This material can be used for smartphone screens, insulated windows, etc.
Thirty years ago, a botanist in Germany had a simple wish: to see the inner workings of woody plants without dissecting them. By bleaching the pigments in plant cells, Siegfried Fink succeeded in creating transparent wood and published his technique in a niche journal of wood technology. The 1992 article was the final word about transparent wood for more than a decade, until a researcher named Lars Bergland stumbled upon it.
Berglund was inspired by Fink's discoveries, but not for botanical reasons. The materials scientist, who works at KTH Royal Institute of Technology in Sweden and specializes in polymer composites, is interested in creating a stronger alternative to transparent plastics. He wasn't the only one interested in the merits of wood. Across the ocean, researchers at the University of Maryland are also busy with a related goal: to use the strength of wood for non-traditional purposes.
Now, after years of experimentation, the research of these groups is beginning to bear fruit. Transparent wood may soon be used for smartphones' ultra-strong screens, soft, glowing light fixtures, and even structural features like discolored windows.
Fu Qiliang, an expert in wood nanotechnology at Nanjing Forestry University in China, said: "I really believe that this material will have a good future. He was a graduate student in the laboratory in Berglund.
Wood is made up of countless small vertical channels, like bundles of straw glued together. These tubular cells carry water and nutrients throughout the tree, leaving behind bags of air when the tree is harvested and the water evaporates. To make transparent wood, scientists first need to modify or remove a gel called lignin, which holds the cell bundles together and gives the trunk and branches much of their earthy brown hue. When lignin is bleached or otherwise removed, it leaves behind a milky white hollow cytoskeleton.
At this point, the skeleton remains opaque because the cell wall bends to light differently than the air in the cell pocket – a value known as the refractive index. Filling the pores with something like epoxy resin that bends the light to a similar point to the cell wall can make the wood transparent.
Scientists study materials that are thin, usually between a millimeter and a centimeter thick. But the cells form a strong, honeycomb-like structure, and the tiny wood fibers are stronger than the best carbon fibers, said Hu Liangbing, a materials scientist who leads the Clear Wood Research Group at the University of Maryland, College Park. With the addition of resin, transparent wood outperforms plastic and glass: in tests that measure how easily a material breaks or breaks under pressure, transparent wood is about three times stronger than transparent plastics such as plexiglass and about 10 times harder than glass.
Hu Liangbing, in the 2023 Materials Research Annual Report, highlighted the properties of transparent wood, saying that the results are amazing, and a piece of wood can be as strong as glass.
This process also works well for thicker woods, but the vision through this material is more blurred because it scatters more light. In their original 2016 study, both Hu Liangbing and Berglund found that millimeter-thick resin-filled wooden skeletons allowed 80 to 90 percent of the light to pass through. When the thickness is close to one centimeter, the light transmittance decreases: Berglund's research team reports,37 mm thick wood (about two coins thick) only penetrates **40% of the light.
The material's slim profile and strength mean that it can be an excellent alternative to products cut from thin, easily broken plastic or glass, such as displays. For example, the French company Woodoo uses a similar lignin removal process in its wooden screens, but leaves a little lignin to create a different color aesthetic. The company is tailoring touch-sensitive digital displays for products including automotive dashboards and billboards.
However, Prodyut Dhar, a biochemical engineer at the West Indian Institute of Technology in Varanasi, believes that most research has focused on transparent wood as an architectural feature, with windows being a particularly promising use. Transparent wood is a much better insulator than glass, so it can help buildings retain heat or keep it out. Hu and his research team also used polyvinyl alcohol, a polymer used in glue and food packaging, to penetrate into the wooden skeleton to create transparent wood that conducts heat at a rate one-fifth lower than glass, a study published in Advanced Functional Materials in 2019.
Researchers have also proposed other ways to improve the ability of wood to retain or release heat, which can be useful for energy-efficient buildings. Céline Montanari, a materials scientist at the Rise Institute in Sweden, and his colleagues have experimented with phase change materials, where they go from storage to heat release as they turn from solid to liquid, and vice versa. For example, by incorporating polyethylene glycol, the scientists found that their wood could store heat when it warms and release it when it cools, and their work was published in the 2019 issue of ACM Applied Materials and Interfaces.
As a result, clear wood windows are stronger than traditional glass and help with temperature control, but the view through them will be blurry and more like frosted glass than regular windows. However, if the user wants diffused light, this blurring can be an advantage: due to the strength of the thicker wood, it can be a partially load-bearing light source, Berglund says, potentially as a ceiling, providing soft ambient light to the room.
Hu Liangbing and Berglund have been working on ways to give transparent wood new properties. About five years ago, Berglund and his colleagues at KTH and Georgia Tech discovered that they could simulate smart windows that could switch from transparent to tinted to block visibility or the sun's rays. The researchers sandwiched an electrochromic polymer, a substance that can change color by electric current, between a transparent layer of wood coated with a conductive electrode polymer. This creates a kind of plank that turns from transparent to magenta as the user passes a small electric current.
Recently, the two organizations have shifted their attention to improving the sustainability of transparent wood production. For example, the resin used to fill wooden scaffolding is often a petroleum-derived plastic product, so it's best to avoid it, says Céline Montanari, who invented a fully bio-based polymer extracted from citrus peels as an alternative. The team first combined acrylic acid and limonene, a chemical extracted from lemon and orange peels and found in essential oils. Then they impregnated it in lignin-delignified wood. Researchers reported in the 2021 journal Advanced Science that the bio-based transparent wood retains its mechanical and optical properties, withstanding about 3,000 megapascals of pressure and transmitting about 90% of light than ordinary wood, even with fruit filling.
Meanwhile, Hu's lab recently reported in the journal Science Advances on a more environmentally friendly method of lignin bleaching that relies on hydrogen peroxide and ultraviolet radiation, further reducing energy requirements in the production process. The research team brushed 05 to 35 mm thick wood chips, then place them in front of a UV lamp to simulate the sun's rays. UV bleaching whitens the pigmented part of the lignin but retains the structural part, thus helping to maintain more of the strength of the wood.
These more environmentally friendly methods help limit the amount of toxic chemicals and fossil-based polymers used in production, but according to an analysis by Dahl and colleagues in Total Environmental Science, glass still has a lower environmental impact than transparent wood so far. Adopting greener production options and scaling up production are two necessary steps to bring transparent wood to the mainstream market, but it will take time, the researchers say. However, they believe it can be done and believe in its potential as a sustainable material.
"When you're trying to be sustainable, you don't just want to match the performance of fossil-based materials," says Céline Montanari. As a scientist, I want to go beyond that. ”
If you like it, please pay attention to "Know the New"!