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Zhu et al. Soft Sci 2024;4:17 https://dx.doi.org/10.20517/ss.2024.05 Page 3 of 38

transmission/processing system, on the other hand, gathers signals from the sensor system and
subsequently transmits and processes them. Lastly, the energy supply system plays a critical role in
providing power to these subsystems, with energy collection and supply mechanisms encompassing options
[24]
such as batteries, triboelectric devices, and piezoelectric devices .

Evidently, the sensor system constitutes the foundation of the entire system. The construction of a
perceptual capability that is clear and precise across various stimuli stands as a pivotal element in
developing an ideal e-skin and, consequently, intelligent systems built upon it. On this basis, multimodal
e-skins with multiple sensing capabilities and e-skin systems empowered with internet of things (IoT)
integration and machine learning (ML) approaches are more capable of meeting the needs in practical
applications. The development of e-skins has been accelerating in recent years, and they are bound to
become an integral part of people’s lives. Therefore, it becomes imperative to curate and analyze the latest
progress in the field of e-skins.

While several notable scholars have previously provided excellent reviews and commentaries on e-skins [25-28] ,
the domain has experienced rapid evolution, necessitating a comprehensive consideration of recent
emerging research. This entails a reevaluation of the current state of research and a forward-looking
perspective toward future developments.

This paper aims to integrate and summarize the strategies and research progress of e-skins. Firstly, we take
stock of several basic capabilities and the main implementation strategies of e-skins, which include strain
sensing, pressure sensing, shear force sensing, temperature sensing, humidity sensing, and self-healing
capability. Subsequently, we will briefly introduce complex e-skin systems and current major applications
[Figure 1] to facilitate readers to swiftly grasp advancements in this field. Finally, we will delve into
discussions regarding existing challenges and potential future directions. We hope this review article serves
as a wellspring of inspiration, assisting readers in forging their pursuits in this emerging research domain.

BASIC SENSING CAPABILITIES OF E-SKINS
The basic sensory capabilities of human skin include five aspects: strain, pressure, shear, temperature, and
humidity, of which the combined perception of strain, pressure, and shear constitutes the “tactility”. In
addition, human skin can heal itself in response to overpowering stimuli.

The basic implementation strategy of e-skins to perceive these stimuli is to utilize the established physical
ground stimulus-response law to design the conductive pathway and structure of the flexible device to
achieve the mapping between the external stimulus and the change of the electrical signals in the conductive
pathway, and to realize the perception of the corresponding stimulus through the detection of the electrical
signals collected by computer. These signals include resistance [29-31] , capacitance [32,33] , piezoelectricity,
semiconductor properties, triboelectricity, and so on.

In this section, we will present the basic implementation strategies and representative works of existing
e-skin systems.

Pressure sensing capability
There are four main categories of pressure sensors: (1) piezoresistive; (2) capacitive; (3) piezoelectric; (4)
triboelectric [34-36] [Figure 2A].