Tail fins of bony fish do not bend away from the compressive force – as would be expected – in the event of a compressive load, but instead bend in the direction of the compressive force. This initially surprising feature enables highly efficient force transmission of the fish to the surrounding medium of water. When the fish moves by flapping its tail, the tail fin assumes the (concave) shape of a ladle (with the open side in the direction of the flapping of the fin). The water volume enclosed by the tail fin is accelerated to the rear, propelling the fish forwards in the process. The adaptive change in shape of the tail fin is a reason for the good force transmission. From a technical point of view, the tail fin of the bony fish is a design which has been optimised in terms of lateral force application in the course of evolution.
This high-tech solution of nature had been overlooked until recently and has been used for the development of bionic products only since the observations and examinations of Leif Kniese and Rudolf Bannasch (EvoLogics GmbH). They analysed the structure-function relationship tail fins of bony fish and found that the bones of the fins each consisted of two cartilaginous longitudinal rays. These two longitudinal rays are connected by elastic connective tissue, which is why they move towards each another in the event of a load.
The tail fin is constructed with a number of such “two-ray basic elements”, which pull radially into the tail fin, beginning where the tail starts. The technical implementation of this structure resulted in the development and patenting of the Fin Ray® Effect.
The technical solution is surprisingly simple. The basic element is an acute-angled triangle which is constructed with flexible sides. The two sides are kept apart by articulated braces connected to the two side parts. This creates a structure based on the two-ray basic element of the bony fish fin with the same deformation properties as the tail fin of the fish.
If this bionic structure with Fin Ray® Effect is pushed sideways against a convexly curved object, it adapts to the shape of the object. Like the biological example of the fish fin, it is bent in the direction of the applied force exercised by the object and snuggles against it in a concave form. Such constructions are very stable and the stability is even increased by the load due to the tension created by the articulated cross braces.
A great number of technical application options is conceivable with the Fin Ray® Effect, based on the bionic basic element. An adaptive gripping tool that is ideally suited to carefully raising sensitive or fragile objects with a convexly curved surface can be produced simply by combining two elements installed on appropriate brackets.
The objects are held securely and tight fit by the Fin Ray® Effect. Since surface loads, not point loads, arise, no pressure points are formed which can damage the object. Elements with Fin Ray® Effect are also used by bionic submarine vehicles such as the Aqua_ray. Flowingelastic movements which appear to be very natural are generated in combination with the fluidic muscle.
Adaptive gripping pliers with Fin Ray Effect® for sensitive objects. © Festo AG & Co. KG
The Fin Ray® Paper Kit
Everybody can create his own Fin Ray® in an easy way with the help of the paper kit. Simply download the pdf, print it, cut it out and glue it together according to the instruction.
Even more easy it is with the Fin Ray® Paper Kit. The cardboard sheet is pre-cutted and fitted with adhesive tape. This way the assembly takes only a few minutes. A package contains 10 pieces.
Fin Ray® is a registered trademark of EvoLogics GmbH in some countries.