A Wearable Sweat pH and Body Temperature Sensor Platform for Health, Fitness, and Wellness Applications
The paper presents the development of a compact system able to measure sweat pH, by means of a functionalized textile and a color sensor, and the skin temperature. The aim is to achieve a wearable miniaturized system capable to estimate the body hydration
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Introduction
Wearable sensors development and diffusion on the market are in rapid expansion [1]. The medical community is particularly interested in the remote monitoring, as a result of budget reduction together with an increasing number of elderly and highrisk patients. Concerning fitness applications, the focus is on real-time monitoring of athletes’ performances, in particular hydration and the sweat quality. The wellness field finally demands real-time data on personal health and environment. The wearable platform presented in this study (Fig. 1) is aimed to monitor the sweat pH and the skin temperature, by using novel high-resolution and accuracy techniques with ultralow power consumption. The sweat pH varies due to various endogenous and exogenous factors [2]. Skin temperature is linked to body thermoregulation [3], and its trend can provide information on the metabolic activity.
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System Description
The developed pH sensor is a combination of a halochromic smart textile (able to change its color depending on the pH value) and a high-sensitivity and low-power color sensor. The main features of the smart textile are the nontoxicity of the pHsensitive molecule, its pH range of color variations compatible with sweat pH typical values, and the solgel technique, which has been used in order to achieve dye
M. Caldara (*) • C. Colleoni • E. Guido • V. Re • G. Rosace Department of Engineering, University of Bergamo, Viale Marconi 5, 24044 Dalmine, Italy e-mail: [email protected] A. Vitali STMicroelectronics, Agrate Brianza, MB, Italy 431 C. Di Natale et al. (eds.), Sensors and Microsystems: Proceedings of the 17th National Conference, Brescia, Italy, 5-7 February 2013, Lecture Notes in Electrical Engineering 268, DOI 10.1007/978-3-319-00684-0_82, © Springer International Publishing Switzerland 2014
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Fig. 1 pH/T sensor platform. Combination of smart textile and electronics (left), electronics prototype picture (top right), and textile color variations between pH 2 and pH 9 (bottom right)
fastness to the cotton fabric and reversibility properties. The fabric color variations are monitored every 5 s, in a reflective way, by using a white LED with a dedicated step-up driver and a high-sensitivity and low-power RGB color sensing. The main measured performances are 205 Hz/lx sensitivity, the quasi-digital output, and a current consumption of only 255 μA (VDD 2.8 V). The temperature sensor is based on reverse-biased Schottky diodes, connected to the same front-end electronics used for the color sensor.
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Experimental
The pH sensor was first characterized by laboratory experiments performed on different fabric samples, by using buffer solutions and artificial sweat (according to EN ISO 105-E04:2009). The pH estimation model obtained from the data is able to estimate the pH within a range of ±0.4 pH from pH 3 up to pH 9 (Fig. 2), which is compatible with the sweat variations reported in the literature [4]. Additional beneficial features are the immediate color change, an excellent
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