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Page 4 of 9 Wang. Soft Sci 2024;4:5 https://dx.doi.org/10.20517/ss.2023.44

biocompatibility, stretchability, sufficient thinness, and mechanical durability. Notably, the epidermal
electrode with a compliant and comfortable interface guarantees high-quality bioelectrical signals where a
low skin impedance can be attained. According to the flexural rigidity equation, flexural rigidity can be
2
calculated as D = 12(1-v ), where E, t, and ν represent Young’s modulus, thickness, and Poisson’s ratio,
respectively, of a thin film . Therefore, reducing thickness is the most effective approach to decrease
[44]
flexural rigidity, thus leading to higher skin compliance [45,46] . Towards this end, lots of electronic tattoo
electrodes have been developed based on conducting polymers and two-dimensional nanomaterials [10,37,47,48] .
Besides reducing thickness to obtain high skin compliance, dry electrodes should also be mechanically
stretchable and durable to secure continuous attachment on the human skin .
[49]

With the introduction of electrospun nanomeshes, skin electronics have evolved from a thin-film form
factor to a gas-permeable, biocompatible ultrathinness form factor [50-52] [Figure 1F]. Ma et al. reported
biocompatible and permeable ECG electrodes using a liquid-metal fiber mat with a stretchability of over
[53]
1,800% strain [Figure 1G] . A self-adhesive electrode has been developed by reducing thickness to 165 nm
employing Au-coated PDMS nanofilm [Figure 1H] . Another efficacious strategy to improve adhesiveness
[44]
is to directly paint/draw inks/gels on the human skin [Figure 1I] [13,54] . A recent example is a paintable
epidermal electrode from thermal-controlled phase change gelatin-based hydrogels, which overcomes the
[55]
limited conformability on hairy areas such as the scalp . Taking advantage of the adhesive properties of
hydrogels, many researchers have been working on simultaneously improving their gas-permeability for
long-term skin applicability. There are two typical approaches: (1) ultrathin enough (a few µm-thick) to be
permeable [20,56] and (2) macroscopic porous structure to be permeable [57,58] .

Most existing wearable electronics are not decomposable and can lead to serious electronic waste (e-waste)
[59]
and burden to Mother Earth . To this end, biodegradable materials have been utilized to develop transient
epidermal electrodes with zero waste footprint [60,61] . Lately, Ye et al. developed a fully biodegradable and
biocompatible ionotronic skin that was made by carboxylated chitosan (CCS) and sulfobetaine methacrylate
(SBMA) polymerized in glycerol and water followed by cross-linking with hydrogen bonds and electrostatic
[62]
attraction . As shown in Figure 1J, the developed ionic epidermal electrodes can accurately record action
potentials and fully degrade in only three days without any residue. Other properties, such as washability ,
[63]
[65]
waterproof , self-healing , and antibacterial characteristics , have also been implemented for specific
[66]
[64]
application scenarios.
APPLICATIONS
It should be noted that a significant application of epidermal electrodes is continuous and long-term
electrophysical monitoring due to its critical role in early disease prevention, screening, diagnosis, and
treatment [28,67] . Generally speaking, the capability of continuous, long-term monitoring requires a
combination of various properties, such as low skin impedance, high conformability, gas-permeability,
robust skin-electrode interface, and mechanical durability. Owing to the advancement of ever-fast materials,
a plethora of such epidermal electrodes have been realized for long-term ECG and EEG
acquirement [44,55,68,69] . Furtherly, the collected high-fidelity electrophysiological signals can be adopted for
BMIs , wireless health monitoring , HMIs , adaptable wearable systems , prosthetics , and muscle
[70]
[18]
[55]
[71]
[72]
theranostics [Figure 1K-O]. As high-fidelity EMG and EEG acquirement is significant for non-invasive
[21]
high-precision HMIs/BMIs [12,72] , it is highly demanding to develop high-performance epidermal electrodes.
Additionally, to enable epidermal electrodes with unsacrificed functionality under extreme conditions, such
as aqueous environments and polar regions, adaptable epidermal electrodes have attracted intensive
attention over the last decade [73-75] . For instance, Wan et al. reported an all-in-one flexible system capable of
working under intense motion, heavy sweating, and varied surface morphology, conducting in situ injection
and photonic curing of a biocompatible and biodegradable light-curable conductive ink .
[71]