Structure-property relationships of bamboo: Surface barrier characteristics of the epidermis and radial mechanical gradients of the culm wall

The natural bamboo culm integrates an external protective boundary with an internal anatomical gradient, yet the distinct roles of these structural components are frequently conflated. Here, we systematically distinguish the surface barrier functions of the native epidermis from the radial energy-dissipation mechanisms across the culm wall. Microstructural and spectroscopic analyses reveal that the intact bamboo epidermis possesses a micro-nano rough hierarchy composed of hydrophobic cuticular wax and biogenic silica, providing a critical transverse moisture barrier that maintains dimensional stability. Removal of this thin protective skin exposes hydrophilic cellular channels, increasing surface wettability and accelerating water uptake. Concurrently, the mechanical capacity and damage tolerance of the culm wall are governed by the radial anatomical gradient from the outer layer (OL) to the inner layer (IL) rather than the epidermis itself. OL exhibits the highest relative crystallinity (32.2%), lowest microfibril angle (7.87°), and highest vascular bundle density, yielding superior tensile strength (146.5 MPa) and volumetric energy absorption (modulus of toughness: 2.9 and 7.1 MJ/m 3 under tensile and flexural loads, respectively). Energy dissipation across the culm wall proceeds through progressive fiber bridging, multi-path crack deflection, and shear delamination rather than catastrophic cleavage. These findings clarify the respective contributions of the epidermal barrier and radial tissue gradient, providing a theoretical foundation for selective surface processing and biomimetic composite design.