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Zhang et al. Soft Sci 2024;4:39 https://dx.doi.org/10.20517/ss.2024.34 Page 9 of 28

growth and proliferation, which demonstrates minimal cytotoxicity when cultured with various cell types.
In vivo assessments further confirm their compatibility, as they typically exhibit low inflammation and
maintain normal cellular morphology in implanted sites. Also, benefiting from the mechanical properties of
hydrogels, they can make conformal contact with tissues without causing significant compression or
damage, which also contributes to their biocompatibility. Overall, the favorable interactions between
hydrogels and biological tissues highlight their potential for applications in advanced neuroelectronic
interfaces and other biomedical areas.

Moisturizing and swelling properties of hydrogels in bioelectronics
Hydrogels possess excellent flexibility and biocompatibility, which provides the fundamental properties for
applications in flexible electronics. However, due to the high water content, almost all hydrogels inevitably
lose water when exposed to an environment with low humidity, which leads to reduced flexibility or even
loss of function. To address the dehydration of hydrogels, highly water-soluble inorganic salts, organic
solvents, surface modification or encapsulation were proposed.

Zhu et al. developed a hydrogel with excellent water retention using a dual hydrophobic coating, which was
[86]
referred to as PAAm-ASO , as shown in Figure 5A and B. Firstly, the hydrogel underwent plasma
treatment to create a rich hydroxyl surface. It was sequentially immersed in a solution of (3-aminopropyl)
triethoxysilane (APTES) in ethyl acetate (EtOAc), a stearic acid (STA) solution, and silicone oil. The robust
coating, which consists of a hydrophobic polymer layer and a hydrophobic oil layer, provides dual shielding
against water evaporation. Since the coating is much thinner than the hydrogel, it does not significantly
affect the mechanical properties of the hydrogel while its water retention property is enhanced.

Inspired by the structure of spider silk, Wu et al. constructed hydrogel fibers with high mechanical
performance and water retention properties by introducing ionic cross-linking and crystalline domains
through the incorporation of Zr based on ionic coordination of inorganic salts and the Hofmeister effect as
4+
4+
[87]
shown in Figure 5C and D . The S-PAZr (S-PVA/PAA/Zr ) hydrogel fibers can maintain 79.91% of the
original weight at 43% RH. In addition, the material can spontaneously absorb water molecules from the
environment when moving from a low-humidity to a high-humidity environment.

Lan et al. also proposed a hydrogel by combining gelatin with pyrrolidone carboxylic acid sodium (PCA-
Na), as shown in Figure 5E and F . With the protonation of the amino groups in gelatin, it can
[88]
electrostatically attract the carboxyl groups of PCA-Na. Therefore, the mechanical properties of the gelatin-
PCA-Na gel can be improved due to the enhanced ionic cross-linking and the tight network structure.
Additionally, the hydrophilic groups of PCA-Na can form hydrogen bonds with water molecules, which
further improves the water retention of the hydrogel. As a result, the hydrogel can be used as a conformal
biological interface for the recording of physiological electrical signals with high fidelity.

Besides the dehydration of hydrogels, the swelling of hydrogels in aqueous environments is also an issue,
especially for in vivo applications, such as implantable electronic devices. The swelling mechanism of
hydrogels results from the balance between the polymer-water interaction forces that promote swelling and
the elastic recoil forces that resist the swelling. To address this issue, researchers have developed anti-
swelling hydrogels by adjusting the hydrophilic-hydrophobic property, cross-linking density and monomer
content.

Dou et al. developed an anti-swelling hydrogel with a protective layer by adjusting non-covalent
interactions as shown in Figure 6A-C . In brief, the chitosan/poly(N-acryloylglycine) (CS/PACG)
[89]