**4. Discussion**

To measure the motility of internal organs such as the stomach, a stretchable, batteryless and wireless strain sensor using resonant frequency modulation was investigated. This sensor can stretch up to 110% from its original length, shifting the resonant frequency by 6.2% at maximum around 13.56 MHz, which exceeds the strain ranges that have been measured wirelessly in previous studies [11–13]. Previous studies have shown that up to 80% strain can be measured wirelessly. However, to measure the stomach motility, the sensor must be able to detect a stretch of up to more than 100%, and thus, we have developed a sensor that can wirelessly measure strains up to 110%. The conductivity of the copper wire used in the present study (5.96 × 105 S/cm) [26] is higher than the conductivity of the materials used in previous studies, by about 9 to 4000 times (silver nano-ink: 1.5 × <sup>10</sup><sup>2</sup> S/cm [11], silver nanowire: 8.13 × 103 S/cm [12], and eGaIn: 3.4 × 104 S/cm [27]). In addition, since the Q increases as the coil resistance decreases [15], the sensor made of a copper wire developed in the present study has the advantage of maintaining a high Q for a wider range of strains than those reported in previous studies. This high Q of the proposed sensor was obtained through embedding the copper wire in silicone rubber sheet, unlike the other commonly employed methods to fabricate stretchable strain sensors based on thin metal films on thin substrates.

The sensor could detect the strain wirelessly at up to 22.5 mm distance by the used transceiver, although there is room for further improvement to increase the operating distance. Thus, our proposed sensor demonstrated that it satisfied the two contradictory requirements for wireless sensing of strains, namely, the high conductivity of the coil sensor and the high stretchability. Moreover, the maximum strain of the developed sensor was not inherently limited, but instead, originated from the limitation of the used experimental setup. One additional point to note is that we only investigated the resonant frequency modulation in response to strains in only one direction. For actual applications, however, bi-axial strains and bending would need to be applied. Previous studies show that there is a correlation

between the diameter of the coil pattern and the wireless sensing performance due to bending. The wireless sensing performance is less affected by the degree of bending of the sensor when the diameter of the coil sensor is smaller [13,28,29]. Thus, to minimize the bending effect caused by the curved surface of the stomach, the outer length of the coil pattern would need to be minimized. Also, to increase the distance of wireless sensing and the sensitivity, parameter optimizations would be needed, which include the reduction of the sensor thickness, the increase in the number of turns of the sensor to increase the degree of inductive coupling between the sensor and the transceiver, and the increase of the outer diameter of the transceiver.

To apply the developed sensor in a real application, misalignment between the sensor and the transceiver should be overcome. To minimize the misalignment effect, the transceiver is suggested to be large enough to cover the entire abdomen as illustrated in Figure 1a. By creating a magnetic field as uniformly as possible throughout the abdomen, the sensor can keep relatively constant coupling with the external transceiver even if the exact location of the sensor is unknown. As shown in Figure 1b, the distance between the sensor and the transceiver can be determined by knowing the distance from the abdominal surface to the sensor, which could be measured by a physician during the surgery or by a medical imaging method. Therefore, it would be possible to detect the deformation of internal organs such as the stomach. To minimize the variations in stomach conditions and posture, the patient would be asked to stay with a constant posture, e.g., laying down on a bed, during the detection. Also, checking the stomach motility would be performed a few days after the gastrectomy, in which duration the food intake is generally restricted and thus, the stomach condition is expected to be quite constant. However, all such aspects need further investigation including animal experiments.

**Author Contributions:** N.C. and S.K. conceived the idea. K.J.L. and N.C. fabricated the coils. K.J.L. and S.K. designed the experimental setup, analyzed the data, and wrote the paper. K.J.L. performed all the simulation and experiment.

**Funding:** This work was supported by the Basic Science Research Program (NRF-2017R1A2B2004598) and the BioMedical Fundamental Technology Development Program (NRF-2017M3A9E2056463) of National Research Foundation (NRF) of Korea.

**Conflicts of Interest:** The authors declare no conflict of interest.
