CAMBRIDGE, MA – An essential lumber product tucked inside the blades of sleek massive wind turbines is experiencing spot shortages of supply: Balsa wood.

The lightweight wood is used to build  sandwich panel construction that combines light weight and strength. Turbine blades contain arrays of balsa wood strips, much of it sourced from Ecuador, which supplies 95 percent of the world’s demand.

For centuries, the fast-growing balsa tree has been prized for its light weight and stiffness relative to density. But balsa wood is expensive. Natural variations in the wood grain can be an impediment in increasingly precise performance requirements of turbine blades s.

Turbine makers are also producing ever-larger blades—the longest now measure more than 200 feet long, nearly the wingspan of an Airbus A380 jetliner. And they must be engineered to operate virtually maintenance-free for decades. So manufacturers are searching for new sandwich construction material options.

The Harvard School of Engineering and Applied Sciences and the Wyss Institute for Biologically Inspired Engineering have developed cellular composite materials that, like wood, are both light weight and stiff. The researchers say the new materials mimic and improve on balsa, and even on the best commercial 3D-printed polymers and polymer composites available.  

“By moving into new classes of materials like epoxies, we open up new avenues for using 3D printing to construct lightweight architectures,” says principal investigator Jennifer A. Lewis, the Hansjörg Wyss Professor of Biologically Inspired Engineering at Harvard SEAS.

“Balsa wood has a cellular architecture that minimizes its weight since most of the space is empty and only the cell walls carry the load. It therefore has a high specific stiffness and strength,” say Lewis. “We’ve borrowed this design concept and mimicked it in an engineered composite.”

Of particular significance is the way that the fibers can be aligned, through control of the fiber aspect ratio—the length relative to the diameter—and the nozzle diameter.

"This marks an important step forward in designing engineering materials that mimic wood, long known for its remarkable mechanical properties for its weight,” said Lorna Gibson, a professor of materials science and mechanical engineering at the Massachusetts Institute of Technology, who was not involved in this research.

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