Scientists seeking stronger glues have been looking to nature for inspiration, trying to figure out what keeps barnacles anchored on rocks and how geckos climb effortlessly up walls. But all that attention to mimicking adhesives may be missing the whole picture, according to a study that finds that when it comes to mussels rooted to rocks or boat hulls, the strength of the glue is only part of the story.
Researchers at the Massachusetts Institute of Technology were initially interested in the adhesive patches that mussels use to anchor themselves, even in rough surf. The glue turns out to use an elegant, simple mechanism to adhere — it is made up of proteins that form bonds like the ones found in water molecules. But by itself, the glue didn’t appear to be strong enough to account for mussels’ tenacity and ability to stick to surfaces.
“When we computed the adhesion strength of these mussels and we compared it to the forces we’d expect from these waves, we found the wave would be way stronger than the strength of the adhesion bonds, and they should break immediately — which they don’t, in nature,” said Markus Buehler, a professor of civil and environmental engineering at MIT who led the research.
So Buehler got interested in the web of “byssus threads” that connect the adhesive to the mussel, tethering the mollusk to a rock or ship’s hull like an anchor’s chain. Perhaps, they thought, those threads had special properties that had not been fully appreciated.
Research scientist Zhao Qin dropped a type of underwater cage into Boston Harbor and returned three weeks later to harvest mussels that could then be systematically studied in the laboratory to understand the strength of the bond of the byssus threads.
In a study published in the journal Nature Communications last Tuesday, the researchers found that the byssus threads’ unusual composition — made up of 80 percent stiff materials and 20 percent soft and stretchy materials — accounted for their remarkable ability to cling to surfaces. The majority of the length of the thread is stiff, researchers found, with the flexible part of the thread attaching to the mussel.
Buehler thinks that engineers can take inspiration from mussels. Usually, he said, engineers approach problems by taking disparate materials, such as steel and cement, and attaching them with something else — a glue, for example, or a bolt. Those junctions between one material and another also tend to be weak spots, where an engineering project is more likely to fail. The simple mussel, Buehler said, mixes together the stiff and weak properties in one continuous material.
“If an engineer would make this, they would take a stiff polymer and glue on a soft polymer, and at some point the interface between the two is going to break,” Buehler said. “This I think for engineers is a good lesson because engineers pick a material and you build something from it; you don’t think of optimizing both” structure and building blocks.
To follow up the work, Buehler is trying to create synthetic versions of the threads, using 3-D printing methods. The technology could be useful in anchoring sensors or other machines in a marine environment, similar to how mussels use it. It also has potential for medical applications in which tiny devices that emit drugs or act as sensors need to be anchored in a blood vessel, staying put as blood pulses around them.