December 6, 2008 | David F. Coppedge

Thanks to Clam Design, Stronger Materials Are Coming

Scientists at Lawrence Berkeley National Labs have produced a lightweight composite material 300 times stronger than its constituents.  How?  By taking inspiration from clams.
    The team, writing in Science,1 described nacre, the shiny mother-of-pearl found inside clam shells.  Because of the way it sandwiches crystalline aragonite with layers of protein (07/26/2004), nacre resists cracks more and more as strain is applied.  Here’s how the scientists marveled at this amazing material:

A prime example is nacre, which consists of 95 vol. % of layered aragonite (CaCO3) platelets bonded by a thin layer of organic material, yet exhibits a toughness (in energy terms) some three orders of magnitude higher than that of calcium carbonate.  The hard aragonite provides strength, but without a means to dissipate strain, nacre would be brittle; however, large inelastic deformation generated by interlayer shearing through the organic phase allows for such strain redistribution, so that toughness is achieved through viscoplastic energy dissipation in the organic layer associated with the controlled, yet limited, sliding of the aragonite layers over each other.  Although there is controversy over the mechanisms that restrain sliding—resistance from the lamellae nanoroughness, plastic deformation of the aragonite at the nanolevel, the organic layer acting as a viscoelastic glue, or from the presence of mineral bridges—the resulting toughness is remarkable.

Previous attempts to mimic this structure have only yielded strength increases about two orders of magnitude (07/05/2007).  The Berkeley team applied “this natural concept of hierarchical design” to ceramics with new techniques that employ controlled freezing of seawater ice crystals in the layering process.  The Lawrence Berkeley press release explained the process and has a picture of the roughened surface of the hybrid ceramic used in the manufacture.  They not only mimicked nacre’s natural lubricant between layers, but its brick-and-mortar structure, as shown in a second picture.  Of nacre, the press release said, “No human-synthesized composite outperforms its constituent materials by such a wide margin.”
    Everything they did was an “an attempt to replicate the microstructural design of nacre,” they said.  Nacre is “the often cited ‘gold standard’ in biomimetic design.”  Using ceramic alumina, they actually exceeded the toughness of nacre.  “We believe that this result illustrates the importance of hierarchical design in promoting toughening mechanisms at multiple length scales as a way to create materials with unique combinations of strength and fracture resistance.”  Nevertheless, they still respect the natural standard: “We believe that of the various hybrid materials that we have fabricated, this structure best mimics nacre.”  They succeeded in sandwiching tough ceramics between microscopic deformable layers with limited shear.  “The result is synthetic materials that, like nacre and bone, are far tougher than what could be expected from the simple mixture of their constituents.”  Even so, natural nacre still has some unsurpassed qualities: “At present, our materials contain too much of the soft phase, and our ceramic layer thicknesses are still somewhat coarse in comparison to nacre,” they said; “indeed, a reduction in the polymer content and refinement of the ceramic layers should improve strength and provide additional nanoscale toughening mechanisms similar to those acting in natural materials.”
    How did they feel about biomimetics in general?  “These results highlight the tremendous potential of the biomimetic approach and suggest promising strategies for structural optimization.”  Materials science will continue to improve – thanks to the lowly clam.
    See also the 07/08/2005 entry.  On a related subject, scientists found, to their surprise, that bone growth is regulated by a neurotransmitter, serotonin, in the small intestine.  The story is told by Science Daily.

1.  Munch, Launey, Alsem, Saiz, Tomsia and Ritchie, “Tough, Bio-Inspired Hybrid Materials,” Science, 5 December 2008: Vol. 322. no. 5907, pp. 1516-1520, DOI: 10.1126/science.1164865

Time for the quiz.  (1) How many times was biological evolution mentioned in these articles?  (2) How many times was the word design used?*
    The enamel in your teeth and the bones in your skeleton are constructed in a similar way.  Think about what might be possible with artificial nacre-imitating materials: ultra-tough lightweight aircraft, bringing more fuel economy without sacrificing safety; lightweight armor; new dental ceramics for oral surgery; better energy-absorbing yet lightweight auto exteriors.  The future looks bright for intelligent design science; tune in, turn on, drop out.  Drop out of the Darwin Party.  Turn on the engine of your new super-fuel-efficient biomimetically-inspired sports car.  Tune in to ID the Future.
*0, 7.

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