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Page 2 of 38 Zhu et al. Soft Sci 2024;4:17 https://dx.doi.org/10.20517/ss.2024.05

INTRODUCTION
The skin, the largest organ in the human body, serves not only as a physical protective barrier but also plays
a crucial role in perceiving the external environment, regulating physiological functions, and actively
participating in immune defense. Throughout the development of human work and life, the skin holds
immense significance, forming the fundamental basis for human perception of the surrounding
[1,2]
environment .
Simultaneously, the advancement of human civilization sees a notable trend in integrating intelligent
machinery, robots, and manufacturing into our daily lives, ushering in a significant transformation toward
the digitization and intelligence of various aspects of human existence. These elements are intricately
[3]
interconnected, collectively driving a profound change in society .

The increasing need for Human-Machine Interfaces (HMI) has led to a notable trend in replicating the
[4-6]
sensory capabilities of human skin, facilitated by advancements in electronics and materials science .
Referred to as electronic skins (e-skins), this innovation holds substantial potential across various domains
such as flexible wearable electronics, health monitoring, healthcare, robotics, smart manufacturing, smart
homes, and more .
[7,8]
An electronic skin (e-skin) system needs to possess the following basic characteristics:

First, akin to human skin, e-skins should be flexible, stretchable and tough enough. They need to adapt well
to the movement of the torso, machinery, or devices in applications while preserving their sensing function,
and thus, some stretchable structures need to be developed to maintain the sensing capability of the active
devices . Obviously, flexible stretchable materials are an important foundation; materials such as hydrogel,
[9]
cellulose, and polydimethylsiloxane (PDMS) are widely used [10-12] .

Second, resembling human skin, e-skins need to be carefully designed to provide various sensing
capabilities. Examples include the ability to sense stimuli such as strain [13-15] , temperature [16,17] , and
humidity [18-21] .

Thirdly, similar to human skin, e-skins should also have good self-healing ability. To better cope with the
complex environment, they may face too strong stimuli, which will significantly extend their lifespan [22,23] .

Fourth, the e-skins applied in some special scenarios should have certain specific characteristics, such as
biocompatibility, self-supply of energy, energy storage, transparency, color-changing capabilities, cold
resistance, recyclability, biodegradability, and so on.

Taking a broad perspective, a human skin system involves the coordinated operation of various subsystems,
including the skin itself, the brain and nervous system, and the heart and blood circulation system.
Similarly, an e-skin system comprises three essential subsystems: a sensor subsystem emulating the
functions of human skin, a signal collection/transmission/processing subsystem mimicking the roles of the
brain and nervous system, and an energy supply subsystem replicating the functions of the heart and blood
circulation.

Within this framework, the sensor system is tasked with detecting pertinent external stimuli or
physiological signals and transforming them into measurable electrical signals. The signal collection/