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Yang et al. Soft Sci. 2025, 5, 46 https://dx.doi.org/10.20517/ss.2025.44 Page 25 of 39

wearable flexible sensor combining ultrafine optical fibers as optical waveguides that can be used to
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accurately detect pulse waves, HRs, and blood pressure . Additionally, using PDMS-encapsulated
microfibers, Wang et al. reported a flexible strain sensor with an extremely high sensitivity and very low
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detection limits . This Sagnac interferometer may provide new ideas for the design of wearable devices for
underwater human physiological signal detection. Pan et al. reported a flexible liquid-filled fiber adapter
(FLFFA) based on the principle of intensity sensing and successfully implemented sensing in areas such as
real-time monitoring of the wrist pulse . The sensor consists of a flexible glycerin-filled tube and two silica
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fibers, and as light propagates through it, part of the light is radiated from the FLFFA into the environment,
causing a loss of light intensity. Recently, Li et al. presented an optical fiber sensor-assisted smartwatch for
accurate continuous blood pressure monitoring, which also uses liquid filling [Figure 10A] . The liquid
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capsule in the smartwatch allows spatial insensitivity and free alignment according to Pascal’s principle
while also improving the coupling between the sensor and the body.

Heartbeat
Accurate heartbeat monitoring is critical for early diagnosis and management of cardiac conditions.
Bonefacino et al. presented a PMMA-based doped polymer FBG and successfully applied it to heartbeat
monitoring of the brachial artery and chest . A new dopant material, diphenyl disulfide, was reportedly
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used in this FBG, which exhibited at least 15 times higher sensitivity than silica glass fibers. Rein et al.
proposed diode fibers for fabric-based optical communications, which were processed by a scalable hot-
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drawing process for electrically connected diode fibers . When light-emitting and light-detecting fibers are
placed 5 mm apart and a finger is placed on them, the reflected light, which is sensitive to the blood
circulation in blood vessels close to the skin, can be recorded. HR measurements revealed that these devices
have potential for use in a whole-fabric physiological state monitoring system. Furthermore, Bae et al.
presented a new method for fabricating stretchable optical waveguides [Figure 10B] that can be combined
with integrated functional devices for multifunctional healthcare monitoring . Based on this platform, the
[50]
communication of photoplethysmogram (PPG) information, including HR, oxygen saturation, respiratory
rate, coughing, and sighing, was successfully demonstrated. Lo Presti et al. proposed a fiber-optic-based skin
interface biosensor (i.e., a smart patch) capable of simultaneously monitoring the HR and respiration rate
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(RR) . The smart patch was shown to estimate the RR and HR with high fidelity under different breathing
conditions and common daily body postures.

Respiratory
Common human respiration monitoring can be realized by monitoring chest and abdominal expansion,
expiratory humidity, or respiratory airflow. Recently, various innovative approaches have been developed.
For example, Yi et al. presented a novel fiber-optic humidity sensor based on tapered microfibers coated
with deposited gelatin, which demonstrated an excellent respiratory mode sensing capability in nasal
breathing humidity monitoring . Bao et al. reported a wearable all-fiber-optic flexible humidity
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transducer for respiratory monitoring that recognizes different respiratory patterns and allows extraction of
the respiratory frequency from the sensor response . Further expanding the scope, Zhang et al. presented
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cantilevered optical micro- and nanofibers (MNFs) for a multifunctional fiber-optic airflow sensor, in
which the cantilever was made of a PDMS substrate encapsulated by MNFs . These optimized sensors
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enable real-time detection and recognition of various respiratory signals, including normal breathing, deep
breathing, and coughing. Zheng et al. developed a multifunctional active plasma platform based on a
moisture-driven metal–hydrogel–metal hypersurface and demonstrated an example of its use as a high-
performance optical respiration sensor . This innovation highlights the potential of combining different
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materials for enhanced sensor performance. As shown in Figure 10C, Mishra et al. presented a wearable
system with a bidirectional stretchable and skin-mountable balloon shape . This spherical waveguide
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