film layer, using layering seen in Figure 5a;
7. Conducting washing tests according to ISO 6330-2012--domestic washing and drying procedures standard. The new set of samples (25-33), on the other hand, was constructed in a similar fashion, however using the layering as given by Figure 5b. For both groups, the PCBs were added onto the centre interposer island, by first dispensing SAC305 solder paste, aligning the PCBs, and consecutively reflowing the solder in the vapor phase reflow oven IBL SLC 300 (IBL, Konigsbrunn, Germany) at a temperature of 250 \({}^{\circ}\)C.  
The finished circuits were laminated to the textile swatches. A fabric piece of 30 cm \(\times\) 22 cm was laminated with TPU film. The three different meander designs were placed on the fabric in the warp direction, and another layer of TPU was laid on top to laminate again. Lamination parameters used were: 170 \({}^{\circ}\)C with 4 bar pressure for 40 s. Lamination was done on a Hotronix(r) Air Fusion IQ(r) (Hotronix(r), Carmichaels, PA, USA) heat press. Before cutting the samples into 3 test strips, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Nr** & **Washing Phase** & **Time, min** & **Temperature, \({}^{\circ}\)C** & **Spin, rpm** & **Water Volume, L** \\ \hline
1 & Main wash & 15 & 40 & 49 & 15 \\
2 & Rine 1 & 3 & Coldwater & 49 & 19 \\
3 & Drain 1 & 8 & Coldwater & 49 & - \\
4 & Rine 2 & 3 & Coldwater & 49 & 19 \\
5 & Drain 2 & 8 & Coldwater & 49 & - \\
6 & Rine 3 & 3 & Coldwater & 49 & 19 \\
7 & Drain 3 & 8 & Coldwater & 49 & - \\
8 & Rine 4 & 3 & Coldwater & 49 & 19 \\
9 & Drain 4 & 8 & Coldwater & 49 & - \\
10 & Spin & 5 & Coldwater & 1100 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Washing test procedure overview. each including one meander design, the laminated pieces were set aside for 12 h to let the TPU film set and become more durable [18; 19]. Each sample was then numbered by sewing. Two different layering systems were used: one TPU layer (Figure 5a) or two TPU layers under and over the circuits (Figure 5b). In the latter option, the second TPU layer was structured in the shape of the corresponding polyimide track to create more protection for the circuit while not losing the overall stretchability. The testing of the samples consists of alternatively performing washing tests and electronics resistance measurements. The washing tests were performed in an Electrolux W465H (Electrolux, Stockholm, Sweden) professional washing machine. No detergent was used, and every washing cycle additionally included 2 kg of microfiber towels (Nabaji, 88% PES, 12% PA). The sample sets did not have any other protection, e.g., they were not washed in a separate bag. The washing cycles were conducted according to ISO 6330-2012--\"Textiles-Domestic washing and drying procedures for textile testing\" standard at 40 \({}^{\circ}\)C.  
The electrical resistance of each track was measured after every washing cycle until 5 washes were performed. After that, measurements were taken after every other washing cycle. To verify the influence of the PCB connection and tracks durability, the resistance was measured of (1) four separate tracks until the middle interposer on the one hand and (2) four separate tracks of the whole circuit (see in Figure 6) on the other hand.