Typical images of the natural and d

Typical images of the natural and delignified bamboo fiber are shown in Figure S7 (Supporting Information). The natural bamboo fiber exhibited a rough and coarse surface due to the heavily packed with noncellulosic components. The treated bamboo fiber consists of highly crystalline cellulose nanofibrils,oriented with a tilt angle about 10along the longitudinal fiberaxis, which is mainly responsible for the tensile properties. As we processed the natural bamboo into densified bamboo, the volume fraction of elementary fibers increased with the full compression of sclerenchyma and parenchymal cells. The densely packed fibers in the densified bamboo also lead to a high degree of alignment of neighboring cellulose nanofibrils and thus drastically increase the interfacial area among them. The removal of hemicellulose and lignin also releases the initial strain in between the cellulose chains that are normally separated by the hemicellulose/lignin matrix. In other word, the closer packing made possible by the removal of the hemicellu- lose/lignin matrix allows the formation of new hydrogen bonds. The highly aligned state of the fibers amplifies their effects due to collective synergy of molecular interlocking, leading to stiffening and effective energy dissipating mechanisms.[13,32] Consequently, the total energy needed to fracture the densified bamboo is much higher than that needed to fracture natural bamboo. As a result, the densified bamboo (8.46 MJ m3) is much tougher than natural bamboo (2.31 MJ m3), corre- sponding to a nearly 4-times increase (Figure S8, Supporting Information).