Natural materials often exhibit excellent mechanical properties. An example of outstanding impact resistance is the pummelo fruit (Citrus maxima) which can drop from heights of 10 m and more without showing significant outer damage. Our data suggest that this impact resistance is due to the hierarchical organization of the fruit peel, called pericarp. The project presented in the current paper aims at transferring structural features from the pummelo pericarp to engineering materials, in our case metal foams, produced by the investment casting process. The transfer necessitates a detailed structural and mechanical analysis of the biological model on the one hand, and the identification and development of adequate materials and processes on the other hand. Based on this analysis, engineering composite foam structures are developed and processed which show enhanced damping and impact properties. The modified investment casting process and the model alloy Bi57Sn43 proved to be excellent candidates to make these bio‐inspired structures. Mechanical testing of both the natural and the engineering structures has to consider the necessity to evaluate the impact of the different hierarchical features. Therefore, specimens of largely varying sizes have to be tested and size effects cannot be ignored, especially as the engineering structures might be upscaled in comparison with the natural role model. All in all, the present results are very promising: the basis for a transfer of bio‐inspired structural hierarchical levels has been set.

The mechanical properties of artificial foams are mainly determined by the choice of bulk materials and relative density. In natural foams, in contrast, variation to optimize properties is achieved by structural optimization rather than by conscious substitution of bulk materials. Pomelos (Citrus maxima) have a thick foam-like peel which is capable of dissipating considerable amounts of kinetic energy and thus this fruit represents an ideal role model for the development of biomimetic impact damping structures. This paper focuses on the analysis of the biomechanics of the pomelo peel and on its structure-function relationship. It deals with the determination of the onset strain of densification of this foam-like tissue and on how this property is influenced by the arrangement of vascular bundles. It was found here that the vascular bundles branch in a very regular manner-every 16.5% of the radial peel thickness-and that the surrounding peel tissue (pericarp) attains its exceptional thickness mainly by the expansion of existing interconnected cells causing an increasing volume of the intercellular space, rather than by cell division. These findings lead to the discussion of the pomelo peel as an inspiration for fibre-reinforced cast metallic foams with the capacity for excellent energy dissipation.

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Fruit walls as well as nut and seed shells typically perform a multitude of functions. One of the biologically most important functions consists in the direct or indirect protection of the seeds from mechanical damage or other negative environmental influences. This qualifies such biological structures as role models for the development of new materials and components that protect commodities and/or persons from damage caused for example by impacts due to rough handling or crashes. We were able to show how the mechanical properties of metal foam based components can be improved by altering their structure on various hierarchical levels inspired by features and principles important for the impact and/or puncture resistance of the biological role models, rather than by tuning the properties of the bulk material. For this various investigation methods have been established which combine mechanical testing with different imaging methods, as well as with in situ and ex situ mechanical testing methods. Different structural hierarchies especially important for the mechanical deformation and failure behaviour of the biological role models, pomelo fruit (Citrus maxima) and Macadamia integrifolia, were identified. They were abstracted and transferred into corresponding structural principles and thus hierarchically structured bio-inspired metal foams have been designed. A production route for metal based bio-inspired structures by investment casting was successfully established. This allows the production of complex and reliable structures, by implementing and combining different hierarchical structural elements found in the biological concept generators, such as strut design and integration of fibres, as well as by minimising casting defects. To evaluate the structural effects, similar investigation methods and mechanical tests were applied to both the biological role models and the metallic foams. As a result an even deeper quantitative understanding of the form-structure-function relationship of the biological concept generators as well as the bio-inspired metal foams was achieved, on deeper hierarchical levels and overarching different levels.

The largest existing biological interface, the surface of living plants, as it stands is capable of converting mechanical energy into electricity based on a combination of contact electrification and electrostatic induction on the plant surface and its inner tissue. Herein, the first design strategies are reported for living plant‐based wind energy harvesting systems that use this effect and that are capable of harvesting simultaneously from multiple leaves of a single plant to upscale the energy output. This is the first study under outdoor‐relevant conditions in a controlled test environment that relates plant‐based energy conversion to wind speed and wind direction as well as parameters such as the environmental humidity. Increasing the wind speed not only leads to higher power but also low winds of 2 m s−1 and less can be converted into storable electricity. The plant‐hybrid generators are moreover capable of converting wind from multiple directions by exploiting the naturally multiplex leaf orientations and the plants can directly power light‐emitting diodes (LEDs) and a digital thermometer. The results draw attention to the opportunity to obtain living plant‐hybrid generators, e.g., for applications in constituting environmental sensor networks.