Researchers in Israel have discovered why sea urchin teeth stay sharp, despite a lifetime of digging into hard limestone.

A new study by Israeli researchers has revealed why the teeth of sea urchins stay sharp, giving engineers vital insights into creating ever-sharp tools or mechanical parts.

Sea urchins are small, spiny globe-shaped creatures that can be found in oceans all over the world. They have five teeth which they use to dig holes to fit their bodies into. Like rodents, their teeth are ground down at the tip, but continue to grow at the other end all through their lives.

In research, scientists at the Weizmann Institute of Science discovered that the teeth of the urchins, which need to be harder and stronger than the rocky limestone they are digging in, are actually made up almost entirely of calcite – the same substance that can be found in much of the limestone.

In a series of studies spanning more than a decade, Prof. Steve Weiner and Prof. Lia Addadi of Weizmann’s Structural Biology Department discovered that the urchins’ secret lies in a combination of ingenious design strategies.  Inside the sea urchins’ teeth are crystals of magnesium calcite, which are smaller, harder and denser than those of pure calcite. These crystals 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. 

Crystals in the center act like sand paper

What holds these crystals at the center of the tip is a matrix of larger and softer calcite crystals. While in most materials like this a matrix of hard fibers contains a softer filling, the reverse is true for the urchins’ tooth: a matrix of relatively soft calcite fibers holds the harder magnesium calcite crystals, which allows these crystals to spread over the entire surface of the tooth. 

The presence of magnesium calcite crystals acts like sand paper helping to grind the rock down. 
The scientists also used X-ray photoelectron emission spectromicroscopy and other high-resolution imaging methods to discover that all the crystalline elements that make up the tooth are aligned in two different arrays, and that these arrays are ‘interdigitated,’ or interlocked like the fingers of folded hands, just at the tip of the tooth where most of the wear occurs. 

The scientists believe that interlocking produces a notched, serrated ridge resembling that of a carpenter’s file. This ridge is self-sharpening: as the tooth is being ground down, the crystalline layers break in such a way that the ridge always stays corrugated.

The researchers, who published their study recently in the US journal, Proceedings of the National Academy of Sciences (PNAS), were helped by postdoctoral fellow Yurong Ma and graduate student Yael Politi.

The study was also done in collaboration with scientists from the University of Wisconsin, the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany, and with the Museum National D’Histoire Naturelle in Paris, France.