Page 139 - Read Online

Page 14 of 27 Tian et al. Soft Sci 2023;3:30 https://dx.doi.org/10.20517/ss.2023.21

Piezoresistive and thermoelectric sensors
The pressure and temperature sensors based on piezoresistive and thermoelectric effects are a common type
of DEOS systems, whose active sensing materials are typically blended by the thermoelectric and other
conductive materials. For instance, Li et al. demonstrated a stretchable bimodal sensor for strain (up to
60%) and temperature monitoring based on piezoresistive and thermoelectric effects with printable
[13]
polyurethane (PU) nanocomposites comprising MXene/AgNWs/PEDOT: PSS/tellurium NWs (TeNW) ,
as illustrated in Figure 6A. Jung et al. designed a multilayer structure consisting of two active layers with
[14]
different mechanisms , namely, the flexible conductive paper substrate printed with PEDOT:PSS and
AgNPs were used as a temperature-sensitive layer, while the micro-pyramid PDMS film deposited with
MWCNTs was performed as a pressure-sensitive layer, as illustrated in Figure 6B. Zhang et al. reported a
PEDOT: PSS-coated PU composite that can detect pressure and temperature instantaneously by
constructing porous microstructures and utilizing thermoelectric properties of PEDOT:PSS and polyaniline
[15]
(PANI) separately , as shown in Figure 6C.

Piezoelectric/Triboelectric and thermoresistive sensors
The second method to realize pressure and temperature sensing is to combine piezoelectric/triboelectric
effects with thermoresistive effects. By this means, output signals exist with no coupling, and piezoelectric/
triboelectric signals provide the sensing system with AC supplies. For instance, Zhang et al. reported a
[27]
strong and flexible vessel-like sensor that consists of a self-supported braided cotton hose substrate , single
wall carbon nanotubes (SWCNTs)/ZnO PVDF function arrays, and a flexible PVDF function fibrous
membrane. The whole system possesses the function of detecting the pressure and temperature of pulsed
fluids, as illustrated in Figure 7A. Shin et al. reported a simple method to realize a multifunctional flexible
motion sensor using ferroelectric lithium-doped ZnO-PDMS , which enables piezoelectric dynamic
[71]
sensing and provides additional motion information to more precisely discriminate different motions, as
shown in Figure 7B. Rao et al. designed a tactile e-skin that can simultaneously detect and distinguish
between temperature and pressure in real time , based on a single-electrode-mode TENG with a specially
[23]
prepared thermoresistive electrode combining BiTO and rGO, as shown in Figure 7C.

Piezo-capacitive and thermoresistive sensors
The third scheme to realize pressure and temperature sensing is combining piezo-capacitive and
thermoresistive effects to implement varying output capacitance and resistance representing pressure and
temperature, respectively. Due to a capacitive detecting mechanism, a common practice is to use a
multilayer structural design, which normally comprises two electrode layers and one dielectric layer with
specific micro-patterns. For example, You et al. presented a deformable artificial multimodal ionic receptor
that can differentiate thermal and mechanical information without signal interference , which is derived
[24]
from the analysis of the ion relaxation dynamics: the charge relaxation time as a strain-insensitive intrinsic
variable to measure absolute temperature and the normalized capacitance as a temperature-insensitive
extrinsic variable to measure strain, as illustrated in Figure 8A. Gao et al. reported a bifunctional self-healing
e-skin with PU and CNTs as the sensing materials by integrating a resistance temperature sensor on top of a
capacitive pressure sensor on the same flexible cellulose substrate , as shown in Figure 8B. An et al.
[72]
developed a transparent and flexible , capacitive fingerprint sensor array with multiplexed, simultaneous
[73]
detection of tactile pressure and finger skin temperature for mobile smart devices, whose networks of hybrid
nanostructures using ultra-long metal nanofibers and finer NWs were formed as transparent, flexible
electrodes of a multifunctional sensor array, as illustrated in Figure 8C. Kim et al. introduced a versatile
droplet-based microfluidic-assisted emulsion self-assembly process to generate three-dimensional
microstructure-based high-performance capacitive and piezoresistive strain/pressure/temperature sensors
for e-skin applications , as illustrated in Figure 8D.
[77]