Around three decades ago, a German botanist expressed a simple desire to observe the internal workings of woody plants without the need for dissection. Through a technique involving the removal of pigments in plant cells, Siegfried Fink successfully developed transparent wood. Published in 1992, Fink’s method remained the definitive guide on see-through wood until Lars Berglund, a materials scientist from KTH Royal Institute of Technology in Sweden, stumbled upon it.
Berglund drew inspiration from Fink’s discovery, not for botanical exploration, but to address the need for a more robust alternative to transparent plastic in the realm of polymer composites. Simultaneously, researchers at the University of Maryland were exploring the use of wood’s strength for unconventional purposes. After years of experimentation, the efforts of these groups are beginning to show promise, with transparent wood potentially finding applications in durable smartphone screens, soft glowing light fixtures, and structural elements like colour-changing windows.
Wood consists of numerous vertical channels resembling a bundle of straws bound together with glue. These tube-shaped cells play a crucial role in transporting water and nutrients throughout a tree. Creating transparent wood involves modifying or eliminating the glue, known as lignin, which holds the cell bundles together and imparts the characteristic brown hues to trunks and branches. After removing or bleaching lignin, a milky-white skeleton of hollow cells remains. To achieve transparency, the air pockets within the skeleton are filled with a substance like epoxy resin, matching the refractive index of the cell walls.
The resulting material is thin, typically less than a millimetre to around a centimetre thick. Despite its slim profile, the honeycomb structure created by the tiny wood fibres imparts remarkable strength, surpassing even the best carbon fibers. With the addition of resin, transparent wood outperforms plastic and glass, exhibiting three times the strength of transparent plastics like Plexiglass and ten times the toughness of glass in tests measuring fracture resistance under pressure.
Transparent wood’s potential applications extend beyond smartphone screens, with architectural features, especially windows, being a promising avenue. Its superior insulation properties compared to glass make it an attractive option for aiding in heat retention or dissipation in buildings. Researchers have also experimented with incorporating polyvinyl alcohol (PVA) into wood skeletons, resulting in transparent wood that conducts heat at a rate five times lower than glass.
Beyond structural enhancements, researchers are exploring additional modifications to enhance wood’s thermal properties for use in energy-efficient buildings. By incorporating phase-change materials, which can store or release heat, transparent wood could offer improved temperature control. Recent advancements include the development of a fully bio-based polymer derived from citrus peels as a sustainable alternative to petroleum-derived resin. Additionally, greener lignin-bleaching methods, such as using hydrogen peroxide and UV radiation, have been explored to reduce environmental impact.
While these eco-friendly approaches aim to minimize the use of toxic chemicals and fossil-based polymers, an analysis suggests that glass still holds an environmental edge over transparent wood in terms of end-of-life impacts. To integrate transparent wood into mainstream markets, researchers emphasize the importance of embracing sustainable production practices and scaling up manufacturing, acknowledging that these steps will take time. Despite the challenges, scientists express confidence in transparent wood’s potential as a sustainable material with properties that surpass those of fossil-based alternatives.
“When you’re trying to achieve sustainability, you don’t only want to match the properties of fossil-based materials,” says Céline Montanari, a materials scientist at RISE Research Institutes of Sweden. “As a scientist, I want to surpass this.”
