Lobster-Inspired Body Armor

Can Crustaceans Really Stop a Speeding Bullet?

Photo by creozavr, Pixabay

Lobsters are the caped crusaders of the sea. Their body armor is tough. Their claws are strong. Their meat is tasty. But maybe there is more to these bottom-dwelling crustaceans than meets the eye. Scientists at MIT and Harvard think so.

A recent study from Ming Guo’s laboratory at MIT, with collaborators from Harvard University, has cracked the wondrous properties of a lobster’s underbelly. Published in the journal Acta Biomaterialia, the researchers determined both the ‘mechanics and mesoscopic structure’ of American Lobster membranes, with an eye towards developing flexible, yet durable, body armors.

Guo, in a discussion with MIT News, stated that he developed the idea for a lobster-inspired body armor while eating a lobster and noticing ‘that the transparent membrane on the animal’s belly was difficult to chew’. While researchers have long studied the rock-hard exoskeleton of crustaceans, their softer, watery insides have remained a mystery.

Underside of an American Lobster. Photo by Paula Ouder.

In the study, the MIT team studied the white, cartilaginous-like material on the underside of lobster tails and found that it possessed astounding material properties. Though it consists of about 90% water, it is as hard as rubber and is ‘fault tolerant’, meaning that a cut into one layer will not negatively impact the performance of other layers. These properties, coupled with its elastic nature, make it ideally suited for flexible armors.

The authors then performed a serious of tests to determine the properties of the bio-derived material. They found that it could withstand forces up to 23.36 MPa (more than 23 million Pascals), which is roughly equivalent to the water spray pressure of high-powered pressure washers or the chamber pressure of a high-powered air gun.

Though consisting of mostly water, the ‘active ingredient’ in this material is chitin, a chemical polymer consisting of many sugars linked together. Similar to cellulose, chitin is among the most abundant chemicals found in nature — it makes up the hard shells of beetles, spiders and ants.

Chitin is impressive because it can easily be stacked into a three-dimensional lattice, which adds strength without severely impacting flexibility. In the research article, the authors mention that the lobster underside consists of ‘tens of thousands of layers’ that are stacked in a manner akin to the wood fibers in plywood.

Microscopy image of a spider’s legs, emphasizing the rigid chitin material from which the hairs extrude. Photo by Janice Haney Carr, Leah Lowrey, Michael Smith, USCDCP.

Now that the material properties have been delineated and published, it is relatively straightforward to play with the numbers and solve the question on all of our minds:

How many lobsters does it take to stop a speeding bullet?

If the lobster ‘chitin’ belly has a tensile strength of 23.36 MPa (a measure of how resistant a material is to breaking under tension), how many Lobster bellies would it take to stop a a speeding bullet?

Let’s imagine that a bullet weighing 30 grams is fired from a pistol at 150 meters per second (about 500 miles per hour). To stop the bullet in a distance of 10 millimeters (a typical thickness of bulletproof vests), how much force would be required?

Initial velocity of bullet = 150 m/s
Final velocity of bullet = 0 m/s (stopped bullet)
Change in distance (delta_x) = 10 millimeters or 0.01 meters

Using these variables, we can solve for the acceleration of the bullet as it hits the material with the equation:

final velocity² = initial velocity² + 2a(delta_x)

Using this equation, we determine that the acceleration as the bullet hits the body armor would be -1,125,000 m/s². Given a bullet mass of 30 grams and solving for F = ma, we then determine that the force required to stop the bullet is 33,750 Newtons.

This is more than three times the bite force of an adult alligator. Yikes! According to the US National Institute of Justice Body Armor Classification, a Kevlar vest requires 16 layers to stop most handgun projectiles. Kevlar has a tensile strength of 3,620 MPa (154x stronger than the lobster belly).

If 16 layers of Kevlar (each with a tensile strength of 3,620 MPa) can stop this bullet, roughly how many layers of lobster bellies could do the same? Well, a rough estimate would say about 2500.

16 layers of Kevlar * 3,620 MPa = 57920 MPa
Divide this value by 23.36 MPa to determine the layers of chitin needed; ~2500

But this is an overestimate because, as a bullet moves through a material, it must impart some energy to move matter out of its way. Not all of the force is used to penetrate the material. Additional energy is released in the form of heat. A better estimation, therefore, would be somewhere between 100 and 2500 layers (probably closer to the lower end of that range — let’s say 300). Every experiment begins with a guess, so I do not feel too guilty about this approximation.

It is safe to say that lobster body armor will probably not hit store shelves anytime soon. This study is still exciting, though, because it brings scientists one step closer to unraveling the mysteries of nature.

Every animal on earth is imbued with remarkable capabilities that allow them to thrive in the darkest depths of the world’s oceans, fly for thousands of miles without resting or fend off predators hiding in the brush. The more we uncover about these extraordinary creatures, the greater our ability to engineer new materials, build more efficient machines and create a more sustainable future.

This article is based on a scientific article by Jinrong Wu, Zhao Qin, Liangliang Qu, Hao Zhang, Fei Deng & Ming Guo, In Press at Acta Biomaterialia.

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