3D-printable bio-sourced nanocomposite hydrogels based on two polysaccharide components: hydroxypropyl guar macromolecules and cellulose nanocrystals.

Hydrogels are soft and wet materials which require enhanced mechanical properties and toughness. For this aim, double-network hydrogels were prepared from soft network of covalently crosslinked hydroxypropyl guar and hard self-assembled network of carboxymethylated cellulose nanocrystals (CNCs) reversibly crosslinked by calcium ions. The gels exhibited a dramatic enhancement of mechanical strength and toughness with increasing content of CNCs and demonstrated remarkable fatigue resistance. For the first time, a 3D-visualization of CNC network within the double-network hydrogel was achieved using cryo-electron tomography. It revealed the presence of thick, fibrillar-like aggregates composed of bundles of two to five stacked nanocrystals, which are linked to adjacent bundles at their ends. These long aggregates can be responsible for significant enhancement of mechanical properties of the hydrogels by CNCs. Also, we first demonstrate the formation of hydrophobic cross-links between individual nanocrystals or bundles when end-to-end connections involve multiple arms. They may play an important role in effective energy dissipation providing higher toughness of the gels. Visualization suggests that added polymer induces microphase separation with the formation of polymer-rich and CNC-rich microdomains. The local concentrating of CNCs within CNC-rich domains should promote the aggregation of nanocrystals and their crosslinking by calcium ions, thereby strengthening the CNC network. The ability of the prepared hydrogels for 3D printing was evaluated for the first time. It was shown that the incorporation of a rigid network into a soft one provides greater shape fidelity for hydrogels.