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Page 6 of 35 Huang et al. Soft Sci 2024;4:40 https://dx.doi.org/10.20517/ss.2024.37

using the poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) solution. This approach
was demonstrated as an effective strategy for integrating electrical conductivity with the inherent properties
of insulating fibers.

Certain fibers, such as polyacrylonitrile (PAN) and cellulose, can be rendered conductive through
carbonization, a method commonly employed to fabricate high-performance carbon fibers [108,109] . For
instance, Gupta et al. reported on the impact of carbonization temperature on the crystallinity and electrical
conductivity of PAN fibers . The conductivity of PAN fibers carbonized at 1,000, 1,800, and 2,200 °C
[110]
-1
-1
increased monotonically from 5.32 to 51.01 S·m and 75.91 S·m , respectively. Thus, by controlling the
carbonization temperature, it is feasible to achieve fibers with the desired level of conductivity.
Additionally, electroless deposition and electrodeposition are also common methods for the surface
functionalization of fibers [111-113] . Electroless deposition employs a potent reducing agent in a solution of
metal ions to reduce the metal ions to their metallic form, which subsequently deposits onto the surface of
materials [114,115] . Electrodeposition, referred to as electroplating, is a method of depositing a metal layer onto
the surface of a material through electrolysis [116,117] . Both electroless deposition and electrodeposition have
their respective advantages. The former does not require an external power source, and it can form a
uniform coating on substrates of complex geometry, making it suitable for surface metallization of complex
shapes or insulating materials [118,119] . The latter allows for the control of metal layer thickness and deposition
rate, making it appropriate for applications requiring a thicker metal layer [120-123] .

Inherently functional fiber making
Previous methods focused on surface treatment of insulating fibers to achieve functional fibers. Extensive
experimental evidence has demonstrated that it is also feasible to produce inherently functional fibers
through various processes, including thermal drawing, spinning, and 3D printing.

The thermal drawing technique, derived from the technology used in optical fiber fabrication, can also be
employed to manufacture functional fibers . Loke et al. have demonstrated the preparation of a preform
[124]
integrating conductors, semiconductors, and polymer insulators, as depicted in Figure 3A . Upon heating
[125]
and stretching the preforms, the functional fiber exhibiting seamless material integration and exacting
dimensional precision was achieved. Various methods can be employed for fabricating these preforms,
including thin-film rolling for cylindrical fiber production, extrusion, lamination, drawing, and integrating
[126]
different material components through hot pressing or advanced 3D printing technology . Furthermore,
various post-processing methods have been developed to overcome the mechanical property mismatch
between different materials [127,128] , such as laser and thermal annealing, as depicted in the second fiber of
Figure 3A.

Spinning is a prevalent process for converting polymers into fibers, including melt spinning, wet spinning,
dry spinning, dry-jet wet spinning, and coaxial extrusion. Melt spinning is a process that involves using the
apparatus shown in Figure 3B to incorporate conductive fillers into easily meltable polymers or to blend
conductive polymers with matrix polymers . Common matrix polymers used in the spinning process
[129]
include polypropylene (PP), PU, thermoplastic PU (TPU), and Ecoflex [130-132] . Probst et al. blended highly
elastic TPU with conductive CNTs to fabricate conductive fibers via the melt-spinning process . These
[132]
fibers exhibit high elongation, mechanical properties comparable to conventional elastic fibers, and
electrical conductivity on par with conductive liquid. Notably, melt spinning focuses on using the blends of
conductive polymers with thermoplastic polymers suitable for this process due to the inadequate stability,
toughness, and processability of pure conductive polymers [129,133] .