Page 189 - Read Online

Nam et al. Soft Sci 2023;3:28 https://dx.doi.org/10.20517/ss.2023.19 Page 13 of 35

surface chemistry, and concentrations. These nanomaterials can potentially induce oxidative stress,
inflammation, and cellular damage. Long-term exposure or systemic absorption of metallic nanoparticles
can lead to adverse health effects. Additionally, the release of metallic ions from nanomaterials may pose
toxicity risks. Ensuring the biocompatibility of metallic nanomaterials in soft wearable electronics requires
careful consideration of material selection, surface modifications, and thorough biocompatibility
assessments to minimize potential risks to users.

With the development of facile metal nanomaterial fabrication, extensive research has been conducted on
nanocomposites with various nanofillers of different dimensions. These metal-based nanocomposites can
enhance both electrical and mechanical properties of materials. While AgNW-based nanocomposites have
performed exceptionally well, recent studies have focused on materials other than silver to improve
biocompatibility.

Soft conductive nanocomposites based on liquid metals
LMs are unique metals and metal alloys that remain in a liquid state near room temperature due to their low
[113]
melting points . The combination of metallic and liquid properties, such as high thermal and electrical
[114]
[115]
[116]
conductivities , low viscosities , and excellent deformability , makes them highly versatile and
valuable in many applications [117-120] . Ga-based alloys, including eutectic Ga-In (EGaIn) and Ga-In-Sn
(Galinstan), have been most widely used due to their low toxicity and high chemical stability compared to
other LM counterparts , such as Hg (which is toxic) and Cs (which is explosively reactive) .
[123]
[121]
[122]
In recent years, researchers have synthesized various LM-based composites in which LMs are dispersed in
an elastomeric matrix as micro- or nanodroplets . These efforts were focused on utilizing the high
[124]
intrinsic conductivity of LMs while preserving the mechanical properties of the elastomer matrix. However,
typical LM-based composites are inherently nonconductive due to the thin oxide layer on the surface of LM
droplets . Therefore, an additional process (e.g., mechanical sintering [126,127] , laser sintering [128,129] , or
[125]
[130]
thermal expansion ) should be introduced to create electrical pathways by rupturing the oxide layer and
coalescing adjacent LM droplets. For instance, Lin et al. embedded EGaIn nanoparticles in PDMS sheets
and created conductive traces by mechanical sintering . The EGaIn nanoparticles with an average
[126]
diameter of ~105 nm were prepared by sonicating bulk EGaIn in ethanol [Figure 5A, left] and cast on top of
the PDMS film. After solvent evaporation, a PDMS prepolymer was poured onto the EGaIn-PDMS film and
cured to form a PDMS-EGaIn-PDMS structure. Then, external pressure was locally applied to merge EGaIn
nanoparticles together [Figure 5A, right]. To demonstrate the concept, they measured the current-voltage
curves before and after mechanical sintering. The electrical conductivity increased by a factor of 4 × 10 after
8
sintering, exhibiting 960 S·cm .
-1
Laser sintering has emerged as another facile process for fabricating conductive LM-based composites. Liu
et al. created conductive EGaIn nanoparticle films on various substrates via laser sintering and investigated
their electrical properties depending on laser fluence, nanoparticle size, substrate material, and film
thickness . The spray-printed EGaIn nanoparticles were treated by a pulsed laser with a wavelength of
[128]
1,065 nm. The oxide layer on the surface of the EGaIn particle was transparent to light with wavelengths
greater than 300 nm, while the gallium-indium core was heated by a photothermal effect. Therefore, the
oxide layer was ruptured due to thermal expansion of the heated EGaIn core, allowing EGaIn nanoparticles
to flow and form conductive networks [Figure 5B, left]. Among various processing parameters, the laser
fluence (energy per unit area) was the most important factor that determined the resistance of the EGaIn
film, while the particle size and film thickness had negligible effects. As more energy was transferred during
sintering, more particles coalesced with neighboring particles, resulting in a decrease in resistance
[Figure 5B, right].