Wood seems to be the gift that keeps on giving. Researchers have tinkered with its chemistry and physical structure to make it transparent, squishy, strong as steel, filter water, and turned it into bioplastic and Styrofoam-like insulation. But they are not done coaxing it to do even more.
In the most recent engineering endeavor, researchers report a way to fold flat sheets of hardwood and mold into 3D shapes that are lightweight but exceptionally tough. The structures are six times stronger than the raw wood that the researchers start with, and as strong as commonly used structural materials such as aluminum alloys.
“For the first time we’re able to mold wood in the way you mold plastic or metal,” says Liangbing Hu, director of the Center for Materials Innovation at the University of Maryland, who led the work reported in Science. “In addition to properties of a material, shape really matters.”
The work could open up brand new uses for wood in furniture, vehicles, and buildings. Plastics and aluminum are commonly used in these applications because they are strong, lightweight, and can be easily molded into various shapes.
But wood is a cheaper, more sustainable alternative, Hu says. It does not carry the environmental footprint of petroleum-based plastics and the energy burden of processing metals. In addition, it is a renewable resource that can help sequester carbon dioxide as it grows.
To broaden wood’s applications, researchers including Hu have fundamentally changed its properties. They usually remove lignin—the glue-like component that gives wood strength and rigidity—to make it see-through or squishy, for instance. The approach retains the natural structure of wood that imparts strength, such as the hollow, parallel channels and fibers that run along its length.
To make the new moldable wood, Hu and his colleagues first removed about half the lignin from a piece of basswood using a special water-based chemical solution. This softens the wood as it swells mildly. Next, they air-dry the wood for 30 hours. As the water evaporates, the cell walls shrivel, and the channels and fibers in the wood close. Then the researchers soak the shrunken wood in water just for a few minutes. “The dry wood is very thirsty for water so sucks it up very quickly,” Hu says.
But the short, quick burst of hydration makes the cell walls swell so fast, it’s almost like a shock wave went through them. The result? Under a microscope, the wood shows a unique structure with wrinkled cell walls, channels partially open, and fibers mostly closed. The accordion-like cell wall structure allows the wood to be compressed and be pulled taut.
The researchers used the technique to make roll up wood; twist it into a helix, fold it into a star, and make a 3D honeycomb structure that looks like corrugated cardboard. They also report folding engineered wood in half and opening it back up 100 times without it breaking. A similarly thick aluminum sheet cracks after just three cycles of folding and unfolding.
Source: Shaoliang Xiao et al. Lightweight, strong, moldable wood via cell wall engineering as a sustainable structural material. Science, 2021.
Video credit: Liangbing Hu