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Mussel-inspired iron-catechol bonds toughen dry elastomers

By Rachel Nuwer January 24, 2018
Mussel-inspired iron-catechol
Schematic of ionomer-like Fe3+-catechol clusters. The black circles represent iron. The red symbols represent the tetrafunctional diamine cross-linkers. The green symbols represent the catechol groups. The blue strands represent the polymer matrix. The large pink circles highlight the iron-catechol clusters. Credit: Science

Mussels are subjected to the repeated force of waves and to harsh seawater conditions, yet they manage to stick to both rocks and each other. They accomplish this by producing adhesive cuticles that feature iron-catechol bonding, a type of strong metal-ligand coordination complex with near covalent bond energies and the ability to reform, if broken. “Because of the high demands on processing and performance of these adhesive materials, mussels have become a nice model for making manmade materials,” says Megan Valentine, an associate professor of mechanical engineering at the University of California, Santa Barbara, and an associate director of the California NanoSystems Institute. “Bioinspired materials can also help us develop new processing strategies that are nontoxic and much more bio-friendly.”

In a recent issue of Science, Valentine and colleagues described just such a mussel-inspired material—a dry, polymer-based elastomer that is both strong and flexible. Valentine believes it could eventually find application in energy-dissipative building materials used in earthquake-prone zones, or for support for robots or other high-impact loading systems. For example, in construction, energy-dissipative joints made from the material would maintain their strength, but would allow for settling or swaying without failing. In electronics, on the other hand, such materials could be used for casings that prevent damage when the product is dropped.

“There’s a real need for materials that are stronger and stiffer for energy absorption,” she continues. “We really wanted to create a material that maintained those properties in a way that didn’t compromise its extensibility.”  

Indeed, while mussel-inspired materials have been explored and developed earlier, these tend to be soft, gooey, water-laden hydrogels suitable for use as surgical glues—not for picking up heavy loads or surviving high impacts. Stronger materials, on the other hand, are usually brittle.

Valentine and her colleagues got around those limitations by doing away with the water in the typical elastomer system. “The dry nature of the network is what makes this an interesting and very new system—and what gives it its strength,” Valentine says.

The researchers began by synthesizing a loosely cross-linked epoxy network. To do this, they created nonreversible bonds between the epoxy groups of a short-chain bisepoxide and a tetrafunctional diamine cross-linker that, at the ratios the researchers used, leads to an excess of amine groups. They introduce catechols into the network as monoepoxide carrying a triethysilyl-protected catechol group, which reacts with the excess amines. They then added aromatic rings and protective groups; HCl removes the slyl groups and allows the catechol groups to complex with iron to form the noncovalent, reversible crosslinks. The silyl caps also prevent undesirable cross-links due to catechol-catechol oxidative coupling. After removing the protective caps with an acid group, they introduced iron(III) as highly soluble ferric nitrate nonahydrate in bicine/NaOH buffered solutions at pH 7.5.

When dry, the resulting network of iron-catechol bonds proved to be 800 times stiffer and 100 times tougher than the pre-iron material, while also maintaining the material’s stretchiness. The researchers also found that a similarly constructed wet network was 25 times less stiff and broke at a five times shorter elongation than the dry network. A series of detailed follow-up x-ray scattering experiments and spectroscopy measurements allowed the team to understand the structural and chemical features that gave rise to those properties.

“While this specific chemistry has been previously used in many materials—including small molecule adhesives, hydrogels, and other hydrated polymers—this work expands the utility of iron-catechol chemistries to dry elastomers,” says Christopher Bettinger, an associate professor of biomedical engineering and materials science at Carnegie Mellon University, who was not involved in the research. “The resulting material offers a unique combination of mechanical properties that has the toughness of a robust O-ring and the extensibility of a rubber band.”

Valentine and her colleagues plan to refine the material for real world application, first by addressing issues of water intrusion. For now, the material must be kept dry in order to be useful for testing or applications, but the team plans to update its chemistry to include more hydrophobic properties. Bettinger added that he would like to see the researchers look into how the material ages, as catechols are sensitive to oxidation, and to investigate whether the polymer network has any adhesive properties.

In addition to working out those remaining issues, Valentine also has bigger goals. Given her and her colleagues’ deep understanding of the new material’s properties, the researchers’ goal is to eventually replicate similar properties in other types of materials, beyond elastomers. “That’s where we’re moving toward,” she says.

Read the abstract in Science.