The strange tail of the seahorse and how it grips

The tail of a seahorse could provide the inspiration for armoured robots that have the ability to grasp, according to researchers at Clemson University in South Carolina. Using 3D printing to trying to mimic the structure of the seahorse’s tail — which unlike almost every other tail in nature is square in cross-section rather than round — led the team to investigate applications for the mechanism that could find applications in robotics and medicine.

“Almost all animal tails have circular or oval cross-sections – but not the seahorse’s. We wondered why,” said mechanical engineering Michael Porter, the lead investigator on the study. What they found came as a surprise. The square armour plates that make up the tail block each other as they move and can only slide against each other in one direction, while round plates can both slide and rotate. As a result, when crushed, the seahorse-tail structure can absorb more energy than a round-section structure before it fails, as the sections tend to retain their shae, while round ones are flattened to ovals.

When  twisted, a square-section structure requires less energy to return to its original shape. And when gripping, the square sections create more points of contact with the surface, making a stronger grip. Seahorses use this to anchor themselves to vegetation and hold on despite strong currents.

In a paper in the journal Science, Porter and colleagues explain how they used 3D-printing to investigate the properties of the structure. “New technologies, like 3D-printing, allow us to mimic biological designs, but also build hypothetical models of designs not found in nature,” said Porter “We can then test them against each other to find inspiration for new engineering applications and also explain why biological systems may have evolved.” Porter collaborated with researchers from the University of California San Diego, where he gained his PhD, and with Dominique Adriaens, an evolutionary biologist from Ghent University. “”Michael decided to use engineering and technology to explain biological features,” said Joanna McKittrick of UC San Diego. This allowed him to simplify the structures so he could study their essential features. “”Then you can build new bioinspired structures and devices.”

These studies revealed that each square unit of the tail is made from L-shaped corner plates, with ‘gliding joints’ that work like ball-and-socket joimnts with three degrees of freedom. These square plates make the seahorse’s tail stiffer, stronger and more resistant to strain at the same time, while usually, strengthening any one of these characteristics will weaken at least one of the others, Porter said. This is what gives the structure its potential for armour. Porter is hoping both to scale-up the structure, so that it can be used in gripping robots, and to scale it down, to build a medical catheter.