Toueh As Shells 



A promising candidate for artificial bone 



By Adam Summers ~ Illustrations by Tom Moore 



Nacre, the tough material 

 of most shells, is made up of layers 

 of calcium carbonate interleaved with 

 layers of organic glue. This cross section of 

 nacre is magnified 2,750X. 



pelvises, the material might be just 

 what the doctor ordered. Nacre is 

 more similar to bone, and would 

 likely make a better match, than tita- 

 nium or stainless steel, when a new 

 joint is needed for an aging hip or a 

 demolished elbow. 



Biomimetics, the art and sci- 

 ence of transferring biological 

 designs into the realm of hu- 

 man use, is far from a straightforward 

 process. The path from theory to 

 product tends to be so convoluted 

 that only a handful of biomimetic 

 products are commercially viable — 

 Velcro sticks out as a rare success. 

 One of the holy grails of biomimetics 

 is artificial bone, which promises to 

 be both useful and marketable. After 

 all, baby boomers are rapidly losing, 

 breaking, and wearing down their 

 natural supply; the demand for re- 

 placement bone within their genera- 

 tion alone is high enough that bio- 

 mimeticists would turn nature inside 

 out to find a solution. 



It so happens that nacre, the brick 

 and mortar of most mollusk shells, 

 can take quite a beating — which is 

 why such otherwise defenseless, soft- 

 bodied creatures go to the trouble of 

 making the stuff. Nacre is mainly 

 made of a ceramic, a hard nonmetal- 

 lic mineral — calcium carbonate in 

 this case. What's intriguing is that 

 unlike your favorite coffee mug or 

 run-of-the-mill grail, nacre is a ce- 

 ramic that is unusually tough. Typi- 

 cally the smallest crack in a ceramic 



object races through the brittle struc- 

 ture to cause a full-blown failure. 

 You may have noticed how seldom 

 you find a mug or a plate that's near- 

 ly, but not completely, broken. 



But nacre is not a simple or homo- 

 geneous piece of clay. Rather, like 

 my favorite pastry, the napoleon, 

 nacre is made of thin sheets of 

 ceramic interleaved with even thin- 

 ner sheets of organic glue. Although 

 one ceramic sheet is easy to fracture, 

 the crack stops when it hits the gluey 

 interface, and more energy must be 

 spent to start the crack in the next 

 layer [see micrograph at top of page]. In- 

 cidentally, those thin sheets are about 

 the same thickness as wavelengths of 

 visible light, which explains why the 

 insides of abalone shells — made of 

 nacre — reflect a rainbow of colors. 



As it happens, your skeleton, too, 

 is made up largely of a ceramic: hy- 

 droxyapatite. When it's healthy, it 

 doesn't shatter nearly as easily as a 

 cup or a bowl either. That is because 

 an organized network of collagen 

 fibers toughens the bone, and a lat- 

 tice of little struts forms a spongy, 

 energy-dissipating framework for 

 most of your bones. If nacre could be 

 sculpted into the shape of patellas or 



Unfortunately, although the struc- 

 ture and remarkable properties 

 of nacre have been known for thirty 

 years, the simplicity of the material 

 is deceptive. So far no one has been 

 able to make synthetic nacre. Most 

 attempts have focused on alternating a 

 layer of ceramic with a wash of glue, 

 and repeating that ad nauseum. The 

 process ends up with a nacrelike 

 material, but the thickness of the 

 ceramic layers is hard to control, and 

 it takes thousands of cycles to produce 

 a slab of appreciable heft. More 

 important, the interface between glue 

 and ceramic — so critical in stopping 

 cracks from propagating through a 

 natural shell — has proved extremely 

 hard to copy from nature. Ceramic 

 bone implants currently on the mar- 

 ket apparently wear well, but they are 

 more brittle than healthy bone. 



Another approach now seems much 

 more promising. Antoni P. Tomsia, a 

 materials scientist at the Lawrence 

 Berkeley National Laboratory in Cali- 

 fornia, and his team have taken advan- 

 tage of the properties of freezing wa- 

 ter to make a finely layered composite 

 that's amazingly tough. Two such 

 properties, seemingly irrelevant to 

 making bone, led to the new tech- 



NATURAL HISTORY June 2006 



