Page 107 - Read Online

Page 16 of 34 Xi et al. Soft Sci 2023;3:26 https://dx.doi.org/10.20517/ss.2023.13

and diabetes levels in 0.5 mL solution absorbed in the laboratory skin model. As shown in Figure 5G, Huang
et al. report a stretchable self-powered biosensor with an epidermal electronic format that can detect in situ
[162]
lactate and glucose concentrations in sweat . The microfluidic channel they developed not only effectively
collects sweat but also provides excellent mechanical performance and stable performance output even
under 30% stretching.

pH sensing can monitor the pH change of biological fluids or environmental samples. Self-powered
wearable sensors for pH sensing usually use materials sensitive to pH, such as polymers or nanomaterials,
which will change their electrical properties according to the change of pH value. As shown in Figure 5H,
Santiago-Malagon et al. described the structure of a self-powered electrochromic device through screen
printing, which can be used to determine metabolites in sweat by the naked eye in the form of 3 × 15 mm
[163]
color strips . The device comprises a lactate oxidase and osmium-polymer-based anode connected to a
coplanar 3 × 15 mm Prussian Blue cathode printed over a transparent poly(3,4-ethylenedioxythiophene)
polystyrene sulfonate electrode. The sensor displays the concentration of lactic acid in the range of 0-10 mM
on the length of the electrochromic display, which has a contrast of 1.43. As a flexible sensor, it can
effectively display the lactic acid content in sweat.

Structure/technique in wearable self-powered sensors
Triboelectric nanogenerator
The TENG is a type of energy harvesting technology that can convert mechanical energy into electrical
energy [38,164,165] . The triboelectric effect is the generation of electrical charge that occurs when two materials
come into contact and then separate. Unlike the traditional electromagnetic generator, the TENG is very
effective in collecting low-frequency mechanical energy. TENGs are increasingly being used in wearable
self-powered sensors due to their ability to generate electricity from the body motion of a wearer . In a
[166]
wearable self-powered sensor, TENGs are typically integrated into the device in a way that allows them to
[167]
harvest energy from the movement of a wearer, such as when the person walks or moves their arms .
Additionally, TENGs are highly durable, can be made using low-cost materials, and are environmentally
friendly. There are several types of TENG-based self-powered sensors [29,168,169] .

TENGs can be integrated into smart gloves or wristbands, where they can harvest energy from the wrist
movements of wearers and power the sensors and displays of the devices [77,170,171] . As shown in Figure 6A, Zu
et al. developed a fully automatic early warning glove through the unique elastic-arched triboelectric
nanogenerator (EA-TENG) and flexible printed circuit technology. EA-TENG is a sustainable self-powered
power supply that can generate electricity by a continuous touch of hands or body and can complete a
charging process with only five taps. In the absence of an external power supply, it can achieve real-time
contactless static monitoring. As shown in Figure 6B, Lee et al. proposed a flexible HNG (FHNG) that
combines solar cells, transparent TENG, and PENGs . FHNG can collect energy from all kinds of energy
[172]
sources in a sustainable way and be used as a wearable self-powered health monitoring device by sensing
health information (such as human motion, arterial pulse rate, or respiratory rate) based on electrical output
when attached to the human body. As shown in Figure 6C, Li et al. prepared a new polycation-modified
carbon point (PCD)-customized PCD/PVA nanocomposite polymer electrolyte (NPE) and used it as the
main triboelectric material to build a new fiber TENG (NPE-TENG) based on NPE for the first time .
[173]
NPE-TENG-based self-powered wearable sensors and smart gloves are developed, which can realize skin-
level tactile perception and joint-related activity monitoring in a fast, real-time, and non-invasive manner.
As a sustainable power supply, NPE-TENG can drive small electronic devices and light up hundreds of
LEDs.