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Zhang et al. Soft Sci 2024;4:39 https://dx.doi.org/10.20517/ss.2024.34 Page 13 of 28

Table 1. Advantages, disadvantages, applications, material composition and signal quality of different non-invasive hydrogel
electrodes
Conduction
Materials Advantages Disadvantages Applications Signal quality Ref.
type
Glycerol, Highly stretchable, Transient nature may Wearable electronic, Ionic conductive Stable for [77]
Hydroxyethylcellulose, HEC adapts to dynamic limit lifespan health monitoring gel dynamic
surfaces patch monitoring
Acrylamide, N,N’- Strong adhesion to May lose adhesion Wearable sensor, Ionic conductive Stable signal, [78]
methylenebisacrylamide non-porous surfaces under extreme stress health patch gel strong interface
contact
Gelatin, chitosan Reduces dynamic Complexity may Bioelectronic, Ionic conductive High clarity, [79]
noise, enhances increase manufacturing wearable sensor gel selective
signal clarity cost frequency
damping
PAM/PVA, CNTs Strong adhesion for Adhesive may cause Non-invasive, long- Ionic conductive High signal [81]
long-term monitoring skin irritation over time term EEG monitoring gel quality, stable
contact
PANI-PEDOT, PAM Low impedance, Complexity in chemical Stretchable Ion-electron co- High-quality, [82]
enhances signal bonding process bioelectronic, conductive low-noise signals
quality dynamic monitoring hydrogel
PVA, EGaIn, TA Exceptional Potential complexity in Wearable electronic, Electronically High signal [83]
toughness and fabrication high-strength sensor conductive quality, low
conductivity hydrogel impedance

HEC: Hydroxyethyl cellulose; PAM: polyacrylamide; PVA: polyvinyl alcohol; CNTs: carbon nanotubes; EEG: electroencephalogram; PANI:
polyaniline; PEDOT: poly(3,4-ethylenedioxythiophene); TA: tannic acid.

Table 2. Advantages, disadvantages, applications, material composition and signal quality of different Invasive hydrogel electrodes
Materials Advantages Disadvantages Applications Conduction type Signal Ref.
quality
PVA, HACC, HA Transparent, enables Potential limitations in Two-photon Ionic conductive gel High-fidelity [85]
optical imaging long-term use neuroimaging, ECoG signals
monitoring
PEG, Conformal contact with Complex integration BMI, neural recording Ion-electron co- High signal [99]
channelrhodopsin-2 brain tissue process conductive hydrogel fidelity
PVA, PEDOT Forms synaptic-like Complex setup, may Synaptic BMI, neural Ionic conductive gel High-fidelity [100]
connections with neuron require precise control interfacing synaptic
signals
PAM, PAA, CNTs Strong adhesion, high May require precise Neural recording and Ion-electron co- High-quality, [102]
conductivity, and coating techniques modulation conductive hydrogel low-noise
durability signals
PVA: Polyvinyl alcohol; HACC: hydroxypropyltrimethyl ammonium chloride chitosan; HA: hyaluronic acid; ECoG: electrocorticogram; PEG:
polyethylene glycol; BMI: brain-machine interfaces; PEDOT: poly(3,4-ethylenedioxythiophene); PAM: polyacrylamide; PAA: polyacrylic acid;
CNTs: carbon nanotubes.

or dipole-dipole interactions generated by the strong dipolar zwitterionic units within the polymer chains,
the reversible physical cross-linking offers advantages in energy dissipation, superelasticity, adaptive
adhesion, and ionic conduction. As a result, this flexible hydrogel electrode can be used for continuous
recording of EEG signals. Also, eye movement and forehead temperature can also be monitored by
integrating with other sensors, which indicates the potential application of the hydrogel in the monitoring
of body movement or analysis of mental states.

Figure 7C and D demonstrates the continuous monitoring of high-quality brain signals with hydrogel and
commercial EEG electrodes by Park et al. . The hydrogel works as the interface and can filter out
[79]
mechanical signals due to the low-frequency noise. In the experiment, the volunteers were asked to blink
their eyes at a fixed frequency and it was demonstrated that the hydrogel can selectively filter 10 Hz noise.