Page 132 - Read Online

Sun et al. Soft Sci. 2025, 5, 18 https://dx.doi.org/10.20517/ss.2024.77 Page 9 of 26

detect the values of pressure, stretching, or bending. One example involved a flexible, high-sensitivity, and
robust pressure sensor made from a PDMS/carbon nanotube composite material, exhibiting fast response
[61]
and low operating voltage [Figure 3C]. A graphene-based piezoresistive pressure sensor with tunable
[62]
sensing performance was fabricated using a template . Additionally, some self-powered tactile sensors
without external power sources harvest energy from the environment to operate and continuously monitor
[63]
pressure changes . By integrating capacitive sensor arrays for static spatiotemporal mapping and friction
generator sensors for dynamic tactile recognition, a flexible bimodal friction-capacitive coupled tactile
[64]
sensor array was created to enable active dynamic tactile sensing [Figure 3D]. A smart neuromorphic
tactile sensor based on a triboelectric nanogenerator (TENG) achieved self-powered pressure sensing
[65]
capabilities . Another commonly used pressure sensor is the capacitive pressure sensor, which detects
touch or contact by measuring changes of capacitance. A recent design featured a pressure sensor with a
multi-size planar structure, inducing charge exchange between adjacent electrodes upon external touch,
providing ultra-high sensitivity. Each pixel could be fabricated within a several-micrometer range and a tiny
pressure of 0.02 Pa would result in a 750% increase in the relative capacitance, equivalent sensitivity of
[66]
-1
3.75 × 10 kPa for 0-0.05 Pa, exceeding all the previous reports to date . A new flexible PDMS-based
5
capacitive tactile sensor array was fabricated to measure normal and shear force distributions. Measurement
of a single sensor shows that the full-scale range of detectable force is about 10 mN, which corresponds to
131 kPa in three directions . Another example involved a flexible capacitive pressure sensor based on
[67]
electrospun PI nanofiber films as the dielectric layer. The sensor with such a dielectric layer exhibited high
sensitivity (2.204 kPa at 3.5-4.1 Pa), wide scale range (0-1.388 MPa), low detection limit (3.5 Pa) and good
-1
cyclic stability (> 10,000 cycles) [Figure 3E].
[68]
Apart from improving the performance of various sensors, another important research challenge is how to
integrate these sensors while maintaining their performance. Achieving the integration of multiple sensor
functions into large-area flexible sensor arrays, while ensuring the preservation of excellent performance,
holds great promise for future applications . By combining various sensing functions, multifunctional
[69]
[70]
sensors have been developed to simultaneously detect temperature, pressure, and tactile stimuli . These
sensors not only enhance the diversity and intelligence of the sensing system, but also provide more sensory
feedback, closely simulating the sensory capabilities of human skin. Such sensors find wide applications in
e-skin, smart prosthetics, bionic hands, robotic skin, wearable devices, and multimodal sensing systems. To
achieve multi-mode sensing, a fully printed flexible trimodal sensor sheet, containing 4 × 4 pressure sensor
units, 2 × 2 temperature sensor units, and 1 proximity sensor unit, could simultaneously achieve
temperature, pressure, and proximity sensing was designed [Figure 3F]. Overall, multi-mode sensing
[71]
devices have made significant advancements.

However, despite the tremendous potential of these multi-mode sensing devices technically, they still face
several key drawbacks and challenges. First, multi-mode sensing devices are designed to simultaneously
detect multiple physical fields, but interactions and signal interference between different fields can occur. In
simple terms, changes in temperature and humidity may influence the measurements of pressure sensors, or
electromagnetic fields and mechanical vibrations may affect the accuracy of sensor readings. This cross-field
interference can result in inaccurate measurements, thereby affecting the overall performance of the sensor.
For example, by employing a pressure-temperature decoupling strategy, a highly sensitive iontronic
bimodal sensor with pressure-temperature discriminability exhibited a maximum force error of 5.9% in the
3-10 N range . Additionally, this single-unit sensor may cause larger measurement errors when arranged
[72]
in an array. In the design process, multi-mode sensing devices must deliver high precision across multiple
domains, often requiring a trade-off between sensitivity, stability, and response time. Due to the different
characteristics of each physical field, optimizing the sensor to maintain high precision and sensitivity across