Materials science in the Amazon

Materials scientist Marc Meyers has studied everything from the structure of the toucan’s beak to abalone shells. Over the years, he has drawn upon the Amazon, its fauna and flora, as a source of inspiration. This past year, he teamed up with engineering alumnus Jeffrey Lehmann for an expedition down the Roosevelt River, named after the American president who almost lost his life exploring it. In 1914, President Theodore Roosevelt co-commanded with Brazilian colonel Candido Rondon the first scientific expedition down the “River of Doubt,” the trip

Piranha's bite

Measuring the piranha's bite

bringing the former president to the verge of suicide and his party to near-starvation.

After more than a year of preparation, Meyers and Lehmann readied themselves with GPS and satellite image printouts of the states of Mato Grosso, Amazon and Rondonia, in Brazil. Dubbing their journey The Roosevelt River Centennial Scientific Expedition, in honor of their presidential predecessor, the group included Meyers, Lehmann and two Brazilian army colonels, Hiram and Angonese, with a vast knowledge of the Amazon. Together, the group would spend 23 days on the water, travelling and conducting research through the thickest jungles of the Amazon. Lehmann came back determined to make the river’s entire 500 miles a World Heritage Site. He also plans to create a documentary film about his experiences in the Amazon. Col. Hiram already completed a book on the experience, and Meyers as well as Lehmann plan to do the same. Meyers came back with the seeds for a number of research projects (see right column).

Lab notebook: A few
future projects

  • The bite force of the piranha
  • The first scientific measurement of the fish’s bite revealed its strength is surprisingly low. Graduate student Vincent Sherman is investigating why.

  • Rocks from Naivete Falls
  • The Amazon is rife with kimberlite, sometimes encasing diamonds.Studying the rock’s properties will help determine how its deep channels resist erosion.

  • Structure of curassow feathers
  • Grad student Tarah Sullivan will test the strength of this amazingly light and strong material with SEM.

  • Crack propagation in exploding trees
  • Meyers will study whether these cracks reach the speed of sound, generating a sonic boom.

  • Fish scales
  • The trip yielded specimens of armored catfish, whose scales will be studied for bioinspiration in engineering concepts

Why is skin so tough?

Engineers explain why skin resists tearing.

Skin is remarkably resistant to tearing, even after it has been cut. A team of researchers from the Jacobs School and the Lawrence Berkeley National Laboratory (LBL) have explained why. Using powerful X-ray beams and electron microscopy, researchers made the first direct observations of the micro-scale mechanisms that allow skin to resist tearing. They identified four specific mechanisms in collagen, the main structural protein in skin tissue, that act together to diminish the effects of stress: rotation, straightening, stretching and sliding. “Straightening and stretching allow the uptake of strain without much stress increase,

curved collagen fibers collagen fibrils

Left: Arrangement of curved collagen fibers before a tear. Right: collagen fibrils at notched side
of the skin are delaminated and align closely to the direction of the tension caused after tearing.

and sliding allows more energy dissipation during inelastic deformation. This reorganization of the fibrils is responsible for blunting the stress at the tips of tears and notches,” said materials science professor Marc Meyers, who led the UC San Diego team that included graduate student Vincent Sherman.

“We hope to replicate these mechanisms in synthetic materials to provide increased strength and better resistance to tearing,” said Robert Ritchie of LBL, where postdoc Wen Yang played a leading role in the synchroton measurements.

The researchers first established that a tear in the skin does not propagate or induce fracture, unlike other materials such as bone or tooth dentin, which are composed of mineralized collagen fibrils. Instead, the tearing or notching of skin triggers structural changes in the collagen fibrils of the dermis layer to reduce stress concentration. Initially, these collagen fibrils are curvy and highly disordered. In response to a tear, they rearrange themselves in the direction in which the skin is being stressed.

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