by Katie Galloway
Ancient Armored Fish May Hold Clues to Lighter, Stronger Bioinspired Armor
If you’ve seen The Dark Knight, you know that early on Batman has an epiphany about his armor. Following a battle with thugs and Rottweilers, he notes, “I need to be lighter, quicker.” But even equipped with the best tech that Wayne Enterprises has to offer, Batman will have to sacrifice a degree of protection to be more mobile. Mobility and protection are often inherent trade-offs in designing armor, so when researchers encountered ancient armored fish that appear to be designed for both, they work to discover their secrets.
Polypterus senegalus (falsely labeled “dinosaur eels”) is an ancient armored fish that grows to about a foot long and lives in shallow, muddy, freshwater lakes in Africa. The skeleton of this 96-million-year-old Cretaceous predator is lined with four-layered interlocking scales that protect it from biting attacks. Recently, a group from MIT set out to discover the structural strategies of these scales that provide protection for the fish. (See their results here.)
From the inside out, the four-layered scales increase in hardness and decrease in thickness. The top layer of ganoine (crystallized apatite) is only 10 micrometers (μm) thick yet it’s harder than iron. The second layer, dentine, is half as hard as iron, more elastic, and five times as thick. By layering these two materials on top of a thick base of more flexible materials (40 μm of isopedine and 300 μm of basal bone plate), the scale, as a whole, is able to prevent and minimize damage from attacks. But why not just make scales entirely out of the hardest material?
The MIT researchers developed a model to test the difference between an all-ganoine scale and the multilayered scale.1 While the all-ganoine scale prevents deeper penetration, it is more vulnerable to catastrophic failure caused by radial cracks. On the other hand, the flexible material in the layered scale does a better job of dissipating energy introduced by a bite, thus preventing radial cracks. At low forces, the outer hard layer resists penetration by deflecting objects. Under greater pressure, the energy from the object pushes beyond the top layer into the softer layers, which distribute the force over a larger area, reducing the overall pressure at the point of contact. (To see this principle in action, press lightly on your fingernail with a pen. The nail easily resists the pen. Push harder and notice that the pressure is distributed across the nail. This is one reason why it’s more common to bruise a nail than to puncture or split one—something to be thankful for when you accidentally smash a finger or a toe!)
By having a scale that is able to resist and absorb bites, the armored fish limits damage and prevents the spread of radial cracks. Additionally, reducing the ganoine content in the scale decreases the weight and stiffness, providing the fish with excellent protection and greater mobility.
Nature continues to reveal amazing designs that inspire us to create new technologies. The knowledge and foresight discovered in natural designs underscores the notion of intelligence behind creation and strengthens the case for a Creator. Bruce Wayne would be wise to learn from this battle-tested fish.
- Benjamin J. F. Bruet et al., “Materials design principles of ancient fish armour,” Nature Materials 7, no. 9 (2008): 748 – 56.