The teeth of the chiton mollusc are its answer to Wolverine's adamantium skeleton – they are made out of magnetite, the hardest material made by any organism.
For more than 400 million years chitons – marauding marine molluscs – have roamed the seas, munching on algae-encrusted rocks with their glittering black, metallic teeth. Chitons roam courtesy of a single, broad muscular foot. They have a shell, made of eight plates, and some species live up to 20 years.
But it's the teeth we're interested in. The teeth are the chiton's answer toWolverine's adamantium skeleton. But these are no fiction. They are the real thing, made out of magnetite, the hardest material made by a living organism.
Lyle Gordon, a bioengineer at Northwestern University in Evanston, Illinois, has come a step closer in understanding just how the humble mollusc grows them.
Gordon's team is working to synthesise super-tough materials in the lab and so have been looking to take a leaf out of the chiton's book for years. Making substances such as magnetite – an iron oxide used in electronics and medical devices – often requires extremely high temperature and pressure, as well as strongly acid or base conditions.
"But these millions-of-years-old animals have been able to make iron oxide in benign ocean water conditions with only the iron available from the algae they eat," Gordon says. "It's an amazing process."
Gordon and his colleagues use an atom probe to study the teeth ofChaetopleura apiculata, a chiton the size of a fingernail. The probe plucks charged atoms from a substance one at a time, allowing a scientist to pinpoint the location and chemical identity of the atoms inside the bulk material. The technique is normally used to study metals, but Gordon suspected that employing the probe on a biologically produced material would not be so different. With the atom probe, Gordon and his team have achieved an unprecedented, nanoscale glimpse of chiton teeth's inner workings.
"What we've found is that there are proteins and sugars buried within the teeth that these animals use to form iron oxide," Gordon says. In the watery gel at a tooth's centre, he says, carbohydrate fibres criss-cross to form a scaffold that binds positively charged ions such as magnesium and sodium. Those ions, Gordon suspects, then bind negatively charged proteins that collect iron, providing a detailed template for the growing tooth's magnetite shell.
The chiton boasts hundreds of teeth on its tongue, the oldest ones on the tip and the youngest furthest back, pushing their way forward every few days as if on a conveyor belt.
Gordon's atom probe method is especially powerful, says Harvard bioengineerDavid Bell, because it can track the complete chronological record of tooth formation. "Using atom probe tomography to analyse teeth – that's never been done before, and the implications are profound," he says.