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Plant-derived Nanocellulose as Structural and Mechanical Reinforcement of Freeze-Cast Chitosan Scaffolds for Biomedical Applications.

Despite considerable recent interest in micro- and nanofibrillated cellulose as constituents of lightweight structures and scaffolds for applications that range from thermal insulation to filtration, few systematic studies have been reported to date on structure-property-processing correlations in freeze-cast chitosan-nanocellulose composite scaffolds, in general, and their application in tissue regeneration, in particular. Reported in this study are the effects of the addition of plant-derived nanocellulose fibrils (CNF), crystals (CNC) or a blend of the two (CNB) to the biopolymer chitosan on structure and properties of the resulting composites. Chitosan-nanocellulose composite scaffolds were freeze cast at 10°C/min and 1°C/min, and their microstructures were quantified in both the dry and fully-hydrated states using scanning electron and confocal microscopy, respectively. The modulus, yield strength, toughness (work to 60% strain) were determined in compression parallel and the modulus also perpendicular to the freezing direction to quantify anisotropy. Observed were the preferential alignment of nanocellulose crystals and/or fibrils parallel to the freezing direction. Additionally, observed was the self-assembly of the nanocellulose into micro-struts and micro-bridges between adjacent cell walls (lamellae), features that affected the mechanical properties of the scaffolds. When freeze cast at 1°C/min, chitosan-nanocellulose fibril scaffolds had the highest modulus, yield strength, toughness, and smallest anisotropy ratio, followed by chitosan and the composites made with the nanocellulose blend, and that with crystalline cellulose. These results illustrate that the nanocellulose additions homogenize the mechanical properties of the scaffold through cell-wall material self-assembly, on the one hand, and add architectural features such as bridges and pillars, on the other. The latter transfer loads and enable the scaffolds to resist deformation also perpendicular to the freezing direction. The observed property profile and the materials' proven biocompatibility highlight the promise of chitosan-nanocellulose composites for a large range of applications, including those for biomedical implants and devices.