# Non-Invasive Hydration Monitoring with a Graphene Dual Sweat Sensor
## 1 Introduction  
Maintaining optimal hydration levels is important for physiological function [1], particularly during physically demanding activities. To avoid the adverse effects of dehydration or overhydration, such as metabolite imbalances, reduced performance, muscle cramps, muscle fatigue, and extended recovery times [2, 3, 4, 5], athletes work withsports scientists to track their hydration status and tailor fluid replacement strategies for optimized performance. Currently, the three methods used to determine hydration status include (I) assessing changes in body weight to detect fluctuations in water content; (II) utilizing bioelectrical impedance analysis, measuring electrolyte concentrations in blood or urine samples; (III) comparing urine density with that of water through urine specific gravity [6]. While each technique offers distinct advantages, they are all limited by the inconvenience of sample or data collection and the inability to offer real-time monitoring, highlighting the need for more efficient solutions. Sweat analysis has gained increasing attention, as sweat contains a diverse range of solutes, including inorganic ions, organic molecules, amino acids, hormones, proteins, peptides, and other secretions [7; 8], that could reflect the body's status. Most importantly, the collection of sweat is non-invasive. Notably, sodium was identified as a key biomarker for hydration status in sweat. The body constantly regulates sodium levels through excretion and reabsorption in the sweat glands and kidneys, thereby controlling water retention [9]. During physical activity, the sweat rate increases with exercise intensity. As sweat production increases, sodium secretion in the sweat glands exceeds the rate of reabsorption, leading to higher sodium concentrations in the sweat secreted, ranging from 10-90 mM [6; 10; 11; 12; 13]. In another words, a higher sweat sodium concentration could indicate more loss of water through sweat, leading to dehydration. In addition to sodium, potassium is one of the three electrolytes that are directly involved in sweat secretion [7]. While the underlying physiological mechanism of how potassium correlates with hydration status may not be clear, it is understood that the concentration of potassium in sweat remains relatively constant and does not fluctuate more than 2 mM when the sweat rate increases [11; 12; 14; 15]. Given this unique characteristic, potassium could serve as a quality control for sweat analysis. If potassium concentration detected in sweat is higher than the typical range (1 to 10 mM), it could be an indication that the sample collected was contaminated or that evaporation has occurred. In this case, the results for other analytes, such as sodium, detected in that sweat sample may not be reliable. Recent developments have focused on wearable biosensors based on electrochemical detection for sweat sodium analysis. While these biosensors can provide instantaneous readings and uninterrupted, second-by-second tracking, they may require other systems such as wireless transmitors, microfluids, and a power source to be built into the system [16; 17]. The most crucial element is for the biosensor to be stable for an extended period of time, requiring no re-calibration during use. As a result of the combination of all these factors, wearable biosensors are often bulky, require a longer fabrication process, and exhibit a higher cost and technical demand [16; 18; 19]. As the sweat sodium concentration does not fluctuate dramatically within seconds, but instead changes gradually over time [12; 20], a sensor that continuously tracks this concentration may exceed the practical requirements for hydration monitoring. Hence, this paper focuses on the development of a sensor that analyzes sweat using a drop-and-test method, where sweat is collected and dispensed onto the biosensor for analysis, weather than on designing a wearable device.