Flectofin® – a hingless flapping mechanism

If there is one astonishing example for a biomimetic project following the ‘bottom up approach’, it is the example of the Bird-of-paradise flower Strelitzia reginae with its extraordinary hingeless flapping (opening) mechanism. This biological system has been successfully analyzed, its functional principle abstracted and implemented in a technical system as a cooperation between biologists, engineers and architects. As a result, the hingeless sun-shading system Flectofin® was developed. As the Bird-of-paradise flowers are in bloom in the Botanic Garden Freiburg at the moment, I would like to introduce this biomimetic project in today’s article.


The biological concept generator Strelitzia reginae

Strelitzia reginae is a plant native to South Africa and is pollinated by birds. To reach its nectar, birds are landing on the perch – a long and thin structure at the end of the blue petals of the flower. The weight of the bird bends the perch downwards (as imitated with my finger in Figure 1) leading the petal laminae to flap sideways and expose the pollen which sticks to the bird and is carried to the next flower. After the bird’s take-off (and release of the weight onto the perch) the closed state of the petals is regained (compare Figure 1), protecting the pollen from environmental impacts such as rain or wind. With the next pollinator landing on the perch, the opening mechanism is repeated unaltered – it can take place a vast number of times without structural or functional loss [1].


Figure 1: The secret behind Flectofin®: Strelitzia reginae – bending of the perch leads to reversible sideways flapping of the petal laminae


Abstraction of the functional principle: lateral-torsional buckling

The biomechanical principle behind this opening mechanism is ‘lateral-torsional-buckling’ and can easily be demonstrated with a wooden model (compare Figure 2). The backbone (the perch of the flower/ the wooden beam) is bent, leading to a reversible sideways deflection of the lamina (laminae of lowers/ plastic foil in the model).  Physically speaking (for the engineers among us): the beam is bent in an uniaxial manner which is causing an unsymmetrical bending motion by torsional buckling [1]. This type of buckling is actually a mode of failure in architecture and is tried to be avoided in constructional engineering. In the case of Flectofin® though, it has been recognized as a sophisticated opening mechanism and analysed thoroughly via modelling (not only with simple wooden structures) but also in Finite Element software [1]. Several abstractions and optimizations finally lead to a technical implementation of this principle: Flectofin®.



Figure 2: Wooden model of the Flectofin® principle


Technical product: Flectofin®

Sun shading systems are prone to failure, as they often consist of many small elements connected by unstable hinges which are moved often and rapidly. The hingeless and durable opening-and-closing mechanism of Strelitzia reginae is therefore a beautiful inspiration to optimize sun-shading systems. The patented end product Flectofin® is fabricated from fibre-reinforced material and consists of two big laminae (similar to the petals of Strelitzia ans our wooden model). A precise arrangement of fibres in the laminae allows for the sideways flapping of the ‘wings’ as its backbone is bent – leading to an opening and shading of windows on sunny days. The closure of the laminae occurs as easily as soon as the backbone is released. Additional biomimetic optimizations such as reducing stress peaks at the edges of the laminae via the tensile triangles contribute to the functionality of Flectofin® [1]. If you are interested to see how this sun-shading system works and learn more about its development, I would like to refer to this video, which was made in the course of Flectofin® being rewarded with the ‘Gips-Schüle-Forschungspreis’ in 2013.

The Bird-of-Paradise flower is an extraordinary organism – a stunning example of beauty and functional optimization!


[1]: Lienhard J, Schleicher S, Poppinga S, Masselter T, Milwich M, Speck T, Knippers J (2011) Flectofin: a hingeless flapping mechanism inspired by nature. Bioinspiration & Biomimetics 6 (4):045001, doi: 10.1088/1748-3182/6/4/045001

Katharina Bunk

My name is Katharina Bunk, I am 26 years old and work as a PhD student in the ‘Plant Biomechanics Group’ in the beautiful city of Freiburg. I studied Biology at the University of Munich followed by the Master program ‘Bionik/ Biomimetics in Energy Systems’ in Villach/ Austria. I am especially interested in Botany and therefore chose Plant Biomechanics as my main field of research.

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