Saltwater mussels are some of the world's clingiest creatures, able to stay stuck to slippery rocks while being thrashed by pounding surf. Now, researchers have designed a polymer that would make these bivalves proud. The stretchy—yet strong—material could lead to a new family of plastics that are tough enough to glue together disparate materials such as wood and metal, and even able to heal themselves when damaged.
Materials scientists have long relied on several strategies for making polymers, which consist of long, chainlike molecules and which can stretch and return to their original shape, like rubber bands. The most common approach forges chemical links, called covalent bonds, between separate polymer chains, turning what looks like a bunch of separate spaghetti strands into a loose, 3D mesh. Such polymers can be stiff, and thereby able to resist being pulled apart. But they typically aren’t strong: If you pull on them too hard they break, like a rubber band yanked too far.
A second strategy places positive and negative electrical charges on separate polymer strands, which then bond together, creating a loose network. These materials can be more flexible than covalently bound polymers, and because the links between opposite charges can reattach after being pulled apart, they can essentially “heal” themselves to regain their original shape.
Mussels have the best of both worlds: They create natural polymer networks with both covalent and charged “ionic” bonds. In recent years, researchers have begun to mimic this approach. They’ve added negatively charged chemical groups called catechols to soft, gellike polymers that already have covalent connections. When they then add positively charged iron atoms to a solution of their polymers, each iron atom grabs multiple nearby catechols on separate polymer strands, creating extra links that help toughen up the soft gels.
The problem is that when these polymers are made in water, as has been the case thus far, the liquid causes the gels to expand like a sponge, says Megan Valentine, a materials scientist at the University of California, Santa Barbara. That makes them nearly fully expanded from the get-go; if you pull on them, they can’t stretch much farther and simply break.
So, Valentine and her colleagues set out to see whether they could adapt the strategy to work with a dry polymer. They started with a gel polymer that harbors a loose network of covalent bonds called polyethylene glycol (PEG). When they synthesized their PEG, they added catechol groups to individual polymer strands. Left to their own devices, catechols readily react with oxygen in either air or water. To prevent this, Valentine and her colleagues temporarily covered the catechols with capping groups. Then, just before strengthening the polymer, they added acid, which tore off the caps. Valentine’s team then spritzed in a small amount of iron atoms, which diffused through the PEG, with each iron atom reacting with multiple catechols, adding a second network of links.
Finally, the researchers dried out their polymer and tested it. They found that the dried polymer was between 100 and 1000 times stiffer than the original PEG yet flexible enough to absorb large amounts of energy before breaking, they report today in Science. This transformed their previous gellike material into one that was strong and flexible like leather. Although this specific polymer isn’t either the strongest or most flexible plastic on the market, adding the secondary network produced a change that’s rarely produced when making a single change to a polymer.
“It’s remarkable to have such an improvement in stiffness,” says Costantino Creton, a materials scientist at the École Supérieure de Physique et de Chimie Industrielles in Paris who was not involved with the work. The question now, he says, is whether the same strategy might work to strengthen other polymers.
It should, says Karen Winey, a materials scientist at the University of Pennsylvania. “There’s no reason why you have to use PEG. It’s quite generalizable.” And because of that, Winey says, “I think it’s really a nice piece of work.”
Valentine adds that she and her colleagues are already exploring this strategy for other polymers. However, she notes, because the new material has already shown that it can withstand forces that would rupture normal PEG-based materials, it might already prove useful in creating tough biomaterials, such as artificial tendons or joints for robots to help prevent wear and tear.