Page 36 - Read Online

Page 18 of 43 Wang et al. Soft Sci 2024;4:41 https://dx.doi.org/10.20517/ss.2024.53

behavior of polymer materials can lead to poor hysteresis properties in resistive fibric strain sensors, a
challenge that must be addressed to enhance their stability and reliability.

Triboelectric fiber-based strain sensors operate on the principle of converting external deformations caused
by the frictional interaction between two fiber-based materials with differing electron affinities into
electrical signals [138,139] . These sensors are widely employed in healthcare applications due to their simple
[140]
structure, high accuracy, and unique self-powered characteristics . A typical triboelectric fiber-based
strain sensor is designed with a helical structure . Upon stretching, the contact state between the two
[141]
triboelectric layers changes, generating an electrical signal. Consequently, helical fiber strain sensors
(HFSSs) are utilized to monitor human respiration [Figure 8A and B]. Additionally, triboelectric fiber-based
strain sensors can be configured in a single-electrode mode [Figure 8C]. Conductive yarn is fabricated by
twisting several polyester microfibers together with a stainless-steel microfiber, resulting in excellent
sensitivity . This sensor can be easily integrated into gloves for gesture recognition [Figure 8D]. It is
[142]
important to note that triboelectric fiber-based strain sensors often develop surface defects due to repeated
friction during prolonged operation, which can negatively affect sensing performance. These surface defects
can be mitigated or prevented through the application of advanced surface treatment technologies.

Capacitive fiber-based strain sensors respond to strain by altering their capacitance. These sensors typically
employ a core-shell structure , which is effective for detecting elongation strain. These sensors can be
[143]
integrated into textiles through sewing or weaving, enabling real-time monitoring of wearer movement
during activities such as walking [Figure 8E and F]. Beyond the core-shell design, capacitive fiber-based
[29]
strain sensors often utilize a double helical structure . For instance, Lee et al. reported a fiber strain-
sensing system based on a helical structure consisting of two conductive fibers , which offers stability,
[144]
durability, and excellent sensitivity. Furthermore, the response of capacitive fiber-shaped strain sensors is
particularly sensitive to changes in the dielectric constant of the surrounding environment. To address this
limitation, it is essential to develop analytical models that account for various environmental conditions.

In addition to fibric strain sensors, fibric pressure sensors are also extensively utilized in motion
monitoring, human-computer interaction, and e-skin due to their simple structure, low power
[145]
consumption, and skin compatibility . Similar to fibric strain sensors, fibric pressure sensors can be
categorized based on their sensing mechanisms into resistive, capacitive, electromagnetic, triboelectric, and
optical pressure sensors.

Resistive fibric pressure sensors detect pressure by altering conductive paths under applied pressure, leading
to a change in resistance. To date, various conductive materials, including CNTs, graphene, MXene, and
metals, have been employed in the fabrication of fibric pressure sensors. Lan et al. developed conductive
Au-MoS composite-coated fibers exhibiting superior electrical conductivity, tensile strength, and
2
stability . By stacking two Au-MoS composite-coated fibers perpendicularly, a pressure sensor is formed
[146]
2
at their cross-contact point [Figure 8G]. Integration of the pressure sensor into a fabric glove allows for
multiple force mapping properties [Figure 8H and I]. The sensitivity of wearable sensors can be significantly
enhanced through the use of advanced fiber materials and innovative microstructural designs. For example,
fibric pressure sensors with high compressibility and sensitivity have been developed using hierarchical
three-dimensional, porous reduced graphene oxide (rGO) fibers as key sensing elements . These sensors
[147]
can effectively monitor human respiration and pulse [Figure 8J and K]. Additionally, highly sensitive fibric
pressure sensors have been fabricated by fabricating stretchable MXene/CNT/polyurethane (PU) fibers via a
wet spinning technique .
[148]