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Page 18 of 34 Xi et al. Soft Sci 2023;3:26 https://dx.doi.org/10.20517/ss.2023.13

[179]
fiber TENG for collecting mechanical energy and self-powered sensors . It is mainly composed of hose,
wire, and electric heating wires filled with low melting point alloys. It has important applications in tactile
sensing and rehabilitation training.

Shoes or other types of footwear can be combined with TENGs, where they can harvest energy from the
footsteps of wearers and power sensors that monitor gait, pressure distribution, and other
parameters [62,180,181] . As shown in Figure 6G, Rahman et al. prepared a new type of wearable and stretchable
multifunctional double-layer TENG by using nanoporous cobalt oxide/silicon and MXene/silicon
[182]
nanocomposites derived from metal-organic framework . The preparation of materials can improve the
charge capture ability and electronegativity, thus obtaining excellent sensitivity. It can be applied to plantar
pressure distribution sensor array and self-powered wearable keyboard.

Piezoelectric nanogenerator
Piezoelectric nanogenerators (PENGs) are devices that can convert mechanical energy (such as motion or
pressure) into electrical energy [35,183] and are usually used for self-powered wearable sensors because they can
generate electricity from the mechanical energy generated by the movement of a wearer [36,184,185] .

Sensors based on PENGs can also be manufactured using organic compounds as piezoelectric
materials [186,187] . Organic piezoelectric materials, such as PVDF, have many advantages over inorganic
[188]
materials, such as higher flexibility, biocompatibility, and ease of processing . The organic PENG sensor
has shown promising prospects in various applications, such as biomedical sensors, energy collectors, and
touch sensors . They have been used to manufacture self-powered wearable devices that can monitor
[189]
heart rate, respiration, and muscle activity. They can also be used in smart fabrics, where they can be
integrated into clothing to generate energy through movement and temperature changes . As shown in
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Figure 7A, Athira BS developed a high-output flexible PENG based on PVDF-barium titanate (BaTiO3)
(ES-PVDF-BT) composite nanofibers . The output of PENGs based on PVDF-BT nanofibers is ten times
[191]
higher than that of the original PENG devices based on PVDF nanofibers. This new PENG has been proven
to be suitable for real-time vibration sensing applications with automatic power supply. As shown in
Figure 7B, Su et al. reported a robust super-hydrophobic antibacterial self-powered electric nanogenerator
(PENG) sensor . The PENG sensor is composed of a flexible electrospun piezoelectric nanofiber film and
[192]
a sprayed cross-fingered CNT electrode. Then, the cross-fingered CNT electrode is encapsulated in a cross-
linked conformal hydrophobic nano-coat around each individual fiber by an induced chemical vapor
deposition (iCVD). The obtained sensors can accurately monitor various human behaviors, such as
breathing and different movements. The iCVD nano-coating provides excellent protection for the sensor,
with self-cleaning, super-hydrophobicity, and an antibacterial contamination rate of more than 90%. As
shown in Figure 7C, Lo et al. fabricated a PENG based on porous nanofibers by near-field
electrospinning . The 3D stacked porous nanofiber structure enhances the stress concentration effect so
[193]
that poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) nanofiber can promote higher
performance of electrical output. A self-powered foot pressure recognition statistics system and a personal
gait biometric recognition system are developed. Through deep learning, the recognition rate of individual
sequential gait piezoelectric signals is 86%.

PENG-based sensors can use inorganic piezoelectric materials, such as zinc oxide (ZnO) or lead zirconate
titanate (PZT), to convert mechanical energy into electrical energy, which can be used to power sensors and
related electronic equipment [194,195] . They can produce high power density, which means they can generate a
[196]
large amount of electrical energy from a small amount of mechanical energy . Generally speaking, they
[197]
have high durability and can resist environmental factors such as temperature and humidity . As shown