The distribution of CNFs in wet hyd

The distribution of CNFs in wet hydrogels is random in the plane, as mentioned above. To exploit the full potential of cellu- lose nanopaper, a preferred orientation of CNFs was achieved via the cold drawing method [6,46,47]. A wet CNF hydrogel mem- brane with a certain size is stretched at a tensile rate by a tensile testing machine up to a certain drawing strain. The membrane in the stretched conformation is placed between paper carrier boards and dried as mentioned above. The cellulose nanopaper with highly oriented CNFs (Fig. 4(f)) has an anisotropic microstructure and the best mechanical properties along the orientation direction. More recently, in view of the natural fibril alignment and the inte- gration of crystalline cellulose chains in wood cell walls, Zhu et al. [92] directly fabricated an anisotropic cellulose nanopaper with highly aligned CNFs from wood via delignification and mechanical pressing. The fabrication process includes two steps. First, a wood slice is immersed in a sodium hypochlorite solution unit it turns white (Figs. 5(a) and 5(b)). Mechanical pressing is applied to densify the lignin-removed wood slice with significant reduction in thickness, which produces a well-aligned cellulose nanopaper (Figs. 5(c) and 5(d)).