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Page 2 of 9 Wang. Soft Sci 2024;4:5 https://dx.doi.org/10.20517/ss.2023.44

machine interfaces (HMIs/BMIs), prosthetics, robotics, and augmented reality (AR) and virtual reality (VR)
[1-5]
communications . In particular, for health monitoring, epidermal electrodes have attracted intensive
attention for non-invasive electrophysiological recording, such as electromyogram (EMG) (amplitude
between 50 and 5,000 µV, frequency between 5 and 500 Hz), electrocardiogram (ECG) (amplitude between
50 and 5,000 µV, frequency between 0.5 and 100 Hz), electrooculogram (EOG) (amplitude between 10 and
3,500 µV, frequency between 0.1 and 30 Hz), and electroencephalogram (EEG) (amplitude less than 100 µV,
frequency between 0.5 and 100 Hz) . Owing to their ultrathinness, lightweight, high conductivity, low
[6-9]
skin-contact impedance, and skin-like mechanical properties, epidermal electrodes can pick up delicate
ionic conduction signals on the epidermis induced by brain, muscle, eyeball, and heart activities [10,11] .
Different types of electrophysiological signals provide valuable insights into the functioning of various
tissues and organs. Together with the capability of electrostimulation, epidermal electrodes play a crucial
role in sleep monitoring, wound healing, fatigue alerts, neurofeedback training, muscle and neurological
disorder theranostic, HMIs, BMIs, etc. [12,13] . Besides digital health, epidermal electrodes are emerging devices
to the realization of on-skin digitalization that aims to create a seamless interface between humans and
[7]
devices and enable remote health monitoring and human-cyber interactions .
MATERIALS
Some pioneering work has been done in epidermal electrodes by introducing structural engineering on
metal and polymeric films [14,15] . Structure engineering is an effective strategy to endow rigid electronic
devices that are conformable and stretchable for skin applicability. Another significant strategy is to design
and utilize intrinsically stretchable materials [16,17] . To achieve high conductivity for epidermal electrodes, a
variety of electrical materials have been employed, such as conducting polymers, ionic liquids, liquid metals,
low-dimensional nanomaterials (e.g., carbon/metallic-based nanomaterials and MXenes), and hydrogels
[Figure 1A-E] [18-22] . Figure 1A demonstrates the utilization of ionic liquid, bis(trifluoromethane) sulfonimide
lithium salt (LiTFSI), integrated with poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)
and ethylene glycol for the fabrication of dermal electrodes . In Figure 1B, micromeshes made of gallium
[18]
[19]
liquid metals are created onto elastomer sponges for EMG electrodes . A solvent-free supramolecular
ion-conductive elastomer has been developed as an ionic tattoo for various electrophysiological monitoring
[20]
[21]
applications [Figure 1C] . Figure 1D shows a sweat-stable EMG electrode made of MXene . Besides
MXene, other prevailing low-dimensional nanomaterials for epidermal electrodes include silver
[26]
[25]
[23]
nanowires , gold nanowires , graphene , and carbon nanotubes . Figure 1E presents bioadhesive
[24]
hydrogel materials for rapid, robust, conformal, and electrically conductive integration between
bioelectronic devices and various wet dynamic tissues . It is worth noting that among them, conducting
[22]
polymers [27,28] , liquid metal, low-dimensional nanomaterials , and their hybrid have been widely
[30]
[29]
exploited for both dry and wet epidermal electrodes [31-34] . Wet electrodes refer to the use of conductive
hydrogels or paste materials, such as the commercialized Ag/AgCl, which have good biocompatibility, skin
adhesion, mechanical softness, and low skin impedance. Therefore, wet electrodes generally can provide
high-fidelity and low-noise signals. On-skin dry electrodes are fabricated by carbon and metal-based
materials and composites. Unlike wet electrodes, dry electrodes rely on external forces or van der Waal
forces to achieve high skin contact, normally with high skin impedance and high-noise signals. However,
their electrical and electromechanical properties can be tailored by various materials and structures [35,36] . Due
to their versatile tunability on electrical and mechanical properties, hydrogels have emerged as
advantageous materials for the development of epidermal electrodes, both as conductive materials and
polymeric substrates [37,38] . Other representative polymeric substrate materials include PDMS [39,40] and
polymeric electrospun nanofibers [41-43] .