EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Within a biomimetic technology pull process, biologists and engineers faced the challenge to develop a plant-inspired actuator without articulated hinges on an architectural scale. Hinges are moveable connection between rigid bodies, which are widespread in animals and technology. However, hinges are subject to wear and tear through friction and thus become prone to failure. In contrast, most plant movements take place without conventional hinges by elastic deformation, which makes them ideal models. The opening and closing movement of grass leaves is a suitable biological concept generator. Morphological, anatomical and mechanical investigations were carried out on the model plant Sesleria nitida. Its leaf halves show pronounced kinetic amplification actuated by turgor changes of fan-shaped groups of motor cells right and left of the leaf midrib. The motor cells or bulliform cells are large bubble-shaped cells in the upper leaf epidermis. The leaf halves fold, when the turgor pressure in the bulliform cells is low and unfold again, when the bulliform cells become fully turgescent. We incorporated all insights on the leaf movement and the underlying functional principle into a finite-element analysis, as a prerequisite for the development of a pneumatic cellular actuator. The first prototype consisted of a row of single cells with compliant hinges positioned on a plate. When increasing the pneumatic pressure applied to each individual cell, the cells become wider at the upper side and the entire structure bends. Since the size of the technical cells can span from centimeters to meters, the pneumatic cellular actuator has the potential for applications on an architectural scale. To illustrate the feasibility of the concept as a bending actuator, it has been successfully integrated into the midrib of the facade shading system Flectofold, where the bending of its midrib controls the hoisting of its wings.