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Page 4 of 35 Nam et al. Soft Sci 2023;3:28 https://dx.doi.org/10.20517/ss.2023.19

and shapes, of nanofillers directly affect how easily the percolation network can be formed. 1D
[43]
nanomaterials with high aspect ratios, such as carbon nanotubes (CNTs) and metallic nanowires [34,44] , are
suitable for achieving highly conductive nanocomposites with minimal filler amounts due to their relatively
low percolation thresholds.

This chapter provides an overview of widely-used conductive nanofillers and their impact on the electrical
and mechanical properties of nanocomposites. We describe the unique features and the advantages and
disadvantages of these composites, which depend on the material types and morphologies of the nanofillers
(see Table 1 for a summary). The chapter also provides general guidelines for selecting materials for high-
performance stretchable conductive nanocomposites, including strategies for modifying fillers and
fabrication methodologies.

Soft conductive nanocomposites based on carbon nanofillers
Carbon-based nanofillers, such as carbon black (CB) [45-47] , graphene [48-51] , and CNTs [52-54] , have unique
features that make them suitable for use in conductive nanocomposites. These materials possess high
electrical conductivity due to the transfer of electrons via delocalized π-orbitals, which enables the
fabrication of high-performance nanocomposites [55,56] . Furthermore, carbon-based nanomaterials are known
for their excellent mechanical properties [57-59] , low production costs , and ease of functionalization [61,62] . In
[60]
this section, we introduce stretchable conductive nanocomposites based on carbon-based nanofillers.

CB particles are spherical particles whose diameters range from 10 to 100 nm. CB has been used as a filler
[63]
for rubbers to enhance tensile strength and/or abrasion resistance . Owing to its electrical conductivity,
elastomers mixed with CB at concentrations of 10% or higher can become electrically conductive. For
instance, Niu et al. synthesized a nanocomposite of CB and polydimethylsiloxane (PDMS) by simply mixing
[45]
the CB powder with PDMS . The cross-sectional scanning electron microscopy (SEM) image of the
resulting nanocomposite showed that the CB particles were uniformly distributed and connected with each
-1
other inside PDMS [Figure 2A, left]. The nanocomposite exhibited electrical conductivity of ~25 S·m and a
percolation threshold of ~10 wt%. The strain-dependent electrical conductivity change was also measured,
and the electrical conductivity increased monotonically as the applied strain increased, especially at the low
strain regime [Figure 2A, right]. By using a soft lithography technique, planar and 3D microstructures of the
nanocomposite could be fabricated, proving its potential for application to microelectronic devices.
However, owing to the 0D structure and the low aspect ratio of CB that lead to a relatively high percolation
threshold and the low electrical conductivity of the CB-based nanocomposites, its application to high-
performance electronics has been limited.

Graphene and its derivatives, such as graphene oxide (GO) and graphite, have been highlighted as another
type of carbon-based filler for their higher electrical and mechanical performances than CB. Graphene is an
allotrope of carbon in which sp hybridized atoms are arranged as a single layer with a 2D honeycomb
2
structure. Hence, graphene can achieve a low percolation threshold and exhibit transparency [64-66] . An
interesting example is the 3D foam-like graphene macrostructure (graphene foam). Chen et al. synthesized
the graphene foam upon a 3D interconnected porous nickel foam by using chemical vapor deposition
[48]
[Figure 2B, left] . Due to its high charge carrier mobility arising from the unique interconnect network, a
graphene foam/PDMS composite showed relatively high electrical conductivity of ~10 S·cm , even at an
-1
ultralow graphene content of ~0.5 wt% [Figure 2B, right]. Moreover, infiltrating the graphene foam with the
polymer matrix improved mechanical properties of the composite. As a result, the resistance of graphene
foam/PDMS composite increased by only ~30% under ~50% strain. In another research conducted by Liu
et al., highly stretchable and transparent graphene electrodes could be fabricated by intercalating graphene