BioEd Online

Scientists have long envied sea urchins' ability to fashion spines from single, large crystals. Now they have cracked the prickly creatures' secret and made a discovery that could spur the development of dental implants and bone grafts.

Each sea-urchin spine is made from a single crystal of calcite, a mineral mostly consisting of calcium carbonate, and can reach several centimetres in length. The crystals have a complex structure bounded by smooth, curved surfaces, unlike calcite crystals grown in the lab, which take on an angular shape with six flat faces, called a rhombohedron.

To find how sea urchins sculpt these exotic crystals, researchers at the Weizmann Institute of Science in Rehovot, Israel, studied what happens when the creatures try to rebuild broken spines. Spines are formed in a two-stage process involving an unstable intermediate called amorphous calcium carbonate, the team reports in this week's Science¹.

The sea urchins package this compound in an envelope of living cells, like icing sugar filling a squeezy bag, before it crystallizes, the researchers explain. "The amorphous calcium carbonate is not ordered; it can assume any shape," says Lia Addadi, who led the study. When marshalled into the correct spiny shape, the compound transforms into a stable crystal, although the researchers still do not know exactly how the reaction occurs.

This two-step process of moulding and setting explains how the creatures grow such large crystals, Addadi's team says. Lab-grown crystals of calcium carbonate form directly from solution without an amorphous stage, so they simply adopt their rhombohedron shape.

Many other animals may use a similar trick, the researchers add. Sea-urchin larvae are already known to use amorphous calcium carbonate during growth; the fact that adults also use it to repair damage suggests that the technique could be widespread among other marine animals such as corals and sponges.

Material benefits

Replicating the strategy in the lab is likely to prove tricky, although the general strategy of using moulding to engineer advanced materials may prove fruitful. Interest in mimicking the processes by which minerals are formed naturally, known as biomimetic synthesis, is growing. "We are interested in finding out what evolution has achieved. I believe we can learn from concepts that have been developed over many thousands of years," Addadi says.

Some material scientists are already using moulding methods to fashion simple crystals, although they have yet to match sea urchins' skill in making complex, curved creations. "Many biomaterials could be moulded into complex shapes through an amorphous precursor," suggests Laurie Gower, an expert on biomaterials at the University of Florida in Gainesville.

Calcium carbonate could be used to tailor-make dental implants or bone grafts. But a bewildering range of applications could become possible if the general strategy can be applied to other materials, says Gower. "The real goal of biomimetic engineering is learning how to draw on nature's ideas. If you could take any material and learn how to mould it or shape it in this way, you could gain far more control over its optical, electronic or mechanical properties."