# Development of New Soft Wearable Balance Exercise Device Using Pneumatic Gel Muscles
## 2 Materials and Methods
### Construction of Wearable Balance Exercise Device  
We designed a soft wearable device for balance exercise using PGMs. The proposed device is shown in Figure 2. The device consists of a soft supporter, a soft pelvic belt, a PGM controller, an inertial measurement units (IMU) sensor, and four PGMs. Our purpose includes the design of a lightweight device for balance exercise because it is important to be able to use the device in a range of settings with a high level of portability and to easily attach the device in clinical site. The proposed device weighed only 0.9 kg, and it took less than 3 min for a beginner to attach the device. A Babo care and Zw-3 (NIPPON SIGMAX Co., Ltd., Tokyo, Japan) were used for soft support of the trunk and pelvis. The soft supporter and pelvic belt were used to support and affix the four PGMs. The two PGMs were attached on the front of body, and the other two PGMs were attached on the back of the body. The PGMs were attached to both sides of the acromion processes and pointed vertically down to the height of the iliac crest. Namely, the right side PGMs cause right side trunk bending from the positional relationship between the same side of PGMs and facet joints because the trunk has many facet joints of thoracic and lumber vertebrae. The system was divided into two sides: the controller side, which sends control signals, and the receiver side, which controls the PGMs. Both sides were controlled by ESP-WROOM-32 (Espressif Systems Pte. Ltd., Shanghai, china) and connected via WiFi  
Figure 1: PGMs used in this study. As shown in Figure 3, five switches were implemented on the controller side. Each blue switch corresponded to the PGM control at each position; the front side of two PGMs contraction, the back side of two PGMs contraction, the left side of two PGMs contraction, and the right side of two PGMs contraction. Therefore, if the left or right side of the PGMs contracted, the device made the user bend the left or right side of the trunk, respectively. Conversely, if the antero-posterior side of PGMs contracted, the device induced trunk flexion or extension in the user. The yellow switch transmitted a signal indicating the start of measurement. The receiver side had a solenoid valve to control PGMs and an M5 Stack FIRE (M5 Stack, Shenzhen, China). Air was supplied to the PGMs from a small CO2 tank, and the PGMs were controlled by turning on/off the solenoid valve according to the received signal. The M5 Stack FIRE was equipped with an  
Figure 3: Hand switch and small CO2 tank. Figure 2: Soft wearable balance exercise device. IMU (MPU 6050, TDK InvenSense, Seoul, Korea) and an SD card writer for outputting time-stamp, acceleration data, and the timing of PGM activation at each position. The left side shows the hand switch for PGM contraction. The yellow switch is the start measurement button. The blue switches are the PGM contraction buttons for each direction. The right side shows the small air tank for PGM contraction. The tank was only 0.19 m in length. Although IMU signal information and COP signal information are different, the IMU sensor can be used to assess postural sway. Previous studies reported that acceleration data during standing and stepping were strongly correlated with COP data [21; 22]. The perturbation during the standing position not only affected COP shift, but also center of mass (COM) shift [23]. Therefore, the IMU sensor could be used to assess postural stability if it was attached close to the COM position without the need for large specialized equipment. This device control mechanism is shown in Figure 4.  
To generate a small perturbation, it is necessary to know the contraction force generated by the PGM. Previous studies reported that the PGM contraction force was related to the stretched length and magnitude of air pressure [20; 24].