Design across hierarchy: morphological and mechanical insights into pili nuts (Canarium ovatum)

• Pili nut endocarp: strong, impact- and puncture-resistant with complex geometry. • Tissue contains only one central vascular bundle: less porous than other nuts. • Fibrous layers and dense sandwich-structure mimic fiber-reinforced composites. • E ∼ macadamia nuts; compressive strength > tensile strength; no fiber-pullout. • Brittle base material → layered in sandwich with weak interfaces. In recent years, the biomimetic potential of hard nutshells has gained increasing attention. For transferring functional principles into bio-inspired applications, a detailed understanding of the structural composition of these natural models is essential. Here, we report our investigations into the hierarchical functional morphology of the pili nut ( Canarium ovatum ) and link it to its mechanical properties across multiple length scales. We distinguish eight hierarchical levels (H) for the entire fruit, comprising structural features at the organ (H0–H2), tissue (H3), cellular (H4), sub-cellular (H5–H6), and molecular levels (H7). Focusing on the endocarp (H3-H6), we characterize the structure by combining 2D and 3D low and high resolution imaging techniques. On the microscale, the endocarp fails in a brittle manner, with cracks propagating straight through the sclerenchymatous cells. Interestingly, we do not observe fiber pull-out and only minor deflections around the fibers. The brittle base material is organized in a sandwich structure a with weak interface. Our detailed morphological parameters at multiple hierarchical levels and our evaluation of their influence on fracture behavior provide a sound basis for understanding the correlation between microstructure and mechanical behavior of the pili nut endocarp, paving the way for future biomimetic designs of robust materials.