Thirty years ago, when the plant researcher in Germany wished to see the internal processes of woody plants without dissecting them. By bleaching the pigments in the plant cells, Siegfried Fink was able to create transparent wood, publishing his technique in the Niche Wood Technology Journal. The research paper from 1992 remained the final word on transparent wood for over a decade until the researcher Lars Berglund stumbled upon it.
The Discovery and Interest in Transparent Wood
Fink’s discovery inspired Berglund, but not for botanical reasons. The materials scientist, working at the KTH Royal Institute of Technology in Sweden, was interested in creating a stronger alternative to transparent plastics. He was not the only one intrigued by the benefits of wood. Across the ocean, researchers at the University of Maryland were busy with a related goal: harnessing the strength of wood for unconventional purposes.
Uses of Transparent Wood
After years of experimentation, the research of these groups began to bear fruit. Transparent wood could find uses in very strong smartphone displays; in soft glowing light fixtures; and even as structural features, such as color-changing windows.
The Composition of Transparent Wood
Wood is made up of countless tiny vertical channels, like a tightly packed bundle of straw glued together. These tubular cells transport water and nutrients throughout the tree, and when the tree is harvested and moisture evaporates, air pockets are left behind. To create transparent wood, scientists first need to modify or remove the glue called lignin that holds the cell bundles together and gives trunks and branches most of their earthy brown hues. After bleaching lignin or removing its color by other means, a milky white structure of empty cells remains.
Characteristics of Transparent Wood
This structure is still not transparent, because the cell walls refract light differently from the air in the cell pockets – a value known as the refractive index. Filling the air pockets with a material such as 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. But the cells create a strong structure like honeycomb, and the small wood fibers are stronger than the best carbon fibers, according to materials scientist Liangbing Hu, who leads the research team working on transparent wood at the University of Maryland in College Park. With the addition of resin, transparent wood surpasses plastic and glass: in tests measuring how easily materials break under pressure, transparent wood came in about three times stronger than transparent plastics like Plexiglas and about ten times more rigid than glass.
Uses of Transparent Wood
Transparent wood could be a great alternative to products made from thin pieces of plastic or glass that can easily shatter, such as display screens. For example, the French company Woodoo uses a similar lignin removal process in their wooden displays, but leaves some lignin to create an aesthetic color difference. The company designs its recyclable and touch-sensitive digital screens for products such as car dashboards and billboards.
However, most research focuses on transparent wood as an architectural feature, with windows being a particularly encouraging use, according to biochemistry engineer Prodyut Dhar from the Indian Institute of Technology. Transparent wood is a much better insulator than glass, so it could help retain or prevent heat in buildings. He and his colleagues have also used polyvinyl alcohol, or PVA – a polymer used in glues and food packaging – to penetrate wood structures, making transparent wood conduct heat at a rate five times less than glass, as reported by the team in 2019 in the journal Advanced Functional Materials.
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In conclusion, it can be said that transparent wood is a sustainable and innovative material that scientists hope to exploit in many future applications.
Source: https://arstechnica.com/science/2023/12/why-scientists-are-making-transparent-wood/
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