**1. Introduction**

Stomach cancer is one of the common cancers in Northeast Asian male population [1,2]. The general treatment is gastrectomy, which removes the stomach or the small intestine partially or totally [3–5]. After surgery, the stomach motility is observed to check normal gastric emptying functions using several clinical methods such as gastric emptying time measurement, ultrasound, scintigraphy, and gastroscopy [6–8]. Recently, methods such as electrogastrography (EGG) and magnetogastrography (MGG) have been introduced to quantitatively measure the stomach motility. However, in case of EGG, the electrodes must be attached onto the operated skin of the patient [9]. In case of MGG, it can be measured only in the place where the huge facility such as Superconducting Quantum Interference Device (SQUID) is located [10]. Both methods are not adequate to continuously measure the stomach condition right after the surgery.

As a potential alternative to those methods, we propose a sensor that can be attached to the stomach directly and detect the stomach deformation wirelessly. Figure 1 illustrates the schematic of monitoring the stomach motility using the proposed sensor. The change in resonant frequency of the sensor could be measure wirelessly by a transceiver with a sufficiently large diameter to minimize the misalignment effect. The wireless strain sensor is attached to the stomach during surgery by stitching, for instance. When the stomach is active, the sensor deforms. The applied strain to the sensor causes the change in inductance, thereby shifting the resonant frequency of the sensor. The transceiver located outside the body senses the change in resonant frequency wirelessly by inductive coupling, and therefore the stomach deformation can be detected.

**Figure 1.** Schematic of the proposed sensor in a wireless sensing application. (**a**) Schematic illustration of the transceiver covering the entire abdomen to wirelessly measure the strain of the sensor attached to the stomach and (**b**) its cross-sectional view.

Several studies have reported sensors that demonstrated the feasibility of measuring the strain wirelessly. Kim et al. developed an LC tag-based RFID tag using silver nano-ink [11]. It operates in the 1.5 to 1.6 GHz frequency range, showing that as the tensile strain increases, the resonant frequency decreases within 4% of strain change. Song et al. developed a microstrip patch antenna made of silver nanowires and polydimethylsiloxane (PDMS), which operates at 2.9 to 3.1 GHz and the resonant frequency increases linearly with increasing strain within 15% of change [12]. Lazarus et al. proved that the modulation of resonant frequency caused by changes in strain can be measured wirelessly using a single-turn coil made of galistan and silicone rubber, which operates at frequencies of 0.97 to 1.29 MHz within 80% of strain change [13].

Unlike these previously developed sensors, the proposed sensor aims to eventually measure the stomach motility, which can stretch up to 100% and more [14]. To develop such a highly stretchable and wirelessly functioning strain sensor, the conducting patterns must have a low DC resistance enough to operate as a coil and at the same time, should have sufficient stretchability [15]. These two requirements are however contradictory, as outlined in many previous studies that the high stretchability can only be achieved by employing very thin conducting lines on very thin substrates [16–18]. Thus, we developed the wireless strain sensor that is patterned with a copper wire in a serpentine shape, based on a flexible substrate to achieve the higher stretchability that can cover the range of stomach motility as well as the wireless functionality.
