Seahorse tail inspires robotic arm team

Researchers at the University of California, San Diego are hoping to create a flexible robotic arm equipped with muscles made out of polymer by mimicking the structure of seahorse tails.

The tail of a seahorse can be compressed to about half its size before permanent damage occurs and this flexibility is due to its structure, made up of bony, armoured plates that slide past each other.

If successfully replicated, the seahorse-inspired robotic arm could be used in medical devices, underwater exploration and unmanned bomb detection and detonation.

UC San Diego engineers, led by materials science professors Joanna McKittrick and Marc Meyers, detailed their findings in the March 2013 issue of the journal Acta Biomaterialia.

‘The study of natural materials can lead to the creation of new and unique materials and structures inspired by nature that are stronger, tougher, lighter and more flexible,’ said McKittrick, a professor of materials science at the Jacobs School of Engineering at UC San Diego.  

Researchers took segments from seahorses’ tails and compressed them from different angles. They found that the tail could be compressed by nearly 50 per cent of its original width before permanent damage occurred.

That’s because the connective tissue between the tail’s bony plates and the tail muscles bore most of the load from the displacement. Even when the tail was compressed by as much as 60 per cent, the seahorse’s spinal column was protected from permanent damage.

McKittrick and Meyers’ research group uses a technique that applies a series of chemicals to materials to strip them of either their protein components or their mineral components. That allows them to better study materials’ structures and properties.

After treating the bony plates in the seahorse’s tail with the chemicals, they discovered that the percentage of minerals in the plates was relatively low – 40 per cent, compared to 65 per cent in cow bone.

The plates also contained 27 per cent organic compounds – mostly proteins – and 33 per cent water. The hardness of the plates varied; the ridges were hardest, likely for impact protection—about 40 per cent harder than the plate’s grooves, which are porous and absorb energy from impacts.

The seahorse’s tail is typically made up of 36 square-like segments, each composed of four L-shaped corner plates that progressively decrease in size along the length of the tail. Plates are free to glide or pivot; gliding joints allow the bony plates to glide past one another and the pivoting joints are similar to a ball-and-socket joint, 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.

The next step is to use 3D printing to create artificial bony plates, which would then be equipped with polymers that would act as muscles.

The final goal is to build a robotic arm that would be a unique hybrid between hard and soft robotic devices.

Sea horses get their exceptional flexibility from the structure of their bony plates, which form its armor. The plates slide past each other. Here the seahorse’s skeleton, as well as the bony plates, are shown though a micro CT-scan of the animal
This image shows how joints are connected in the armored plates in the seahorse’s tail