Bamboo has a heterogeneous structur

Bamboo has a heterogeneous structure. As shown in Figure S1A (Supporting Information), raw bamboo primarily consists of highly rigid fiber bundles and lower density paren- chyma cells. Scanning electron microscopy (SEM) imaging shows how the fiber bundles are highly aligned and parallel to the growth direction whereas parenchyma cells are parallel or perpendicular to the growth direction (Figure 2A and Figure S1B: Supporting Information). The fiber bundles, paren- chyma cells, and vessels adhere to each other via a low strength polymer matrix composed of lignin and hemicellulose. There are obvious intrinsic defects in the natural bamboo structure, such as vessels along the growth direction, pit aperture on the longitudinal wall of fiber (Figure 2B), and porous parenchyma cells (Figure 2C). Furthermore, there also exist irregular small channels (lumens) in the center of the elementary bamboo fibers and clear intercellular space that can be observed between the adjacent fibers (Figure S1C, Supporting Information)