Q BLUE ECONOMY INSPIRED BY MUSSELS
Scientists are learning how to make materials that are stronger and more stretchy, by studying the threads marine mussels use to secure themselves to rugged intertidal surfaces
Words: Julie Cohen A 16
wide range of polymer- based materials, from tyre rubber and wetsuit neoprene to Lycra clothing and silicone, are elastomers – valued for their ability to fl ex and stretch without breaking
then return to their original form. Making such materials stronger usually
means making them more brittle. That’s because, structurally, elastomers are practically shapeless networks of polymer strands – often compared to a bundle of disorganized spaghetti noodles – held together by a few chemical cross-links. Strengthening a polymer requires increasing the density of cross-links between the strands by creating more links. This causes the elastomer’s strands to resist stretching away from each other, giving the material a more organized structure but also making it stiffer and more prone to failure. Inspired by the tough, fl exible polymeric
byssal threads that mussels use to secure themselves to surfaces in the rugged intertidal zone, a team of researchers affi liated with the University of California, Santa Barbara, (UCSB) Materials Research Laboratory, has developed a method for overcoming the trade-off between strength and fl exibility in elastomeric polymers. “We have made tremendous advances in understanding how biological materials maintain strength under loading,” says report author Megan Valentine, associate professor in UCSB’s Dept of Mechanical Engineering. “In this paper, we demonstrate our ability
to use that understanding to develop useful human-made materials.”
Previous efforts – also inspired by the
mussel’s cuticle chemistry – have been limited to wet, soft systems such as hydrogels. By contrast, the UCSB researchers incorporated the mussel-inspired iron coordination bonds into a dry polymeric system. Such a dry polymer could potentially be substituted for stiff but brittle materials, especially in impact- and torsion-related applications. “We found that the wet network was 25
times less stiff and broke at fi ve times shorter elongation than a similarly constructed dry network,” explains co-author Emmanouela Filippidi, a postdoctoral researcher at UCSB’s Valentine Lab. “That’s an interesting result, but an expected one. What’s really striking is what happened when we compared the dry network before and after adding iron. Not only did it maintain its stretchiness but it also became 800 times stiffer and 100 times tougher in the presence of these reconfi gurable iron-catechol bonds. That was unexpected.” To achieve networks having architecture
and performance similar to those of the mussel byssal cuticle, the team synthesized an amorphous, loosely cross-linked epoxy network and then treated it with iron to form dynamic iron-catechol cross-links. In the absence of iron, when one of the covalent cross-links breaks, it is broken forever, because no mechanism for self-healing exists. But when the reversible iron-catechol coordination bonds are present, any of those iron-containing broken cross-links can reform, not necessarily in exactly the same place but nearby, so maintaining the material’s resilience even as its strength increases.
The material is both stiffer and tougher
than similar networks lacking iron-containing coordination bonds. As the iron-catechol network is stretched, it doesn’t store the energy, so, when the tension is released, the material doesn’t bounce back like a rubber band but, rather, dissipates the energy. The material then slowly recovers to resume its original shape, in much the same way as a viscoelastic material such as memory foam does after the pressure on it is released. “A material having that characteristic,
called an ‘energy-dissipative plastic,’ is useful for coatings,” says co-author Thomas Cristiani, a UCSB graduate student. “It would make a great cellphone case because it would absorb a large amount of energy, so a phone would be less likely to break after an impact.” The dry system the researchers used is
important for two reasons. In a wet system, the network absorbs water, causing the polymer chains to stretch, so there is not much extra fl exibility left. But, with a dry material, the amorphous spaghetti-like strands are initially very compact, with a lot of room to stretch. When the iron cross-links are added to strengthen the polymer, the stretchiness of the dry material is not compromised, because those bonds can break, so the polymer chains are not locked in place. Additionally, removing the water from the network results in the catechol and iron being closer together and able to form regions of high connectivity, which improves the mechanical properties. “This difference between response in wet
and dry systems is huge and makes our approach a game-changer,” says Valentine.
Mussels successfully incorporate iron coordination bonds into dry polymeric systems
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