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Page 12 of 34 Xi et al. Soft Sci 2023;3:26 https://dx.doi.org/10.20517/ss.2023.13

PVDF strip device that can monitor joint motion and environmental vibration. Wang et al. demonstrated
[114]
the stretchable, flexible, and wearable TENG . This device can work effectively during stretching or
bending processes. It can be used as a wearable self-powered sensor for real-time human motion
monitoring, such as knee joint bending and human posture. Wang et al. demonstrated a self-powered
frictional tactile sensor with a multi-layer structure of polydimethylsiloxane (PDMS) frictional layer and
[115]
PDMS/eutectic gallium indium alloy composite electrode . They can be used to detect pulse waves.
Overall, the use of stretchable materials in self-powered wearable IoT sensors offers a range of advantages,
including improved comfort and fit, durability and reliability, sensing properties, and energy harvesting
capabilities. These advantages make stretchable materials ideal for developing next-generation wearable
technologies.

Sensor working mode
Self-powered wearable sensors can work in two kinds of sensing modes: physical sensing and chemical
sensing. Physical sensing is a self-powered wearable sensor operating in the physical sensing mode, which
can measure changes in physical parameters, such as strain, pressure, temperature, humidity, or
motion [33,55,96,116-118] . These sensors usually detect changes in physical parameters measured by changes in
electrical, magnetic, or optical properties of materials or equipment [119,120] . Chemical sensing is a self-
powered wearable sensor operating in the chemical sensing mode, which can detect changes in chemical or
biochemical properties, such as the concentration of specific molecules or ions in biological fluids,
environmental samples, or gases [121-123] . Some self-powered wearable sensors can work in physical and
chemical sensing modes, allowing simultaneous detection of changes in multiple parameters to more
comprehensively monitor physiological or environmental conditions [124,125] . The selection of sensing mode
depends on the specific application and the parameters to be measured.

Physical sensing
Physical sensing is a type of sensor that involves measuring physical parameters [94,126] . In self-powered
wearable sensors, physical sensing is usually achieved by changing the electrical, magnetic, or optical
[41]
properties of materials or equipment used to detect physical parameters . Physical sensing is a
multifunctional sensing mode that can be used in a wide range of applications, from medical care to sports
and fitness to environmental monitoring . The selection of physical sensing technology depends on the
[98]
specific application and physical parameters to be measured [49,95] .

Pressure sensing is a sensing mode that can detect pressure and can be used to monitor blood pressure or
detect changes in airflow and other applications [24,127,128] . Self-powered wearable sensors for pressure sensing
usually use piezoelectric or capacitive materials, which generate electrical signals in response to pressure
changes [77,128-131] . As shown in Figure 4A, Tan et al. designed an artificial intelligence-enhanced blood
pressure monitoring wristband . The sensor of the wristband is based on the PENG and has a high signal-
[46]
to-noise ratio of 29.7 dB. Through the deep learning model, the wristband can predict the blood pressure
reading with an error of less than 4 mmHg. This wristband can monitor the blood pressure of subjects for
three consecutive days.

Strain sensing is common in physical sensing. It is used to measure the deformation or strain of materials
and can be used to monitor muscle movement, detect the irregularity of gait, or measure the change of body
posture [55,106,132,133] . Self-powered wearable sensors for strain sensing usually use piezoelectric materials, which
generate electric charges in response to mechanical strain. As shown in Figure 4B, Lee et al. designed a
geometrically asymmetric paired electrode TENG . It enhances pressure-induced electrical output
[134]
through microelectrodes on the microstructure to monitor deformation. As shown in Figure 4C, Wang