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Page 2 of 8 Xiao et al. Soft Sci. 2025, 5, 40 https://dx.doi.org/10.20517/ss.2025.51

INTRODUCTION
With the rapid advance of flexible electronics, wearable sensors that form high-performance interfaces with
human skin are gradually replacing conventional rigid devices and have become indispensable in health
monitoring , active rehabilitation , and intelligent human-machine interaction (HMI) . Among the
[1-3]
[4,5]
[6-8]
various soft electronic materials under development, hydrogels stand out because their physicochemical
characteristics closely resemble those of human skin [9-14] . When used as epidermal electrodes, hydrogel
devices can record diverse biosignals - such as electroencephalograms (EEG) and surface electromyograms
(sEMG) - and supply them as high-value inputs to HMI systems [15,16] .

High-performance HMI platforms rely on electrophysiological signals with a high signal-to-noise ratio
[17]
(SNR) . Achieving such quality depends on a low-impedance hydrogel-skin interface that minimizes signal
attenuation and external interference . Excellent electrical conductivity of hydrogel electrodes is demanded
[18]
to support efficient signal transmission. Simultaneously, sufficient mechanical flexibility is required to
conform intimately to the microtopography of the skin . Moreover, strong adhesive properties are
[19]
necessary to prevent electrode displacement during movement and enable stable and continuous biosignal
monitoring [Figure 1A] . However, conventional hydrogels often suffer from poor conductivity, weak
[20]
adhesion, and mechanical mismatch with skin, compromising signal quality . Therefore, next-generation
[19]
hydrogel electrodes must be engineered with conductive additives, ions, adhesion enhancers, and
multidimensional structural optimizations to ensure long-term, stable collection of high-quality
physiological data for advanced HMI applications.

PERFORMANCE OPTIMIZATION STRATEGIES AND INTERFACE ENGINEERING OF
HYDROGEL ELECTRODES
The three-dimensional network of hydrogels offers intrinsic porosity for incorporating functional fillers,
enabling optimization of conductivity, adhesion, and mechanical compliance to enhance skin-electrode
interfaces [21,22] . Common strategies include adding fillers such as tannic acid (TA) and MXene, which
[23]
improve both conductivity and adhesion, ensuring a low-impedance and stable skin-electrode interface .
For example, Wan et al. integrated MXene nanosheets into a self-healing hydrogel made of TA-
phenylboronic acid-grafted hyaluronic acid (HA-PBA) [Figure 1B] , resulting in a stable skin interface that
[24]
supports multimodal biosignal acquisition and additional functions such as ultraviolet (UV) protection and
photothermal therapy via MXene’s photothermal conversion ability. Furthermore, Wu et al. introduced TA
into a polyacrylamide (PAM) hydrogel and immersed it in sodium chloride (NaCl) solution, enhancing
ionic conductivity and enabling high-performance signal acquisition from hairy skin surfaces [Figure 1C] .
[17]
In addition to TA, polydopamine (PDA) is also a commonly used functional filler to improve the adhesive
and mechanical properties of hydrogel electrodes. For example, Han et al. added PDA nanoparticles to a
polyvinyl alcohol (PVA)/polyvinylpyrrolidone (PVP) hydrogel [Figure 1D], improving adhesion and
enabling stable attachment to various surfaces, including skin, metal, and glass [Figure 1E] . This electrode
[19]
showed low interface impedance, ensuring stable EEG signal acquisition [Figure 1F]. PDA and TA enhance
adhesion through hydrogen bonds and π-π interactions with skin proteins, improving interfacial stability
during movement [17,19] . Liquid metal is another filler that boosts conductivity and flexibility, making
hydrogels suitable for dynamic, stretchable applications . Hu et al. incorporated liquid metal into a dual-
[1]
network hydrogel of PAM and gelatin, achieving high conductivity and intelligent adhesiveness [Figure
[25]
1G] , resulting in superior sEMG signal acquisition compared to commercial electrodes. This hydrogel
maintained stable signal acquisition for up to 48 h, showing promise for wearable bioelectronics
[Figure 1H].