The lobster's external carapace is more effective than ever, but apparently it is not the most resistant part of the shellfish body. In the translucent and segmented membrane that covers the undertail of these animals, the MIT engineers have identified an extremely resistant material to cuts and abrasions, but at the same time also incredibly flexible: a protection that allows the lobsters not to scratch the belly against the parts rough on the bottom, and to resist the bites of predators.

A model to aspire to. According to scientists, this membrane - composed of 90% water and the rest of chitin - could be the most resistant hydrogel among those discovered in nature: with a reliability that comes close to that of the industrial tires used for tires, garden rods and conveyor belts.

Its properties and composition could inspire the design of the armor and exoskeletons of the future, to be used in the medical or military field: casings that are at the same time ultra-strong, but which leave the mobile parts of the body such as elbows and knees free. Details of the research have been published in the journal Acta Materialia.

Stress test. When the lobsters swim, they give the tail rapid "whipping" to escape predators. In addition to the rigid carapace, therefore, they must have some coating that allows a certain elasticity in the movements. This detail inspired the researchers, who subjected the membrane in question to various stresses.

The mechanical tests revealed that the hydrogel initially lets itself be easily pulled until it reaches twice the initial length. At that point it begins to stiffen and becomes progressively more resistant. This feature is quite unique, due to the materials found in nature, which are usually left to go as they are pulled.

Plywood. Scratch resistance tests have seen the membrane respond to cuts much better than most known gums, while microscopic analysis has revealed the secret of these exceptional properties: the lobster hydrogel is made up of tens of thousands of layers of chitin fibers, each placed at an angle of 36 degrees from the underlying level.

The structure resembles that of industrial plywood, typically consisting of several layers of wood, whose fibers are oriented at right angles to the upper and lower ones. This arrangement is commonly used to obtain very resistant materials, but it is the first time it has been observed in a hydrogel.

For researchers, the membrane hides part of the secret of the evolutionary success of lobsters, on Earth for more than 100 million years. The structure of this envelope will also inspire the design of flexible robots and smart fabrics.