Bioinspired Thermo-Responsive Xyloglucan-Cellulose Nanocrystal Hydrogels

Thermo-responsive hydrogels present unique properties, such as tunable mechanical performance or changes in volume, which make them attractive for applications including wound healing dressings, drug delivery vehicles, and implants, among others. This work reports the implementation of bio-based thermo-responsive hydrogels comprised of xyloglucan (XG) and cellulose nanocrystals (CNCs). Thermo-responsive properties were obtained by enzymatic degalactosylation of tamarind seed XG (DG-XG), which reduced the galactose residue content by ~50%, and imparted a reversible thermal transition. XG with comparable molar mass to DG-XG was achieved by ultrasonication treatment (XGu) for direct comparison of behavior. The hydrogels were prepared by simple mixing of DG-XG or XGu with CNCs in water. Phase diagrams were established to identify the ratios of DG-XG or XGu to CNCs (from 1:300 to 20:1 by mass) that yielded a viscous liquid, a phase separated mixture, a simple gel, or a thermo-responsive gel. Gelation occurred at a DG-XG or XGu to CNC ratio higher than that needed for the full surface coverage of CNCs, and required relatively high overall concentrations of both components (tested concentrations up to 20 g/L XG and 30 g/L CNCs). This is likely a result of the increase in effective hydrodynamic volume of CNCs due to the formation of XG-CNC complexes. Investigation of the adsorption behavior indicated that DG-XG formed a more rigid layer on CNCs compared to XGu. Rheological properties of the hydrogels were characterized and a reversible thermal transition was found for DG-XG/CNC gels at 35°C, where the mechanical properties of the gel could be tuned by adjusting the CNC content