Biomimetic Cement, Self-Healing Polymer Could Revolutionize Materials Game

New innovations in materials science by two of the US’ most respected engineering and research institutions could revolutionize the materials we use for a host of applications across industries.

First, what if the inherent weaknesses of a material actually made houses and buildings stronger during wildfires and earthquakes?

Thanks to the genius of biomimicry, Purdue University researchers have 3D-printed cement paste, a key ingredient of the concrete and mortar used to build various elements of our infrastructure, that gets tougher under pressure, much like the shells of arthropods such as lobsters and beetles. The technique could eventually contribute to more resilient structures during natural disasters.

The idea uses designs inspired by arthropod shells to control how damage spreads between the printed layers of a material — like trying to break a bunch of uncooked spaghetti noodles as opposed to a single noodle.

"The exoskeletons of arthropods have crack propagation and toughening mechanisms that we can reproduce in 3D-printed cement paste," said Pablo Zavattieri, Purdue professor of civil engineering.

3D-printed cement-based materials would give engineers more control over design and performance, but technicalities have stood in the way of scaling them up. Purdue engineers are the first to use 3D printing to create bioinspired structures using cement paste, as shown in a published paper and the frontispiece for an upcoming print issue of the journal, Advanced Materials.

"3D printing has removed the need for creating a mold for each type of design, so that we can achieve these unique properties of cement-based materials that were not possible before," said Jeffrey Youngblood, professor of materials engineering at Purdue.

The team is also using micro-CT scans to better understand the behavior of hardened, 3D-printed, cement-based materials and take advantage of their weak characteristics, such as pore regions found at the "interfaces" between the printed layers, which promote cracking. This finding was recently presented at the 1st RILEM International Conference on Concrete and Digital Fabrication.

"3D printing cement-based materials provides control over their structure, which can lead to the creation of more damage- and flaw-tolerant structural elements like beams or columns," said Mohamadreza "Reza" Moini, a Purdue Ph.D. candidate in civil engineering.

The team was initially inspired by the mantis shrimp, which conquers its prey with a "dactyl club" appendage that grows tougher on impact through twisting cracks that dissipate energy and prevent the club from falling apart.

Some of the bioinspired cement paste elements designed and fabricated by the team using 3D printing techniques include the "honeycomb," "compliant" and "Bouligand" designs, called "architectures," each of which allowed for new behaviors in a 3D-printed element once hardened. The Bouligand architecture, for example, takes advantage of weak interfaces to make a material more crack-resistant, whereas the compliant architecture gives cement-based elements a spring-like quality, even though they are made of brittle material.

The team plans to explore other ways that cement-based elements could be designed for building more resilient structures.

Meanwhile, over at MIT, chemical engineers have designed a material that can react with carbon dioxide from the air, to grow, strengthen and even repair itself. The polymer — which could someday be used as construction or repair material or for protective coatings — continuously converts the greenhouse gas into a carbon-based material that reinforces itself.

The current version of the new material is a synthetic gel-like substance that performs a chemical process similar to the way plants incorporate carbon dioxide from the air into their growing tissues. The material might, for example, be made into panels of a lightweight matrix that could be shipped to a construction site, where they would harden and solidify just from exposure to air and sunlight, thereby saving on the energy and cost of transportation.

The finding was described in another paper in the journal ***Advanced Materials***, earlier this month, by Professor Michael Strano, postdoc Seon-Yeong Kwak, and eight others at MIT and at the University of California at Riverside.

“This is a completely new concept in materials science,” says Strano, the Carbon C. Dubbs Professor of Chemical Engineering at MIT. “What we call carbon-fixing materials don’t exist yet today” outside of the biological realm, he says, describing materials that can transform carbon dioxide in the ambient air into a solid, stable form, using only the power of sunlight, just as plants do.

As the researchers point out, developing a synthetic material that not only avoids the use of fossil fuels for its creation but actually absorbs carbon dioxide from the air has obvious environmental and climate: “Imagine a synthetic material that could grow like trees, taking the carbon from the carbon dioxide and incorporating it into the material’s backbone,” Strano says.

The material the team used in these initial proof-of-concept experiments uses one biological component — chloroplasts, the light-harnessing components within plant cells, which the researchers obtained from spinach leaves. The chloroplasts are not alive but catalyze the reaction of carbon dioxide to glucose. Isolated chloroplasts are quite unstable, meaning that they tend to stop functioning after a few hours when removed from the plant. In their paper, Strano and his co-workers demonstrate methods to significantly increase the catalytic lifetime of extracted chloroplasts. In ongoing and future work, Strano says the chloroplast is being replaced by catalysts that are nonbiological in origin for increased stability.

The material the researchers used — a gel matrix composed of a polymer made from aminopropyl methacrylamide (APMA) and glucose, an enzyme called glucose oxidase, and the chloroplasts — becomes stronger as it incorporates the carbon. It is not yet strong enough to be used as a building material, though commercial applications such as self-healing coatings and crack fillings are realizable in the near term, they say; additional advances in backbone chemistry and materials science are needed before construction materials and composites can be developed.

One key advantage of such materials is their ability to self-repair upon exposure to sunlight or some indoor lighting, Strano says. If the surface is scratched or cracked, the affected area grows to fill in the gaps and repair its own damage.

While there has been widespread effort to develop self-healing materials that could mimic this ability of biological organisms — and promising advancements have been made in textiles development — the researchers say these have largely required outside inputs to function; heating, UV light, mechanical stress, or chemical treatment were needed to activate the process. By contrast, these materials need nothing but ambient light, and they incorporate mass from carbon in the atmosphere, which is ubiquitous.

“Materials science has never produced anything like this,” Strano says. “These materials mimic some aspects of something living, even though it’s not reproducing.”

Because the finding opens up a wide array of possible follow-up research, the U.S. Department of Energy is sponsoring a new program directed by Strano to develop it further.

“Our work shows that carbon dioxide need not be purely a burden and a cost,” Strano says. “It is also an opportunity in this respect. There’s carbon everywhere. We build the world with carbon. Humans are made of carbon. Making a material that can access the abundant carbon all around us is a significant opportunity for materials science. In this way, our work is about making materials that are not just carbon neutral, but carbon negative.”

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