Transparent and mechanically robust fish gelatin hydrogels through covalent–noncovalent interfacial regulation by dialdehyde carboxymethyl chitin nanofibers

Polysaccharide reinforcement enhances the mechanical strength of fish gelatin (FG), narrowing its performance gap with mammalian gelatins (bovine and porcine gelatins). However, the improved mechanical strength is often accompanied by reduced optical transparency due to interfacial incompatibility-induced aggregation, phase separation, and light scattering. Herein, a covalent–noncovalent interfacial regulation strategy based on dialdehyde carboxymethyl chitin nanofibers (dCMCh-NF) was developed to optimize protein–polysaccharide interactions in FG hydrogels. At a low loading of 0.1 wt%, dCMCh-NF effectively reinforced the FG network. The FG/dCMCh8-NF hydrogel exhibited a hardness of 695.52 ± 4.51 g, significantly higher than that of pure FG (497.30 ± 19.19 g) and comparable to that of mammalian gelatin. Meanwhile, its light transmittance reached 85.70 ± 1.32%, markedly exceeding that of FG/CMCh-NF (71.37 ± 0.47%). Structural characterization and molecular dynamics simulations revealed that aldehyde groups on dCMCh-NF formed Schiff-base linkages with FG as the primary interaction, while hydrogen bonding and electrostatic interactions served as secondary stabilizing forces. This dual crosslinking suppressed local aggregation, reduced phase separation, and facilitated the formation of a homogeneous and compact network, thereby minimizing light scattering. This work establishes covalent–noncovalent interfacial regulation as an effective strategy for overcoming the inherent transparency-strength trade-off in polysaccharide-reinforced FG hydrogels.