Human sweat monitoring, which unlike blood monitoring can be easily performed on the skin, can provide valuable personal healthcare data, as well as being suitable for use in diagnosis of disease and other conditions such as hydration status during the sports activities [1]. However, despite the tremendous interest in sweat sensing, the development of low-cost, high-efficiency, multi-biometric sensing-capable platforms is still required for the large-scale utilization of wearable biosensors for extensive monitoring with other physiological signals [2]. In this study, a new method of elastomeric substrates was fabricated through the synthesis of biopolymer materials lignin powder and polydimethylsiloxane, and then laser-induced graphene was formed on the fabricated substrate by short-wavelength UV laser irradiation. Such substrates are rapidly converted to LIG through a one-step process under ambient conditions, are highly sensitive to resistance changes, and have superior electrical properties due to the three-dimensional structural properties in which graphene flakes are stacked. Such LIG-based electrodes are easy processing of complex patterns and have high conductivity. In addition, it has properties that are harmless to the human body, so it shows excellent applicability as a substrate for sensors attached to the skin. Based on this, we suggest fabricating an ion-selective sensor with excellent sensitivity (63.3 mV/decade (Na+, n=6), K: 59.2 mV/decade (K+, n=7)) and high linearity (R2=0.99988 and 0.99855, respectively). Compared with previous results, this patch provided not only optimized selective ion sensing but also motion monitoring, such as monitoring of muscle contractions and pulse waves. We optimized this patch with the Arduino Bluetooth system to make a wearable device for exercise performance monitoring. With the two ion ISEs and reference electrode (or electrode for strain sensing) of this patch, indicators of the intensity of exercise can be clearly measured, including: 1) sodium concentration in sweat, 2) potassium concentration in sweat, 3) muscle contraction, and 4) heart rate. Due to the hydrophobic properties of the manufactured LIG, the patch showed stable results over a long period without any damage due to sweat absorbed through the microfluidic channels, which are composed of hydrophilic film. We demonstrated the mechanical robustness of this device as a strain sensor under repeated stretching cycles. In addition, this LIG-based patch showed utility for real-time vital sign monitoring by in situ monitoring of sodium and potassium level in sweat and pulse rate. This work envisions the production of a multifunctional sensing platform that is very original and is completed at a low manufacturing cost as a biocompatible, wearable sensor for performing individual exercise analysis and health check.

[1] Shamsuddin, A.K.M et al. Changes in the index of sweat ion concentration with increasing sweat during passive heat stress in humans. Eur. J. Appl. Physiol., 94(3), 292-297, (2005).

[2] Mitsubayashi, K.et al. Analysis of metabolites in sweat as a measure of physical condition. Anal. Chim. Acta, 289(1), pp.27-34, (1994).