Skip to main content
Biomaterials

Biomaterials

Bioinspired mechanical gradients make engineered wood

04 Apr 2019 Isabelle Dumé
Taking inspiration from trees
Taking inspiration from trees for density gradients and fibre adjustment on two different length scales (in the tree branch hole and wood ray). Courtesy: M Frey, ETH Zurich

A technique that shapes delignified wood while it is wet and then densifies and dries it can be used to turn wood into a versatile engineering material that could make for a sustainable alternative to glass fibre composites. Applications in the automotive industry are possible thanks to its excellent mechanical properties.

“We began by first removing the lignin matrix (which acts as an adhesive between the wood fibres, or tracheids) in the plant cell walls and in between the cells (the cell corners),” explains Marion Frey of ETH Zurich in Switzerland, who is lead author of this study.  “When wet, delignified wood is malleable thanks to the presence of water in the region between cells, which allows for shear movement between the now decoupled cells.”

Mimicking trees

The researchers exploited this property to orient the wood fibres in the direction they wanted and shape the wood into 3D shapes with the fibres perfectly aligning to the shape. This technique mimics the way trees adapt the direction in which fibres are aligned in response to changing environmental conditions or external loading.

“Upon drying, delignified wood shrinks and the distance between neighbouring cells decreases,” explains Frey. “This combined with deformation in the cell walls (and especially at the corners of cells), causes mechanical interlocking and hydrogen bonding between the cells, which leads to connected neighbouring fibres. This compensates for the removed lignin matrix and results in a dry cellulose material with high stiffness and strength.”

Tuning strength and stiffness

Frey and colleagues say they are able to tune this strength and stiffness by creating mechanical density gradients. They do this by densifying the material locally or by stacking its cellulose layers. Trees also adapt densities naturally to reduce stress concentrations under certain conditions.

Shaped delignified wood

The researchers studied the stiffness and strength of the material at different densities by conducting tensile tests on densified cellulose materials with fibre volume contents (FVCs) between 20% and 85%. They measured tensile elastic moduli in the range of 5 to 35 GPa and tensile strengths of between 50 to 270 MPa by changing the FVC. “It is important to note that our material is entirely matrix free,” says Frey. “The high stiffness and strength come entirely from the mechanical interlocks mentioned above.”

The wood produced can be “frozen” into the desired structure and be made water-proof by dip-coating it in a suspension of titanium dioxide nanoparticles dispersed in PDMS/THF.

Sustainable alternative to glass fibre composites

“Our work opens a new avenue for multifunctional bio-based materials with gradients encoded into their architecture,” Frey tells Physics World. “After protecting the matrix-free wood from water uptake, it could be used as a sustainable alternative to glass fibre composites. We foresee, in particular, applications in the automotive industry.”

The team, reporting its work in Advanced Science, says that it is now busy further investigating the effect of different delignification treatments on the formability and mechanical performance of the final composite. “We are also looking into functionalizing the material – with magnetic particles, for instance – and upscaling the concept for industrial applications,” reveals Frey.

Copyright © 2024 by IOP Publishing Ltd and individual contributors