Thirty years ago, when the plant world in Germany wished to see the inner workings of woody plants without dissection. By bleaching pigments in plant cells, Siegfried Fink was able to create transparent wood and published his technique in a specialty journal on niche wood technology. The 1992 paper remained the final word on transparent wood for over a decade until researcher Lars Berglund found it.
The Discovery of Transparent Wood
Fink’s discovery inspired Berglund, but not for plant-related reasons. The materials scientist, working at the KTH Royal Institute of Technology in Sweden, was focused on creating a stronger alternative to transparent plastics. He was not the only one interested in the benefits of wood. Across the ocean, researchers at the University of Maryland were busy with a related purpose: harnessing the power of wood for unconventional uses.
Applications of Transparent Wood
After years of experimentation, the research from these groups began to bear fruit. Transparent wood could find uses in extremely durable smartphone screens; in soft, illuminated lighting devices; and even as structural features, like color-changing windows.
The Composition of Transparent Wood
Wood is made up of countless small vertical channels, like a tight bundle of straw glued together. These tube-shaped cells transport water and nutrients throughout the tree, and when the tree is harvested and moisture evaporates, pockets of air remain. To create transparent wood, scientists first need to modify or remove the glue known as lignin, which binds the cell bundles together and gives trunks and branches most of their earthy brown hues. After bleaching the color of lignin or removing it through other methods, a milky white structure of empty cells remains.
Properties of Transparent Wood
This structure is still opaque, as the cell walls refract light differently than the air in the cell pockets – a value known as the refractive index. Filling the air pockets with a substance like epoxy resin that refracts light similarly to the cell walls makes the wood transparent.
The material scientists worked with is thin – usually less than a millimeter to about one centimeter thick. However, the cells create a strong honeycomb-like structure, and the small wood fibers are stronger than high-grade carbon fibers, according to materials scientist Liangbing Hu, who leads the research group working on transparent wood at the University of Maryland in College Park. When resin is added, transparent wood outperforms plastics and glass: in tests measuring the ease of material breaking under pressure, transparent wood came in nearly three times stronger than clear plastics like plexiglass and ten times stronger than glass.
Applications of Transparent Wood
Transparent wood could be a fantastic alternative to products made from thin pieces of plastic or glass that easily crack, such as display screens. For example, the French company Woodoo uses a similar lignin removal process in their wooden screens but leaves some lignin to create a different color effect. The company designs their recyclable, touch-sensitive digital screens for products like car dashboards and billboards.
However, most research focuses on transparent wood as an architectural feature, with windows being a particularly promising use, according to chemical engineer Prodip Dhar from the Indian Institute of Technology in Varanasi. Transparent wood is a much better insulator than glass, so it can help retain or repel heat in buildings. He and his colleagues also used polyvinyl alcohol, or PVA – a polymer used in adhesives and food packaging – to penetrate the wood structures, making transparent wood conduct heat at a rate five times lower than glass, the team reported in 2019 in the journal Advanced Functional Materials.
In short,
Transparent wood is a potential sustainable material used in various applications, ranging from robust smartphone screens to insulated windows and more. Scientists believe that transparent wood has a promising future and could be a strong alternative to traditional plastic and glass materials.
Source: https://arstechnica.com/science/2023/12/why-scientists-are-making-transparent-wood/?comments=1
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