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Strong as a hedgehog

by ace
Strong as a hedgehog

Have you seen the peel of the Brazil nut?

Not the brown three-sided wrapper that surrounds each chestnut, shaped like a sfiha (or does someone help me with a better analogy?).

Much less the remains of dark film left over in the chestnuts, a food so Brazilian and, at the same time, unfamiliar in its original appearance.

Does anyone know the hedgehog, the spherical fruit of the chestnut tree, which houses one or two dozen seeds (chestnuts), does not break in the fall of trees that can reach 50 meters in height and is usually broken by the teeth of a single animal , the cotia?

Marília Sonego did not know him. Until an uncle brought a copy home from Porto Velho, and, with the researcher's father, embarked on a saga to saw it in two parts with a view to producing an ashtray. “I was in the master's degree, looking for a topic for my doctoral research, already interested in biological materials. I was intrigued by all that difficulty to open the hedgehog ”, says today almost a doctor by the Postgraduate Program in Materials Science and Engineering at the Federal University of São Carlos (UFSCar), with the defense of the thesis scheduled for next March .

Four years later, Marília Sonego has a great chance of being the greatest connoisseur of the chestnut fruit mesocarp in the world. His doctorate first sought to characterize the layer responsible for the hedgehog's remarkable mechanical resistance – that mesocarp, located between a more external layer that rots on ripening and a very thin endocarp to support anything. Generally, this is a work carried out by specialists in the field of botany, but, as she did not find the description in the existing literature, the materials engineer rolled up her own sleeves.

In addition, the work sought strategies to use the structures found in new materials, in a process known as bioinspiration or biomimicry. “In nature, materials are subject to the same laws and face the same problems as us in the laboratory, in industry, in architecture… They face, for example, gravity, friction, degradation by sunlight… The difference is that nature had billions of years to find the solutions, through trial and error, and understanding the strategies it developed can help a lot ”, says the researcher.

In the effort to characterize the hedgehog, Sonego used microscopy and tomography equipment, as well as experiments to verify the chemical composition and mechanical tests to measure the performance of the mesocarp under compression and traction and other properties related to the toughness of the material. In compression, the hedgehog of the chestnut from Pará proved to be more difficult to break than the shells of all the other nuts studied, including macadamia, second place; almonds, hazelnuts and walnuts.

Image examinations and chemical analyzes revealed details of the hedgehog's structure at different levels: from macroscopic to molecular, passing through the cell (microscopic) and the so-called fibrillar level (nanoscopic). It was at the cellular level that Sonego found the central inspiration for the material proposed at the end of the research. “All scales have their strategies, which are connected, and it is this hierarchical organization that explains how relatively weak components can result in a system with exceptional properties. But this is a complexity that is difficult to reproduce artificially, and I had to make choices ”, reveals the researcher.

The main results found can be summarized in two characteristics. One is the combination of two types of cells present, the fibers, elongated, and the sclereids, spherical, both hollow and with thick cell walls. The other is the positioning of the fibers in three layers with different orientations, such as a sandwich with two layers vertically and a central layer horizontally.

The combination of fibers and sclereids can be compared to a lattice (formed by fibers) with spaces filled with foam (the hollow sclereids). This is a strategy that allows the presence of more resistant material where it is necessary to support a greater load, with the rest filled by less dense elements, which reduces the final weight of the structure.

In addition, this organization suggests a mechanism that hinders the propagation of cracks similar to what we see in exposed brick walls. In them, the positioning of the bricks in displaced rows causes the crack to tend to deviate from the bricks, which require more energy to be broken. Thus, the crack travels a longer way, which slows down the fracture. In the hedgehog, the crack would avoid breaking the cell wall of fibers and sclereids, propagating through the interfaces between them.

The positioning of the fibers in different orientations results in an opposite effect to that observed in a banana peel. In the banana, the fibers are positioned in one direction, from one end to the other (longitudinal), which makes it difficult to break around the fruit (latitudinal), but allows us to peel it easily, pulling the peel towards the fibers. In the hedgehog, as there are fibers in all directions, there is resistance in all of them.

Considering these characteristics, the researcher proposed a material organized in several layers of fibers of a polymer (PLA) reinforced by carbon fiber. These layers were produced by 3D printing, to reach the different orientations of the fibers, vertical and horizontal. The spaces between the fibers were filled by a foam with hollow glass spheres imitating the sclereids, and the whole set was connected with the use of a resin (epoxy).

The resulting material was also subjected to tests to verify its mechanical performance, which showed some good results and evidenced necessary improvements, such as changes in the quantity and size of the balls and a reduction in the diversity of applied materials.

“The characterization stage was long. For the first year, for example, I was studying biology! I only got to the composite proposition stage in the last year, and my expectations were low due to the hedgehog's high complexity. But we got some good results, and now I know what the next steps are. I only used commercial materials, for example, and one possibility is to develop these materials here at the university ”, records the researcher.

Sonego's research was carried out in partnership with Luiz Antonio Pessan, his doctoral advisor, professor in the Materials Engineering Department at UFSCar, and with Claudia Fleck, researcher at Technische Universität Berlin, in Germany, where the Brazilian carried out some of the experiments. The study received funding from the São Paulo State Research Support Foundation (Fapesp).

The different stages also involved other collaborations, in Brazil and Germany. Part of the results were already published in July last year in the specialized magazine Bioinspiration & Biomimetics, and a second article is accepted and should be published soon in Scientific Reports, of the Nature group. The work was also presented at congresses in Germany, Australia and Canada, in addition to Brazil.

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