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He et al. Soft Sci 2024;4:37 https://dx.doi.org/10.20517/ss.2024.32 Page 7 of 27

Figure 3. The illustrations of the fabrication procedure of MOFs-based hydrogel by direct mixture and in-situ growth. MOFs: Metal-
organic frameworks.

hydrogel’s liquid precursor or solution . During the subsequent gelation process, the hydrogel matrix
[11]
encapsulates and enwraps the pre-existing MOF crystals, resulting in the creation of MOF-hydrogel
composites. This approach guarantees a uniform distribution of MOFs throughout the hydrogel matrix,
offering control over the dispersion and interaction of MOF particles .
[35]
An alternate strategy in this synthesis approach involves the addition of MOF crystals to the hydrogel
system after the introduction of a gelation trigger and the formation of the pre-polymerized sol. In this
dynamic process, MOF crystals are incorporated into the evolving hydrogel network post-gelation
initiation . The stepwise addition of MOF crystals enables temporally controlled integration, affecting the
[12]
structural and mechanical properties of the resulting MOF-hydrogel composites. This approach facilitates
precise adjustments, improving the flexibility and functionality of the composite materials for targeted
applications.

In situ growth of MOF on hydrogel
The in-situ growth method has become a novel and dynamic technique for preparing MOF-hydrogel
composites, gaining attention in recent years due to its distinctive advantages. Distinguishing itself from
traditional methodologies, this technique utilizes chemical or physical methods to connect, polymerize, and
deposit materials, capitalizing on the synergistic advantages of combining two distinct matrices . This
[11]
approach, commonly used in the synthesis of composite materials, utilizes the intrinsic properties of the
matrix material along with external conditions such as high temperature and pressure to create growth
centers on the matrix. In the case of MOF-hydrogel composites, the in-situ growth method harnesses nano
effects or external conditions to trigger the growth of MOFs within the pores of the hydrogels. The MOF
precursors are integrated into the hydrogel matrix, supplying the essential components for in situ growth.