Sea urchins dig themselves hiding holes in the limestone of the ocean floor using teeth that don't go blunt. Weizmann Institute scientists have now revealed their secrets, which might give engineers insights into creating ever-sharp tools or mechanical parts.
The urchins dig holes to fit their globular bodies using their five teeth, which, like those of rodents, are ground down at the tip but continue to grow on the other end throughout the animals' lives. The amazing part, however, is that the teeth, which need to be harder and stronger than the rocky limestone being dug out, are themselves made almost entirely of calcite the same calcite that makes up much of the limestone. How is this possible? In a series of studies spanning more than a decade, Profs. Steve Weiner and Lia Addadi of Weizmann's Structural Biology Department have discovered that the urchins' secret lies in a combination of ingenious design strategies. The latest of these studies, conduc ted with postdoctoral fellow Yurong Ma and graduate student Yael Politi and in collaboration with Prof. Pupa Gilbert and Dr. Rebecca Metzler of the University of Wisconsin, Drs. Barbara Aichmayer, Oskar Paris and Peter Fratzl from the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany, and Dr. Anders Meibom from Museum National D'Histoire Naturelle in Paris, France, was reported recently in the Proceedings of the National Academy of Sciences (PNAS), USA.
The scientists found that the sea urchins' teeth contain crystals of magnesium calcite, which are smaller, harder and denser than those of pure calcite; they are concentrated at the grinding tip of the tooth, particularly in the tip's center, where the most force is being exerted in the course of grinding. What holds these crystals at the center of the tip is a matrix of larger and softer calcite crystals. While in most such materials a matrix of hard fibers contains a softer filling, the reverse is true for the urchin
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