RESUMO
In the present work a comprehensive characterization of the hierarchical architecture of the walnut shell (Juglans regia L.) was carried out using scanning electron microscopy (SEM), atomic force microscopy (AFM) and confocal laser scanning microscopy (CLSM). Furthermore, micromechanical properties (hardness, HIT and elastic modulus, EIT) of plant tissues were evaluated at cell wall level by applying the instrumented indentation technique (IIT). The complex architecture of the material was described in terms of four hierarchical levels (HL): endocarp (H1), plant tissues (H2), plant cells (H3) and cell wall (H4). Our findings revealed that the walnut shell consists of a multilayer structure (sclerenchyma tissue, ST; interface tissue, IT; porous tissue, PT; and flattened parenchyma tissue, FPT), where differences in the microstructure and composition of plant tissues generate parallel gradients along the cross-section. The indentation tests showed a functional gradient with a sandwich-like configuration, i.e., a lightweight and soft layer (PT, HIT = 0.04 GPa) is located between two dense and hard layers (ST, HIT = 0.33 GPa; FPT, HIT = 0.28 GPa); where additionally there is an interface between ST and PT (IT, HIT = 0.16 GPa). This configuration is a successful strategy designed by nature to improve the protection of the kernel by increasing the strength of the shell. Therefore, the walnut shell can be considered as a functionally graded material (FGM), which can be used as bioinspiration for the design of new functional synthetic materials. In addition, we proposed some structure-property-function relationships in the whole walnut shell and in each of the plant tissues.
Assuntos
Juglans , Parede Celular , Juglans/químicaRESUMO
The objective of this study was to evaluate the effect of the presence of calcium oxalate (CaOx) crystals on the micromechanical properties of sclerenchyma tissue from the pecan nutshell (Carya illinoinensis). The microstructure of the cross-section nutshell was examined using light microscopy (LM) and atomic force microscopy (AFM). Using an instrumented indentation system, indentation tests with maximum loads of 500 mN were made on the biological material where the variables studied were the number of crystals present in the evaluated area and the size of individual crystals. Microscopic analysis revealed that the pecan nutshell consists of sclerenchyma tissue with multiple CaOx crystals randomly distributed throughout the material, exhibiting prismatic shapes and various sizes. The results of the indentation tests showed that the examined areas where there were crystals (1, 2 or up to 3) presented values of hardness and elastic modulus significantly higher (P < 0.05) compared to the sclerenchyma (without crystals). Likewise, there were no significant differences (P > 0.05) between the values of the micromechanical properties of the areas evaluated as a function of the number of crystals. On the other hand, it was observed that the size of the crystals did not show a direct correlation with the mechanical properties evaluated as expected. In conclusion, the biomineralization phenomenon is a successful strategy designed by nature to improve the rigidity of the pecan nutshell, where the CaOx crystals strengthen the structure by increasing the micromechanical properties.
Assuntos
Carya , Oxalato de CálcioRESUMO
In this work, the porosity of the layers of calcified chicken eggshell (vertical crystal layer VCL, palisade layer PL and mammillary layer ML) was evaluated using atomic force microscopy (AFM) and image processing (IP). AFM topographic images were obtained from different locations for each layer and along the cross-section of calcified eggshell. Roughness parameters, surface area values, pore size and shape, surface porosity, area occupied by pores and pore density were obtained from AFM and IP. It was observed that the thickest layer (PL) exhibited the highest degree of porosity (surface porosity = 2.75 ± 1.68%, pore density = 162 ± 60 pores/µm2) when compared to the other two layers. In general, the pores located in all layers ("bubble pores") had circular shape and similar sizes. Measurements revealed a porosity gradient along the cross-section which varied with position, i.e., increasing surface porosity from the VCL towards the region of the PL closer to the ML, and decreasing surface porosity from this location towards the ML innermost surface. This suggests that the calcified eggshell has a sandwich-like structure where porosity may influence gas exchange and mechanical properties. The combination of AFM with IP presented here provides a simple and precise method to study porosity in calcified chicken eggshell, and this methodology could be used to examine other types of porous biological materials.
RESUMO
The morphology and micromechanical properties of the mineral crystals embedded in the pecan nutshell (Carya illinoinensis) were characterized. Qualitative and quantitative morphological analyses carried out revealed that the crystals were comprised of calcium oxalate (CaOx) and have a wide range of sizes, with prismatic shapes, distributed heterogeneously in the sclerenchyma tissue. From indentation tests, it was found that CaOx crystals are stiffer structures compared to stone cells (sclerenchyma tissue), showing hardness and elastic modulus values of 0.53⯱â¯0.19â¯GPa and 9.4⯱â¯1.80â¯GPa, respectively. Additionally, the values of fracture toughness (0.08⯱â¯0.02â¯MPaâ¯m0.5) and the brittleness index (9336â¯m-0.5) revealed that these types of structures are extremely brittle. The results obtained suggest that the main function of the CaOx crystals is to provide structural support to tissue. The presented methodology demonstrates the potential of the instrumented indentation technique (IIT) for in situ micromechanical characterization of mineral crystals located in plant tissues.
