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Page 8 of 38 Zhu et al. Soft Sci 2024;4:17 https://dx.doi.org/10.20517/ss.2024.05

Huang et al. prepared polyvinylidene fluoride (PVDF)/carbon nanotubes (CNTs) incorporated into
polyacrylonitrile/CNTs [PVDF/CNT @PAN/CNT (DPCPC )] single-layer binary fiber nanocomposite
X
X
X
membranes (SBFNMs) using the co-electrospinning method and fabricated the interpenetrating structures
[72]
of PVDF@CNT and PAN@CNT nanofiber interpenetrating structures by adding CNTs . The prepared
monolayer fabrics possess excellent self-powered capability and exhibit excellent synergistic effects of
piezoelectricity and triboelectricity, while the triboelectric conversion is realized by the friction between the
binary fibers within the membrane [Figure 4A-D]. The drawbacks of traditional piezoelectric materials are
avoided, and significantly higher piezoelectric capabilities are realized.

Strain sensing capability
Due to the limitations of the underlying principles, capacitive and triboelectric principles are unsuitable for
strain sensors, and strain sensors of the relevant principles tend to exhibit unfavorable performance with
low sensitivity , with resistive strain sensors (RSSs) being the mainstream.
[73]

The sensing mechanism of RSSs utilizes the deformation of the device under the action of stress to change
the structure of the conductive network inside the sensor device, thus causing corresponding changes to
convey the strain information. The characteristics of the conductive network as a function of strain are the
core of this type of sensor.

RSSs measure strain by analyzing the relationship between strain and resistance based on the principle that
deformation of the material under stress leads to regular changes in the microscopic conductive
mechanisms such as crack expansion-repair, fracture, and tunneling effects of the conductive network
within the material, resulting in a corresponding change in the macroscopic resistance of the material [74,75] .

Based on the substrate used, RSSs can be further classified into two categories: (1) flexible conductive
polymer composites (FCPCs)-based; (2) conductive gel-based.

For FCPCs-based strain sensors, there are two main implementation strategies :
[76]

(1) Filled-type FCPCs-based RSSs are made by randomly filling a stretchable polymer matrix with
conductive fillers to make a conductive elastomer, which is combined with electrodes at both ends to form a
tensile strain sensor . Changes in the conductive network cause changes in the resistance signal during
[77]
stretching, and the strain can be detected based on the correspondence therein . It is favorable for sensing
[78]
[79]
in a large dynamic range and also has the advantages of low cost and simple process . In addition, scholars
have begun to consider using sophisticated structural design to further improve performance, such as
micro-cracks and micro-contacts [80,81] .

Na et al. designed highly durable and recyclable PDMS/vertical graphene (VG)-structured RSSs with ultra-
sensitive sensing properties [Figure 5A] [82-84] , which present a gauge factor (GF) of over 5,000 with only a
very small hysteresis and maintained a stable performance after more than 10,000 cycles . This study fully
[82]
demonstrates that prefabricated cracks in the cluster network structure possess highly reversible resistance
changes, especially after the current path is broken and the state can still be restored.

Qiao et al. reported total graphene artwork strain sensors (TGASSs) based on a laser engraving technique
that allows the sensor to be conveniently transferred to any other surface . The sensor exhibits excellent
[83]
performance, including ultra-high sensitivity of up to 673 and a strain range of up to 10% with better
stability, and its performance can also be adjusted by tweaking the pattern. It can be used to detect weak
physiological signals such as pulse, breathing and sound [Figure 5B].