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Process converts wood into a squishy sensor

Scalable approach may provide energy storage solutions

by Carmen Drahl
March 26, 2018 | A version of this story appeared in Volume 96, Issue 13

A hand holds a dandelion seed, with a lightweight wood carbon sponge balanced atop the dandelion.
Credit: Courtesy of Chaoji Chen
The wood carbon sponge is lightweight enough to balance atop a dandelion seed head.

Carbon-based materials that bounce back when you press them are in demand for flexible electronics and energy storage devices alike. Most of the available options have limitations, though: They come from nonrenewable sources, have inferior electrical properties, or are difficult to manufacture on scale. Now, researchers have revealed a compressible material with promising electrical properties from a sustainable, if somewhat surprising, source: wood.

University of Maryland, College Park, engineer Liangbing Hu presented the work last week at the ACS national meeting in New Orleans.

To make the spongy material, Hu, postdoctoral researcher Chaoji Chen, and colleagues were inspired by honeycombs, which have a wavy lattice that stands up to compression. They hypothesized they might be able to chemically change wood’s architecture to achieve a similar effect. Indeed, by stripping away lignin and hemicellulose from a block of balsa wood, they transformed the wood’s rigid, boxlike cell walls into a lattice of arch-shaped layers of cellulose, confirmed by scanning electron microscopy. Balsa wood’s thin cell walls likely contributed to the success, Chen told C&EN. “We tried other kinds of wood but failed to achieve the compressible structure,” he said.

Credit: Courtesy of Chaoji Chen/University of Maryland
Standard wood carbon, generated by heating wood at high temperatures (left), collapses under reasonable pressure, whereas the new wood carbon sponge (right) bounces back through about 10,000 cycles of compression.

The wood sponge rebounded from about 10,000 cycles of compression, and it functioned reliably in a prototype strain sensor that could be used to track physical activity. “When we press the sponge, the contact area between the curved layers increases,” thus boosting electrical conductivity, Chen explained. The researchers published this work on March 1 (Chem 2018, DOI: 10.1016/j.chempr.2017.12.028).

Herbert Sixta, an expert on wood chemistry at Finland’s Aalto University who was not present at Hu’s talk, called the team’s publication “one of the most innovative I have read in a long time.” Sixta pointed out, however, that the researchers employed boiling sodium hydroxide and hydrogen peroxide to remove lignin and hemicellulose, noting that greener routes are available.

“We will test greener methods in the future,” Chen said. In the meantime, the team has filed a provisional patent application and intends to commercialize the technology through Inventwood, a company Hu cofounded.


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