Transparent wood, a groundbreaking material stronger than plastic and tougher than glass, is being developed by researchers at KTH Royal Institute of Technology and the University of Maryland for applications in smartphone screens, insulated windows, and more. Inspired by Siegfried Fink’s 1992 technique of bleaching wood to achieve transparency, this innovation involves removing lignin and filling the wood with resin to match the refractive index of the cell walls. The result is a superior insulator with enhanced mechanical strength, offering significant potential for architectural and technological uses. Researchers are also focusing on sustainable production methods.
Lars Berglund, a specialist in polymer composites, aimed to develop a robust alternative to transparent plastic. Concurrently, University of Maryland researchers explored wood’s strength for unconventional purposes. After years of experimentation, their efforts are yielding promising results. Transparent wood may soon be used in super-strong smartphone screens, glowing light fixtures, and structural features like color-changing windows.
Qiliang Fu, a wood nanotechnologist at Nanjing Forestry University and a former graduate student in Berglund’s lab, envisions a promising future for this material. Wood’s structure, comprising countless vertical channels resembling tightly bound straws, necessitates the removal or modification of lignin to create transparency. After lignin removal, filling the remaining air pockets with a substance like epoxy resin, which bends light similarly to the cell walls, renders the wood transparent.
The material, typically less than a centimeter thick, forms a sturdy honeycomb structure. According to Liangbing Hu, a materials scientist leading the University of Maryland’s transparent wood research, tiny wood fibers are stronger than the best carbon fibers. With added resin, transparent wood surpasses plastic and glass in strength, being approximately three times stronger than Plexiglass and about ten times tougher than glass.
“The results are amazing, that a piece of wood can be as strong as glass,” Hu noted, highlighting transparent wood’s features in the 2023 Annual Review of Materials Research. Thicker wood can be used, but it becomes hazier as it scatters more light. Initial studies by Hu and Berglund found that millimeter-thin sheets allowed 80 to 90 percent of light through, while 3.7-millimeter-thick wood transmitted only 40 percent.
Transparent wood’s slim profile and strength make it an excellent alternative for thin, easily shattered plastic or glass products like display screens. The French company Woodoo uses a similar lignin-removing process for its wood screens, leaving some lignin for a distinct color aesthetic. Their recyclable, touch-sensitive digital displays are designed for products including car dashboards and advertising billboards.
Most research focuses on transparent wood as an architectural feature, with windows being particularly promising. Transparent wood insulates far better than glass, aiding in heat retention or exclusion in buildings. Hu’s team used polyvinyl alcohol (PVA) to create transparent wood that conducts heat five times slower than glass, as reported in Advanced Functional Materials in 2019.
Researchers are tweaking wood’s ability to hold or release heat, useful for energy-efficient buildings. Materials scientist Céline Montanari and colleagues at RISE Research Institutes of Sweden incorporated phase-change materials like polyethylene glycol, enabling wood to store and release heat. Their work, published in ACS Applied Materials and Interfaces in 2019, found that the wood could store heat when warm and release it as it cooled.
Transparent wood windows, stronger and better at temperature control than traditional glass, offer a hazy view similar to frosted glass. This haziness can diffuse light, potentially allowing thicker wood to act as a partially load-bearing light source. Berglund suggests it could serve as a ceiling providing soft, ambient light.
Hu and Berglund continue to explore new properties for transparent wood. Five years ago, Berglund and colleagues at KTH and Georgia Institute of Technology mimicked smart windows that switch from transparent to tinted. They sandwiched an electrochromic polymer between transparent wood layers, creating a pane that changes from clear to magenta with a small electrical current.
Recently, both groups focused on improving the sustainability of transparent wood production. Montanari’s team developed a bio-based polymer from citrus peels as a resin replacement. Combining acrylic acid and limonene, they impregnated delignified wood, creating bio-based transparent wood that maintained mechanical and optical properties, withstanding around 30 megapascals of pressure and transmitting 90 percent of light. This was reported in Advanced Science in 2021.
Hu’s lab reported a greener lignin-bleaching method in Science Advances, using hydrogen peroxide and UV radiation, reducing production energy demands. They brushed wood slices with hydrogen peroxide and exposed them to UV lamps, bleaching pigment-containing lignin parts while retaining structural integrity.
These environmentally friendly methods reduce toxic chemicals and fossil-based polymers in production. Despite this, glass currently has lower end-of-life environmental impacts than transparent wood, according to a Science of the Total Environment analysis by Dhar and colleagues. Embracing greener production and scaling up manufacturing is essential to mainstream transparent wood, but researchers are confident it will become a sustainable material.
“When you’re trying to achieve sustainability, you don’t only want to match the properties of fossil-based materials,” Montanari says. “As a scientist, I want to surpass this.”
