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Page 12 of 27 Tian et al. Soft Sci 2023;3:30 https://dx.doi.org/10.20517/ss.2023.21

Evaluation parameters
Flexible pressure and temperature e-skin sensors are supposed to possess excellent performance, including
compliance, robustness, and timeliness. Therefore, based on several kinds of combined mechanisms,
quantitative comparison can be realized since the output signals containing data information can be read
out in numerical form. The main quantitative parameters comprise sensitivity, range, response time, and
[7]
stability .

The sensitivity parameters are the most essential for the characterization of entire sensing systems. Both
-1
resistive and capacitive sensors have similar forms and units (Pa ) for their sensitivity parameters, as shown
in Equations (1), (2), and (7). However, the unit of sensitivity can be expressed in V/Pa or V/℃ for sensors
based on potential mechanisms due to a linear relationship between input stimuli and output response, as
shown in Equations (3), (4), and (5) [62,69] . Most sensing systems exhibit non-linear stimuli-response changes.
Hence, non-linear sensing curves are often segmented to calculate sensitivities for different scopes of
stimuli. With the pressure or temperature increasing, sensitivity parameters decrease for most systems.

The sensing range defines the maximum and minimum ranges that a sensor system can measure under
normal conditions. Maximum and minimum sensing data originate from sensors in a limited state, so their
accuracy is doubtful. For e-skin sensors with pressure and temperature sensing, the focus is primarily on
tactile sensation and body temperature, such as pulse, respiration, gesture, and motion. From this point of
view, the sensitivity is more significant than the sensing range, as e-skin sensors have a relatively narrow
desired sensing range.

The response time is also a vital evaluation parameter, which implies how rapidly the output of the sensor
changes with the varying external stimuli. It can be calculated by measuring the time difference between the
[7]
input time of stimuli and the stable (or 90% position) time of output response . Stability, symbolizing the
robustness of the sensing system, can be measured by the shifting of output response after thousands of
cycles of external stimuli. The more cycles the sensors bear, the less irreversible deformation occurs,
resulting in better robustness.

MULTIPLE MECHANISM INTEGRATION AND STRUCTURAL DESIGN OF PRESSURE AND
TEMPERATURE SENSORS
In a brief review, we have listed details about material selection and basic mechanisms for signal detection
of pressure and temperature. However, systematic design and construction are undoubtedly important, as it
integrates multiple materials, mechanisms, and structures to achieve composite sensing functions, and even
manufacturing methods are briefly considered. In regard to pressure and temperature sensing, two
strategies are adopted so as to design highly-integrated pressure and temperature sensors, respectively,
new structures with traditional materials and new materials with traditional structures. Actually,
common structures used in sensing systems include multilayer films [23,24,27,71,72] , porous structures [11,15] ,
multipixel , 3D structures , nanofibers , pyramid (pyramid-like) microstructures [14,23,75] , and serpentine
[11]
[73]
[74]
configurations [45,76] . Herein, we start with the integration of sensing mechanisms dividing flexible pressure
[3]
and temperature sensors into two categories, namely, the same and different output signals . Because of
extensive applications of electric outputs and decoupling-free, sensing systems with different electric output
signals (DEOS) are mainly concerned and presented in detail, while sensing systems with the same electric
output signals (SEOS) are introduced briefly. Typically, some pressure and temperature (or more physical
and biochemical sensing functions) sensor devices based on the DEOS system and the SEOS system are
compared in Table 2.