07 January 2011
The mystery of how sea urchins maintain sharp teeth as they grind holes into rock has been solved by researchers in the US and Israel. By revealing the detailed structure of the tooth to understand its self-sharpening mechanism, the work raises the possibility of one day creating self-sharpening tools.
Sea urchins use their teeth to scrape algae off rocks and carve holes for sanctuary from predators and waves. It was known that urchin teeth remain sharp with use, and that their teeth comprise a complex arrangement of calcite (calcium carbonate) crystals, forming plates and fibres that are cemented together by a polycrystalline matrix of nanoparticles. However, the limestone rock which they grind is also mainly composed of calcite creating a puzzle: how can the tooth be stronger than the rock?
Sea urchins bare their teeth for researchers
© Pupa Gilbert
'You always use a harder tool to either grind or break or fracture parts of another object,' says Pupa Gilbert at the University of Wisconsin, Madison, US who, with colleagues, has figured out the answer by looking at the fine details of the complex tooth structure of the California purple sea urchin. 'In this case the tooth is harder, but it's harder because it's designed very smartly and because of the subdivision into nanoparticles that make it much more robust.' The key to this design is how and where the tooth breaks.
The team used a number of high resolution imaging and found that when subjected to stress, the tooth fractures at discontinuities in the material. 'Around the surface of all the plates and all the fibres there is a thin organic layer about a tenth of a micron in thickness,' Gilbert says. 'That is basically the weak link in the chain - that's where the tooth breaks.'
The tooth's surface breaks so as to shed a layer of material in a very specific location at the nanoscale. Since the tooth constantly grows, this wearing and shedding action continually exposes the stronger and fresh 'stone' part of the tooth.
'It's beautiful characterisation and we haven't had this level of resolution of what's really going on inside of the sea urchin tooth before,' says Lara Estroff who investigates bioinspired materials at Cornell University in Ithaca, New York, US. She is particularly excited about the idea of now synthesising a material with pre-programmed fault lines.
Gilbert speculates that the self-sharpening principle could be useful for designing layered self-assembling nanomaterials in a variety of morphologies that always have a fresh surface to do a specific job.