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and high catalytic efficiency, to covalently cross-link gel with recombinant human type

III collagen (rhCol III). This enzymatic approach facilitated the formation of ε-(γ-

glutamyl)lysine isopeptide bonds between protein molecules under mild reaction
conditions, thereby circumventing the cytotoxicity associated with chemical cross-

linking agents. Meanwhile, this approach preserved both the biocompatibility and 3D

printability, offering an efficient strategy for fabricating whole-protein hydrogel

scaffolds with excellent mechanical stability and cell-adhesive properties. Amin

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Shavandi et al. fabricated a biocompatible Gel-COS hydrogels through a dual cross-
linking strategy, including co-enzyme-mediated cross-linking via Gox/HRP and

phenolized polyelectrolyte complex (PHEC). The cross-linking was initiated by

spontaneous electrostatic interaction between positively charged CS and COS of

negatively charged alginate, followed by mild co-enzymatic cross-linking triggered by

H2O2 gradually released from the GOx reaction. This hydrogel provided a novel

therapeutic option for treating disabling diabetic foot and venous leg ulcers through its
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combined protective, healing, and antimicrobial properties. Lay Poh Tan et al.

developed a 3D printed GelMA hydrogel with adjustable flow properties, shear-
thinning behavior, and dual-stage cross-linking that maintained cell viability while

producing long-lasting structures for tissue engineering applications.

Figure 4. Schematic process of the biocatalytic mesoporous silica nanocomposite hydrogels.
Adapted from [54] .

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