By connecting cellulose nanofibers from wood in a special arrangement, researchers have made a stiff, strong thread. The new biomaterial bests the strength of natural dragline spider silk and specific strength of steel, which are some of the strongest known materials. The bio-based material could be a sustainable alternative to petroleum-based materials in many load-bearing applications. The fibers could find use in light-weight composites for cars and bikes, as well as high-performance medical implants, say its developers, who reported the advance in a recent issue of ACS Nano.

The key to the strength of many materials found in nature is the optimized complex architecture of their nanoscale to macroscale components. In wood, for example, assembly of thin cellulose nanofibrils in cell walls makes the walls strong and stiff.

“Nature has optimized the architecture of natural materials after centuries of evolution,” says Nitesh Mittal, lead author and a doctoral student in mechanics and wood science at KTH Royal Institute of Technology in Stockholm, Sweden. “One of the biggest challenges in the field of materials science is to realize the potential of nanoscale building blocks into macroscale materials.”

Researchers have tried to mimic the nanoscale organization of wood-based cellulose to make strong materials for decades. But it has been a challenge to get the components to stick together and stay aligned. As a result, natural or artificial cellulose composites have been 3–15 times weaker than cellulose nanofibrils.

So Mittal as part of an international research team led by KTH’s Daniel Söderberg came up with a process that relies on structuring components by controlling the flow of nanofibers suspended in water. The researchers started with cellulose nanofibers that are 2–4 nm in diameter and up to 1.2 µm long. They suspended the nanofibers in water that was fed into a 1-mm-wide steel channel. De-ionized water and low-pH water are pumped into the channel from both sides.

The nanofibers first encounter de-ionized water, which supports electrostatic repulsion on the surface of the nanofibers, aligning them along the flow while keeping them from sticking to the surface of the steel channel. The second flow of low-pH water further aligns the nanofibers and promotes molecular attractions between the nanofibers, accomplished by two different methods. In one approach, the nanofibrils bind by supramolecular interactions (hydrogen bonds and van der Waals forces), and in another approach through cross-linking so that the nanofibrils form covalent bonds. These molecular attractions cause the nanofibers to self-assemble into a perfectly aligned and  densely-packed thread. Once the hydrogel fibers form at the channel exit, they are air dried.

The researchers can tune the thread thickness from 5 µm to 20 µm and make it even meters long. The threads have a stiffness of 86 GPa and tensile strength of 1.57 GPa. That makes them 8 times stiffer and  stronger than dragline spider silk, which is considered to be the strongest known bio-based material. The specific strength of the fibers exceeds those of metals, alloys, and silica-based E-glass fibers.

One downside to using nanocellulose is that their mechanical properties degrade in humidity, which could limit practical uses. But, says Mittal, “researchers are working on it and they are trying to combine it with other building blocks to bring down its humidity sensitivity.”

“This is a great breakthrough that shows what we can do with cellulose nanofibers at a macroscopic scale, a record high strength approaching that of its building block,” says Liangbing Hu, a professor of materials science and engineering at the University of Maryland. The material would need a low-cost, rapid manufacturing method so it can be commercialized, he says.

The KTH research team is now working with Swedish company RISE Bioeconomy on scaling up production of the material, and preliminary results look positive, Mittal says.

Read the article in ACS Nano.