Study Reveals Soft Membrane Underneath Lobster’s Belly Appears to be Remarkably Tough

When a lobster is turned over on its back, one can see that the underside of its tail is divided into segments linked by a translucent membrane that appears quite weak when compared to the armor-like covering that protects the rest of the crustacean.

However, engineers at MIT and others have found that this soft membrane is amazingly tough, with a microscopic, layered, plywood-like structure that makes it extremely tolerant to cuts and scratches. This deceptively tough film shields the lobster’s belly as the animal runs over the rocky seafloor.

Additionally, the membrane is stretchy to a degree, which allows the lobster to whip its tail from side to side, rendering it difficult for a predator to chew through the tail or pull it apart.

This flexibility is due to the fact that the membrane is a natural hydrogel, made up of 90% of water. Chitin, a fibrous material present in several shells and exoskeletons, constitutes the majority of the rest.

The study results demonstrate that the lobster membrane is the toughest material of all natural hydrogels, including natural rubber, collagen, and animal skins. The membrane is almost as strong as industrial rubber composites, such as those used to produce garden hoses, conveyor belts, and car tires.

The tough but stretchy membrane of the lobster could serve as a guide for designing more flexible body armor, specifically for highly movable parts of the body, like elbows and knees.

We think this work could motivate flexible armor design. If you could make armor out of these types of materials, you could freely move your joints, and it would make you feel more comfortable.

Ming Guo, d’Arbeloff Career Development Assistant Professor, Department of Mechanical Engineering, MIT

The full paper explaining the study results was published online on February 14^th in the journal Acta Materialia. (The journal posted an uncorrected proof on January 31^st.) The co-authors of Guo include Jinrong Wu and Hao Zhang of Sichuan University, Liangliang Qu and Fei Deng of Harvard University, and Zhao Qin, who is a research scientist in the MIT Department of Civil and Environmental Engineering and another senior author of the paper.

Flexible Protection

Guo began investigating the properties of the lobster membrane after dinner with a visitor to his lab.

“He had never eaten lobster before, and wanted to try it,” Guo recalls. “While the meat was very good, he realized the belly’s transparent membrane was really hard to chew. And we wondered why this was the case.”

Although there has been much research dedicated to the lobster’s unique, armor-like shell, Guo discovered that there was limited knowledge about the crustacean’s softer tissues.

“When lobsters swim, they stretch and move their joints and flip their tails really fast to escape from predators,” stated Guo. “They can’t be entirely covered in a hard shell—they need these softer connections. But nobody has looked at the membrane before, which is very surprising to us.”

Hence, Guo and his team started to characterize the properties of the bizarre material. They dissected each membrane into thin slices, and each one was subjected to a variety of experimental tests. They kept some slices in a small oven to dry and later measured their weight. From these measurements, they predicted that 90% of the lobster’s membrane is made up of water, making it a hydrogel material.

They placed other samples in saline water to reproduce a natural ocean environment. They carried out mechanical tests with some of these samples, by placing each membrane in a machine that stretches the sample, while measuring the force applied accurately. They observed that the membrane was initially soft and expanded easily, until a point where it reached almost twice its initial length, and at this point, the material began to stiffen and gradually became tougher and more resistant to stretching.

“This is quite unique for biomaterials,” Guo notes. “For many other tough hydrogels, the more you stretch, the softer they are. This strain-stiffening behavior could allow lobsters to flexibly move, but when something bad happens, they can stiffen and protect themselves.”

Lobster’s Natural Plywood

When a lobster moves across the seafloor, it can scratch against abrasive rocks and sand. The scientists were curious about how resilient the lobster’s membrane would be to such small cuts and scratches. They used a small scalpel to scrape the membrane samples, and then expanded them in the same way as the intact membranes.

We made scratches to mimic what might happen when they’re moving through sand, for example. We even cut through half the thickness of the membrane and found it could still be stretched equally far. If you did this with rubber composites, they would break.

Ming Guo, d’Arbeloff Career Development Assistant Professor, Department of Mechanical Engineering, MIT

After that, the scientists used electron microscopy to focus on the membrane’s microstructure. They observed a structure which was very analogous to plywood. Each membrane, measuring nearly a quarter of a millimeter thick, is made up of tens of thousands of layers. A single layer consists of countless numbers of chitin fibers, which looks like filaments of straw, all aligned at the same angle, precisely 36° offset from the layer of fibers on top. In the same way, plywood is usually made of three or more thin layers of wood, the grain of each layer is arranged perpendicular to the layers above and below.

“When you rotate the angle of fibers, layer by layer, you have good strength in all directions,” stated Guo. “People have been using this structure in dry materials for defect tolerance. But this is the first time it’s been seen in a natural hydrogel.”

Under the supervision of Qin, the team also performed simulations to observe how a lobster membrane would respond to a simple cut if its chitin fibers were arranged like plywood, rather than in completely random orientations. In order to perform this, they initially simulated a single chitin fiber and assigned it certain mechanical properties like stiffness and strength. They subsequently mimicked millions of these fibers and compiled them into a membrane structure made up of either completely random fibers or layers of precisely oriented fibers, analogous to the actual lobster membrane.

It is amazing to have a platform that allows us to directly test and show how identical chitin fibers yield very different mechanical properties once they are built into various architectures.

Zhao Qin, Research Scientist, Department of Civil and Environmental Engineering, MIT

At last, the scientists developed a small notch via both the random and layered membranes, and programmed forces to expand each membrane. The simulation envisioned the stress throughout each membrane.

“In the random membrane, the stress was all equal, and when you stretched it, it quickly fractured,” stated Guo says. “And we found the layered structure stretched more without breaking.”

“One mystery is how the chitin fibers can be guided to assemble into such a unique layered architecture to form the lobster membrane,” stated Qin. “We are working toward understanding this mechanism, and believe that such knowledge can be useful to develop innovative ways of managing the microstructure for material synthesis.”

According to Guo, the materials designed to imitate lobster membranes could not only be useful in flexible body armor but also in soft robotics and tissue engineering. At any event, the results throw new light on the survival of one of nature’s most resilient creatures.

We think this membrane structure could be a very important reason for why lobsters have been living for more than 100 million years on Earth. Somehow, this fracture tolerance has really helped them in their evolution.

Ming Guo, d’Arbeloff Career Development Assistant Professor, Department of Mechanical Engineering, MIT

This work was supported, in part, by the National Natural Science Foundation of China and State Key Laboratory of Polymer Materials Engineering.