Photo of a lobster showing its tough but flexible underbelly membrane.Traditional medieval suits of armor can weigh up to 50 kg and even modern body armor sacrifices mobility for protection. But now researchers think the soft underbelly of lobsters could hold a clue to developing a material that is tough and strong as well as flexible [Wu et al., Acta Biomaterialia (2019), https://doi.org/10.1016/j.actbio.2019.01.067].
“The knowledge learned from the soft membrane of the American lobster sheds light on the design of a synthetic material that is both tough and flexible, which could eventually enable armor for full body protection without sacrificing mobility,” says first author of the study, Jinrong Wu of Sichuan University.
Together with colleagues at Massachusetts Institute of Technology and Harvard University, the researchers studied the membrane found in the joints and abdomen of lobsters, which is tough enough to protect the creature’s underbelly from the rough sea bottom and predators while being sufficiently stretchy to allow the crustacean to whip its tail back and forth.
“The lobster membrane is actually a hydrogel, which has a water content of up to 90%,” explains Wu.
Within this soft, flexible hydrogel is a small amount of the strong fibrous material chitin, which also makes up the lobster’s outer shell or cuticle and many other hard natural materials. The chitin takes up a plywood-like structure of aligned fibers in layers that are slightly offset, rotated by approximately 36°, with respect to each other. Each of these layers is only weakly bound to its neighbors. The combination of highly aligned fibers in weakly interacting layers gives the material its unique combination of strength, toughness, and flexibility.
“Such high toughness and tensile strength, together with such a low initial modulus, provide the basis for the lobster membrane to be extremely flexible in the relaxed state, while becoming much stiffer to protect it from tearing at large deformations,” points out Wu.
Although similar in strength to conventional rubber composites, the weakly interacting layers in the lobster membrane suppress crack propagation so that even large defects don’t lead to catastrophic failure. While this type of structure has been observed in hard biomaterials, it has not been identified in soft natural materials before, says Wu.
The researchers believe that their findings will inspire the design of synthetic materials that are strong, tough, and defect-insensitive.
“Since the structure of the lobster membrane is quite simple and straightforward, it [should be possible] to fabricate next-generation multifunctional soft materials with similar structures,” says Wu.
These novel soft materials could lead to flexible, full body armor or high performance composite materials for industrial applications. The researchers are now working on developing such materials and understanding how chitin fibers assemble into such highly ordered architectures.