Hydrogen bond accessibility engineering enables water-rich and mechanically robust chitosan hydrogels

Natural polysaccharide hydrogels are attractive for biomedical applications due to their inherent biocompatibility and high water content. However, their mechanical performance is often limited by inefficient hydrogen bond utilization arising from restricted accessibility of bonding sites and competitive occupation by water molecules. Here, we propose a hydrogen bond accessibility engineering strategy to reconstruct the hydrogen bond network in chitosan-based hydrogels without altering their chemical composition. Surface activation of chitin nanofibers exposes additional hydrogen bond sites, while sequential solvent exchange and controlled freezing treatments reduce water occupation and promote solid-solid interactions, thereby enabling network densification. This strategy enhances interfacial coupling between nanofibers and the chitosan matrix, resulting in a highly interconnected load-bearing network. Consequently, compared with the primordial Cs-gel, the optimized hydrogel achieved an elastic modulus of 1880.4 kPa (25.7-fold higher), a 5.74-fold increase in fracture toughness, and a reduction in tensile hysteresis loss after 50 loading-unloading cycles from 40.6% to 25.9%, while maintaining a water content above 87%. Furthermore, the hydrogel was processed into diverse functional formats, including implantable patches, surgical sutures, and anisotropic 3D-printed constructs. This work establishes hydrogen bond accessibility as a key design principle for engineering mechanically robust, water-rich hydrogels.