MIT’s New Plant-Derived Material Is Tough As Bone and Hard As Aluminum

A new woody composite, engineered by a team at MIT, is tough as bone and hard as aluminum, and it could pave way for naturally-derived plastics. This image shows a tooth printed by the team resting on a background of wood cells. Credit: Figure courtesy of the researchers, edited by MIT News

The material could pave the way for sustainable plastics.

The strongest part of a tree lies not in its trunk or its sprawling roots, but in the walls of its microscopic cells.

A single wood cell wall is constructed from fibers of cellulose ­— nature’s most abundant polymer, and the main structural component of all plants and algae. Within each fiber are reinforcing cellulose nanocrystals, or CNCs, which are chains of organic polymers arranged in nearly perfect crystal patterns. At the nanoscale, CNCs are stronger and stiffer than Kevlar. If the crystals could be worked into materials in significant fractions, CNCs could be a route to stronger, more sustainable, naturally derived plastics.

Now, an MIT team has engineered a composite made mostly from cellulose nanocrystals mixed with a bit of synthetic polymer. The organic crystals take up about 60 to 90 percent of the material — the highest fraction of CNCs achieved in a composite to date.

The team hit on a recipe for the CNC-based composite that they could fabricate using both 3-D-printing and conventional casting. Credit: Figure courtesy of the researchers

The researchers found the cellulose-based composite is stronger and tougher than some types of bone, and harder than typical aluminum alloys. The material has a brick-and-mortar microstructure that resembles nacre, the hard inner shell lining of some mollusks.

The team hit on a recipe for the CNC-based composite that they could fabricate using both 3D printing and conventional casting. They printed and cast the composite into penny-sized pieces of film that they used to test the material’s strength and hardness. They also machined the composite into the shape of a tooth to show that the material might one day be used to make cellulose-based dental implants — and for that matter, any plastic products — that are stronger, tougher, and more sustainable.

“By creating composites with CNCs at high loading, we can give polymer-based materials mechanical properties they never had before,” says A. John Hart, professor of mechanical engineering. “If we can replace some petroleum-based plastic with naturally-derived cellulose, that’s arguably better for the planet as well.”

Hart and his team, including Abhinav Rao PhD ’18, Thibaut Divoux, and Crystal Owens SM ’17, have published their results on February 10, 2022, in the journal Cellulose.

Gel bonds

Each year, more than 10 billion tons of cellulose is synthesized from the bark, wood, or leaves of plants. Most of this cellulose is used to manufacture paper and textiles, while a portion of it is processed into powder for use in food thickeners and cosmetics.

In recent years, scientists have explored uses for cellulose nanocrystals, which can be extracted from cellulose fibers via acid hydrolysis. The exceptionally strong crystals could be used as natural reinforcements in polymer-based materials. But researchers have only been able to incorporate low fractions of CNCs, as the crystals have tended to clump and only weakly bond with polymer molecules.

The team sculpted the composite into the shape of a tooth to show that the material might one day be used to make wood-derived dental implants — and for that matter, any plastic product — that are stronger, tougher, and more sustainable. Credit: Figure courtesy of the researchers

Hart and his colleagues looked to develop a composite with a high fraction of CNCs, that they could shape into strong, durable forms. They started by mixing a solution of synthetic polymer with commercially available CNC powder. The team determined the ratio of CNC and polymer that would turn the solution into a gel, with a consistency that could either be fed through the nozzle of a 3-D printer or poured into a mold to be cast. They used an ultrasonic probe to break up any clumps of cellulose in the gel, making it more likely for the dispersed cellulose to form strong bonds with polymer molecules.

They fed some of the gel through a 3-D printer and poured the rest into a mold to be cast. They then let the printed samples dry. In the process, the material shrank, leaving behind a solid composite composed mainly of cellulose nanocrystals.

“We basically deconstructed wood, and reconstructed it,” Rao says. “We took the best components of wood, which is cellulose nanocrystals, and reconstructed them to achieve a new composite material.”

Tough cracks

Interestingly, when the team examined the composite’s structure under a microscope, they observed that grains of cellulose settled into a brick-and-mortar pattern, similar to the architecture of nacre. In nacre, this zig-zagging microstructure stops a crack from running straight through the material. The researchers found this to also be the case with their new cellulose composite.

They tested the material’s resistance to cracks, using tools to initiate first nano- and then micro-scale cracks. They found that, across multiple scales, the composite’s arrangement of cellulose grains prevented the cracks from splitting the material. This resistance to plastic deformation gives the composite a hardness and stiffness at the boundary between conventional plastics and metals.

Going forward, the team is looking for ways to minimize the shrinkage of gels as they dry. While shrinkage isn’t much of a problem when printing small objects, anything bigger could buckle or crack as the composite dries.

“If you could avoid shrinkage, you could keep scaling up, maybe to the meter scale,” Rao says. “Then, if we were to dream big, we could replace a significant fraction of plastics, with cellulose composites.”

Reference: “Printable, castable, nanocrystalline cellulose-epoxy composites exhibiting hierarchical nacre-like toughening” by Abhinav Rao, Thibaut Divoux, Crystal E. Owens and A. John Hart, 10 February 2022, Cellulose.
DOI: 10.1007/s10570-021-04384-7

This research was supported, in part, by the Proctor and Gamble Corporation, and by the National Defense Science and Engineering Graduate Fellowship.

3D PrintingMaterials ScienceMITPolymersPopularSustainability
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  • Andres Albarran

    This is beautiful it eases my mind to know thaT im not as nuts as i think i might be considered for believing plant molecules can be made to consciously fuse in a method of preservation which is a form of fossilization. creating exotic metals by harnessing and exploiting the aspect of evolution it’s still a long road before everyone sees it as such. it’ll have to be a Beyond Fusion Warhead which is created with Psychotropic particles that have been rewritten to have the structure of apolar neurons it needs to occur for it to be possible at the planetary level as of now it’s theoretical as i believe there’s not enough nerds on this planet the combustion of such a warhead would cause the evolution of radioisotopes and the boost in neurons every species needs.

    • Andres Albarran

      We’d become more peaceful The neurons would manifest in the way of a brand-new outer most wall for each cell in the human body as they would be daltonic state. depending on the variety of tropics used would determine how big of a change would occur i don’t believe physical appearance couldnt be altered at this point yet an alteration with such a method in the future should be the goal as of now our Sprint stance would become a paw print. the warhead would be consciously self encrypted so it’s not something that can be controlled nor created with intentions to harm just to harm for protection purposes in the wild it’d take a very capable individual a warrior that can vocalize the song of a humpback. The possible ingredients are the DEA’s list of schedules yet the possibility of none scheduled is highly likely.

  • Georgios Syriotis

    Was ist mit der Recycling!