Advances in Science and Technology Vol. 58

Abstract: Silks spun by arthropods exhibit a set of unique properties that have emerged as the result of over four hundred million years of evolution. Silks show the most optimized combination of tensile strength and deformation at breaking, yielding the highest work of fracture of any known material. These outstanding features have thrust an increasing interest in reproducing or even improving the properties of natural silks. However, the advances in the field are hampered by an incomplete knowledge on the relation between microstructure and mechanical properties as well as by uncertainties related to the influence of processing in the performance of the fiber. In this work we present some of the most significant contributions of our groups to the field, stressing the possibility of controlling the tensile properties of silks and the contribution of this basic knowledge to the production of artificial regenerated fibers. Spider silk shows a large variability that it is thought to allow the spider to adapt the fibers to its immediate requirements, but represents a major drawback for its study or application. The development of the wet stretching process has allowed the modification of silk fibers in a controlled and reproducible way for the first time. Besides, recent improvements in the spinning of regenerated silkworm silk fibers have led to artificial fibers with properties that approach those of natural silks. These progresses allow envisaging the production of bioinspired fibers in a not too distant future.

Abstract: In this article I discuss the backgrounds, some technical insights, and the novel developments of a bioengineering approach to semi-synthetic minimal cells that is currently pursued within the EU project SYNTHCELLS. Originally developed by Pier Luigi Luisi and coworkers at the Swiss Federal Institute of Technology (ETH, Zurich), the project aims to the construction of liposome-based bioreactors, which display living properties, although at a minor complexity level.

Abstract: A butterfly's fore- and hindwings act as one low aspect ratio wing. The variation in the feathering angle is not as large as that of other insects such as a dragonfly and a damselfly. A butterfly varies the lead-lag angle of the forewing and the angle between the thorax and the abdomen at take-off. This implies the possibility that the insect moves all parts of its body to fly. This is an advantage that an insect has over a conventional aircraft. Moreover, a new method to investigate an insect’s flight control ability is introduced. An attached plate disturbs the insect, and a remarkable flight pattern can be observed. The flight control ability of the insect can be elucidated by analyzing the insect’s flight pattern.

Abstract: Myliobatidae is a family of large pelagic rays including cownose, eagle and manta rays. They are extremely efficient swimmers, can cruise at high speeds and can perform turn-on-a-dime maneuvering, making these fishes excellent inspiration for an autonomous underwater vehicle. Myliobatoids have been studied extensively from a biological perspective; however the fluid mechanisms that produce thrust for their large-amplitude oscillatory-style pectoral fin flapping are unknown. An experimental robotic flapping wing has been developed that closely matches the camber and planform shapes of myliobatoids. The wing can produce significant spanwise curvature, phase delays down the span, and oscillating frequencies of up to 1 Hz, capturing the dominant kinematic modes of flapping for myliobatoids. This paper uses dye flow visualization to qualitatively characterize the fluid mechanisms at work during steady-state oscillation. It is shown that oscillatory swimming uses fundamentally different fluid mechanisms than undulatory swimming by the generation of leading-edge vortices. Lessons are distilled from studying the fluid dynamics of myliobatoids that can be applied to the design of biomimetic underwater vehicles using morphing wing technology.

Abstract: The deployment of leaves with plane surface and straight parallel folds, as observed in leaves of hornbeam and beech, was investigated by using numerical methods. In both species the veins are angled at 30° to 50° from the midrib, when the leaves are outstretched. Although a higher angle allows the leaf to be folded more compactly within the bud, it has very small leaf area in the early stage of unfolding. The midrib of leaf grows very slowly at first and then it does with an almost constant speed. From the numerical simulation, it was found that the midrib grows with the minimum unfolding energy. The deployment of flowers was also investigated from mechanical point of view. A potato flower has five or six petals with triangle gussets between petals. The bud volume becomes largest when the number of petals, N, is five. However, the energy for unfolding of the model with N = 5 or 6 is smaller than those of other models, if the energy can be represented by the total kinetic energy during unfolding.

Abstract: A bat-like aircraft is proposed, using a smart joint mechanism to actuate the morphing of the wings. The smart joint stays in its deformed shape after cooling, which can be up to 5% of 25 mm length joint. The morphing of the wing shapes of three different bat species is evaluated using a planar lifting line analysis. The morphing improves the lift coefficient over 1000% and the lift to drag ratio over 300% at an angle of attack of 0.6°. The results compare well with what is expected from the type of flight and morphology that has been documented for the bats.

Abstract: Compliant mechanisms fulfil a desired force and displacement characteristic. The development of such structures having a defined kinematical motion and subjected to several constraints, like deformability, stiffness and activation force is highly challenging. The present work deals with a methodology for analysing compliant mechanisms considering geometrically nonlinear deformations. By assembling pre-calculated nonlinear beam elements a new beam truss approach is introduced. The accuracy and quality of the mechanical model are verified by selected examples and compared to existing methods.

Abstract: Many biological materials exhibit a hierarchical structure over more than one length scale. Understanding how hierarchy affects their mechanical properties emerges as a primary concern, since it can guide the synthesis of new materials to be tailored for specific applications. In this paper the strength and stiffness of hierarchical materials are investigated by means of a fractal approach. A new model is proposed, based both on geometric and material considerations and involving simple recursive formulas.