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Page 2 of 19 Hussain et al. Soft Sci. 2025, 5, 21 https://dx.doi.org/10.20517/ss.2025.02

[3]
ancient Greek physician, linked sweat to fever and other health conditions . However, systematic analysis
did not begin until the 19th century, when researchers linked basic sweat components such as salt (sodium
[4-6]
chloride) to functions such as temperature regulation . The early 20th century marked significant progress
in sweat analysis, focusing on electrolyte content such as sodium and potassium, which improved the
understanding of conditions such as dehydration and heat-related illnesses. In the 1950s, sweat analysis took
a major step forward with the introduction of the sweat chloride test for diagnosing cystic fibrosis (CF) ,
[7,8]
as researchers discovered abnormally high chloride levels in sweat from CF patients. This test remains a
standard diagnostic tool [9,10] . The late 20th century saw significant technological advancements, including
the development of gas chromatography, mass spectrometry, and ion-selective electrodes, enabling detailed
analysis of sweat for a broader range of substances, such as metabolites, hormones, and drugs [11-16] . The
1980s and 1990s introduced sweat patches for monitoring substance use, offering a new approach to drug
testing [17-19] . In the 21st century, the field expanded with wearable sweat sensors incorporating electronics
and optical materials to continuously track biomarkers such as glucose, lactate, and electrolytes in real
time [13,19-22] . These innovations have made sweat analysis a promising non-invasive alternative for
monitoring health, fitness, and chronic diseases, as modern devices can detect multiple analytes
simultaneously, providing valuable insights for diagnostics and personalized medicine.

Wearable biosensors have emerged as promising tools for non-invasive monitoring of physiological
parameters, with sweat analysis being particularly advantageous due to its ease of collection and wealth of
[23]
biological information . Traditional sweat sensors, including electrochemical [24,25] , and colorimetric paper-
based systems, have shown potential but face significant limitations related to sensitivity, selectivity, and
mechanical robustness. Electrochemical sensors, while highly sensitive, often suffer from interference due to
the complex composition of sweat, requiring frequent recalibration, which reduces their long-term
reliability. Additionally, these sensors often incorporate rigid conductive wires and require an external
power supply, complicating the design and assembly, and making them susceptible to mechanical failure
due to the natural stretching of human skin [24,26-28] . Colorimetric paper-based sensors rely on changes in
color intensity rather than a full wavelength shift, making the results difficult to interpret with the naked
[27]
eye . Textile-based colorimetric sensors have also been developed for lactate sensing in sweat, but the
intensity-based color change requires sophisticated instrumentation to correlate accurately with lactate
concentrations . Fluorescent dye-based sweat sensors face additional challenges, such as low efficiency,
[29]
cross-contamination of sweat, and irreversible color changes, limiting their practicality [30,31] . Recent
advancements in microfluidic technology have significantly improved sweat sensor performance by
overcoming these limitations [32-34] . Microfluidic devices address issues such as inefficient fluid collection,
cross-contamination, and irreversible detection signals seen in traditional sensors [35,36] . Conventional
methods, often reliant on absorbent materials such as paper or textiles, are prone to mixing old and new
sweat samples, which hinders accurate measurements [37,38] . Additionally, backflow and contamination of
chemical reagents compromise sensor reliability. However, by incorporating super-wettable materials and
surface modifications, microfluidic systems can channel sweat more efficiently . Moreover, microfluidic
[39]
devices enable real-time monitoring of sweat loss and chemical concentrations by utilizing the natural
secretion pressure from sweat glands and the capillary forces within microchannels. This enhances fluid
collection efficiency while minimizing evaporation and contamination. These innovations not only allow
for dynamic feedback and intermittent analysis but also provide a level of control and reliability that
traditional sensors lack. As a result, microfluidic sweat sensors have emerged as powerful tools for
continuous health monitoring and real-time physiological assessments.

Cholesteric liquid crystal network (CLCN) is a class of one-dimensional photonic materials that reflect only
circularly polarized (CP) light based on the handedness of their helical structure . This unique property is
[40]