Scientists look to mussels for super-strong polymers

If you’ve ever gone down to the seashore and tried to pull mussels off rocks (and hey, who hasn’t?), then you’ll know how tenacious their holdfasts can be - although they can be tugged back and forth, it’s almost impossible to actually remove them. Recently, scientists at Germany’s Max Planck Institute for Colloids and Interfaces analyzed how the delicious mollusks are able to to achieve such a feat of natural engineering. What they discovered could find its way into human technology.

Mussels attach themselves to rocks with a fibrous appendage called the byssus. The individual byssal threads are stiff but stretchy, in order to dissipate the energy of crashing waves. They are produced by the mussel through a process not unlike injection molding. Because they are constantly being blasted with water-borne debris, they have a protective outer cuticle. This cuticle is described as “a biological polymer”, and while it exhibits epoxy-like hardness, it can also stretch up to 100% without cracking.

When viewed under a scanning electron microscope, the byssal cuticles have a knobby appearance. This is because they contain numerous submicron-sized granular inclusions, which are distributed in a continuous matrix. It is believed that when the cuticle is stretched, submicron-sized tears occur in this matrix, hindering the formation of larger cracks.

The cuticles were found to have a high concentration of iron ions, and a modified type of the amino acid commonly called dopa. Dopa is known for its ability to bond with iron ions, creating metal-protein complexes that have a high breaking point, but that also have the ability to pull themselves back together once they have broken. The scientists discovered that the cuticles have a higher density of the dopa-iron complexes around the granular inclusions, while the spaces between the inclusions have less of the complexes - this means that the granules serve as the rigid structure of the cuticle, while the areas between them function in a sacrificial manner, allowing bonds to break before catastrophic failure.

“Nature has evolved an elegant solution to a problem that engineers are still struggling with; namely, how to combine the properties of abrasion resistance and high extensibility in the same material”, says Peter Fratzl, director of the biomaterials department at Max Planck. “Conceivably, this same strategy could be applied in engineered polymers and composites.”