Page 115 - Read Online

Page 8 of 32 Keum et al. Soft Sci 2024;4:34 https://dx.doi.org/10.20517/ss.2024.26

dependent conductivity is highly sensitive to mechanical deformation, and improvements in mechanical
cycling stability would be required [47,48] . Therefore, utilizing intrinsically stretchable conductors is desirable
for the fabrication of highly conductive stretchable devices. In particular, conductive polymers offer the
flexibility of adjusting molecular structure and electrical/mechanical properties, and their solution
processability provides advantages for the mass production of flexible electronic devices . As an example,
[47]
poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) has been reported as a promising
conducting polymer . PEDOT:PSS exhibits excellent coating uniformity and relatively low processing
[44]
-1
costs, along with desirable properties such as low sheet resistance (~46 Ω·sq ) and high transmittance
(~82%) . Moreover, it is amenable to various processing techniques, such as inkjet printing,
[49]
[50]
photolithography, and screen printing, facilitating strategic patterning . For the realization of stretchable
interconnects and electrodes, the patterning process is a crucial part. Kraft et al. reported inkjet-printed
stretchable interconnects based on conductive composites consisting of PEDOT:PSS, ionic additives, and
[39]
surfactants . The printed interconnects showed a maximum conductivity and sheet resistance of
7 × 10 S·m and 75 Ω·sq , respectively. The stretchable interconnects sustained strains above 100% and
-1
-1
4
showed mechanical stability of up to 1,000 cyclic tests. As a potential application, the developed stretchable
interconnects were utilized in stretchable circuits and interconnection of micro-LEDs [Figure 5A].
However, for polymer-based conducting materials, they have relatively lower electrical conductivity
compared to typically metallic materials. They also can suffer from thermal instability, leading to
performance degradation in high-temperature and air-ambient environments. Therefore, future research
will involve addressing these challenges.

Carbon-based nanomaterials
Similar to metallic particle fillers, carbon-based nanomaterials such as 0D-shape carbon black (CB),
1D-shape CNTs, and 2D-shape graphene can be utilized as conductive fillers in stretchable composites due
to their relatively low production cost and good mechanical properties . The formation of percolation
[13]
networks depends on the size and shape of carbon-based nanomaterials, which significantly impacts the
preservation of electrical properties under stretching. Due to their high aspect ratio, 1D-structured CNTs
are widely utilized as conductive fillers in stretchable composites. For instance, Gong et al. reported a
method to fabricate stretchable electrodes by forming anisotropic CNT wrinkle films coated on latex
balloon substrates, achieving up to 500% tensile strain and maintaining a gauge factor of ~0.09 with
[51]
resistance change within 200% strain . Wang et al. demonstrated the fabrication of conductive and
stretchable graphene honeycomb (GHC) with a porous structure using a 3D-printing process
[40]
[Figure 5B(i)]. It was demonstrated that the GHCs exhibit density and electrical conductivity of
-3
-1
3.25 mg·cm and 72 S·m , respectively. In addition, the porous structure of the GHC enabled low resistance
changes and mechanical stability in stretching conditions up to 60%. By utilizing the GHCs sandwich as the
interconnect circuits, stretchable LED displays as pixel-type structures were demonstrated [Figure 5B(ii)].

Hybrid materials
To further enhance the performance and stability of intrinsically stretchable materials, a combination of
various conductive materials can be a feasible pathway. This approach has the advantages such as the facile
development of high-performance stretchable materials by combining and mixing different materials to
complement their limitations, in terms of mechanical durability and electrical properties. For example, a
mixture of inorganic- (cf. LM EGaIn) and organic-based (cf. conductive polymer) materials can be adopted.
In the case of EGaIn, which is frequently used as a stretchable electrode, realizing high-performance
stretchable electrodes or interconnects can be limited due to the fluidity, high surface tension, surface oxide
film formation, and conductivity change during stretching [42,52] . Lee et al. presented a wafer-scale conductive
film with enhanced chemical/mechanical durability by encapsulating PSS on particulate EGaIn using a
photolithography process [Figure 5C(i)]. They successfully demonstrated a large area multilayer
[42]