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Seahorse tails could inspire new generation of robots

03 July 2015

Inspiration for the next 'big thing' in robotics and biomedicine could come from a seahorse's tail, according to a new study reported in the journal, Science.

3D-printed model based on the design of a seahorse tail (photo: Clemson University)

The research, led by Clemson University's Michael Porter, an assistant professor of mechanical engineering, centres on the curious shape of seahorse tails. These are organised into square prisms surrounded by bony plates that are connected by joints.

The study built on work Porter started in the McKittrick and Meyers Lab at the University of California, San Diego in collaboration with Dominique Adriaens, professor of evolutionary biology at Ghent University.

Researchers found that a seahorse’s tail bends in a way such that it can grasp objects in front of its line of sight. Study co-author Ross Hatton, assistant professor of mechanical engineering at Oregon State University and specialist in robotics helped Porter develop kinematic models that prove the tail’s geometry is optimized for this forward grasping.

Many other creatures, ranging from New World monkeys to rodents, have cylindrical tails. The researchers wanted to know whether the square-prism shape gives seahorse tails a functional advantage.

To find out, the team created a 3D-printed model that mimicked the square prism of a seahorse tail and a hypothetical version that was cylindrical.

When researchers twisted the 3D-printed square seahorse tail model, they found that its plates interfered with one another, limiting its range of movement by about half when compared to the model made of round segments. In addition, after it was twisted, the square model returned to its original shape faster, while expending a minimum amount of energy.

Researchers theorise this might protect the tail from damage. By contrast, a tail made from round segments twists easily and requires more energy to return to its original shape. Researchers also found that the tail’s square segments created more contact points with the surface that it is gripping when compared to a tail with round segments.

Researchers also compressed the models made of 3D-printed segments and compared their behavior to solid rings with square and circular cross-sections. They found that a seahorse's tail has joints at the exact locations where solid rings fail when crushed.  This allows the structures to absorb more energy on impact.

Even more impressive, the square model outperformed the round one in all crushing tests. That’s because square segments deform linearly. By contrast, round segments open up under the applied load, changing their shape from circular to elliptical. 

This is important, because one of the seahorse’s main predators are water birds, which capture their prey with their beaks and crush them in the process.

The seahorse’s tail is typically made up of about 36 square-like segments, each composed of four L-shaped corner plates that progressively decrease in size along the length of the tail. The plates are free to glide or pivot. Gliding joints allow the bony plates to slide past one another.

Pivoting joints are similar to ball-and-socket joints, with three degrees of rotational freedom. The plates are connected to the vertebrae by thick collagen layers of connective tissue. The joints between plates and vertebrae are extremely flexible with nearly six degrees of freedom.

Porter believes the seahorse tail could inspire new forms of armour, as well as search-and-rescue robots that move on the ground like a snake and are able to contract to fit into tight spaces. His next step is to build a robot to prove the concept.

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