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Nam et al. Soft Sci 2023;3:28 https://dx.doi.org/10.20517/ss.2023.19 Page 25 of 35

Figure 8. Pressure sensors. (A) Schematic illustration of resistive sensing mechanism (left) and relative resistance change of the
GNP/PU film with finger pressures (right). Reproduced with permission from ref [70] . Copyright 2017, American Chemical Society;
(B) Schematic illustration of the flexible pressure sensor based on PVDF/PANI nanofibers (left) and relative current response for
different gaits (right). Reproduced with permission from ref [180] . Copyright 2021, American Chemical Society; (C) Schematic illustration
of the PPy/AgNP hybrid film through interfacial photopolymerization (left) and detection of respiration rate with different breathing
patterns using the film as a pressure sensor (right). Reproduced with permission from ref [183] . Copyright 2021, Elsevier B.V; (D) Materials
and device structure of the FPS with and without applied pressure (left). Blood pressure obtained from the FPS and conventional
photoplethysmogram sensor (right). Reproduced with permission from ref [187] . Copyright 2015, WILEY-VCH Verlag GmbH & Co. KGaA,
Weinheim; (E) Schematic illustration of stacked LM-PVA films under pressing (left) and relative capacitance change with increasing
pressure (right). Reproduced with permission from ref [188] . Copyright 2020, Elsevier B.V; (F) Preparation of the F-TENGs and structural
representation of the F-TENG-based tactile sensor arrays (middle). Image of the sensor array with an H-shaped object and
corresponding pressure distributions (right). Reproduced with permission from ref [189] . Copyright 2020, Wiley-VCH GmbH. AgNP: Silver
nanoparticle; AgNWs: silver nanowires; BP: blood pressure; CNT: carbon nanotube; FPS: flexible piezoresistive sensor; F-TENG: fiber-
shaped triboelectric nanogenerator; GF: gauge factor; GNP: graphite nanoplate; PANI: polyaniline; PDMS: polydimethylsiloxane; PPy:
polypyrrole; PU: polyurethane; PVA: poly(vinyl alcohol); PVDF: polyvinylidene fluoride; UV: ultraviolet.

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(74 MJ·m ). Two LM-PVA films were stacked, forming a parallel-plate capacitor. When external force was
applied, the LM droplets were compressed, increasing the conductive area, reducing the gap between films,
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and increasing capacitance [Figure 8E, left]. The sensor demonstrated three levels of sensitivity (MPa )
based on the range of applied pressures [Figure 8E, right].

Ning et al. developed a pressure sensor array using fiber-shaped TENGs (F-TENGs) . Firstly, they
[189]
fabricated a single fiber with a diameter of 0.63 mm by consecutively depositing AgNWs, CNTs, and PDMS
on a stretchable spandex fiber [Figure 8F, left]. The fiber exhibited high stretchability of up to 140% and
could be knotted, folded, and woven into textiles. It showed negligible performance change even after
agitation in a beaker with detergent and water. When used as a pressure sensor, a taping force of 1 N
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generated the maximum open circuit voltage of 10 V. The sensitivity of the sensor was 5.2 and 0.39 mV·Pa
for tapping forces less than and greater than 4 kPa, respectively. For the wearable device applications,
F-TENGs were woven into an 8 × 8 sensor array [Figure 8F, middle]. Each of the eight F-TENGs was placed
as weft and warp yarns with a spacing of three common fibers. Then, the 16 voltage signals were recorded
using the multichannel data acquisition system. This sensor array could detect the pressure distribution