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World’s strongest biomaterial now comes from a tree

A new method creates superstrong fibers out of cellulose

by Katharine Gammon
June 19, 2018

Schematic showing cellulose nanofibers flowing through a channel to aliign them into a superstrong material.
Credit: ACS Nano
Cellulose nanofibers are injected into a channel that focuses them and aligns the fibers into a superstrong material. In this schematic, the fibrils’ relative size has been magnified around 300-fold.

Spider silk has long been considered the strongest biological material in the world and has inspired generations of materials scientists to understand and mimic its properties. However, new findings knock spider silk off its pedestal, reporting that engineered cellulose fibers, derived from plant cell walls, are the strongest biobased material (ACS Nano 2018, DOI: 10.1021/acsnano.8b01084). The material is more than 20% stronger than and eight times as stiff as spider silk. It could eventually be used in lightweight biobased composites for cars, bikes, and medical devices, the researchers say.

L. Daniel Söderberg of KTH Royal Institute of Technology and his colleagues took inspiration from trees in their search for lightweight, strong, renewable materials. The outer cell walls of woody trees provide strength and stiffness, helping trees to stand tall. Those cell walls contain cellulose nanofibers, which are aligned and embedded in a matrix of lignin and hemicellulose. That alignment transmits the exceptional strength and stiffness of individual nanofibers to the macroscale properties associated with wood, says study coauthor Nitesh Mittal. Even so, wood is not as strong as the nanofibers themselves because defects in alignment occur, which weaken the material.

Scanning electron micrograph of engineered cellulose fiber made from cellulose nanofibers.
Credit: ACS Nano
Scanning electron micrograph of a cellulose fibril surface shows the near-perfect alignment of tree-derived cellolose nanofibers.

Mittal’s team tried to mimic this structure using commercially available cellulose nanofibers from spruce and pine trees, 2 to 5 nm in diameter and up to 700 nm long. Using a process called hydrodynamic focusing, they squeezed the nanofibers together using streams of water into larger fibers 6 to 8 µm in diameter and up to a meter long.

Using electron microscopy, the team confirmed that the resulting structure mimicked the unique arrangement found in tree cell walls but was even better: The nanofibers aligned nearly perfectly, without defects, in a tight thread.

Further testing revealed that the material had a tensile strength of 1.57 gigapascals, stronger than natural dragline spider silk fibers, the previous reigning champion, whose strength ranges from 0.6 to 1.3 GPa. The strongest cellulose fibers were also 1.2 to 1.5 times as strong as wet-spun carbon nanotubes and graphene fibers, non-biobased materials prized for their strength. The cellulose fibers’ tensile stiffness was 86 GPa, eight times as stiff as silk, allowing it to be used in artificial joints or surgical sutures that require a stretchy but strong material. Artificially assembling these nanofibers makes something stronger than what nature produces, Mittal notes; scientists working on spider-silk-based materials have not made anything stronger than what comes from the spider.

In the future, materials like these could form parts in load-bearing applications like cars and bikes, where most materials come from unsustainable sources or processes that produce large carbon emissions. They could also be used for tissue engineering applications, and the material is strong enough to be used as for body parts like limbs, according to Mittal.

Zhenan Bao, a materials scientist at Stanford University, says the new work is interesting. “The tensile strength they got is impressive for biosynthetic materials,” she says. “The method they used is very simple to realize. This work shows the great potential of controlling structures to achieve remarkable mechanical properties.”


The drawback to this and all wood-based materials is that they are humidity sensitive, Mittal says, which limits their practical application. The big challenge for those working with such materials is to identify other biobased building blocks to mix with nanocellulose that could increase the usefulness of the material.



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