Plant-derived compound is strong as bones and tough as aluminum

Cellulose fibers are nature’s most abundant polymer, and the main structural component of all plants and algae. Inside each fiber are cellulose nanocrystals, or CNCs, which are chains of organic polymers arranged in near-perfect crystal patterns.

Cellulose fibers, commonly used in the production of paper and fabric. Image: Choksawatdikorn – Shutterstock

If crystals could be worked into materials in significant fractions, CNCs could be a route to stronger, more sustainable and naturally derived plastics.

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Based on this concept, a team at the Massachusetts Institute of Technology (MIT) designed a compound made essentially of cellulose nanocrystals mixed with a little synthetic polymer. Organic crystals absorb about 60-90% of the material, the highest fraction of CNCs achieved in a composite to date.

According to the researchers responsible for the discovery, published this Thursday (10) in the journal Cellulose, the compound is stronger and stronger than some types of bone, in addition to being harder than typical aluminum alloys. The material has a brick-and-mortar microstructure that resembles nacre, the hard inner lining of the shell of some molluscs.

Plant cell-based dental implants

According to the study, the CNC-based composite could be manufactured in both 3D printing and conventional casting. They printed and cast the compound onto penny-sized pieces of film, which they used to test the material’s strength and hardness.

The team sculpted the compound into the shape of a tooth to show that the material could one day be used to make plant-derived dental implants. Credit: Massachusetts Institute of Technology

They also shaped the composite into the shape of a tooth to show that the material could one day be used to make cellulose-based dental implants — and, in this case, any plastic products — stronger, stronger and more sustainable.

“By creating composites with CNCs at high load, we can arrive at polymer-based materials of mechanical properties that they never had before,” says John Hart, professor of mechanical engineering and co-author of the study. “If we can replace some petroleum-based plastic with naturally derived cellulose, that’s arguably better for the planet too.”

Every year, more than 10 billion tons of cellulose are synthesized from the bark, wood or leaves of plants. Most of this cellulose is used to make paper and fabric, while some is powdered for use in food and cosmetics thickeners.

How was the discovery

Hart and his team sought to develop a composite with a high fraction of CNCs, which they could mold into strong, durable shapes. They started by mixing a synthetic polymer solution 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 be fed through a 3D printer nozzle or poured into a mold.

They used an ultrasonic probe to break up any clumps of cellulose in the gel, making it more likely that the dispersed cellulose would form strong bonds with polymer molecules.

They then let the printed samples dry. In the process, the material shrunk, resulting in a solid compound composed mainly of cellulose nanocrystals.

“We basically deconstruct wood and rebuild it,” says Abhinav Rao, another co-author of the research. “We took the best components of wood, which are the cellulose nanocrystals, and reconstructed them to achieve a new composite material.”

Interestingly, when the team examined the structure of the compound under a microscope, they observed that cellulose grains settled in a brick-and-mortar pattern, similar to nacre architecture. In nacre, this zig-zag microstructure prevents a crack from going straight through the material. The researchers found that this is also the case for their new cellulose compound.

They tested the material’s resistance to cracking, using tools to initiate first nano-cracks and then micro-scales. They found that, at various scales, the composite arrangement of cellulose grains prevented the cracks from splitting the material. This resistance to plastic deformation gives the compound a hardness and rigidity on the border between conventional plastics and metals.

Now the team is looking for ways to minimize shrinkage of the gels as they dry. While shrinkage is not a big issue when printing small objects, anything larger can bend or crack as the composite dries.

“If you could prevent shrinkage, we could keep increasing it, maybe to the gauge scale,” says Rao. “So by dreaming big, we could replace a significant fraction of plastics with cellulose composites.”

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