Fish fins from Actinopterygii (ray-finned fish) ‚Äúprobably represents the most elaborate and refined adaption to efficient interaction with water that has ever evolved‚Äù (Videler 1993) and they can inspire new robotic materials that combine large morphing deformation and high grasping force. Fish fins do not contain muscles, yet fish can change the orientation, shape and effective surface of their fins to generate powerful hydrodynamic forces in 3D. Individual fish fins are composed of stiff rays covered with skin. The rays are mineralized and consist of bony segments connected with collagenous tissues. The fish can exerts push/pull muscular forces at the base of individual rays, which leads to large morphing deformations along their entire length. The actuation forces to ‚Äúmorph‚Äù individual rays is relatively small, yet rays are stiff when subjected to external flexural loads. This unique combination of high flexural stiffness and morphing efficiency contrasts with state-of-the-art engineering morphing materials, which are either very compliant to achieve large shape changes (metamaterials, elastomers, hydrogels) or very stiff but with large actuation forces and small changes in shape (piezo actuated sandwich beams). Here we use mechanical modeling and mechanical testing on 3D printed ray models to demonstrate that the segmented architecture of the rays is key to unique combinations of morphing efficiency and flexural stiffness which would not be possible with continuous designs. We also show that these mechanisms are the most efficient for very high contrast of stiffness between bony elements and surrounding tissues, and for relatively thin bony segments (both of these characteristics are observed in natural fish fins). These insights can inspire new material designs for underwater propulsion, aerospace morphing materials, medical devices, or new materials and structures for soft robotics. 

The very soft and the very hard: Segmented rays in fish fins enable large morphing amplitudes combined with high flexural stiffness

The Virtual Technical Meeting of the Society of Engineering Science 2020 is the first fully virtual meeting of the SES community to envision and connect during the hiatus created by Covid-19, providing a platform for the exchange of ideas, and to showcase the latest research in the field of engineering science.

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