## 4 Evaluation of the Developed Unit
### Discussion  
This study aimed to develop a device that changes the temperature, humidity, and friction of its surface under different environmental conditions. After experimental validation, the developed device was shown to be effective at tuning these three conditions. The design requirements were satisfied through avoiding the use of electrical motors and attaining a thin structure consisting only of flexible elements. Figure 12: Friction coefficient ratios after friction change to those in the nominal condition, depending on whether the developed device includes granular objects (grey) or not (white). Figure 13: Experimental setup to evaluate the effect of the presence of granular objects under external load: (**a**) with granular objects and (**b**) without granular objects. The rate of temperature increase measured was approximately 0.44 \({}^{\circ}\)C/min, and it could be lowered by more than 1.5 \({}^{\circ}\)C after 1 min of water-spraying and 2 min of blowing dry air under any of the environmental conditions tested. Thermal comfort is achieved at temperatures between 28.6 and 32.0 \({}^{\circ}\)C [34]; this device takes 10 min to reach thermal comfort when placed in a room at temperatures in the range between 28.6 and 32.0 \({}^{\circ}\)C. Thus, the rates of the temperature increase and decrease were deemed sufficient. The optimal relative humidity for healthy skin is approximately 60% RH [35]. Although the extent of humidity change by the device is affected by environmental conditions, the humidity change function could tune humidity sufficiently close to 60% RH. The reduction in kinetic friction achieved by the device is effective for dressing and undressing, whereas the increase in static friction is effective for holding and supporting the human body. The developed variable-friction mechanism satisfied the functions required for the device. The mechanisms used to tune the temperature, humidity, and friction worked in tandem. Consider a case where the temperature needs to be reduced. If the humidity is high, the dry-air blowing unit decreases both the temperature and humidity; this action also reduces friction, thereby improving garment ventilation. If the humidity is low, both the liquid-injection and dry-air blowing units are activated to decrease the temperature. In this case, to achieve a comfortable humidity level, the humidity should be increased while the temperature is reduced. Thus, the working mechanism of the developed device exhibits good configuration in this context. Increasing the humidity also increases friction, which is generally required when humidity is low. Moreover, when humidity is low, skin moisturization is deemed comfortable, which can be achieved using the liquid-injection function. Conversely, decreasing the humidity also decreases the friction between the device and human skin; both of these are necessary when the humidity is high. Thus, a reduction in friction improves comfort via the ventilation of the worn garments. In summary, the developed device is fully functional and comfortable when used on human skin. The strategies utilized worked effectively to increase or decrease the temperature, humidity, and friction. However, the environmental conditions of temperature and humidity influence the degree of the increase or decrease. The higher the environmental temperature and the lower the environmental humidity, the higher the degree of temperature cooling. This is owing to the use of vaporization heat for cooling. Moreover, the lower the environmental temperature, the higher the degree of heating. The lower the environmental temperature, the higher the degree of humidification. The higher the environmental temperature and environmental humidity, that is, the higher the environmental absolute humidity, the higher the degree of dehumidification. The lower the environmental humidity, the higher the degree of increase in static friction. However, the difference in degree was small when the environmental humidity was above 50%.