Natural hydrogel in American lobster: A soft armor with high toughness and strength.

Homarus americanus, known as American lobster, is fully covered by its exoskeleton composed of rigid cuticles and soft membranes. These soft membranes are mainly located at the joints and abdomen to connect the rigid cuticles and greatly contribute to the agility of the lobster in swimming and preying. Herein, we show that the soft membrane from American lobster is a natural hydrogel (90% water) with exceptionally high toughness (up to 24.98 MJ/m3) and strength (up to 23.36 MPa), and is very insensitive to cracks. By combining experimental measurements and large-scale computational modeling, we demonstrate that the unique multilayered structure in this membrane, achieved through the ordered arrangement of chitin fibers, plays a crucial role in dissipating energy during rupture and making this membrane tough and damage tolerant. The knowledge learned from the soft membrane of natural lobsters sheds light on designing synthetic soft, yet strong and tough materials for reliable usage under extreme mechanical conditions, including a flexible armor that can provide full-body protection without sacrificing limb mobility. STATEMENT OF SIGNIFICANCE: A body armor to provide protection to people who are at risk of being hurt is only enabled by using a material that is tough and strong enough to prevent mechanical penetration. However, most modern body armors sacrifice limb protection to gain mobility, simply because none of the existing armor materials are flexible enough and they all inhibit movement of the arms and legs. Herein, we focus on the mechanics and mesoscopic structure of American lobsters' soft membrane and explore how such a natural flexible armor is designed to integrate flexibility and toughness. The knowledge learned from this study is useful to design a flexible armor for full-body protection under extreme mechanical conditions.