Recent developments in sustainable materials have highlighted the potential of protein-based hydrogels and nanocomposites reinforced with cellulose nanocrystals (CNCs), for advanced applications requiring mechanical robustness and biocompatibility. CNCs, derived from plant biomass, are renewable and feature high aspect ratios, large surface areas, and excellent mechanical strength (modulus of 50-200 GPa). These attributes, combined with their amenability to surface modification, make CNCs ideal nanofillers for reinforcing biopolymer matrices. In this work, DOPA (3,4-dihydroxyphenylalanine) surface-functionalized CNCs were incorporated into protein-based matrices, including soy protein isolate (SPI) and silk-amyloid-mussel foot (SAM) protein, to fabricate mechanically enhanced hydrogels and nanocomposites. The strong interactions between modified CNCs and the protein matrix led to significant improvements in tensile strength, modulus, and adhesion. Notably, DOPA-coated CNC-reinforced hydrogels exhibited ultimate stress and strain values of 4881 ± 859 kPa and 770 ± 31.7%, respectively, with up to 4.7-fold and 2-fold improvements over unmodified systems. Additionally, adhesion to biological tissues improved markedly, with shear adhesive stress reaching 880 ± 247 kPa on porcine skin and 1216 ± 297 kPa on bovine bone. Micromechanical modeling further elucidated the reinforcing mechanisms enabled by CNC surface functionalization, offering insight into the tunable nature of protein-CNC interactions. These findings not only underscore the promise of CNCs as reinforcing agents in biopolymer networks but also point toward their application in fields such as bio-adhesives, food packaging, and tissue interfacing materials.
