[1] Nacre, the iridescent material lining the shells of certain mollusks, has long intrigued engineers for a paradoxical reason: though composed almost entirely of a brittle mineral, aragonite, it is remarkably resistant to fracture. [2] A block of pure aragonite shatters readily, yet nacre can be thousands of times as tough, a disparity that composition alone cannot explain. [3] The answer, researchers came to recognize, lies in architecture. [4] Nacre consists of microscopic mineral tablets stacked in overlapping layers and bonded by exceedingly thin films of organic material, an arrangement often likened to a brick wall in which the mortar is far more compliant than the bricks. [5] When the material is stressed, the soft interfaces allow the rigid tablets to slide fractionally rather than crack, dissipating energy that would otherwise drive a fracture straight through the mineral.
[6] For a time this “brick-and-mortar” account was taken to be sufficient, but recent work suggests it is incomplete. [7] The tablets themselves are not smooth; their surfaces bear nanoscale asperities that resist sliding and force the layers to interlock, and the organic films appear to unfold under load in a manner that absorbs additional energy. [8] Some investigators now argue that nacre's toughness emerges from the interplay of mechanisms operating across several scales at once, no one of which suffices in isolation. [9] The lesson has not been lost on engineers, who have begun to fabricate synthetic laminates that mimic nacre's layered geometry, though reproducing its performance has proved harder than the elegant model might suggest.
