## 2 Materials and Methods
### Experimental Analysis of SPA for Lower Limb  
After the 3D model design and simulation stages, the SPA chambers were fabricated for the experimental analysis. The experiment was conducted using the SPA chamber unit to resemble the actual compression. For the validation of the SPA system model, the obtained simulated and experimental results were compared for a better understanding of the fabricated SPA chambers. A sensor-based SPA system was developed, consisting of SPA, a SPA controller, and monitoring software as shown in Figure 4. The exterior layer of the chambers was laminated with a woven-based textile shell with a dimension of 210 (width) \(\times\) 460 (length) mm\({}^{2}\) to form the main part of the SPA unit. This was wrapped onto a mannequin leg for compression treatment simulation as depicted in Figure 4a. According to [PERSON] et al., the mannequin leg demonstrated similar contact pressure patterns to the human leg [34]. The force applied to the mannequin leg was measured to provide an accurate pressure reading. Force-sensitive resistor (FSR) was attached to the medial side of the lower leg, about 20 cm above the ankle, where the gastrocnemius muscle's tendinous and muscular parts separate as shown in Figure 4b. Next, the pressure exerted on the skin by the SPA system on the force sensor was assessed. A portable air compressor was attached to the pressure sensor and solenoid valve to operate the SPA system. Pressure sensors (model no. MPS20N0040D, Soldered Electronics, Osijek, Croatia) were used to monitor the supply pressure. The SPA controller comprised an air pump, Step-Down voltage regulator (LM2596 SIMPLE Switcher Power Converter 150-kHz, Digi-Key Electronics, Thief River Falls, MN, USA), pressure sensor, data acquisition control board (Arduino Pro Mini, Digi-Key Electronics, Thief River Falls, MN, USA), and solenoid valve. The hardware working model was built as shown in Figure 4c.  
Figure 3: FEM simulation setting for SPA chamber. Fixed support surrounding the object and earth gravity applied in the negative y direction.  
The experimental platform developed to calculate the 3D deformation profile of the chamber concerning their inflation height is shown in Figure 5. The test rig consists of a distance laser meter (Class 2635 nm, \(<\)1 mV, Accuracy \(\pm\) 1 mm) that was fixed onto the retort stand vertically to secure its position. Since the greatest deformation for uniformly distributed pressure occurred at the center of the simulation result, the laser head was aimed at the chamber's center. The SPA chamber was placed horizontally on a flat table to measure the deformation. The laser point coincides with the center of the chamber.  
Figure 4: The experimental setup to evaluate pressure transmission: (**a**) monitoring system for mannequin leg with SPA chamber; (**b**) the FSR sensor embedded inside the skin layer at gastrocnemius muscle; (**c**) schematic diagram of SPA testing system, pressure transmission monitor; (**d**) schematic diagram of SPA with six airtight chambers; and (**e**) schematic diagram of lower-limb SPA system.  
The initial height, \(h_{0}\) of the chamber was measured. The SPA chambers were inflated in increments of 1.0 mL air volume for 23 s inflation and 7 s holding and 7 s deflation. The deformed height, \(h_{i}\), at every 1 s interval was recorded. This test was repeated for five complete cycles at the designated position, and the average values for deformation were included in the result.