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Bio-inspired autonomous devices for movements and stress generation

Subject Area Polymeric and Biogenic Materials and Derived Composites
Synthesis and Properties of Functional Materials
Term from 2009 to 2015
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 127651422
 
Final Report Year 2016

Final Report Abstract

Smart materials with a rather simple “Bauplan”, in which sensor and actuator are intrinsically embedded in the hierarchical structure of the systems are highly needed for various applications. Such materials can react autonomously to external changes and hence do not require complex feedback control systems. Sources of inspiration for the design of such devices can be found in nature and here we investigated different plants to derive bio-inspired honeycombs for movements and stress generation. In the project, two plant systems were investigated, which possess a common principle for actuation, but are substantially different with regard to the resulting deformation pattern, the exerted forces and velocity of movement. Both systems were analysed and the underlying deformation mechanisms were transferred to bio-inspired actuator systems. The seed capsules of the ice plant Delosperma nakurense possess a honeycomb mesh in each of the five valves by which the capsules open, which undergoes large deformations but is able to generate only very small forces, while in papaya, the secondary phloem contains parenchyma and bast fibres that forms a tissue with a honeycomb structure and is able to generate high stresses. The transfer of the actuation principles from the biological model systems to biomimetic actuation systems was achieved in terms of developing various demonstrators. These demonstrators pick up specific features of the plant model systems and show a great variety of functions and levels of abstraction. In the first project phase, specific focus was paid with regard to the characterisation and understanding of the movement of the ice plant seed capsules and the underlying principles of the capsule opening. In the course of this research, a locking and packing mechanism could be unravelled as well. A peculiar feature of the capsules was the observation that the latter open only after being wetted by liquid water, while high relative air humidity did not affect the swellable material. In the second project phase, the ice plant system was further analysed with regard to the principles of the hydro-actuation and the transfer of the extracted principles to first bio-inspired demonstrators was achieved. In this phase, research activities started on Carica papaya applying basic anatomical, biophysical and mechanical methods, in order to add a second plant actuation system with a different deformation pattern. During these investigations the actual driving force for stabilizing the stem that does not produce a woody secondary xylem (i.e. wood) and for the ability to re-erect, could be identified. A third complementary project part was started focusing on developing bio-inspired honeycomb devices based on the plant systems, which were actuated by swellable hydrogels. In the third project phase, various bio-inspired demonstrators were developed, which were derived from the observed actuation principles of ice plant seed capsules and papaya stems. Two main bio-inspired actuation concepts were followed. Honeycombs filled with swellable hydrogels, which deform the mesh based on different external stimuli, and honeycomb systems, in which the active element is incorporated in the honeycomb cell walls, which leads to an actuation based on a bilayer deformation principle. Due to the versatility of the underlying principles and the straightforwardness of the mechanisms, such devices may potentially find application in various fields, ranging from biomedicine to energy saving systems in buildings.

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