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environments can also pose risks, particularly for implantable devices, as it may affect the mechanical
integrity and performance. Researchers have proposed anti-swelling hydrogels through adjustments in
hydrophilic-hydrophobic balance and cross-linking density, which help maintain structural stability during
prolonged exposure to bodily fluids. Together, these approaches highlight the importance of optimizing
hydrogel properties to ensure their reliability and performance in bioelectronic applications, ultimately
supporting their use as effective interfaces for physiological signal monitoring.

In addition, SNR is crucial in the performance of electrodes used for signal monitoring, particularly in
brain-machine interfaces. Hydrogel-based electrodes offer distinct advantages and face specific challenges,
especially concerning long-term monitoring. The high biocompatibility and low modulus of hydrogels give
them a softness similar to that of soft tissues, allowing them to better conform to biological tissues and form
close contact, which is beneficial for long-term interface stability. Hydrogels contain large amounts of water
and ions, enabling charge transfer through ionic conductivity. This conductive mechanism is consistent
with the ion conduction mechanism in biological tissues, which is beneficial for electrical signal
transmission and reducing interface impedance caused by conductivity differences. Low impedance can also
reduce energy loss during signal transmission, which will result in clearer electrophysiological signals.
Compared to rigid electrodes, this soft interface does not exert additional mechanical stimulation on tissues
which can be helpful to reduce noise caused by interface movement. Therefore, hydrogel electrodes can
achieve good interface integration and long-term stability with biological tissues through their
biocompatibility, softness, ionic conductivity, and low interface impedance. As a result, it can effectively
reduce interference in electrophysiological signals and improve the SNR.

HYDROGEL-BASED BRAIN-MACHINE INTERFACES
Tables 1 and 2 show the main features of non-invasive and invasive hydrogel electrodes in different
bioelectronic applications. These tables give a complete picture of how hydrogel electrodes can be used in
biosignal monitoring, health management, and neural interfaces. They can be used to improve performance
and find the best materials.

Non-invasive hydrogel interfaces
General non-invasive hydrogel interfaces
Hydrogel is gaining recognition as an ideal material for semi-dry EEG electrodes due to its unique ability to
improve signal quality, comfort, and practicality. It provides excellent skin conformability, ensuring a stable
and low-impedance contact between the electrode and the skin, which enhances the accuracy of EEG
recordings. Unlike traditional wet electrodes, hydrogel electrodes are more comfortable for long-term wear,
reduce skin irritation, and are easier to apply and remove without the mess of conductive pastes or gels.
They also maintain consistent electrical properties, which can offer better signal integrity, even in dynamic
or mobile settings. With their durability, ease of use, and reduced maintenance requirements, hydrogel-
based electrodes offer a more hygienic, cost-effective, and user-friendly alternative for both clinical and
home-based EEG monitoring . Therefore, the monitoring of EEG signals is widely investigated by the
[32]
incorporation of hydrogels with commercial electrodes or EEG caps to realize multi-channel EEG signals
with non-invasive method. In these frameworks, the hydrogels are pre-prepared and are attached on the
EEG caps for electrodes to use the interface.

Chen from the Institute of Materials at the Chinese Academy of Sciences reported a hydrogel , which is
[92]
composed of a physically cross-linked interpenetrating network based on calcium ions and zwitterionic [2-
(methacryloyloxy)ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide (SBMA) and 2-hydroxyethyl
methacrylate (HEMA), for the recording of EEG signals as shown in Figure 7A and B. Due to the ion-dipole