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 iron1 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, biomedical2 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 substrate3, 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 force4 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, mitigating5 the power6 of the waves.
Composition of byssus threads
The byssus threads of mussels are the fine filaments7 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 protein8 closely related to collagen9, a main constituent of skin, bone10, cartilage11 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 clay12 in the cage. Back in the lab, the mussels, threads and substrates13 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 adhesion14," 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.
Applications
The researchers say the findings could help in the design of synthetic15 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 conditions16.
The research was published in the 23 July 2013 issue of the journal Nature Communications.
Find out about New Zealand green-lipped mussels.
- iron: A chemical element with the symbol Fe.
- biomedical: Using biological science for medical developments.
- substrate: 1. In ecology, the surface or sediment where an organism lives and grows. 2. In biochemistry, the substance on which an enzyme works.
- force: A push or a pull that causes an object to change its shape, direction and/or motion.
- mitigate: To make less severe or to lessen the intensity. For example, mitigating climate change involves reducing greenhouse gas emissions and enhancing ‘sinks’ (such as forests, oceans and soils) that store the gases.
- power: 1. The rate at which work is done (defined as work divided by time taken). 2. Mechanical or physical energy, force or momentum.
- filament: A long thin strand of a polymeric substance.
- protein: Any of a large class of complex compounds that are essential for life. Proteins play a central role in biological processes and form the basis of living tissues. They have distinct and varied three-dimensional structures. Enzymes, antibodies and haemoglobin are examples of proteins.
- collagen: Any of various tough, fibrous proteins found in bone, cartilage, skin, and other connective tissue. Collagens have great strength and their job is to form strong insoluble fibres that connect cells. Collagen is converted into gelatin when it is boiled.
- bone: A specialised form of connective tissue. The presence of the mineral hydroxyapatite helps to give bone its strength and density.
- cartilage: Fibrous, dense connective tissue that acts as a cushion between bone joints.
- clay: A naturally occurring fine-grained material formed from the chemical weathering of feldspar minerals found in rocks.
- substrate: 1. In ecology, the surface or sediment where an organism lives and grows. 2. In biochemistry, the substance on which an enzyme works.
- adhesion: A binding force that allows unlike particles or surfaces to cling to one another.
- synthetic: Made in a laboratory or factory by a chemical process, usually to imitate a natural process.
- condition: An existing state or situation; a mode or state of being.