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Zhao et al. Soft Sci 2024;4:18 https://dx.doi.org/10.20517/ss.2024.04 Page 7 of 32

nanostructured surface with wedge-shaped wettability-patterned channels. This design allows for efficient
sweat transfer without retention inside the channels, resulting in higher sweat-collecting efficiency and
[58]
faster sensing time [Figure 4D] . In order to facilitate efficient small-volume sweat collection and analysis
during rest, Nyein et al. incorporated hydrophilic fillers into the sensing channel. This integration enhances
the rapid uptake of sweat, reducing the necessary time for sweat accumulation and enabling real-time
measurements [Figure 4E]. A microfluidic channel with pillars in the sweat collection center is designed to
optimize the sweat flow conditions and enable a better sampling procedure for quick detection of reservoir
[16]
filling .
Sweat inducing methods
Regarding techniques for stimulating sweat production, exercise, such as running and cycling, is a
commonly used method for sweat analysis [Figure 5A and B], where various devices monitor biological
markers in sweat during physical activity [59,60] . However, some limitations remain, such as the inability to
engage in strenuous exercise, inconvenience for individuals with mobility issues, and challenges for the
elderly. Therefore, there is a need to explore alternative methods of inducing sweat for research and
analysis.

Alternatively, wearable sensors can be engineered to use iontophoresis to locally induce sweating. This
makes it possible to obtain equilibrium sweat without exercising, which makes it perfect for medical checks
when a person is sedentary. Hydrogels with pilocarpine medication are applied and activated below the
skin, prompting sweat gland activation, and then the wearable sensors detect the released perspiration
[Figure 5C] . A tattoo-based alcohol sensor is developed by Kim for noninvasive detection of alcohol in
[61]
perspiration. The wearable alcohol sensing platform combines flexible wireless electronics with an
iontophoretic-biosensing temporary tattoo device [Figure 5D] . Recently, Xu et al. proposed a novel
[62]
iontophoresis-based sweat collection powered by a triboelectric nanogenerator (TENG). When a TENG is
tapped, it will generate an electric field to facilitate the penetration of the skin and activate sweat production
by delivering carbachol through iontophoresis. This study indicates a direct correlation between the amount
of sweat and the applied force, where external force ranging from 1 to 9 N resulted in sweat volumes
ranging from 2.7 to 5.9 μL·cm in a 30-minute period [Figure 5E]. However, due to potential skin injury
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from repeated iontophoretic current application and noisy artifacts from excessive contact, this method is
more suitable for situations where it is preferable to employ it once rather than for ongoing monitoring
[63]
purposes .

Recent advancements in wearable sweat devices have successfully integrated sweat sampling, storage, and
analysis into multifunctional platforms. Here, we also present an extensive overview of the advantages and
persistent obstacles related to various sweat sampling methods in Table 1 [16,44-46,49,50,52,54-58,64] .

SWEAT-BASED BIOSENSORS
Eccrine sweat, excreted directly onto the skin surface, is easily accessible and noninvasive. It consists mainly
of water, along with metabolites such as glucose, lactate, uric acid, electrolytes such as Na , K ions, etc.,
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making it relevant for assessing human health status [10,26,42,65] . Here, the concentration of common analytes in
sweat (sweat volume, various ions, metabolites, nutrients, hormones, proteins, and so on) and their
association with related diseases is summarized in Table 2 [6,9,16,18,27,33,61,66-97] . For instance, an abnormal sweat
rate may lead to dehydration, while deviations in K concentration from the normal range can indicate
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conditions such as hypokalemia, hyperkalemia, irregular heartbeat, arrhythmia, muscle cramps, and even
renal failure [66-68,72,74] . The real-time detection of various physiological markers (glucose, lactate, uric acid,
Na , K and NH ) can also provide insights into the emotional fluctuations of the human body. These
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