Design and fabrication strategies of cellulose nanocrystal-based hydrogel and its highlighted application using 3D printing: A review

To eliminate the potential toxicity and biological incompatibility from hydrogels prepared using synthetic polymers, researchers have paid tremendous efforts to design hydrogels using nature-obtainable biopolymers due to their outstanding biocompatibility, low cytotoxicity, and no secondary hazards. Among the biopolymers, cellulose nanocrystals (CNCs) have attracted ever-increasing interest from both academic and industrial sides because of their whisker nanostructure, high axial stiffness, high tensile strength, and abundant hydroxyl groups on the surface. CNCs can provide the three-dimensional (3D) hydrogels with enhanced mechanical properties and designed functions and, therefore, offering CNC-based composite hydrogel wide applications in the fields such as biomedical, tissue engineering, actuator, etc. In this review, we begin with the design rationales of the "CNC-only" hydrogel and CNC-based hydrogels, to illustrate the interactions between CNCs themselves or with the surrounding hydrogel backbones. Then, as a fashionable method, the extrusion-based 3D printing technique for fabricating and shaping CNC-based composite hydrogels was elaborately introduced, followed by a brief review of 3D printed CNC-based hydrogels in different fields. Finally, limitations and future directions of CNC-based hydrogels were discussed. We aim to provide a deeper understanding of CNC-based composite hydrogels in the aspects of rational design, fabrication strategy and highlighted applications in 3D printing.