Mussels' Sticky Secrets Revealed by MIT Researchers

Researchers at the Massachusetts Institute of Technology have unraveled the secret to what makes mussels so sticky and the find may prove to be a boon to bio-inspired industrial design.

Unlike other sticky seafarers, such a barnacles, which essentially cement themselves onto a surface, mussels dangle loosely from a surface, attached by thin filaments known as byssus threads.

By not being tightly affixed to a surface, the mussels are able to drift further out in to the water to collect nutrients.

Marine scientists have long been curious about this aspect of mussel behavior because even when being thrashed around by swift ocean currents, the mussels' byssus threads are almost never torn away from their base.

MIT research scientist Zhao Qin and civil and environmental engineering professor Markus Buehler conducted a study of byssus threads to better understand just how such a thin membrane an withstand significant impact forces.

As it turns out, byssus threads are composed of soft, stretchy material on one end and a much stiffer material on the other. Despite having different functionality, the materials are both made of a protein very similar to collagen, the main constituent of bone, cartilage, tendons and skin.

For their experiment, the researcher placed an underwater cage in Boston Harbor for three weeks. During that time the mussels attached themselves to glass, wood, ceramic and clay surfaces in the cage. The captured mussels were later taken back to the laboratory where a tensile machine tested their strength by slowly pulling on the affixed creatures. They then recorded the force applied during the tests.

Mussels anchor their byssus threads to a surface with a sort of natural glue. The glue is strong, but not strong enough to always withstand the impact of waves.

"We figured there must be something else going on," said Buehler. "The adhesive is strong, but it's not sufficient."

The key, the researchers found, is the distribution of stiffness in the mussels' threads. About 80 percent of the of the length of the byssus thread is made of stiff material and 20 percent is made of soft, stretchy material. That distribution is what allows the bivalves to be subjected to large impact forces from waves, perhaps critically, the researchers suggest. The stiff portions attach to the rock, while the soft portions attach to the mussel itself.

"It turns out that the ... 20 percent of softer, more extensible material is critical for mussel adhesion," Qin said.

In the lab, Qin and Buehler tested different ratios, but found that the 80:20 ratio yielded the most stable results. Having a majority of stiff material in the byssus thread prevents the mussels from being pulled too far out by waves, which Qin suggests keeps the mussel from incurring damage.

The researchers suggest the discovery could aid in the design of synthetic structures that would benefit from a combination of stiff and stretchy. Surgical sutures used on blood vessels or intestines, for example, which are subject to pulsating or irregular flows of liquid, could benefit form the industrial design based off the find.

Qin and Buehler's research is published in the journal Nature Communications.