I. Introduction
In recent years, owing to population aging and global warming, the number of patients with heat stroke has increased. Heat stroke leads to death in the worst cases, particularly in the elderly population. Therefore, early detection and medical treatment have gained increasing attention in recent years [1, 2]. Heat stroke progresses unnoticed and often develops indoors [3]. For these reasons, the constant measurement of biological signals in daily life is effective for the prevention and early detection of heat stroke. One of the main causes of heat stroke is an imbalance between water volume and electrolyte concentration in the body. As shown in Fig. 1, during perspiration, electrolytes are normally reabsorbed by sweat glands, resulting in a low perspiration concentration. However, this mechanism does not function well during heavy sweating and perspiration when a high concentration of electrolytes is released [4]. Perspiration is highly viscous and difficult to evaporate, rendering it inefficient for body temperature regulation. In addition, a decrease in electrolyte concentration in the body leads to dehydration due to urination for osmotic pressure adjustment. To achieve early detection of heat stroke, it is important to separately measure the volume of perspiration and electrolyte concentration in perspiration.