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Page 6 of 38 Wei et al. Soft Sci 2023;3:17 https://dx.doi.org/10.20517/ss.2023.09

can realize continuous manufacturing of functional fibers and large-scale industrial production. Conjugated
spinning is to apply a high voltage of opposite polarity to two spinnerets, and the nanofibers ejected from
[82]
the two spinnerets are attracted and wound together to obtain continuous yarn . As shown in Figure 2D,
Ma et al. reported a triboelectric yarn with helical hybridized nano-micro core-shell fiber bundles, which
was fabricated by a continuous conjugated spinning technology . Polyvinylidene fluoride (PVDF) and
[83]
polyacrylonitrile (PAN) hybrid nanofibers were uniformly wound on the conductive yarn at a certain angle
to form a tightly wrapped core-shell structure. Melt spinning is a yarn preparation method that uses a
polymer melt as raw material to spin through a melt spinning machine . Wang et al. developed an
[84]
activated carbon fiber by using melt spinning technology, presenting a good application prospect in the field
[85]
of flexible electronics . Wet spinning is to dissolve the polymer in a solvent, and then the solution is
[86]
ejected from the spinneret and solidified into fiber in the coagulation bath . Using wet spinning
technology, Wu et al. prepared a highly stretchable and conductive CNTs/MXene-TPU hybrid fiber
electrode with a porous structure . TPU molecular chain was the main skeleton, while CNTs and MXene
[61]
form conductive networks, which could be further applied to flexible strain sensors.

Thermal drawing
Thermal drawing is another way that can be available for the industrial production of fibers, which is to
draw preforms into miniaturized fibers [87-90] . During the drawing process, fibers of different sizes can be
achieved by setting different parameters. In the study of Marion et al., the preform made of two
thermoplastic elastomers was thermally drawn into fibers, and the conductive copper wire was fed into the
hollow channel of the preform . Subsequently, the elastomer fiber was twisted to produce helical metal
[91]
electrodes embedded in a stretchable yarn, as shown in Figure 2E. Zhang et al. proposed thermoelectric
micro/nanowires by thermally drawing inorganic thermoelectric materials in a flexible fiber-like
substrate . The thermoelectric fibers were highly flexible, ultralong, and mechanically stable and could
[92]
meet the requirements of large-scale preparation. Thermal drawing technology has been regarded as a
platform for the industrialized manufacturing of fiber electronic devices. By combining metals, insulators,
and semiconductors all in one fiber, fibers with optical, electrical, acoustic, and optoelectronic functions can
be easily produced.

Coaxial extrusion
Coaxial extrusion is also a common fiber manufacturing technology. Typical coaxial extrusion technology is
that conductive materials and insulating materials are extruded at the same time to form core electrodes and
triboelectric layers, respectively, after drying . In the study of Wu, the liquid melts and silicone rubber
[93]
were respectively injected into the central channel and the outer channel of a coaxial needle .
[94]
Subsequently, by coaxial extrusion, stretchable conductive core/shell fibers were prepared for strain sensing
and self-powered smart textiles, as shown in Figure 2F. 3D printing technology is an emerging technology
in recent years, which is also another form of coaxial extension . Chen et al. have prepared stretchable
[95]
elastic fibers with a coaxial core-sheath structure by a 3D printing method, which consisted of a conductive
core and an insulative sheath .
[63]

Textile forming
The functional fibers produced by continuous production have good machine manufacturing properties
and can be made into multimodal electronic textiles by various textile processing technologies (including
weaving , knitting , sewing , non-woven , etc.). Owing to its salient merits of mechanical deformation
[98]
[97]
[96]
[49]
and adaptability, the textile can be integrated into ordinary clothing to achieve multimodal sensing.