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Nam et al. Soft Sci 2023;3:28 https://dx.doi.org/10.20517/ss.2023.19 Page 21 of 35

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conductivity was further improved to 103,100 S·cm (parallel to AgNW alignment) and 32,900 S·cm
(vertical to AgNW alignment) by cold welding. The nanomembrane maintained its conductivity up to
1,000% strain (in the vertical direction to AgNW alignment) and 400% strain (in the parallel direction to
AgNW alignment). As the AgNWs were exposed to the surface of the nanomembrane, they could be
patterned via conventional photolithography. Thanks to the high softness and ultrathin structure (~250 nm)
of the nanomembrane, it could be applied to high-performance EMG sensors [Figure 6H].

Strain sensors
Strain sensing is important for wearable electronics as it enables the devices to detect and respond to
changes in body position and movement. Wearable electronics, including fitness trackers and smartwatches,
are designed to be worn on the body and provide information about physical activity and health. The
addition of strain sensors allows these devices to accurately detect movement and exertion. Strain sensing is
also crucial for the development of wearable robotics and prosthetics as it helps to control their movement
and enables more natural and intuitive movement .
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Strain sensors are divided into resistive, capacitive, and piezoelectric types according to their working
mechanisms. In resistive strain sensors, the applied strain is measured by detecting the resistance variation
during stretching deformations . Capacitive strain sensors measure the capacitance change resulting from
[164]
variation in the distance between two electrodes induced by strain. Piezoelectric strain sensors consist of
piezoelectric materials that convert the mechanical deformation into the electrical potential.

Most of the strain sensors utilizing the soft conductive nanocomposites belong to the resistive type due to
their simple structure, wide sensing range, and excellent reproducibility . In resistive strain sensors, two
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main principles are involved: macroscopic deformation and nanoscopic deformation [Figure 7A].
Macroscopically, when a soft elastic material is stretched to a longer structure, it becomes thinner due to its
Poisson’s ratio, which increases its electrical resistivity. Nanoscopically, the limited conductive pathway and
higher inter-nanomaterial resistivity increase the overall electrical resistivity when stretched. Additionally,
the creation of microcrack structures through pre-stretching of the nanocomposites further enhances the
operational range and sensitivity of sensors [166-168] . As the material deforms, the microcracks within it open or
change their alignment, causing a significant microscopic alteration in the conductive pathways. This
change magnifies the sensitivity of the strain sensor.

Low hysteresis is a crucial property for strain sensors, as well as high strain sensitivity. In 2014, Amjadi et al.
demonstrated that a sandwich-structured nanocomposite consisting of AgNWs embedded between two
PDMS layers was suitable for strain sensors due to its stretchability, sensitivity, and linearity . This
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nanocomposite endured repeated strains of up to 40% with no hysteresis, preventing post-strain wrinkles
that could deform the conductive pathway structure. Additionally, Amjadi et al. also demonstrated a
wireless smart glove system using the nanocomposite strain sensor on the fingers [Figure 7B] . The
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computer-simulated finger avatar successfully mimicked the motion of real fingers with the smart glove.

Ensuring conformal integration between wearable strain sensors and the target region of the body is crucial
for accurately representing body motion through strain sensing data. The moduli of the sensor and tissue
should be matched to prevent erroneous readings. To address this challenge, conducting organic
composites has been proposed, as demonstrated by Stoyanov et al. . They developed an elastomer-like
[170]
conductor based on PANI that is covalently bonded to maleic anhydride-grafted SEBS and then mixed it
with SEBS. This block copolymer elastic conductor (BEC) exhibited excellent elastic and electrical
properties, with a low Young’s modulus reaching down to 290 kPa and high conductivity increasing up to