Bio-inspired, Flexible Structures and Materials - Biotechnologies and Biomimetics for Civil Engineering - page 283

kinetics of the double Flectofin would need to be actuated from a common
backbone. This system is shown in Fig. 12.5 , with a configuration of two fins that
theoretically interpenetrate (A). Therefore, they rest in position (B) where they
push against each other and share a large contact area which highly increases their
stability. As shown in Fig. 12.5 , due to their concave curvature in their inactive
state the fins bend outwards when the backbone is actuated (C-F). The leaning
against each other of the fins in their 'rest position' also serves to stiffen the fins
against wind deformation. As a positive side effect of the symmetrical deforma-
tion, the eccentric forces in the backbone that are induced by the bending of the fin
counteract each other. This limits the torsion in the backbone and results in a more
filigree profile.
It was found that the elastic kinetic system relies on perfect symmetry which is
difficult to produce on a larger scale. Therefore additional geometric changes were
studied to make the elastic kinetics more robust. Here, a study of the Aldrovanda
snap-trapping mechanism (Fig. 12.6 ) led to the idea of using curved line folding
logics to force the fins into a consistently outward folding motion. The Aldrovanda
traps consist of two sickle-shaped lobes that are connected to a lens-shaped central
portion by a curved living hinge. In the symmetry axis of the trap and in the middle
of the central portion there is another distinct element to be found—a midrib that
acts as the ''driver'' for the closing movement. When a prey (e.g., small crusta-
ceans like water fleas) stimulates the trap by touching its sensory hairs, a sudden

Simulation of a double Flectofin , B position of the planar fins, B real position of the fins
pushing against each other, E opened fins due to bending of the backbone (Lienhard et al. 2011 )

Fig. 12.5