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He et al. Soft Sci 2024;4:37 https://dx.doi.org/10.20517/ss.2024.32 Page 5 of 27

What is hydrogel
Hydrogel is a kind of hydrophilic gel with 3D network structure. In water, it expands rapidly and absorbs a
large amount of water while maintaining its structure and stability without dissolving. However, if the
water-absorbed hydrogel is exposed to air, its volume will gradually shrink with the water evaporation. The
material substrates used determine the grid structure of hydrogels, resulting in varying swelling properties,
durability and stability among different hydrogels. In general, the higher the degree of crosslinking, the
better the durability and stability of the hydrogel, but its water absorption capacity will be reduced . As
[37]
shown in Figure 2, this exceptional water-retention capability positions hydrogels as pivotal materials in
various scientific and technological applications [38-51] . Park et al. developed a hydrogel with a specific
[52]
response to nitric oxide . The hydrogel responds only to nitric oxide and then expands and has proved to
have great potential in the direction of drug delivery. Yao et al. developed a hydrogel coating, which has a
wide range of applications. The hydrogel coating can significantly reduce friction and performs well in
[53]
durability tests. Meanwhile, it has a stimulus-responsive structure .
Based on the classification of synthetic materials, hydrogels can be categorized into natural polymer
hydrogels, synthetic polymer hydrogels, and hybrid hydrogels. Natural polymers offer superior
compatibility, but their stability is poor and they are prone to degradation. Synthetic polymer hydrogels
have good stability, but most of the crosslinking agents used to synthesize polymer hydrogels are toxic to
humans. With the development of research, the hybrid hydrogel with good biocompatibility and stability
has been paid growing attention. The aggregated state of a hydrogel is a nuanced equilibrium between solid
and liquid phases, endowing it with dynamic and versatile behaviors. In its swollen state, a hydrogel retains
a specific shape, behaving akin to a solid material, yet simultaneously, it exhibits fluidic properties as solutes
[54]
can diffuse within its matrix, manifesting liquid-like characteristics . This dual nature of hydrogels makes
them inherently adaptable and well-suited for applications in fields ranging from biomedical engineering to
environmental science. The ability of hydrogels to maintain their structural integrity while absorbing and
retaining water renders them invaluable for applications where controlled water release, drug delivery, or
tissue engineering are essential [26,55-63] . Moreover, the unique dual-phase behavior of hydrogels provides a
platform for tailored designs in responsive materials, where environmental stimuli can trigger changes in
the structure and properties of hydrogels. Hydrogels with various functionalities possess significant promise
for advancing technologies in drug delivery systems, wound healing, and biosensing applications, among
others [64,65] .

What are MOF-hydrogel composites and the synthesis of MOFs-based hydrogel
Adding fillers to enhance the mechanical strength or performance (such as flexibility and conductivity) of
[66]
hydrogels is a common material design strategy . The mechanical strength and other performance are
difficult to balance. The addition of MOFs makes the mechanical strength and performance (such as
[10]
catalytic performance and heavy metal ions adsorption performance) of hydrogels even better . This
unique marriage of MOFs and hydrogels imparts enhanced mechanical strength to the resulting composites
compared to standalone hydrogels. The hierarchical structure and interconnected channels within MOF-
hydrogel composites not only accommodate the hydrogel matrix but also offer an extended surface area,
providing ample space for functionalization and the incorporation of active species.

In the synthetic process of MOF-hydrogel composites, two primary methods have gained prominence. The
first entails the direct integration of pre-synthesized MOFs into a hydrogel matrix, preserving the unique
properties of both components. The second involves the in-situ growth of MOFs within an already-formed
hydrogel, facilitating a more intimate integration of the two materials. These methods offer versatile
approaches to tailor the composition, structure, and properties of MOF-hydrogel composites, allowing for
the optimization of their performance in specific applications [Table 1].