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Nam et al. Soft Sci 2023;3:28 https://dx.doi.org/10.20517/ss.2023.19 Page 15 of 35

Liu et al. thermally treated EGaIn nanoparticles and created a biphasic Ga-In (bGaIn), a printable
[133]
conductor suitable for scalable manufacturing of highly stretchable electronic circuits . bGaIn was made
by spray-printing EGaIn nanoparticles [Figure 5D, i], heating them in a furnace for 30 min at 900 ℃, and
cooling them. When heated, particles at the top surface transformed into crystalline solids due to oxidation
and phase segregation [Figure 5D, ii], while underlying particles coalesced into conductive liquid paths
[Figure 5D, iii]. The resulting bGaIn was transferred to a stretchable substrate, such as PDMS
[Figure 5D, iv]. The solid oxide particles were evenly spread over the substrate, and the liquid EGaIn
showed strong wettability to the solids, therefore exhibiting mechanical stability and excellent
electromechanical performance. It showed an initial conductivity of 20,600 S·cm , and resistance increased
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only 6% at 100% strain on PDMS. In addition, bGaIn was employed to build robust connections with
conventional rigid electronic components. When an electronic component was mounted on bGaIn, the
liquid parts of bGaIn flowed to encompass the electronic component, while solid bGaIn particles secured
the liquid bGaIn to the electronic component, resulting in the formation of a reliable electrical connection.
As a consequence, stretchable circuit board assemblies, which operated even under large strains, could be
fabricated by transfer-printing bGaIn traces and connecting electronic components, including resistors,
capacitors, and light-emitting diodes [Figure 5D, bottom].

Although the LM itself is known to be highly biocompatible when used in wearable electronics, the non-
breathable encapsulation is required to prevent leakage of the LM from the composite, which can cause skin
irritations over long-term wearing [134-136] . Meanwhile, Ma et al. developed a highly permeable and
[137]
superelastic conductor named “liquid metal fiber mat” (LMFM) . The LMFM was fabricated by coating
EGaIn on electrospun SBS, followed by repeating pre-stretching cycles to a strain of 1,800%. During the pre-
stretching process, EGaIn was transferred into a mesh-like porous structure along the SBS microfibers and
formed a vertically wrinkled structure [Figure 5E, left]. The lateral meshes and vertical wrinkles contributed
to ultrahigh electrical stability and excellent permeability. For instance, the LMFM with an initial
conductivity of ~100 S·cm exhibited only a small increase in resistance when stretched to 1,800%
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[Figure 5E, right]. The air permeability (79.5 mm·s ) and moisture permeability (724 g·m ·day ) of the
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LMFM were higher than those of commercial nylon cloth (11.8 mm·s and 621 g·m ·day ) and a medical
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patch (7.0 mm·s and 31 g·m ·day ). To evaluate the importance of permeability for long-term wearable
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applications, on-skin tests were conducted. In contrast to non-permeable materials, such as SBS, Ecoflex,
and PDMS film, permeable LMFM did not cause any skin irritations (no erythema or edema).
LMs have also been combined with hydrogels to fabricate flexible, printable, and rewritable electronic
circuits. For example, Park et al. demonstrated composites of EGaIn droplets and poly(ethylene glycol)
diacrylate (PEGDA) hydrogel, in which EGaIn microparticles were vertically phase-segregated . Friction
[138]
on the composite surface with a high EGaIn population induced the removal of the covering PEGDA and
rupturing of the oxide layers in the EGaIn microparticles, resulting in the formation of conductive
pathways. When water was applied to the hydrogel, the surface was swollen, and the conductive pathway
was removed [Figure 5F, left top]. After the water was dried, the surface was de-swollen and returned to its
initial nonconductive state [Figure 5F, left bottom]. The writing/erasing/rewriting cyclic performance was
examined, and the result showed that the hydrogel could endure over 20 rewritable cycles, with a
tremendous change in electrical resistance from ~1 to ~10 Ω [Figure 5F, right]. Besides, the liquid-like
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properties of EGaIn imparted extra softness to the composite, making it suitable for a mechanically flexible
3D electrode.

LM-based nanocomposites have been extensively researched due to their outstanding stretchability and
metal-like conductivity. By forming a percolation network through the coalescence of neighboring LM