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Page 18 of 38 Zhu et al. Soft Sci 2024;4:17 https://dx.doi.org/10.20517/ss.2024.05

strong self-healing properties, with a healing efficiency of 94.3% in 1 h for the PDMS/MWCNT matrix
alone.

Shi et al. report a heterogeneously integrated e-skin system of dynamically covalently bonded polyimide
(encapsulation and matrix) + liquid eutectic LM (electrical and sensing) + chip (information processing),
which simultaneously provides full recyclability, excellent mechanical stretchability, self-healing and
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reconfigurability . The polyimide matrix with an active bond exchange reaction network and the fluidity
of LM enable this e-skin system to self-heal from damage and re-adapt to the current application scenario.
All materials and components involved are recyclable.

Brief summary
This section summarizes the six most common basic capabilities of e-skins: pressure, strain, shear,
temperature, humidity, and self-healing.

Overall, resistive sensing mechanisms are the most used, thanks to their easy and reliable detection and the
rich possibilities in device architecture design. In addition, e-skins employing triboelectric and
piezoelectricity can be developed into self-powered e-skins, which can be used in the future as power supply
units in integration with other e-skins.

From the material point of view, e-skins can be classified into two main categories: FCPCs and conductive
gels. FCPCs are classified into polymer matrix and conductive materials. For the former, flexible polymer
materials, such as PDMS, TPU, SBS, cellulose, paper, etc., are widely used , in which PDMS is popular for
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its excellent flexibility, transparency, and process adaptability. Commonly used conductive materials include
one-dimensional materials, such as silver nanowires (AgNWs), copper nanowires (CuNWs), and CNTs,
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two-dimensional materials, such as reduced GO (rGO) and MXene , and highly conductive materials
such as Ag NPs, LM, and carbon black (CB).

E-skins constructed using conductive gel materials have a higher sensing range but tend to exhibit larger
response time, and lower durability, which comes from their traditional characteristics such as low modulus
and low toughness. However, they tend to be irreplaceable in terms of biocompatibility and adhesion, and
scientists have also been trying to improve their performance and shortcomings. Their performance has
been greatly improved in recent years. A typical example is the “Salting Out-Alignment-Locking” design
recently adopted by Sun et al. to greatly improve the tensile strength, modulus and toughness of gelatin
hydrogels. The tensile strength, modulus, and toughness of gelatin hydrogels can be increased up to 940,
2,830, and 1,785 times, respectively .
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The structure, material, mechanism and main characteristic parameters of some classic e-skins have been
listed in Table 1.

COMPLEX E-SKIN SYSTEMS
In fact, the perception of human skin of external stimuli is seldom a single sense or sensing unit acting
alone. Typically, for example, when a drop of water is dropped on our skin, most of the time, it is perceived
through a combination of temperature and humidity sensing, plus pressure sensing if the drop is heavy
enough and falling fast. On the other hand, all responses and processing of stimulus signals require the
intervention of information processing units such as the nervous system so that more advanced human-
environment interactions can be achieved (for e-skins, it is HMI). Therefore, single-sensing systems are
insufficient, and the scientists have been conducting research on complex e-skin systems such as
multimodal, IoT-integrated, and ML-enabled e-skin systems.