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Page 22 of 27 Tian et al. Soft Sci 2023;3:30 https://dx.doi.org/10.20517/ss.2023.21

properties, such as non-toxic, antibacterial, highly flexible, and highly biocompatible, and additional
properties, such as hydrophobicity and breathability, in order to fit the human skin to a high degree during
long wear. In addition, green materials for use in electronic devices are highly sought after, and their
biodegradable properties meet the requirements for sustainability. With regard to the integration of
multiple sensing, the traditional combination of capacitive, resistive, and potential detection is still the
mainstream design approach for flexible pressure and temperature sensors, but nonelectrical signals, such as
optical, electromagnetic, frequency-based, and biochemical detection, also provide new ideas for the
integration of multiple mechanisms, which are highly relevant to the application scenario. Interestingly,
natural flora and fauna can also provide valuable design guidance for sensing mechanisms and structures.
Structural design is the most direct aspect of sensor uniqueness. Flexible pressure and temperature e-skin
sensors are used in a wide range of applications, including, but not limited to, underwater operations,
medical treatment, emergency rescue, and many more, so the exploration of structural diversity is essential.
Several interesting bionic structures have now been proposed and applied to sensor design. Finally, high-
level manufacturing methods can empower flexible sensors. Large-scale, highly processable, and cost-
effective manufacturing methods can produce highly flexible, thin, and light e-skin sensors that are highly
compatible with the desired structure, but the greatest challenges exist in the stable mechanical and
electrical connections along with device packaging under the complex heterogeneous integration, which is
essential for the stability and durability of sensors .
[101]
Facing huge opportunities, flexible pressure and temperature sensors towards e-skin are greatly desired to
overcome several bottlenecks containing crosstalk between two responsive signals, integrated transistor
circuit implementation, and self-powered supply requirements. Firstly, crosstalk between temperature and
pressure apparently has an impact on the overall performance, especially temperature changes that
influence the properties of active materials with unavoidable disturbances to the pressure output. Two
different methods, namely sensing mechanism and output data processing, have been discussed to solve this
problem. The DEOS system-based two types of mechanisms can effectively alleviate signal crosstalk, and
[3]
machine learning algorithms are also researched for distinctions from one mixed output electric signal .
Secondly, flexible sensors integrated transistor circuits show promising research space as a vital element of
sensors in interdisciplinary fields. With the integrated readout circuit, pressure and temperature sensing
data can be monitored in a real-time fashion. In addition, communication with the upper computer is so
necessary in many situations that a wireless transceiver module is integrated into the circuit. However,
several problems, including the extra signal noise and flexible circuit fabrication, need to be taken seriously
during the design and manufacturing process of circuits [96,100] . Thirdly, the issue of energy supply for sensing
systems is essential, which drives more and more researchers to pursue kinds of clean energy for sensing
systems. Interestingly, hybrid nanogenerators provide a solution for flexible pressure and temperature
sensing arrays. Based on similar mechanisms with sensing, it is relatively achievable to integrate nano-scale
energy harvesters with sensing systems, which embody tremendous latent capacity in wearable
electronics . Additionally, the low-power performance of the sensing element is also worthy of attention as
[98]
a method of alleviating design sophistication on energy supply issues. Currently, e-skin with intelligent
sensing capabilities is being explored. High-precision sensor arrays are only at the forefront of the vast e-
skin system, and to truly realize the sensory capabilities of human skin requires the integration and
penetration of multiple disciplines, such as the integration of high-speed data collectors and machine
learning to assist in sensing contact surface material properties to process complex information,
[102]
highlighting the wide range of applications for flexible e-skins in the fields of flexible prosthetics , haptic
[104]
perception , and text recognition . Therefore, challenges co-exist with opportunities in the design of
[103]
flexible pressure and temperature sensors towards e-skin. Besides pressure and temperature, other tactile
parameters containing proximity, humidity, sound, magnetic field, and some biochemical signals are
expected to be detected in a highly integrated multifunctional flexible sensing system equipped with a self-