Hierarchical levels of organization of the Brazil nut mesocarp

Abstract

Aiming to understand Nature´s strategies that inspire new composite materials, the hierarchical levels of organization of the Brazil nut (Bertholletia excelsa) mesocarp were investigated. Optical microscopy, scanning electron microscopy (SEM), microtomography (MicroCT) and small-angle X-ray scattering (SAXS) were used to deeply describe the cellular and fibrillary levels of organization. The mesocarp is the middle layer of the fruit which has developed several strategies to avoid its opening and protect its seed. Fibers have a different orientation in the three layers of the mesocarp, what reduces the anisotropy of the structure. Sclereids cells with thick cell walls fill the spaces between the fibers resembling a foam-filled structural composite. The mesocarp has several tubular channels and fractured surfaces which may work as sites for crack trapping and increase toughness. The thick and lignified cell wall of sclereids and fibers and the weak interface between cells can promote a longer and tortuous intercellular crack path. Additionally, fibers with high strength and stiffness due to microfibrils oriented along the main cell axis (µ = 0° to 17°) were identified in the innermost layer of the mesocarp. Such an understanding of each hierarchical level can inspire the development of new cellular composites with improved mechanical behaviorThe mesocarp layer of Bertholletia excelsa fruit, the seed of which is known as brazil nut, is an impressive structure. It is a shell capable to resist falls as high as 50 m and compression forces higher than 10 kN 1 . Such a strong natural structure has great potential as a source for bioinspiration to produce new high performance composites 1,2 . The impressive properties of the mesocarp arise from its hierarchical structure, which emerged from millions of years of evolution.This hierarchy is the main factor to explain how relatively weak components organized in a complex way can result in a system with outstanding properties 3 . Four hierarchical levels of structuring have been described for plants: macroscopic, cellular, fibrillar, and molecular 3 . Each hierarchical level has its own contribution to the overall properties of the natural composite.On the largest length-scale, the "macroscopic" level, trunk, leaves, roots, flowers, and fruit are distinguished, and it is analyzed how the association of different tissues leads to different geometries and functions in the plant. At this level, the mesocarp is described as a spherical or elliptical shell of 10-12 cm diameter with a wall thickness of approximately 1 cm. Its function is to protect the seeds against predators and impact on the ground when the fruit falls from the tree 1,4,5 . The mesocarp has a rough surface with a peduncle and an opercular opening on opposite sides of the fruit. The opening is a hole with a diameter of approximately 2 cm, which is, however, not large enough to allow the dispersion of the seeds, as it was millions of years ago.On the cellular hierarchical level, the cells and vegetable tissues are ...

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“…The mesocarp is a hard, impermeable, and fibrous structure composed of sclereids and fibers (another sclerenchyma tissue) in a composite-like arrangement, and a vascular system (Sonego et al, 2019). Sclereid cells are ∼30 μm in diameter, have a thick lignified cell wall and with an isometric shape, while fibers are ∼20 μm in diameter, are elongated, and organized in bundles (Sonego et al, 2020). Sonego et al (2019) applied nanoindentation to the cross and longitudinal sections of the fiber and sclereid cells in the mesocarp.…”

Pit pairs in ginkgo seed shell guide the crack

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“…Schmier et al [9] have investigated six different levels (H1-H6) of the shells, scaling from the entire spheroidal shell (H1), the tissue level with bounded sclereids and vascular bundles (H2), individual sclereids (H3), thick cell walls (H4), more than 30 individual layers (H5) and the molecular levels of the cellulose (H6). Some of these shells (such as pine, Macadamia, and Brazil nuts) break along the shaped cells leading to the deflection of crack path [4,7,8]. Walnut and pistachio have three-dimensional puzzle sclereid cells extending cracks into cells, and show higher ultimate tensile strength with stronger topological interlocking [3][4][5].…”

“…The above-mentioned research only analyzed the relationship between the microstructure and properties of nut shells on a single level (such as cell level, fiber level, etc.). However, one natural nut shell is a complex collection of multi-level materials, and besides the cell and fiber level, there are pits [8]. These multi-level assemblies may play a significant role in strengthening and toughening nutshells.…”