Page 25 - Read Online

Page 12 of 38 Zhu et al. Soft Sci 2024;4:17 https://dx.doi.org/10.20517/ss.2024.05

Shear sensing capability
The detection of shear forces by human skin does not only refer to the perception of lateral forces applied to
[102]
the skin surface but also to the perception of wind and proximity . The bionic micro-spine or
hemispherical structures, commonly used in other sensors, can only be used to sense the magnitude of the
force, not the direction of the force, and are inapplicable to the needs of shear force sensors. Sensor
structures commonly used in the field of shear force sensing are fluff structures [102-104] .

Yu et al. developed a capacitive shear force sensor with tilted micro-hair arrays (TMHAs) by mimicking the
[105]
hair structure of human skin [Figure 7A] [105,106] . The asymmetric microhair structure dielectric layer was
obtained by the two-photon polymerization (TPP) method, which would allow different deformations due
to shear forces in various directions to discriminate the direction of the force. The sensor can determine the
direction of static and dynamic shear forces, exhibit a large response scale from 30 Pa to 300 kPa with a
relative capacitance change ΔC/C < 2.5%, and maintain high stability even after 5,000 cycles.
0

In addition, the recognition of the direction of shear force can be realized using the strategy of multi-sensor
array + array structure design + signal processing . Zhu et al. proposed a haptic sensor based on a multi-
[107]
touch mechanism using a structure of PDMS/MWCNTs with conductive and curved surfaces, which
sequentially contact the resistive columns on the micro-honeycomb electrodes (MHEs) according to the
pressure to maintain proper piezoresistive characteristics [Figure 7B], and the whole sensor is characterized
by high sensitivity, high linearity, good robustness, and wide dynamic range . By adjusting the contact
[106]
distribution density and the curvature of the PDMS/MWCNTs contact, the sensitivity and detection range
can be tuned (500 kPa + 64.68 kPa for high sensitivity mode, 1,400 kPa + 25.88 kPa for wide range mode).
-1
-1
The possibility of realizing normal and force measurements was demonstrated by fabricating a triaxial
tactile sensor capable of sensing normal force (0-3.5 N) and shear force (0-1.5 N) with sensitivities of 58.097
and 36.137 N .
-1

Temperature sensing capability
Temperature sensing capability provides early warning of burn/frostbite risk and is important for the skin
system to maintain intact function. There are four main types of sensing mechanisms for temperature
sensors: resistive [108,109] , capacitive , piezoelectric, and triboelectric [111,112] . Resistive temperature sensors are
[110]
the most common type [113,114] ; their basic principle is that the resistance of the device changes with
temperature due to the thermal transport/scattering mechanism of the thermosensitive material and the
change in geometry with temperature [115,116] . One of the difficulties of temperature sensors is achieving a high
level of resolution.

Li et al. prepared a PEDOT-TPU composite fiber (PTCF) temperature sensor by growing poly(3,4-
ethylenedioxythiophene) (PEDOT) in situ on the surface of thermoplastic PU (TPU) fibers using in situ
polymerization . The sensor exhibited a sensitivity as high as 0.95% °C and a high linearity between 20
[117]
-1
and 40 °C. Additionally, the best part is the temperature resolution of up to 0.2 °C. The temperature-sensing
fibers can be easily embedded in textiles. By sewing the fibers in an S-shape into normal textiles, strain
disturbances can be virtually avoided, even when the textile is stretched to 140%.

Temperature changes can lead to variations in ion mobility, so gel materials that contain a large number of
ions and are predominantly ion-conductive can also be designed as temperature sensors [118-120] . Generally
speaking, as the temperature rises, the enhanced ion dissociation increases the concentration of charge
carriers, further boosting ion conductivity and enhancing conductivity. The relationship between ion
conductivity and surrounding temperature can be characterized using the Vogel-Tamman-Fulcher (VTF)
equation :
[121]