Can't see the wood for the trees? Scientists are increasingly modifying wood to make it transparent, offering a range of environmental benefits for the planet.
Wood has been used for construction and furniture for thousands of years, thanks to its "versatility, renewability and aesthetic appeal", said the University of Maryland researchers in the latest Annual Review of Materials Research. Now, as the climate crisis and resource scarcity accelerate interest in sustainable materials, scientists are devising methods to chemically or physically modify the structure of wood.
These advancements "open the door" to potentially replacing non-renewable materials, said Interesting Engineering. Transparent wood could soon be used in "super-strong screens for smartphones", said Knowable Magazine, as well as "in soft, glowing light fixtures" and even "color-changing windows".
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"I truly believe this material has a promising future," said Qiliang Fu, a wood nanotechnologist at Nanjing Forestry University in China.
What happened?
In 1992, a German botanist named Siegfried Fink "had a simple wish", said Jude Coleman in Knowable Magazine: "to see the inner workings of woody plants without dissecting them".
By bleaching away pigments in plant cells, he managed to create "transparent wood", publishing his technique in "a niche wood technology journal". That paper "remained the last word on see-through wood for more than a decade", until Swedish materials scientist Lars Berglund "stumbled across it", said Coleman.
Berglund, of the KTH Royal Institute of Technology in Sweden, was interested in using the technique to create "a more robust alternative to transparent plastic". In the US, researchers at the University of Maryland were busy on "a related goal". Three decades later, the research of both groups is "starting to bear fruit".
How does it work?
Wood is made up of "countless little vertical channels", said Coleman, like "a tight bundle of straws bound together with glue", which transport water through the tree.
Scientists can remove the glue – or lignin – that holds the bundles together and gives branches their brown hues. When you bleach lignin's colour away or remove it, a "milky-white skeleton of hollow cells remains". It is still opaque, but filling those hollow cells with a substance like epoxy resin bends light differently, rendering the wood transparent.
The process previously involved soaking away the lignin with sodium chlorite, explained New Scientist. But this is wasteful, chemical-intensive, and can weaken the wood.
In 2021, the Maryland researchers came up with a method to modify the lignin, rather than removing it, which is "quicker and uses fewer materials", as well as leaving the wood stronger.
This moved the world "a step closer to windows that are far better insulators than traditional glass ones", said New Scientist.
What could we use it for?
Transparent wood is about three times stronger than transparent plastics like Plexiglass, and about 10 times tougher than glass. That makes it an excellent alternative to products made from thin, easily shattered plastic or glass – such as phone screens.
Transparent wood modified under the newer method "allows more than 90 per cent of light to pass through it", said New Scientist, and is more than 50 times stronger than transparent wood with the lignin completely removed.
"It could be used for load-bearing windows and roofs," said materials scientist Liangbing Hu, who leads the Maryland research group.
Transparent wood could also help regulate heat more efficiently, according to a study by the RISE Research Institutes of Sweden in March. The researchers developed a wood composite thermal battery, capable of storing both heat and cold.
Modifying wood "will provide the basis for a range of new wood-based materials", said the Maryland researchers in their report – "or a carbon-neutral future."
