One Shell Fits All

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Science  23 Jan 2009:
Vol. 323, Issue 5913, pp. 438
DOI: 10.1126/science.323.5913.438d

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Nature has evolved remarkably complex structures from a limited number of materials in order to obtain multifunctionality. For example, the shell of a turtle needs to be stiff in order to protect the internal organs from trauma. However, it also needs to be sufficiently flexible to enable efficient respiration and locomotion. As such, the shell needs to be compliant at low cyclical loads but simultaneously stiff and tough to resist sudden large impacts. Krauss et al. show that turtle shells contain densely packed inner and outer cortical sheets, and these two layers in turn sandwich sparsely located porous trabecular bone tissue. Attached to the spinal column are rigid ribs, which are separated by regions of suture (shown above). The suture has a seesaw or wavy geometry and is not mineralized, but instead is composed of aligned collagen fibers that are angled relative to the direction of the suture. This arrangement allows the fibers to remain in tension when the shell is loaded in either tension or compression. At low loading, the suture compresses only slightly, allowing flexibility in the movement of neighboring bones, but at high loading, the bones can lock together to strongly resist deformation. — MSL

Adv. Mater. 10.1002/adma.200801256 (2008).

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