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Zhao et al. Soft Sci 2024;4:18 https://dx.doi.org/10.20517/ss.2024.04 Page 3 of 32

Figure 1. Schematic diagram of flexible sweat electronics.

loss. It avoids contamination and interference with natural sweating. During the study, participants engaged
in exercise within a plastic isolation box on a cycle ergometer, dressed simply. After exercise, everything
they touched was rinsed using deionized water to collect the electrolytes produced by their bodies. The
[44]
difference in body mass before and after exercise was measured to determine the total sweat loss . Another
sweat collection method involved gathering samples from patients’ backs in a sauna. The process included
cleaning the back with a moist towel, draping it with a hole and petroleum jelly, and collecting fluids in a
pocket as individuals took a sauna bath. Sweat was then collected using a syringe and filtered through a
[45]
cartridge-style filter [Figure 2A] . Absorbent patches were also introduced as an alternative to traditional
collection methods. These patches efficiently absorbed sweat, allowing for local analysis through
+
+
centrifugation. Subsequently, the sweat sample was tested using a laboratory device for Na and K detection
[Figure 2B] . However, all these approaches are impractical for field research and treadmill protocols due
[46]
[47]
to the controlled laboratory environment, complex processes, and other limitations . Recently, Brueck
et al. have proposed a calorimetric-based flow-rate detection system for sweat detection [Figure 2C]. The
whole system includes Macroduct for sweat sampling, a calorimetric sensor for sensing, a printed circuit
board (PCB) for system control, and a rigid lithium battery for power, which is very bulky and lacks
portability .
[48]
Advancements in skin electronics have overcome challenges in previous sweat-based platforms, enabling
real-time sampling, sensing, energy harvesting, and data display. This review will comprehensively explore
the advancement of flexible sweat-based electronics and the progress achieved so far.

DEVELOPMENT OF FLEXIBLE SWEAT SAMPLING METHODS
Skin-electronics-based sweat sampling methods
Novel sweat collection methods for flexible electronics have been proposed to overcome limitations of