Wearable Sweat Loss Measuring Devices: From the Role of Sweat Loss to Advanced Mechanisms and Designs

Abstract

Wearable sweat sensors have received significant research interest and have become popular as sweat contains considerable health information about physiological and psychological states. However, measured biomarker concentrations vary with sweat rates, which has a significant effect on the accuracy and reliability of sweat biosensors. Wearable sweat loss measuring devices (SLMDs) have recently been proposed to overcome the limitations of biomarker tracking and reduce inter- and intraindividual variability. In addition, they offer substantial potential for monitoring human body homeostasis, because sweat loss plays an indispensable role in thermoregulation and skin hydration. Previous studies have not carried out a comprehensive and systematic review of the principles, importance, and development of wearable SLMDs. This paper reviews wearable SLMDs with a new health perspective from the role of sweat loss to advanced mechanisms and designs. Two types of sweat and their measurement significance for practical applications are highlighted. Then, a comprehensive review of advances in different wearable SLMDs based on hygrometers, absorbent materials, and microfluidics is presented by describing their respective device architectures, present situations, and future directions. Finally, concluding remarks on opportunities for future application fields and challenges for future sweat sensing are presented.

Keywords: health monitoring; insensible sweat; sensible sweat; wearable sweat devices.

Figure 1

The significance of measuring two types of sweat loss and summary of wearable sweat loss measuring devices based on hygrometer (open chamber method and closed chamber method), absorbent materials (paper: Reproduced with permission.^{[ 33 ]} Copyright 2019, Springer Nature, sponge: Reproduced with permission.^{[ 34 ]} Copyright 2014, John Wiley & Sons, textile: Reproduced with permission.^{[ 35 ]} Copyright 2017, The Royal Society of Chemistry, and hydrogel: Reproduced with permission.^{[ 36 ]} Copyright 2021, Elsevier), and microfluidics (colorimetric signal: Reproduced with permission.^{[ 37 ]} Copyright 2019, AAAS, electrical signal: Reproduced with permission.^{[ 38 ]} Copyright 2018, American Chemical Society, and calorimetric signal: Reproduced with permission.^{[ 39 ]} Copyright 2021, Springer Nature).

Figure 2

Human skin structure and two types of sweat loss in the skin surface (sensible sweat and insensible sweat).

Figure 3

Facial topographic map of inSSR. From left: unmapped subject, anterior view, oblique view, lateral view. Reproduced with permission.^{[ 57 ]} Copyright 2015, John Wiley & Sons.

Figure 4

Absolute regional median sweat rates of male athletes at: a) exercise intensity 1 [55% VO₂ max]; b) exercise intensity 2 [75% VO₂ max] in moderately warm conditions. Reproduced with permission.^{[ 60 ]} Copyright 2011, Springer Nature.

Figure 5

The relationship between change in mean body temperature and sweat rate for thermoregulation. Under the onset threshold (office work, sleeping, yoga, etc.), insensible sweat is the main body sweat loss, where the relationship is characterized by an initially relatively flat portion. Beyond the onset threshold (eating hot food, sauna, running), sensible sweat emerges and becomes dominant, where the relationship is the linear portion. Ultimately, SSR reaches a maximal level, leading to a plateau despite mounting mean body temperature^{[ 44 ]} (modified from ref. [90]). Reproduced with permission.^{[ 90 ]} Copyright 2018, John Wiley & Sons.

Figure 6

Schematic of stratum corneum and water transport properties for skin hydration (modified from ref. [99]). Reproduced with permission.^{[ 99 ]} Copyright 2012, Hindawi Publishing Corporation.

Figure 7

Equivalent schematic of sweat glands and the secretion mechanisms of water and some typical sweat components passage into the secretory coil, including chloride, sodium, potassium, bicarbonate, glucose, etc. Reproduced with permission.^{[ 115 ]} Copyright 2020, Springer Nature.

Figure 8

Schematic illustration of operating mechanisms and structures of conventional hygrometer‐based sweat devices. Natural ventilation: a) open chamber method, b) closed chamber method. Forced Ventilation: c) condenser chamber method, d) ventilated chamber method.

Figure 9

Typical wearable hygrometer‐based SLMDs using the open chamber method. a) Schematic of the first prototype relying on two humidity sensors integrated onto textile substrates and its validation study compared with VapoMeter. Reproduced with permission.^{[ 153 ]} Copyright 2010, IEEE. b) A wearable SLMD with a rigid open chamber and its validation study compared with DermaLab. Reproduced with permission.^{[ 95 ]} Copyright 2017, Elsevier.

Figure 10

Typical wearable hygrometer‐based SLMDs using the closed chamber method. a) A watch‐type SLMD on human wrist. b) Operating mechanism of the watch‐type SLMD. Reproduced with permission.^{[ 151 ]} Copyright 2018, Springer Nature. c) Structure and on‐body test of a microporous PDMS sweat rate sensor. Reproduced with permission.^{[ 157 ]} Copyright 2019, IEEE. d) A NFC‐enabled SLMD based on cellulose sheet. Reproduced with permission.^{[ 158 ]} Copyright 2013, IEEE. e) Structural diagram of a bifunctional wearable sensor for EEG and sweat rate. f) The simultaneous measurement of EEG signal and sweat rate. Reproduced with permission.^{[ 160 ]} Copyright 2020, IEEE.

Figure 11

Typical wearable SLMDs by using filter paper materials. a) Design of a sweat volume colorimetric platform. b) Calibration curve of the colorimetric platform for measuring SSV. Reproduced with permission.^{[ 162 ]} Copyright 2020, American Chemical Society. c) Conceptual illustration and photograph of a finger‐shaped wearable colorimetric patch. Reproduced with permission.^{[ 33 ]} Copyright 2019, Springer Nature. d) Mechanism and on‐body test of a conductive paper‐based sensor. Reproduced with permission.^{[ 66 ]} Copyright 2019, John Wiley & Sons.

Figure 12

Typical wearable SLMDs by using sponge as absorbent materials. a) A stretchable wireless SLMD based on dielectric detection. b) Image of measurement of dielectric property. c) Body test with comparison to the gravimetric method evaluated using reference absorbent substrates. Reproduced with permission.^{[ 34 ]} Copyright 2014, John Wiley & Sons.

Figure 13

Typical wearable SLMDs by using textile as absorbent materials. Textile: a) a wearable digital flowmeter based on a patterned textile surface. Reproduced with permission.^{[ 35 ]} Copyright 2017, The Royal Society of Chemistry; b) structure and mechanism of another novel type of digital flowmeter. Reproduced with permission.^{[ 68 ]} Copyright 2019, The Royal Society of Chemistry; c) a conductive thread‐based textile SLMD integrated into clothing. Reproduced with permission.^{[ 163 ]} Copyright 2018, MDPI.

Figure 14

Typical wearable SLMDs by using hydrogel as absorbent materials. a) A wearable SSV monitoring patch and the construction and mechanism. Reproduced with permission.^{[ 164 ]} Copyright 2020, The Royal Society of Chemistry. b) Design of a wearable strain sensor for measuring SSV. c) Calibration curve of the strain‐enabled SLMD and validation study compared with conventional gravimetric method. Reproduced with permission.^{[ 36 ]} Copyright 2021, Elsevier.

Figure 15

Colorimetric wearable microfluidics‐based SLMDs from Rogers and co‐workers. a) An epidermal system with an orbicular serpentine microchannel for monitoring sweat loss and its operation mechanism. Reproduced with permission.^{[ 178 ]} Copyright 2016, AAAS. b) Another epidermal system with a circular ratcheted microchannel. Reproduced with permission.^{[ 37 ]} Copyright 2019, AAAS. c) A soft, flexible skin‐integrated for colorimetric analysis of sweat and its validation study. Reproduced with permission.^{[ 176 ]} Copyright 2019, American Chemical Society. d) Schematic of a patterned adhesive used in a waterproof, blockage‐reducing sweat patch and its validation study. Reproduced with permission.^{[ 183 ]} Copyright 2019, AAAS. e) A resettable SLMD with hydration feedback and the operation mechanism and validation study. Reproduced with permission.^{[ 184 ]} Copyright 2019, Springer Nature. f) Configuration of a wearable SLMD that has a hard/soft hybrid structure for robust measurement of sweat loss. Reproduced with permission.^{[ 186 ]} Copyright 2020, John Wiley & Sons.

Figure 16

Electrical wearable microfluidics‐based SLMDs. Admittance: a) structure and mechanism of a thin, wireless, battery‐free sweat analysis system. Reproduced with permission.^{[ 191 ]} Copyright 2018, John Wiley & Sons; b) schematic of a wearable SLMD with a spiral microchannel and a pair of electrodes inside. Reproduced with permission.^{[ 38 ]} Copyright 2018, American Chemical Society; c) exploded illustration of the various components of a multimodal sweat sensing platform. Reproduced with permission.^{[ 69 ]} Copyright 2019, The Royal Society of Chemistry; d) structure and mechanism of a wearable patch with capability in continuous collection and measurement of both type of sweat loss; e) sweat loss data measured at 8 different regions. Reproduced with permission.^{[ 67 ]} Copyright 2021, Springer Nature. Capacitance: f) structural diagram of a low‐cost, wearable capacitive SLMD; g) the validation study and on‐body test compared with Macroduct. Reproduced with permission.^{[ 193 ]} Copyright 2020, American Chemical Society; h) structural diagram of a wireless wearable SLMD with high sensitivity; i) fully assembled device for measuring sweat rate and its on‐body tests. Reproduced with permission.^{[ 194 ]} Copyright 2020, The Royal Society of Chemistry.

Figure 17

Calorimetric wearable microfluidics‐based SLMDs. a) Theoretical principle of the calorimetric flow rate sensor. Reproduced with permission.^{[ 197 ]} Copyright 2017, AIP Publishing. b) Separate parts and overall device of a wireless SLMD for calorimetric measurement of sensible sweat loss. Reproduced with permission.^{[ 71 ]} Copyright 2018, MDPI. c) Structural illustration of a miniaturized, on‐skin, flexible SLMD for wireless sensing of sweat loss. d) Image of the thermal flow rate sensor on a finger. e) Operating mechanism of the calorimetric SLMD for measuring sweat rate. Reproduced with permission.^{[ 39 ]} Copyright 2021, Springer Nature.