Two scientists from the Massachusetts Institute of Technology (MIT) have solved the puzzle as to how marine mussels remain attached to wood, stone, concrete or iron in spite of continuously impacting waves. They say the results, which show a clever distribution of materials in anchoring byssus threads that are able to absorb energy from impacting waves, could be used to create better glues, biomedical interfaces and quake-resistant building materials.
Combination of stiff and flexible fibres
Unlike barnacles and limpets, which cement themselves tightly to the surfaces of rocks, piers or ships, mussels use a combination of stiff and flexible fibres attached in just the right way to securely attach themselves to a substrate, say the researchers. How this was achieved so effectively has been a mystery as the strength of both the connection with the surface and the threads themselves are less than the force of the impacting waves.
In a press release from MIT, research scientist Zhao Qin and professor of civil and environmental engineering Markus Buehler explain how they isolated the specific structural and composite material design principles in the byssus threads that allow for emphatic energy absorption, mitigating the power of the waves.
Composition of byssus threads
The byssus threads of mussels are the fine filaments commonly known as ‘beards’, which chefs remove before cooking. The researchers found that the threads are composed of a specific combination of soft, stretchy material on one end and much stiffer material on the other. Both materials, despite their different mechanical properties, are made of a protein closely related to collagen, a main constituent of skin, bone, cartilage and tendons.
The researchers placed an underwater cage in Boston Harbour for 3 weeks, during which time mussels attached themselves to the surfaces of glass, ceramics, wood and clay in the cage. Back in the lab, the mussels, threads and substrates were mounted in a tensile machine designed to test their strength by pulling on them with controlled deformation and recording the applied force during deformation.
"Many researchers have studied mussel glue before," Dr Qin says, referring to the sticky substance that anchors byssus threads to a surface. But the static strength of the glue and of the thread itself, "is not sufficient to withstand the impact by waves," he says.
"We figured there must be something else going on," says Professor Buehler. "The adhesive is strong but it's not sufficient."
Ratio of stiff to soft material in threads
The key is apparently the distribution of stiffness along the mussel’s threads. About 80% of the length of the byssus threads is made of stiff material, while 20% is made of softer, stretchy material. 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. "It turns out that the 20% of softer, more extensible material is critical for mussel adhesion," says Dr Qin.
Using computer simulations, Dr Qin and Professor Buehler 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. Having more stiff material in byssus threads prevents the mussels from being pulled too far out by waves, which "would make it easier to hit other objects" and be damaged, says Dr Qin.
The researchers say the findings could help in the design of synthetic materials where energy absorption is required. For example, surgical sutures used in blood vessels or intestines are subjected to pulsating or irregular flows of liquid. The use of materials that combine stiffness and stretchiness, as byssus threads do, might provide advantages. There may also be applications for materials to attach instruments to buildings, sensors to underwater vehicles or sensing equipment in extreme conditions.
The research was published in the 23 July 2013 issue of the journal Nature Communications.
Find out about New Zealand green-lipped mussels.