nique for fabricating nacre. First, sea- 

 water doesn't freeze uniformly: pure 

 water crystals segregate themselves 

 from salt and other suspended impuri- 

 ties. Second, the growth of this pure 

 ice can be controlled to produce 

 broad, flat crystals; the crystals natural- 

 ly organize themselves in such a way 

 that distinct layers of pure water-ice 

 crystals and layers of salt or other par- 

 ticles are formed. The result looks a 



that span the spaces left behind by the 

 sublimated ice add support, and a 

 quick blast of heat — not unlike the 

 firing of clay in a kiln — further 

 strengthens the lattice. Finally, an 

 epoxy is added to the dried block of 

 hydroxyapatite in a vacuum; the 

 epoxy infiltrates the spaces between 

 the plates where the ice used to be 

 and mimics the organic glue layer of 

 nacre [sec diagrams below]. 



t ft ft ft ft ft 



Artificial nacre can be made by freezing a slurry of water and ceramic particles, 

 which forces the particles into distinct layers between growing, self-organizing 

 ice crystals (see schematic diagram, above left). The pure ice crystals are 

 freeze-dried, leaving vertical voids between pillars of ceramic (above middle). 

 Glue is then forced into the voids and allowed to harden (above right). The 

 final product, shown in the photomicrograph at right, approaches natural nacre 

 in strength and toughness, but the layers of the natural substance are 

 substantially thinner. The photomicrograph is magnified 260X. 



lot like nacre but on a much larger 

 scale. Tomsia's team discovered that by 

 increasing the rate at which water 

 freezes, they could make the layering 

 progressively finer. 



To make synthetic bone, the team 

 adds granules of hydroxyapatite to the 

 water, then freezes the mixture at a 

 very low temperature. The result is a 

 finely layered composite of ice and 

 mineral. Now they can remove the 

 water by freeze-drying the composite, 

 which leaves a complex, layered struc- 

 ture of hydroxyapatite. The structure 

 has rough surfaces, as does natural 

 nacre. Some hydroxyapatite granules 



One advantage of Tomsia's system is 

 that the final product closely matches 

 the shape of the freezing container. 

 That makes it possible to mold the 

 blocks according to the bone that 

 must be replaced. Furthermore, since 

 varying the freezing rate can change 

 the thickness of the layers, composites 

 can be formed that have, say, a core 

 that is more dense than its shell. Un- 

 fortunately, a practical method of 

 making this material in bulk and 

 molding it to exact specifications has 

 yet to be tested. Tomsia's group is also 

 working to achieve even thinner layers 

 in their faux nacre. 



Nature offers an infinite variety of 

 biological designs, free for the 

 taking. But exploiting nature's solu- 

 tions to structural problems requires a 

 team with disparate talents and a large 

 dose of patience. Bone continues to 

 pose a challenge to bioengineers and 

 biomechanists. As Tomsia's method 

 and other competing products take 

 the stage, I wish them all the time- 

 honored words of good luck: "Break 



a leg." And if someday the phrase 

 turns from theatrical encouragement 

 into literal description, I might find 

 myself very grateful to be patched up 

 with the architectural stuff of seashells 



Adam Summers (asummers@uci.edu) is 

 an assistant professor of bictngmetnnz md oj 

 ecology ami evolutionary biology at the I 'ni- 

 rersity of California, Iruine. 



June 2006 NATURAL HISTORY 



