*2.1. Fabrication of the Sensor*

Figure 2 illustrates the fabrication process of the sensor. In the whole process, no specific semiconductor fabrication equipment was required. Ecoflex was chosen as the substrate material since it is flexible, highly stretchable and biocompatible [19–21]. Ecoflex mixture of part A and part B (Ecoflex 00-30, Smooth-On Inc., Easton, PA, USA) was prepared at a ratio of 1:1. The 15 g of mixture was poured into a petri dish to make a 1 mm thick substrate and left for 10 min at room temperature to remove air bubbles and make the surface even. The substrate was then partially cured in an oven at 60 ◦C for 3 min. Partially cured substrates enabled copper wires to be easily embedded in them (Figure 2a). Prior to the transfer step, the wire pattern was prepared using a custom-made winding mold. A copper wire with a diameter of 160 μm was wounded manually on the pillars printed by a 3D printer (Projet 3500, 3D systems, Rock Hill, SC, USA) to make a serpentine pattern (Figure 2b). The coil pattern was transferred to the top of the partially cured substrate and then 15 g of silicone rubber mixture was poured again onto the substrate with the serpentine pattern transferred (Figure 2c). It was then cured completely in the 60 ◦C oven for 10 min. The resulted coil had a single turn in a serpentine pattern embedded in silicone rubber in a thickness of 2 mm (Figure 2d). The outer dimension of the fabricated coil was 35 mm × 35 mm, the radius and angle of a single arc were 1.57 mm and 216◦, respectively, taken from the previous study to maximize the stretchability [22] (Figure 2e). Figure 3 shows the fabricated sensor in non-stretched and stretched states. When the sensor was stretched by 100% in one direction, the coil pattern was stretched by 63% in the same direction and shrunk by 30% in the orthogonal direction. The resistance and the inductance of the sensor was 1.45 ± 0.04 Ω and 192 ± 3 nH, respectively. The Young's modulus of the sensor was measured to be 40.5 ± 1.1 kPa.

**Figure 2.** Fabrication process of the wireless strain sensor: (**a**) curing the substrate partially, (**b**) patterning the copper wire in serpentine shape, (**c**) transferring the pattern and (**d**) curing the device completely. (**e**) Detailed design of the serpentine pattern of the coil.

**Figure 3.** Photographs of the wireless strain sensor in (**a**) non-stretched and (**b**) 100% stretched states.

To make the sensor resonate at a specific frequency, additional capacitors were connected in parallel to the sensor. Ceramic capacitors were used as they have a low parasitic inductance [23]. 13.56 MHz was selected as the resonating frequency of the sensor since it belongs to the industrial, scientific and medical (ISM) bands. The capacitance value to tune the sensor to resonate at 13. 56 MHz was 708 pF on average.
