An international team of researchers has developed a new biodegradable material which is stronger than steel and even than dragline spider silk. The results appear in the journal ACS Nano.
The new material is made of cellulose nanofibers (CNFs), the essential building blocks of wood and other plant life.
“We successfully transferred the unique mechanical properties of CNFs to a macroscopic, lightweight material that could be used as an eco-friendly alternative for plastic in airplanes, cars, furniture and other products,” said Dr. Daniel Söderberg, a scientist at KTH Royal Institute of Technology in Stockholm, Sweden.
“This new material even has potential for biomedicine since cellulose is not rejected by your body.”
Dr. Söderberg and colleagues started with commercially available CNFs that are just 2 to 5 nm in diameter and up to 700 nm long.
The nanofibers were suspended in water and fed into a small channel, just one 1 mm wide and milled in steel. Through two pairs of perpendicular inflows additional deionized water and water with a low pH-value entered the channel from the sides, squeezing the stream of nanofibers together and accelerating it.
This process, called hydrodynamic focusing, helped to align the nanofibers in the right direction as well as their self-organization into a well-packed macroscopic thread.
No glue or any other component is needed, the nanofibers assemble into a tight thread held together by supramolecular forces between the nanofibers, for example electrostatic and Van der Waals forces.
To optimize the process, the researchers used the X-ray light source PETRA III at the Deutsches Elektronen-Synchrotron (DESY) in Germany.
“The X-rays allow us to analyze the detailed structure of the thread as it forms as well as the material structure and hierarchical order in the super strong CNFs,” explained Dr. Stephan Roth, of DESY.
“We made threads up to 15 μm thick and several meters in length.”
The measurements showed a tensile stiffness of 86 GPa (gigapascals) for the material and a tensile strength of 1.57 GPa.
“The bio-based CNFs fabricated here are 8 times stiffer and have strengths higher than natural dragline spider silk fibers,” Dr. Söderberg said.
“If you are looking for a biomaterial, there is nothing quite like it. And it is also stronger than steel and any other metal or alloy as well as glass fibers and most other synthetic materials.”
Nitesh Mittal et al. Multiscale Control of Nanocellulose Assembly: Transferring Remarkable Nanoscale Fibril Mechanics to Macroscale Fibers. ACS Nano, published online May 9, 2018; doi: 10.1021/acsnano.8b01084