When you think of lobsters and armor, the obvious connection would be their hard shells. An international team of scientists, however, is more interested in the flexible translucent membrane on the underside of the crustaceans' tails. It has a unique structure, which could find uses in human technology.

Led by MIT's Prof. Zhao Qin, the researchers determined that the membrane is actually a hydrogel – it's composed of 90 percent water, with the other 10 percent made up mostly of chitin, which is a fibrous material commonly found in shells and exoskeletons. In fact, the membrane actually turned out to be the world's toughest natural hydrogel, beating out materials such as animal skin and natural rubber, and roughly matching the strength of industrial rubber composites.

Additionally, it was found that the membrane actually becomes stiffer and tougher when stretched beyond approximately twice its relaxed length. By contrast, most other tough hydrogels become softer when stretched. Also, even when a scalpel was used to cut half-way through the thickness of the membrane, it could still be stretched just as far without breaking. According to the scientists, this wouldn't be possible with rubber composite materials.

These qualities come thanks to the membrane's microstructure, which is similar to that of plywood. Approximately a quarter of a millimeter thick, the lobster material is made up of tens of thousands of stacked layers of chitin fibers. All of the fibers in each layer are oriented in the same angle, that angle being offset by 36 degrees to that of the layer above. In plywood, the wood grain of each layer sits at a right angle to that of the layers above and below.

When the scientists tested a similar lab-produced hydrogel, but one in which the chitin fibers were randomly oriented, it soon broke when stretched.

"One mystery is how the chitin fibers can be guided to assemble into such a unique layered architecture to form the lobster membrane," says 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."

It is hoped that the research could ultimately lead to more flexible body armor – particularly for covering joints like the knees and elbows – along with applications in fields such as soft robotics and tissue engineering.

A paper on the research, which also involved scientists from Harvard University and China's Sichuan University, was recently published in the journal Acta Materialia.

Source: MIT