Plastic-free wood becomes highly transparent through alkali treatment, with clarity shaped by its internal structural direction

Osaka, Japan - Researchers at The University of Osaka have developed a highly transparent material made entirely from natural wood without adding plastic and uncovered why some wood becomes clearer than others. Their study reveals that transparency depends not only on chemical treatment but also on the direction of the wood’s internal microscopic structure. The findings open new possibilities for sustainable, plastic-free transparent materials for energy-efficient buildings and next-generation devices.

Wood is normally opaque because it contains lignin and countless microscopic air cavities called lumens, which scatter light. Removing lignin turns wood white and translucent but achieving true transparency has been challenging.

The research team focused on delignified wood treated with potassium hydroxide (KOH). They discovered that alkali treatment removes most of the remaining hemicellulose and changes the chemical state of carboxyl groups in the cell walls. These changes soften the wood’s internal cellulose microfibril skeleton. When the treated wood is dried, the softened cell walls collapse more completely, reducing internal air gaps and dramatically decreasing light scattering. As a result, the material becomes highly transparent—without polymer impregnation or plastic additives.

The team demonstrated that the transparency of the material depends on the wood’s anisotropic (direction-dependent) structure. Wood has three main directions: longitudinal, radial, and tangential. After alkali treatment and drying, tangential sections of wood became significantly more transparent than radial sections. At a wavelength of 550 nm, radial samples transmitted about 59% of light, while tangential samples reached about 69%. When reinforced with a transparent polymer for comparison, tangential composites achieved over 90% transmittance. This optical anisotropy arises from how cellulose microfibrils are arranged inside the cell walls. The tangential sections undergo more complete collapse of internal lumens during drying, leading to a denser structure and higher transparency.

Transparent wood has attracted global attention as a sustainable alternative to glass and plastics. However, the reason why delignified wood sometimes remains cloudy was not fully understood. By clarifying the mechanism behind transparency enhancement and identifying the importance of structural direction, this study provides design principles for future bio-based transparent materials. Such materials could contribute to energy-efficient architectural panels, lightweight optical components, flexible wood-based electronics, and other environmentally friendly technologies.

“Our results show that transparency is not only a matter of removing lignin,” said senior author Professor Masaya Nogi at SANKEN. “The intrinsic anisotropic structure of wood, specifically the cellulose microfibril skeleton, plays a decisive role. Understanding this directional effect gives us new freedom to design sustainable transparent materials from natural resources.”

###

The article, “Anisotropic Transparency of Alkali-Treated Wood,” was published in Macromolecular Materials and Engineering at DOI: https://doi.org/10.1002/mame.202500389

About The University of Osaka
The University of Osaka was founded in 1931 as one of the seven imperial universities of Japan and is now one of Japan's leading comprehensive universities with a broad disciplinary spectrum. This strength is coupled with a singular drive for innovation that extends throughout the scientific process, from fundamental research to the creation of applied technology with positive economic impacts. Its commitment to innovation has been recognized in Japan and around the world. Now, The University of Osaka is leveraging its role as a Designated National University Corporation selected by the Ministry of Education, Culture, Sports, Science and Technology to contribute to innovation for human welfare, sustainable development of society, and social transformation.
Website: https://resou.osaka-u.ac.jp/en