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Page 14 of 34 Xi et al. Soft Sci 2023;3:26 https://dx.doi.org/10.20517/ss.2023.13

thermoelectric properties under repeated bending and stretching. The self-driven thermoelectric sensor
could realize temperature recognition and respiratory monitoring. It also showed an enhanced output signal
when stretching. Zhang et al. reported a compressible and stretchable magnetoelectric sensor (CSMS) with
[139]
an arch air gap, which can realize self-powered breath monitoring driven by exhaled/inhaled breath .
High-sensitivity, self-powered, and electromagnetic electrical sensors can be further used as non-invasive,
miniaturized, and portable respiratory monitoring systems to warn of potential health risks.

Pulse wave sensors are a type of physical sensor that can detect the heart rate and other cardiovascular
[140]
parameters of the wearer . In addition to the heart rate, these sensors can also provide information about
the wearer’s blood pressure and arterial stiffness . Pulse wave sensors can be integrated into various
[141]
wearable devices, such as smart watches, fitness trackers, and health monitors [142-144] . As shown in Figure 4F,
Xu et al. developed a self-powered ultra-sensitive pulse sensor based on a TENG for non-invasive multi-
indicator cardiovascular monitoring. The detailed characteristic peaks in the pulse waveform of the region
[145]
can be clearly identified . As shown in Figure 4G, Laurila et al. reported the development of a scalable
manufacturing process for a highly inconspicuous piezoelectric ultra-thin e-tattoo arterial pulse wave
sensor, which only uses transparent and biocompatible polymer-based materials . The performance of the
[146]
sensor is optimized in many ways to enhance its performance. Through the actual measurement of the
radial artery, its ability to detect pulse waves has been proved.

Chemical sensing
Chemical sensing is a type of sensing that involves the detection of chemical or biochemical properties, such
as the concentration of specific molecules or ions in biological fluids, environmental samples, or
gases [126,147,148] . In self-powered wearable sensors, chemical sensing is usually achieved by using chemical or
biochemical recognition elements (such as enzymes, antibodies, or synthetic receptors), which selectively
combine and detect target molecules or ions [95,96,149,150] .

Biomarker sensing can monitor the changes of the biomarker level in disease diagnosis [60,151] . Self-powered
wearable sensors for biomarker sensing usually use antibody-based sensing technology, which selectively
combines specific biomarkers and generates measurable electrical signals [36,152-154] . As shown in Figure 5A,
Kanokpaka et al. designed a triboelectric sensor based on a self-powered molecularly imprinted polymer .
[155]
The PVDF/graphene flexible electrode-modified poly (3-aminophenylborate) imprinted lactic acid
molecules showed changes in surface properties after the adsorption of lactic acid. When a higher lactic acid
concentration is detected, more lactic acid adsorption results in a lower energy barrier and lower potential.
Self-powered triboelectric lactic acid sensors can directly supply power to LED lamps without an external
power supply and verify the feasibility of wearable sensors on human skin. As shown in Figure 5B, Lv et al.
developed a flexible spiral structure of nitrogen-doped carbon cloth-modified MoN and manufactured a
flexible all-solid-state asymmetric supercapacitor . After 10,000 cycles and a retention rate of over 90%, it
[86]
still exhibits excellent electrochemical performance. As shown in Figure 5C, Shajari et al. manufactured a
flexible, self-powered, non-evaporation, non-bubble, non-surfactant, and expandable capillary microfluidic
device to reliably collect sweat from different parts of the human body . The sensor can detect the cortisol
[156]
released by sweat glands to monitor people’s psychological stress levels. The sensor was used to obtain the
longitudinal and personalized distribution of sweat cortisol in different body positions. As shown in
Figure 5D, Gai et al. designed a wireless self-powered wearable sweat analysis system, which effectively
converts the mechanical energy of human motion into electrical energy through the HNG module
(HNGM) . The HNGM showed stable output characteristics at low frequencies of 15 mA current and 60 V
[39]
voltage. Through the real-time body sweat analysis provided by HNGM, it had been proven to be able to
selectively monitor the biomarkers (Na and K ) in sweat and wirelessly transmit the sensing data to the user
+
+
interface through Bluetooth.