Structure–property relationship in Fomes fomentarius fruiting bodies: mechanical properties across scales of a biological lightweight material

Hierarchical structuring is widespread in biological materials and leads to efficient designs combining high porosities with high strength and failure resistance. Wood and bone are prominent examples for this. Another promising blueprint for lightweight bioinspired applications is the fruiting body of Fomes fomentarius (F. fomentarius), a cellular, hierarchically structured, light, strong, and failure-resistant biological tissue. A main part of this doctoral these was the examination of the four segments of the F. fomentarius fruiting body (crust, trama, hymenium, and mycelial core) with special emphasize on the hierarchical structure of the hymenium. The comprehensive analysis included structural, chemical, and mechanical characterization with particular attention to cell wall composition, such as chitin/chitosan and glucan content and distribution of trace elements. The hymenium exhibited the best compressive mechanical properties even though having the highest porosity. Our results suggest that this outstanding strength is due to the high proportion of skeletal hyphae and the hiitughest chitin/chitosan content in the cell wall, next to its honeycomb structure. In addition, an increased calcium content was found in the hymenium and crust, and the presence of calcium oxalate crystals was confirmed by SEM/EDX. Interestingly, layers with different densities as well as layers of varying calcium and potassium depletion were found in the crust. To receive a better understanding of the mechanisms leading to failure in the segment of the hymenium further compression tests of wet and dry specimens were conducted ex situ and in situ using phase-contrast enhanced microcomputed tomography. Specimens are loaded parallel or transverse to the long axis of the tubes. The failure mechanisms of the hymenium under compression are evaluated between defined loading steps with three dimensional optical flow and morphological image analysis is used to correlate the micro- and mesostructure with local strains and structural damage. Parallel loading results in plastic buckling and delamination in dry samples, while their wet counterparts show telescopic shortening from the first loading step and fewer fatal cracks, probably due to greater hyphae elasticity. Key findings include that displacements are about five times larger in transverse than parallel loading, with greater displacements in regions of lower density. Higher hyphae density is found in struts compared to vertices, which leads to uniform displacements without significant local strain concentration. While cracks are observed in both struts and vertices, surprisingly, fracture is more catastrophic in the struts. Another main part of the PhD thesis dealt with the hierarchical structure of the hymenium and its contribution to stiffness at different length scales. An interdisciplinary approach was chosen by combining a multiscale finite element simulation with experimental methods to evaluate the stiffness at three length scales. At the micro-scale, high-resolution phase-contrast-enhanced micro-computed tomography was used to characterize the internal fiber architecture. The mesostructural features were investigated using laboratory-based micro-computed tomography. At the macro level, tensile tests, supported by digital image correlation, provided data on the mechanical properties. Nanoindentation measurements were carried out to validate the simulation results at the micro level. Similar to wood fibers and spider silk, the stiffness of the micrometer-sized building blocks, the hyphae, is in the GPa range, which enables the robustness of the macroscopic structure with high porosity at the same time. The mechanical properties of laboratory-grown F. fomentarius differ greatly from those of the natural product. Artificially cultivated fungal mycelium has some advantages, but its mechanical properties need to be improved. By combining the fungal mycelium mat with compostable biopolymers in a 3D printing process, the tensile strength and stiffness could be improved, which is a promising way to develop more durable and sustainable fungal mycelium products. This thesis shows important insights into the structure-property relationships of fungal materials and underline their potential as sustainable models in bioinspired materials engineering.