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Page 8 of 35 Huang et al. Soft Sci 2024;4:40 https://dx.doi.org/10.20517/ss.2024.37

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highly conductive MXene fibers with excellent electrical conductivity (7,713 S·cm ) and good mechanical
properties.

Additionally, the dry-jet wet spinning technology combines both dry and wet spinning characteristics.
Unlike conventional wet spinning, in dry-jet wet spinning, the fine filament stream exits the nozzle and
traverses a defined air gap without encountering any solidifying medium, as illustrated in Figure 3E. This
[141]
intervention is beneficial for curtailing fiber stretching and enhancing fiber orientation . Consequently,
dry-jet wet spinning can significantly reduce solvent consumption, boost production efficiency, and is
applicable to a broad spectrum of polymers, including those that present processing challenges in traditional
wet spinning setups.

Electrospinning involves continuously drawing fibers using static electricity [Figure 3F] [142,143] . Although
electrospinning shares a reliance on solvent evaporation for fiber solidification with dry spinning,
traditional dry spinning is limited in producing nanoscale fibers. Furthermore, advances in spinneret design
have led to the development of coaxial extrusion techniques, such as coaxial electrospinning and coaxial wet
spinning [Figure 3G] [144,145] . Coaxial extrusion can create multi-layered fibrous structures, typically with a
conductive layer at the core and an insulating layer as the sheath.

Moreover, 3D printing technology, with its exceptional design flexibility and manufacturing adaptability,
enables on-demand customization. For example, composite fibers of boron nitride nanofiber/polyvinyl
alcohol (BNNS/PVA) have been successfully produced via 3D printing, boasting high mechanical strength
[146]
and excellent thermal conductivity . The functional fibers can be further integrated into textiles with
diverse structures for personal cooling applications. As shown in Figure 4A and B, the BNNS/PVA ink is
continuously extruded through a metal needle into a methanol bath at 0 °C for cooling, forming a
continuous printed fiber. In this process, PVA serves as a dispersing agent to achieve uniformly dispersed
BNNSs, as shown in Figure 4C.

Similar to the coaxial extrusion, coaxial fibers can also be prepared by 3D printing. As shown in Figure 4D,
a conductive core ink is formulated by blending graphene powder with a PDMS prepolymer, while an
insulating sheath ink is concocted by combining polytetrafluoroethylene (PTFE) particles with the same
PDMS prepolymer. The core and sheath printing inks are extruded through the coaxial printing nozzle of
the 3D printer. The printed fibers were heat-cured at 170 °C, and then highly elastic coaxial fibers with 300%
strain were obtained . Moreover, 3D printers can design various 3D shapes and even directly print a
[147]
complete device [148,149] . Figure 4E shows the step-by-step process of 3D printing to fabricate a pressure
sensor. In this process, a base layer was printed using silicone rubber. A sensing layer was printed using a
composite material with specific silver content. The ratio of silver to polymer can be adjusted to achieve the
desired conductivity and pressure sensitivity. The electrode layer was printed using a composite material
with high Ag content to ensure optimal conductivity. An isolation layer was employed in the device to
separate the sensing layer and the electrode layer. Notably, the support layer should be removed through
hydrolysis after printing.

In general, the preparation of multifunctional fibers often requires the participation of various technologies.
Consequently, in the specific preparation process, these technologies should be strategically selected and
combined based on the actual requirements of the application.

Fabrication of functional fabrics
Functional fibers can be obtained using the fabrication techniques previously presented, preparing for the