Mussels hang loosely from piers and rocks, attached by fine filaments known as byssus threads, which allows them to drift a bit to absorb more nutrients. So why aren't they washed away by waves? The secret is that they secrete a protein closely related to collagen, which is a combination of soft, stretchy material on one end and much stiffer material on the other. This allows mussels to withstand impact forces that are nine times greater than the forces exerted by stretching in only one direction.
Researchers at Massachusetts Institute of Technology, Cambridge, used computer modeling and laboratory tests in a tensile machine designed to test the strength of the mussel glue by pulling on them with controlled deformation and recording the applied force during deformation.
They discovered that only by measuring the system's performance in simulated wave conditions could they determine how the distribution of stiffness along the threads is key, suggesting that the distribution of intrinsic properties as well as the overall architecture of the mussel attachment are important.
The distribution of stiffness in the mussels' threads enables them to be subjected to very large impact forces from waves. About 80 percent of the length of the byssus threads is made of stiff material, while 20 percent is soft and stretchy. This precise ratio may be critical, the researchers found: The soft and stretchy portions of the threads attach to the mussel itself, while the stiffer portion attaches to the rock.
In their simulations, they systematically tested other ratios of the material composition and found that the 80:20 ratio of stiff to soft leads to the smallest reaction force. Having more of the softer material increases the reaction force because the material cannot effectively slow down deformation. Moreover, having more stiff material in byssus threads has other advantages, as it prevents the mussels from being pulled too far out by waves, which could lead to injury.
These findings, they say, could help in the design of synthetic materials to be used as surgical sutures in blood vessels or intestines, which are subjected to pulsating or irregular flows of liquid. It could also perhaps inspire new research into the attachment of tendons to bone.
