Biomechanics of tendrils and adhesive pads of the climbing passion flower Passiflora discophora.

The climbing passion flower Passiflora discophora features branched tendrils with multiple adhesive pads at their tips allowing it to attach to large-diameter supports and to flat surfaces. We conducted tensile tests to quantify the performance of this attachment system. We found that the force at failure varies with substrate, ontogenetic state (turgescent or senescent), and tendril size (i.e. tendril cross-sectional area and pad area). The tendrils proved to be well balanced in size and to attach firmly to a variety of substrates (force at failure up to 2N). Pull-off tests performed with tendrils grown on either epoxy, plywood, or beech bark revealed that senescent tendrils could still bear 24, 64, or 100% of the force measured for turgescent tendrils, respectively, thus providing long-lasting attachment at minimal physiological costs. The tendril main axis was typically the weakest part of the adhesive system, whereas the pad-substrate interface never failed. This suggests that the plants use the slight oversizing of adhesive pads as a strategy to cope with 'unpredictable' substrates. The pads, together with the spring-like main axis, which can, as shown, dissipate a large amount of energy when straightened, thus constitute a fail-safe attachment system.

Hierarchical Structure of the Cocos nucifera (Coconut) Endocarp: Functional Morphology and its Influence on Fracture Toughness.

In recent years, the biomimetic potential of lignified or partially lignified fruit pericarps has moved into focus. For the transfer of functional principles into biomimetic applications, a profound understanding of the structural composition of the role models is important. The aim of this study was to qualitatively analyze and visualize the functional morphology of the coconut endocarp on several hierarchical levels, and to use these findings for a more precise evaluation of the toughening mechanisms in the endocarp. Eight hierarchical levels of the ripe coconut fruit were identified using different imaging techniques, including light and scanning electron microscopy as well as micro-computer-tomography. These range from the organ level of the fruit (H0) to the molecular composition (H7) of the endocarp components. A special focus was laid on the hierarchical levels of the endocarp (H3-H6). This investigation confirmed that all hierarchical levels influence the crack development in different ways and thus contribute to the pronounced fracture toughness of the coconut endocarp. By providing relevant morphological parameters at each hierarchical level with the associated toughening mechanisms, this lays the basis for transferring those properties into biomimetic technical applications.

Strength-size relationships in two porous biological materials.

According to the Weibull theory for brittle materials, the mean experimental strength decreases with test specimen size. For the brittle parts of an organism this would mean that becoming larger in size results automatically in reducing strength. This unfavorable relationship was investigated for two porous, biological materials that are promising concept generators for crack deflective and energy dissipative applications in compressive overloading: the quasi-brittle coconut endocarp and the brittle spines of the sea urchin Heterocentrotus mamillatus. Segments in different volumes were prepared and tested in uniaxial compression experiments. Failure of both materials is Weibull distributed underlining that it is caused by statistically distributed flaws in the structure. However, the coconut endocarp has a much higher Weibull modulus (m = 14.1-16.5) than the spines (m = 5). The more predictable failure of the endocarp is probably attributed to a rather homogeneous microstructural design and water bound in the structure. In terms of the spines it was found that the Weibull modulus is structure dependent: More homogeneous spines feature a higher Weibull modulus than spines with a heterogeneous structure. Whereas the nearly dense endocarp exhibited, although less pronounced, the expected decrease in strength with increase in size, the spines showed a failure independently of size. This remarkable behavior may be explained with their highly porous internal structure. Small and large spines consist of struts of similar size, which constitute the porous internal structure, potentially limiting the flaw size to the size of the strut regardless of the spine size. STATEMENT OF SIGNIFICANCE: Scaling is an important aspect of the biomimetic work process, since biological role models and structures have rarely the same size as their technical implementations. The algorithms of Weibull are a standard tool in material sciences to describe scaling effects in materials whose critical strength depends on statistically distributed flaws. The challenge is to apply this theory (developed for homogeneous, isotropic technical materials) to brittle and quasi-brittle biological materials with hierarchical structuring. This study is a first approach to verify whether the Weibull theory can be applied to the coconut endocarp and to sea urchin spines in order to model their size/volume/property-relations.