In the realm of metamaterials creation, the prevailing notion has consistently been “more robust is superior.”

Metamaterials are engineered substances featuring microscopic configurations that impart remarkable qualities to the overall material. There has been a substantial emphasis on crafting metamaterials that exhibit greater strength and rigidity compared to traditional alternatives. However, there exists a compromise: The more rigid a material, the less adaptable it becomes.

Engineers at MIT have recently discovered a method to produce a metamaterial that is both resilient and elastic. The foundational substance is usually quite stiff and fragile, but it is fabricated in detailed, elaborate patterns that yield a structure that is both sturdy and pliable.

The secret to the new material’s dual characteristics lies in a blend of rigid microscopic struts and a softer woven design. This microscopic “double network,” created using a plexiglass-like polymer, resulted in a material that could extend over four times its original size without completely failing. In contrast, the polymer in other variations has minimal to no elasticity and fractures easily once damaged.

The researchers believe that the innovative double-network configuration can be utilized for other materials, such as to produce flexible ceramics, glass, and metals. Such resilient yet pliable substances could be fashioned into tear-resistant fabrics, adaptable semiconductors, electronic chip packaging, and robust yet yielding scaffolds for cellular growth in tissue repair.

“We are venturing into this new domain for metamaterials,” states Carlos Portela, the Robert N. Noyce Career Development Associate Professor at MIT. “One could print a double-network metal or ceramic and reap numerous benefits, including requiring more energy to break them, while also being considerably more elastic.”

Portela and his collaborators disclose their discoveries today in the journal Nature Materials. His co-authors from MIT include primary author James Utama Surjadi, along with Bastien Aymon and Molly Carton.

Inspired gel

Together with other research teams, Portela and his associates have primarily created metamaterials by printing or nanofabricating microscopic lattices using conventional polymers akin to plexiglass and ceramics. The particular configuration, or architecture, they print can bestow remarkable strength and impact resilience to the resulting metamaterial.

Several years back, Portela pondered whether a metamaterial could be derived from a naturally stiff substance, patterned in such a manner that it would transform into a significantly softer, stretchable alternative.

“We recognized that the domain of metamaterials hasn’t truly explored the realm of soft matter,” he remarks. “To date, we’ve all been in search of the stiffest and most robust materials available.”

Instead, he sought a method to engineer softer, more elastic metamaterials. Foregoing the printing of microscopic struts and trusses typical of conventional lattice-based metamaterials, he and his team developed an architecture comprising interlaced springs or coils. They discovered that, although the material they employed was stiff like plexiglass, the resultant woven metamaterial was soft and spring-like, similar to rubber.

“They were elastic but excessively soft and compliant,” Portela recollects.

In their quest to reinforce their softer metamaterial, the team drew inspiration from an entirely different substance: hydrogel. Hydrogels are soft, elastic, Jell-O-like materials primarily made up of water and some polymer framework. Researchers, including teams at MIT, have devised methods to create hydrogels that are both soft and elastic, yet robust. They achieve this by merging polymer networks with significantly differing properties, such as a naturally rigid network of molecules that is chemically cross-linked with another molecular network that is fundamentally soft. Portela and his associates speculated whether such a double-network design could be adapted for metamaterials.

“That was our ‘aha’ moment,” Portela states. “We asked ourselves: Can we derive inspiration from these hydrogels to construct a metamaterial exhibiting similar rigid and elastic characteristics?”

Strut and weave

For their recent investigation, the team created a metamaterial by merging two microscopic architectures. The first is a hard, grid-like framework of struts and trusses. The second is a coil arrangement that intertwines around each strut and truss. Both networks are fabricated from the same acrylic plastic and are printed simultaneously using a high-precision, laser-based printing method known as two-photon lithography.

The researchers produced samples of the new metamaterial inspired by the double-network, each ranging in size from several square microns to several square millimeters. They subjected the material to a series of stress assessments, wherein they attached either end of the sample to a specialized nanomechanical press and measured the force required to separate the material. They also recorded high-resolution videos to monitor where and how the material stretched and tore during the process.

The findings indicated that their new double-network design was capable of extending three times its own length, which also differed by being 10 times further compared to a conventional lattice-patterned metamaterial produced with the same acrylic plastic. Portela explains that the elastic resistance of the new material arises from the interplay between the rigid struts and the more complex, coiled weave as the material is stressed and extended.

“Imagine this woven network as a tangle of spaghetti entwined around a lattice. As we disrupt the solid lattice network, those fractured sections come along for the ride, resulting in all this spaghetti becoming intertwined with the lattice pieces,” Portela clarifies. “That encourages more entanglement between woven fibers, which translates to greater friction and enhanced energy dissipation.”

In essence, the softer structure woven throughout the material’s rigid lattice absorbs more stress due to multiple knots or entanglements created by the fractured struts. As this stress disseminates unevenly throughout the material, an initial crack is unlikely to traverse directly through and instantly split the material. Furthermore, the team discovered that by introducing calculated holes, or “defects,” in the metamaterial, they could further disperse stress the material experiences, enhancing its elasticity and increasing its resistance to tearing.

“One might assume this would degrade the material,” remarks study co-author Surjadi. “However, we found that as we began to introduce defects, we doubled the extent of stretch we could achieve and tripled the energy we dissipated. This results in a material that is both rigid and durable, which is normally a contradiction.”

The team has created a computational framework that enables engineers to predict how a metamaterial will behave based on the design of its stiff and elastic networks. They envision that this blueprint will be instrumental in developing tear-resistant fabrics and materials.

“We also aspire to apply this technique to more fragile materials to provide them with multifunctional abilities,” Portela expresses. “While we have only discussed mechanical properties thus far, what if we could also render them conductive, or responsive to temperature? For this purpose, the two networks could be fabricated from different polymers that react to temperature in various manners, allowing a fabric to open its pores or become more yielding when warm and transform into a more rigid state when cold. That’s an avenue we can investigate now.”

This research received partial support from the U.S. National Science Foundation and the MIT MechE MathWorks Seed Fund.