Arapaima Fish Scales are One of Nature’s Toughest Flexible Materials

Arapaima gigas is a large Amazonian fish (weighing up to 150 kg) living primarily in seasonal lakes infested with ferocious piranhas. The freshwater giant has armor-like scales that can deform, but do not tear or crack, when a piranha attacks. According to a new study published in the journal Matter, these scales are one of the toughest flexible materials in nature.

Arapaima gigas. Image credit: Shizhao / CC BY-SA 3.0.

Arapaima gigas. Image credit: Shizhao / CC BY-SA 3.0.

Arapaima’s adaptation naturally solves a problem that engineers face when attempting to develop synthetic armors.

Its scales have a tough, yet flexible, inner layer bound by collagen to its mineralized outer layer of scales.

Similarly, bullet-proof vests are made of several layers of flexible webbing sandwiched between layers of hard plastic. But man-made materials are bound using a third adhesive material, whereas the fish’s scales are bound on an atomistic level; they grow together, weaving into one solid piece.

“A window may appear strong and solid, but it has no give. If something attempted to puncture it, the glass would shatter,” said Professor Robert Ritchie, a materials scientist in the Department of Materials Science Engineering at the University of California Berkeley.

“When nature binds a hard material to a soft material, it grades it, preventing this shattering effect. And in this case, the binding structure is mineralized collagen.”

Other fish use collagen like arapaima does, but the collagen layers in arapaima scales are thicker than in any other fish species.

For fish scales to provide protection from predators without severely compromising mobility, they must be lightweight, flexible, and tough. The arapaima fish scale is a superb example of this, enabling its survival in piranha-infested lakes of the Amazon. These elasmoid scales comprise two layers: a laminate composite of parallel collagen fibrils arranged in a Bouligand-like pattern and a highly mineralized surface layer that prevents initial penetration damage. Yang et al measure its J-integral fracture toughness and find that the crack-growth toughness is 100–200 kJ/m2, representing a very high fracture resistance for a natural material. This toughness results from multiple deformation mechanisms acting in concert in the twisted plywood structure of the scale, involving the collagenous lamellae at varying orientations retarding crack advance through stretching, reorientation, delamination and shear, and fracture. The toughness values obtained for the arapaima scales indicate that they are among the toughest of nature’s flexible biological materials. Image credit: Yang et al, doi: 10.1016/j.matt.2019.09.014.

For fish scales to provide protection from predators without severely compromising mobility, they must be lightweight, flexible, and tough. The arapaima fish scale is a superb example of this, enabling its survival in piranha-infested lakes of the Amazon. These elasmoid scales comprise two layers: a laminate composite of parallel collagen fibrils arranged in a Bouligand-like pattern and a highly mineralized surface layer that prevents initial penetration damage. Yang et al measure its J-integral fracture toughness and find that the crack-growth toughness is 100–200 kJ/m2, representing a very high fracture resistance for a natural material. This toughness results from multiple deformation mechanisms acting in concert in the twisted plywood structure of the scale, involving the collagenous lamellae at varying orientations retarding crack advance through stretching, reorientation, delamination and shear, and fracture. The toughness values obtained for the arapaima scales indicate that they are among the toughest of nature’s flexible biological materials. Image credit: Yang et al, doi: 10.1016/j.matt.2019.09.014.

Professor Ritchie and colleagues hypothesize that this thickness is the secret to the fishes’ defense.

They tested this by soaking cracked arapaima scales in water for 48 hours, then slowly pulling the edges apart while adding pressure to a central point.

As they added pressure, they observed that the part of the mineralized, hard outer layer expanded, cracked, then gradually peeled off.

The scales then localized the crack, containing it and preventing damage from spreading in the twisting structural collagen layer.

If the pressure did break through to the collagen, it deformed the layer instead of breaking it.

“If humans can develop a flexible hierarchical structure that behaves like the collagen layer in the fish scales, better, potentially impermeable, synthetic armors can be made,” Professor Ritchie said.

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Wen Yang et al. Arapaima Fish Scale: One of the Toughest Flexible Biological Materials. Matter, published online October 16, 2019; doi: 10.1016/j.matt.2019.09.014

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