Tuesday, August 6, 2013

Mussel Bungee Cords

A rather interesting insight and unexpected.  It even informs on design options for the architecture of fabrics made from synthetics.  The mussel rides securely on a bungee cord in the active intertidal region in which even the rocks are moving a lot.  This is the environment that happily grinds up concrete in short order.  Yet shell fish do prosper her or more correctly the mussel prospers here as anyone who walks along the edge knows quite well.

It is really quite neat.

We also forget the critical importance of the mussels in early coastal diets of the early rise of man.  I suspect that they alone provide enough protein to allow an initial denser population during the earliest development.

Researchers discover secret to mussels’ magical staying power

The study’s findings could help in the design of synthetic materials with similar properties.
Stephanie Verkoeyen | Wednesday, July 24, 2013

Unlike barnacles, which cement themselves to surfaces, mussels dangle from a collection of fine filaments known as byssus threads, which allows the creatures to drift out into the water to absorb nutrients. This process would also seem to expose them to a risk of dislodgement via crashing waves, yet this almost never happens.

Despite an outwardly frail appearance, in the dynamic environment of waves and currents these threads can withstand impact forces nine times greater than forces exerted by stretching in a single direction.

Zhao Qin and Markus Buehler unraveled the secret to these natural bungee cords. Their findings appear in the journal Nature Communications this week.

The researchers found that byssus threads are composed of about 20 percent of soft, stretchy material and 80 percent of stiffer material. Despite different mechanical properties, both materials are made of a collagen-like protein.

Using a combination of computer modeling and laboratory tests the team carried out a series of experiments to test the threads. Mussels were placed in an underwater cage in the Boston Harbor for three weeks. During this time, the mussels attached themselves to various surfaces in the cage, including glass, ceramics, wood and clay. Once brought to the lab, the mussels, threads and substrates were mounted in a tensile machine to test their strength by pulling with controlled deformation.

Many studies have looked at mussel glue, the sticky substance that anchors byssus threads to a surface. However, the static strength of the glue, or the thread itself, is insufficient to withstand wave impact. Only by measuring the system’s performance in simulated wave conditions were Qin and Buehler able to determine the mechanism behind this amazing tenacity.

The stiffness distribution along the attachment is key, which suggests that the distribution of material properties and overall architecture of the byssus threads are important.

This distribution enables the mussels to endure very large impact forces from waves. The exact ratio of stretchy to stiff material is critical. The soft, stretchy portion of the threads attach to the mussels themselves, while the stiffer material adheres to the rock.

“Like the rest of the field, I certainly never suspected an architectural features of the byssi themselves to be so central to the dynamic resilience of these organisms,” said Guy Genin, a professor of mechanical engineering and materials science at Washington University, in a press release. ”The magic of this organism lies in the structurally clever integration of this compliant region with the stiff region.”

Throughout their simulations, Qin and Buehler methodically tested other composition ratios and discovered that the 80-20 ratio of stiff to soft results in the smallest reaction force. More of the softer material increases the reaction force by increasing deformation, while more of the stiff material prevents the mussels from being pulled too far out by waves, making it easier to hit other objects and sustain damage.

These results could help in the design of synthetic materials with similar properties. There may also be applications for attaching instruments to buildings, or sensors to underwater vehicles.

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