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

A closed-loop platform consisting of monitoring and therapy takes personalized healthcare to the next level.
In interesting research, Song et al. developed an all-in-one, bioderived, air-permeable, and sweat-stable
MXene electrode that can simultaneously record EMG signals and achieve electrostimulation and
electrothermal therapy for muscle theranostics . Specifically, the as-prepared MXene electrodes exhibit
[21]
3
high breathability, are ultralightweight (~0.25 mg/cm ), and have a low and stable electrode-skin interfacial
impedance in various environments, enabling the long-term reliable monitoring of electrophysiology.

SUMMARY
In this work, we highlight recent key developments of epidermal electrodes. Materials, properties, and
applications have been discussed individually. Remarkable progress has been made in this area due to the
enormous efforts devoted by researchers worldwide. It is believed that epidermal electrodes have
contributed a significant part to digital health and on-skin digitalization. However, there are some
remaining issues waiting to be addressed before these devices can be seamlessly integrated into our daily
lives.

Concurrent realization of combined promising properties, such as low skin impedance, robust electronic
bonding, high skin compliance, mechanical durability, and gas-permeability . It requires the development
[49]
of advanced materials and fabrication techniques and an in-depth understanding of the soft-rigid interface
interactions during constant dynamic skin motions. For example, it is important to develop unconventional
gas-permeable materials to overcome the intrinsic tradeoff between mechanical durability and thinness
geometry. To solve this issue, lots of efforts have been devoted to developing fiber-based or fiber-reinforced
ultrathin, gas-permeable electronics . Another approach in materials development is to design bulky
[51]
gas-permeable gels that have high bioadhesive properties. In this case, delicate cross-linking networks
should be considered to maintain excellent long-term stability, such as anti-drying and mechanical
robustness.

System intelligence. Skin sensor-artificial intelligence (AI) networks are paramount to the development of
both digital health and on-skin digitalization. The AI algorithms enable epidermal electrodes not only to
detect the health status for health management in real-time but also to enhance the interactions between
[76]
[77]
humans and machines . A recent work by Ouyang et al. demonstrated a system-on-a-chip with Bluetooth
Low Energy for data transmission and a compressed deep-learning module for autonomous operation .
[78]
The system achieved applications in studies of sleep-wake regulation and for the programmable closed-loop
pharmacological suppression of epileptic seizures in mice via feedback from EEG recording. Besides the
incorporation of data management technologies, other issues, such as processing capacity, long-term
stability, and data security, should also be taken into consideration.

Multichannel and multifaceted operation. Multichannel bioelectrical sensing is vital to achieve
comprehensive electrophysiology information for high-precision diagnosis and treatment and precision
control for HMIs/BMIs [7,11,79-81] . For example, Tian et al. reported a large area bioelectronic interface for
electrophysiological recordings that enable coverage of the full scalp and the full circumference of the
forearm . The large-area sensing arrays enabled multifunctional control of a transhumeral prosthesis by
[79]
patients who have undergone targeted muscle-reinnervation surgery, in long-term EEG, and in
simultaneous EEG and structural and functional magnetic resonance imaging. Additionally, the fusion of
multifaceted functionalities, such as biophysical and biochemical monitoring and self-powering, is
appealing to realize a full-fledged epidermal electrode system.