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Page 12 of 35 Nam et al. Soft Sci 2023;3:28 https://dx.doi.org/10.20517/ss.2023.19

1D nanowires have a higher aspect ratio than 0D nanoparticles and thus can achieve a highly percolated
[34]
network and higher conductivity . Among metallic nanowires, silver nanowires (AgNWs) have been
widely used because very long AgNWs can be synthesized in large quantities. For example, Liang et al.
fabricated a highly conductive and stretchable nanocomposite using a water-based AgNW ink with ~50 μm
pattern resolution [Figure 4C, left] . The AgNW-based conductive ink exhibited a high conductivity of
[100]
4
-1
-1
4.67 × 10 S·cm with only 10.4 wt% AgNWs and still maintained a conductivity higher than 10,000 S·cm ,
even at 70% tensile strain [Figure 4C, right]. It outperformed AgNP-based conductive inks, whose
-1
conductivity was 5,710 S·cm at 0% strain and 20 S·cm at 140% strain.
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AgNW-based nanocomposites have demonstrated high initial conductivity and outstanding electrical
stability under strain, but they often lack cyclic durability due to their fragile nature under repetitive stress.
Therefore, Jiang et al. proposed new porous nanomesh-type elastomeric conductors to mitigate this
durability issue [Figure 4D, left] . By forming hydrogen bonds between poly(vinylpyrrolidone) (PVP)
[101]
ligands grafted on the AgNW surface and PU nanofibers, a nanocomposite with a layer-by-layer structure
was fabricated. The porosity of the nanocomposite imparted permeability and was attributed to low flexural
rigidity. The nanocomposite exhibited both high conductivity of 9,100 S·cm and a high stretchability of
-1
310%. The resistance increased only by 75% after 1,000 stretching cycles of 50% strain [Figure 4D, right].

Although AgNWs are frequently utilized for fabricating highly conductive and stretchable nanocomposites,
they lack biocompatibility to be used for in vivo applications due to their cytotoxicity caused by leaching
[102]
silver ions . In order to suppress such shortcomings, research has been conducted to develop gold-based
biocompatible nanofillers [103-105] . However, since the typical diameter of AuNWs is less than 30 nm, it is
difficult to achieve the same level of conductivity as AgNW-based nanocomposites. Instead of AuNWs, Lim
et al. developed a novel 2D material, whiskered gold nanosheets (W-AuNSs), for conductive, stretchable,
and biocompatible nanocomposites [Figure 4E, left] . Due to its unique structure with long ribbon-shaped
[106]
whiskers, the W-AuNS formed a percolation network at 1.56 vol%, much lower than that of AuNP
(5.02 vol%) and AuNS (2.74 vol%). When fabricated into a nanocomposite with thermoplastic PU (TPU), it
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exhibited a conductivity of 1,600 S·cm [Figure 4E, right] and a stretchability of 200%.
In addition to the synthesis of pure gold nanofillers, wrapping a gold (Au) shell on silver nanomaterials
presents a promising strategy to achieve biocompatibility. For example, Choi et al. developed ultralong
Ag-Au core-sheath nanowires by coating an Au shell on AgNW and applied them to implantable devices
[Figure 4F, left] . The Ag-Au core-sheath nanowire had the advantages of both the high aspect ratio of
[107]
AgNWs and the biocompatibility of Au [Figure 4F, right]. By dispersing the Ag-Au nanowire in
poly(styrene-butadiene-styrene) (SBS), a nanocomposite with high conductivity (41,850 S·cm ) and high
-1
stretchability (266%) could be fabricated.

Recently, Sunwoo et al. fabricated a biocompatible, low-impedance nanocomposite comprising
[37]
Ag-Au-platinum (Pt) core-shell-shell nanowires and Pt nanoparticles dispersed in SEBS . Additional Pt
shells in the Ag-Au-Pt nanowires effectively reduced Ag ion leaching by 99%, leading to significantly
improved cell viability compared to Ag-Au nanowires. The embossed surface morphologies of the Ag-Au-
Pt nanowires, along with the in situ synthesized Pt nanoparticles, increased the effective surface area and
reduced contact impedance while maintaining high conductivity (11,000 S·cm ) and excellent stretchability
-1
(480%).

As briefly discussed above, metallic nanomaterials, such as silver nanomaterials [108,109] , metal oxides , and
[110]
even gold nanomaterials [111,112] , may exhibit cytotoxic effects depending on factors such as sizes, shapes,