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Nam et al. Soft Sci 2023;3:28 https://dx.doi.org/10.20517/ss.2023.19 Page 23 of 35

In another study conducted by Araromi et al., ultra-sensitive strain sensors were developed by forming
microstructures in an anisotropic conductive material consisting of orthogonally-stacked aligned-carbon-
[172]
fiber/epoxy nanocomposite layers . The researchers carved the anisotropic conductive material into a
periodic structure that is deformable under external stress. The microstructure exhibits high initial
resistance under no stress, but under compression, the conduction pathways are shortened, resulting in low
resistance. Conversely, under extension, the conduction pathways are saturated, resulting in higher
resistance. The sensor exhibited ultra-high sensitivity, with a gauge factor greater than 9,000 at high linearity
2
2
(R > 0.98) and greater than 85,000 at lower linearity (R > 0.96). This material was used to develop a textile-
based sensor-integrated sleeve to detect hand motion. The customized sleeve strain sensor successfully
detected various hand and wrist motions.

Sun et al. developed an ultrasensitive strain sensor utilizing a microcrack structure . MWCNTs and
[167]
AgNWs were deposited on the electrospun TPU nanofiber, and the composite was slowly pre-stretched to
100% strain to create the microcrack structure. The microcracks divided the conductive layer into separate
islands, forming an island-bridge structure. When the sensor was stretched, the bridges became longer, and
the area of the islands increased, resulting in an increase in the resistance. Notably, as conductive pathways
dramatically decreased due to crack propagation at high strains, resistance increased exponentially with the
applied strain. Based on these mechanisms, the sensor achieved both high sensitivity and a wide operational
range, exhibiting a gauge factor greater than 110,000 within the strain range of 135% to 171%. The sensor
successfully captured weak signals, such as airflow impacts, and strong signals, such as large-scale human
motions.

Paper-based sensors have gained significant attention as next-generation flexible sensors due to their
various advantages, including disposable, inexpensive, lightweight, and easily available nature [173-177] . For
example, Yun et al. developed a paper-based strain sensor composed of a paper substrate and a CB/
graphene/sodium carboxymethyl cellulose (CMC) conductive layer . By utilizing ionic CMC as a bridge
[173]
to enhance the interaction between CB and graphene, electrical conductivity and corresponding sensitivity
of the sensor were improved. It exhibited high sensitivity with a gauge factor exceeding 70 and could
demonstrate mechanical durability (> 10,000 cycles). In addition, the sensor overcame its vulnerability to
water by coating it with hydrophobic silica nanoparticles, enabling it to maintain its sensing performance
even under wet conditions.

Pressure sensors
Pressure sensors fabricated with soft nanocomposites are another key component of the bio-integrated
smart electronics [178,179] . Pressure sensors enable the detection of a wide range of body motions, from subtle
finger touches to large limb motions, through one of the following mechanisms: piezoresistive, piezoelectric,
capacitive, and triboelectric. Similar to the case of strain sensors, various types of nanofillers have been
applied to develop highly sensitive soft pressure sensors .
[104]
Since the nanofillers introduced in this review are all conductive, pressure sensors made with these
conductive nanofillers mostly correspond to the piezoresistive type. Piezoresistive pressure sensors rely on
the resistance change caused by the deformation of their shape under applied pressure. If a dielectric layer is
introduced between two conductive nanocomposites, capacitive-type pressure sensors can be fabricated. In
this case, the capacitance changes as the two electrodes come closer under applied pressure. Also, if different
material layers, capable of generating charges through rubbing or contacts, are incorporated with the
[156]
conductive nanocomposites, triboelectric-type pressure sensors can be fabricated . Piezoelectric-type
pressure sensors require piezoelectric materials, either inorganic or organic, of which an electric dipole is