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Page 24 of 35 Nam et al. Soft Sci 2023;3:28 https://dx.doi.org/10.20517/ss.2023.19

formed under external forces and charges are transported through the external circuit.

In the example provided by Wu et al., a piezoresistive type pressure sensor was developed by using a
[70]
GNP/ PU nanocomposite film covered with PDMS layers . When a mechanical force was applied to the
surface of the GNP/PU film, the film was elongated, which increased the total length of the current
path and resulted in higher resistance [Figure 8A, left]. The sensor demonstrated high sensitivity, which
could detect forces as low as 5 mN, and was able to sense various movements with a quick response time of
less than 0.5 s. The sensor was also successful in detecting random hits by fingers [Figure 8A, right].

Piezoresistive-type pressure sensors have also been fabricated by using conductive polymers, such
[182]
as PANI [180,181] and PPy . For example, Yang et al. developed a flexible pressure sensor based on PANI
for wearable applications . Firstly, PANI was oxidatively polymerized in situ on electrospun
[180]
polyvinylidene fluoride (PVDF) nanofibers. The resulting hierarchical PANI/PVDF nanofiber
(HPPNF) film was then sandwiched between two electrode planes to function as a piezoresistive device
[Figure 8B, left]. Although made of polymers, the pressure sensor showed a high sensitivity of 53 kPa -1
within the range of 58.4 to 960 Pa and exhibited a rather fast response time of 38 ms and high cycle stability
(> 50,000 cycles). It could efficiently detect gait motion signals and identify different phases of walking,
running, and jumping [Figure 8B, right].

Wang et al. developed a PPy/AgNP-based piezoresistive pressure sensor for monitoring respiration states
[183]
on a mask . First, they fabricated a PPy/AgNP hybrid film by irradiating an aqueous solution of pyrrole
and silver nitrate with ultraviolet (UV) light [Figure 8C, left]. After 3 h of UV irradiation, short chains
of PPy and AgNPs were formed. Then, additional PPy short chains, residual pyrrole, and Ag ions
aggregated at the surface of AgNPs to form the final PPy/AgNP hybrid film at the air/water interface.
The thickness of the film was 220 nm, and the AgNPs had an average diameter of 50 nm. To apply this
hybrid film as a pressure sensor, it was placed between two micropatterned PDMS films. The sensor
demonstrated stable resistance change during 1,000 bending cycles and could be stretched repeatedly to
20% strain. Its sensitivity ranged from 0.4 to 0.58 kPa for pressures of 100-400 Pa. When attached
-1
to a mask, it detected changes in breathing patterns, whether shallow or deep [Figure 8C, right].

Continuous monitoring of BP in daily life is beneficial for understanding the relationship between the
lifestyle and a variety of diseases of an individual, such as cardiovascular , cerebrovascular , and
[184]
[185]
respiratory diseases . However, conventional BP sensors are bulky, incompatible with the elastic and
[186]
curved skin, and often inaccurate, which interrupts continuous and long-term monitoring of BP. Therefore,
Luo et al. developed a skin-attachable, continuous BP monitoring system by combining a flexible
piezoresistive sensor (FPS) and ECG sensors . Inside the FPS, the number of contact points between CB
[187]
nanoparticle-decorated fabric and the underlying gold electrode changed depending on the applied
pressure, which imparted piezoresistive characteristics to the flexible device [Figure 8D, left]. The system
calculated BP using the time difference between the wrist pulse signals from the FPS and the ECG signals.
As a result, the researchers obtained BP values that coincided with those measured from a conventional
device [Figure 8D, right]. Notably, the FPS-based device only consumed 3 nW to operate, allowing
continuous and real-time BP monitoring with minimum power consumption.

Lou et al. prepared a highly resilient capacitive pressure sensor by layering LM-PVA films . The LM-PVA
[188]
film was fabricated by uniformly dispersing LM droplets in a PVA matrix, which were bound through
hydrogen bonding between the hydroxyl group of PVA and the oxide layer of the LM droplets. The LM
droplets improved the mechanical properties of the film, resulting in a 12.3-fold increase in toughness