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Page 8 of 35 Nam et al. Soft Sci 2023;3:28 https://dx.doi.org/10.20517/ss.2023.19

Among conductive carbon-based nanomaterials, CNTs are considered one of the most suitable fillers to
form a percolation network owing to their high aspect ratios [43,72] . For instance, Sekitani et al. developed
printable elastic conductors with single-walled CNTs (SWCNTs) dispersed in a fluorinated copolymer
[52]
matrix . A jet-milling process enabled a uniform dispersion of fine SWCNTs without shortening their
length [Figure 2E, left], whereas typical dispersing methods, such as ultrasonication, grinder-milling, and
ball-milling, made SWCNT bundles finer but shorter, which reduced the electrical conductivity of their
percolated network. The conductive ink prepared by mixing SWCNTs, an ionic liquid, and a fluorinated
copolymer was screen-printed and dried on a PDMS substrate. The printed elastic conductors exhibited the
maximum conductivity of 102 S·cm (at 15.8 wt% of SWCNTs) and the maximum stretchability of 118% (at
-1
1.4 wt% SWCNTs) [Figure 2E, right]. Taking advantage of their excellent electrical conductivity,
stretchability, and processability, an active-matrix display was successfully fabricated with the elastic
conductor. Due to its stretchability, the display could be conformally mounted on a hemispherical surface.

Further improvement in the material performance could be made by integrating multiple carbon-based
nanofillers. For example, Tringides et al. fabricated a conductive alginate hydrogel loaded with graphene
flakes (GFs) and CNTs . The hydrogel was modified to possess micropores by lyophilization before
[59]
crosslinking. GFs were connected to CNT bundles dispersed in the porous matrix, improving the
probability of forming a percolation network [Figure 2F, left]. Hence, the highly porous alginate hydrogel
retained an ultrasoft modulus and improved electrical conductivity. The carbon-based hydrogel exhibited a
-1
conductivity of 35 S·m and a percolation threshold of ~0.9 wt% carbon [Figure 2F, right]. In addition, the
charge storage capacity (CSC) of the hydrogel could be tuned by varying the relative ratio of GFs and CNTs,
which suggested the possibility for further optimization of sensing and/or stimulation performance.
Viscoelastic electrode arrays that have tissue-like mechanical properties were fabricated and exhibited
excellent long-term electrical stability even after 10,000 cycles of 11% biaxial strain.

As introduced above, carbon nanofillers have shown potential for use in soft wearable bioelectrodes.
However, concerns arise regarding their biocompatibility and cytotoxicity when in contact with living
organisms. Possibilities for the skin irritation, inflammation, oxidative stress, and even cancer have been
reported for chronic use of CB [73,74] , graphene , and CNTs . The cytotoxicity of these materials is related
[76]
[75]
to the factors such as sizes, concentrations, and surface modifications. To address these concerns,
researchers have been actively exploring surface modification methods, encapsulation techniques, and
handling protocols to enhance biocompatibility and minimize cytotoxic effects. Regulatory guidelines also
play a crucial role in ensuring the safe use of these materials in soft wearable electronics.

Carbon-based nanocomposites offer several benefits, such as mechanical robustness, chemical stability, and
easy mass production. However, their intrinsic conductivities are lower than those of metal-based
nanocomposites, which can limit their use in certain device applications. To overcome this limitation,
researchers are exploring various material strategies to improve the electrical and electromechanical
properties of carbon-based nanocomposites.

Soft conductive composites based on conducting polymers
CPs have become increasingly popular as filler materials for stretchable conductors, particularly for
bioelectronics applications [77-79] . This is due to their attractive features, such as reasonable conductivity ,
[80]
[81]
mechanical ductility , tunable electrochemical properties , and easy processability [83,84] . CPs have fibril
[82]
structures and single polymer dimensions similar to nanomaterials. In this section, we introduce some
representative stretchable conductors that use CPs, such as poly(3,4-ethylenedioxythiophene):poly(styrene
sulfonate) (PEDOT:PSS), polyaniline (PANI), and polypyrrole (PPy).