Page 40 - Read Online

Page 22 of 43 Wang et al. Soft Sci 2024;4:41 https://dx.doi.org/10.20517/ss.2024.53

touch-induced temperature variations [Figure 9D].

Thermoelectric fibric temperature sensors utilize the thermoelectric effect between two different conductors
to measure temperature. When one end of the thermocouple is exposed to a temperature gradient, the
[108]
potential difference between the conductors varies with the temperature . Wang et al. employed
electrospinning technology to fabricate an elastic poly(styrene-block-butadiene-block-styrene) (SBS)
nanofiber mat, which was subsequently drop-coated with a PEDOT:PSS solution to form a uniform shell
layer. This process resulted in a temperature-sensitive layer with advantages including linearity, high
[166]
sensitivity, and stability .
Wearable electric heaters have broad applications across various fields, including healthcare and personal
[167]
[168]
thermal management . Fibric electric heaters allows for more efficient integration of conductive materials,
as well as their inherent flexibility . Moreover, they can be woven into a variety of flexible and breathable
[169]
fabrics . Fibric electric heaters can be achieved by printing metallic materials directly onto the surface of
[23]
elastic fibers to create. For instance, helical silver electrodes can be printed onto the surface of PU fibers and
encapsulated with polydimethylsiloxane (PDMS) [Figure 9E and F]. The helical structure provides the
[15]
fibers with excellent stability, softness, and stretchability, allowing them to be woven into items such as
kneepads and gloves [Figure 9G]. Additionally, coating fibers or textiles with a layer of metal or metal oxide
nanowires represents another viable approach for the development of fibric electric heaters. For instance,
Cheng et al. designed a copper nanowire (CuNW) composite fiber with a unique hierarchical structure
[170]
[Figure 9H]. This fiber not only exhibits excellent heating performance but also demonstrates remarkable
tolerance to mechanical impacts, such as bending, twisting, and stretching. It has been applied as a heating
layer for infant warming coats [Figure 9I and J]. Currently, the preparation of stretchable conductive fibers
is complex and these fibers are prone to failure under significant deformation, which limits their
application. To address this issue, Li et al. enhanced the wettability of PU multi-filaments using dopamine.
They then coated the filaments with striated polypyrrole (PPy), resulting in composite fiber bundles with
excellent stability and strain-insensitive properties. These composite fibers were integrated into smart
textiles and demonstrated considerable potential for use in wearable thermal therapy devices [Figure 9K
[171]
and L].

Optical fiber sensors for environmental monitoring
SPR-based optical fiber sensors
Optical fibers, as a typical fibric device, offer significant advantages in long-distance monitoring,
miniaturization, and the flexible manipulation of light. Sensing based on the surface plasmon resonance
(SPR) effect can be achieved by fabricating functional coatings on the surface of optical fibers . When the
[110]
incident light and surface plasmon polaritons (SPP) satisfy the wavevector matching condition, a portion or
most of the incident light energy couples into the SPP [Figure 10A]. When the refractive index of the
surrounding environment changes, characteristics such as the SPR wavelength and the half-peak width
detected by the spectrometer will also change in response [Figure 10B]. Nanomaterials such as AuNPs ,
[172]
[174]
[175]
[173]
CNTs , silicon oxide (SiO ) , and GO are commonly used in these coatings. Due to the extreme
2
sensitivity of SPR effect to the refractive index of the surrounding environment, it is frequently utilized in
biological and chemical detection applications.
In general, the performance of fiber optic SPR sensors is heavily influenced by the material and thickness of
the metal film. Gold is widely utilized for SPR sensing owing to its stable chemical properties and high
dielectric constant . Li et al. proposed a sensing system based on gold-plated single-mode optical fibers
[176]
with a tilted fiber Bragg grating (TFBG) etched in the fiber core , as shown in Figure 10C. The spectral
[177]