Ever-sharp urchin teeth may yield tools that never need honing

Dec 22, 2010 by Terry Devitt
A California purple sea urchin is pictured in hand. Photo: Jeff Miller

(PhysOrg.com) -- To survive in a tumultuous environment, sea urchins literally eat through stone, using their teeth to carve out nooks where the spiny creatures hide from predators and protect themselves from the crashing surf on the rocky shores and tide pools where they live.

The rock-boring behavior is astonishing, scientists agree, but what is truly remarkable is that, despite constant grinding and scraping on stone, urchin never, ever get dull. The secret of their ever-sharp qualities has puzzled scientists for decades, but now a new report by scientists from the University of Wisconsin-Madison and their colleagues has peeled back the toothy mystery.

Writing today (Dec. 22, 2010) in the journal , a team led by UW-Madison professor of physics Pupa Gilbert describes the self-sharpening mechanism used by the California purple sea urchin to keep a razor-sharp edge on its choppers.

The urchin's self-sharpening trick, notes Gilbert, is something that could be mimicked by humans to make tools that never need honing.

"The sea urchin tooth is complicated in its design. It is one of the very few structures in nature that self-sharpen," says Gilbert, explaining that the sea urchin tooth, which is always growing, is a biomineral mosaic composed of calcite with two forms — plates and fibers — arranged crosswise and cemented together with super-hard calcite nanocement. Between the crystals are layers of organic materials that are not as sturdy as the calcite crystals.

"The organic layers are the weak links in the chain," Gilbert explains. "There are breaking points at predetermined locations built into the teeth. It is a concept similar to perforated paper in the sense that the material breaks at these predetermined weak spots."

Ever-sharp urchin teeth may yield tools that never need honing
Sea urchin teeth. Photo: courtesy Pupa Gilbert

The crystalline nature of sea urchin dentition is, on the surface, different from other crystals found in nature. It lacks the obvious facets characteristic of familiar crystals, but at the very deepest levels the properties of crystals are evident in the orderly arrangement of the atoms that make up the biomineral mosaic teeth of the sea urchin.

To delve into the fundamental nature of the crystals that form sea urchin teeth, Gilbert and her colleagues used a variety of techniques from the materials scientist's toolbox. These include microscopy methods that depend on X-rays to illuminate how nanocrystals are arranged in teeth to make the capable of grinding rock. Gilbert and her colleagues used these techniques to deduce how the crystals are organized and melded into a tough and durable biomineral.

Knowing the secret of the ever-sharp sea urchin tooth, says Gilbert, could one day have practical applications for human toolmakers. "Now that we know how it works, the knowledge could be used to develop methods to fabricate tools that could actually sharpen themselves with use," notes Gilbert. "The mechanism used by the urchin is the key. By shaping the object appropriately and using the same strategy the urchin employs, a tool with a self-sharpening edge could, in theory, be created."

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Raveon
4.5 / 5 (2) Dec 22, 2010
"Now that we know how it works,...."

Now that they know how it works, maybe the writer of the article could have told us. I am assuming that layers at the angle of the edge continually shear away exposing a new edge. This principle is already known, how to make it is another question. The depleted uranium core of armor piercing projectiles operates on a similar principle If I'm not mistaken.
ormondotvos
5 / 5 (1) Dec 22, 2010
I thought beaver teeth were self sharpening...

http://www.fishin...nife.cfm
meerling
5 / 5 (1) Dec 23, 2010
I suspect Raveons guess is basically right as to the teeth, but as to the d.u.p. ammo, no.
D.U.P. (depleted uranium projectile) is a depleted uranium core surrounded by an aluminum shell that keeps it from deforming before it strikes the target. If it wasn't for deformation risk before impact, there would be no reason for the aluminum windscreen.
It is in no way similar to the self sharpening of urchin teeth.
Not trying to be a pain, just trying to make sure nobody gets the wrong idea.
fmfbrestel
5 / 5 (1) Dec 23, 2010
"On more properly military grounds, depleted uranium is favored for the penetrator because it is self-sharpening and pyrophoric.[27] On impact with a hard target, such as an armored vehicle, the nose of the rod fractures in such a way that it remains sharp. The impact and subsequent release of heat energy causes it to disintegrate to dust and burn when it reaches air because of its pyrophoric properties.[27" -- Wikipedia

Sorry Meerling, But DU is used precisely because it is self-sharpening. And it is also self-sharpening in much the same way as these teeth -- It sheers off in predictable ways such that there is always a sharp point at the front.

The other reason it is used is because all the material that sheers off burns at high temperatures. But without the sheering effect of DU it would not be nearly as effective at piercing armor.
fmfbrestel
5 / 5 (3) Dec 23, 2010
However, claiming that this property could lead to products that never need sharpening is a little disingenuous. While a material with similar properties would stay sharp, it would also be constantly eroding itself away. So it would stay sharp until it didn't exist any longer. So it would only be beneficial if the sheering took less material from the blade then manual sharpening does.
Scalziand
5 / 5 (3) Dec 27, 2010
However, claiming that this property could lead to products that never need sharpening is a little disingenuous. While a material with similar properties would stay sharp, it would also be constantly eroding itself away. So it would stay sharp until it didn't exist any longer. So it would only be beneficial if the sheering took less material from the blade then manual sharpening does.

You are ignoring the gains in productivity from not having to take a tool offline for sharpening.
gmurphy
not rated yet Dec 29, 2010
This self-sharpening process is not so different from what is seen in an angle-grinder, imho.
Skeptic_Heretic
not rated yet Dec 29, 2010
This self-sharpening process is not so different from what is seen in an angle-grinder, imho.

except an angle grinder becomes observably less useful for its task on each shearing event.