The digging technologies of the sea urchins will serve as inspiration for the design of digging machines

Sea urchins living on the seabed do not seem to belong in the world of advanced technology. But scientists at the Weizmann Institute of Science recently discovered, to their surprise, that these spiny creatures create their hiding places using advanced technologies

When they dig themselves burrows in the seabed, the round-bodied hedgehogs use their five teeth, which, like the teeth of rodents, wear away at their ends and continue to grow on the other side, from the root, throughout the life of the hedgehog. Surprisingly, these teeth, which should be stronger than the limestone in which the hedgehogs dig their burrows, are mainly made of calcite, the same mineral that most limestone is made of. How then do these teeth manage to cut and dig in the seabed?

In a series of studies carried out over more than a decade, Prof. Steve Weiner and Prof. Leah Eddy from the Department of Structural Biology at the Weizmann Institute of Science discovered that the secret of sea urchins lies in a combination of original and surprising design technologies. The last study in the series, conducted by post-doctoral researcher Yorong Ma and research student Yael Politi, in collaboration with Prof. Popa Gilbert and Dr. Rebecca Metzler from the University of Wisconsin, Dr. Barbara Eichmeier, Oscar Farris and Peter Pratzel from the Max Institute in Potsdam, Germany, and Dr. André Meibom of the National Museum of Natural History in Paris, recently published in the Journal of the American National Science Association (PNAS).

In the first step, the scientists found that the teeth of sea urchins contain magnesium calcite crystals that are smaller, denser and stronger than pure calcite. These crystals are concentrated at the molar end of the tooth and especially in the center, where most of the force is applied. What holds the crystals in the correct position is a pattern of calcite crystals which are larger and softer. While most materials of this type are constructed as a matrix of hard fibers containing the softer filling, hedgehog teeth appear to be constructed differently: a matrix of relatively soft calcite fibers holds magnesium calcite crystals which are harder. This fact allows these crystals to spread along the surface of the tooth tip. The presence of magnesium calcite crystals creates a sandpaper effect which helps to grind the rock.

Using high-resolution X-PEEM electron microscopy, the scientists discovered another fascinating aspect of tooth design in sea urchins. It turns out that all the crystalline materials that make up the tooth are arranged in two different layers, and these layers "lock" on top of each other like the fingers of two joined hands. The scientists believe that in this way the sea urchins produce for themselves a jagged tool that resembles a saw. The edge sharpens itself when the tooth is ground. At the same time, the crystalline layers always break in such a way that the edge remains jagged.

A better understanding of the structure of sea urchin teeth may lead to the design of improved mechanical tools, such as efficient grinding and cutting tools that do not tarnish, just like the self-sharpening teeth of sea urchins.

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