*2.4. Experimental Setup*

To verify whether the change in sensor strain results in resonant frequency shifting, the experimental test bench was constructed as shown in Figure 6a. An impedance analyzer (4294A, Agilent, Santa Clara, CA, USA) was directly connected to the sensor to measure the impedance, and the strain applied to the sensor was controlled by an XY stage (CROSS 130-HSM, OWIS®, Staufen, Germany). Since the applied strain and the impedance should be measured at the same time, a LabVIEW-based program (LabVIEW 2016, National Instrument, Austin, TX, USA) was used to control the XY stage and the impedance analyzer simultaneously. Because the available travel range of the XY stage was limited, the sensor was stretched up to 110% at maximum. From each impedance measurement, the resistance, inductance, and capacitance of the sensor were evaluated using the equivalent circuit model, in which the resistance and inductance are connected in series and capacitance connected to those in parallel [25]. Figure 6b shows the experimental setup to observe whether the change in strain of the sensor can be measured wirelessly and to investigate the maximum distance of wireless detection. The deformation of the sensor was measured wirelessly using a transceiver, which was a circular planar coil with 15 turns, an outer diameter of 65 mm and a wire diameter of 1 mm, resulting in a self-inductance of 10 μH and the quality factor of 71. The impedance of the transceiver was measured as the strain applied to the sensor varied from 0% to 100%. The distance between the sensor and the transceiver was varied from 10 mm to 50 mm to observe the maximum distance for wireless sensing. The centers of the sensor and the transceiver were aligned manually at every measurement step. Since the purpose of the wireless sensing setup was to observe the modulation of resonant frequency at the transceiver with varying the vertical distance, the influence of coil misalignment was excluded.

**Figure 6.** Experimental setup configurations for (**a**) tensile impedance testing and (**b**) wireless sensing.
