Wood is strong and versatile, but it rots, gets eaten by bugs, and blocks sunlight. Glass is better, but it can shatter and isn’t particularly insulating.
Engineers at the University of Maryland are working on transparent wood that could solve both problems—and even be stronger than glass. Their process decomposes the wood’s lignin molecules and refills the resulting pores with polymer, making the wood clear.
Optical Properties
Transparent wood, also known as “frosted” wood, has become a leading topic of research in recent years. Currently, the most popular method of fabricating transparent wood is to completely remove the lignin through an intensive chemical process, resulting in a clear material with desirable optical, mechanical, and thermal properties. However, this approach has some drawbacks such as a blurred appearance due to the loss of the original cell wall structure and annual growth patterns as well as low mechanical strength.
In order to address these issues, a new approach has been developed that allows maintaining the natural aesthetics and structural features of wood while achieving high optical transmittance and UV-blocking capability. The approach uses a specialized coating that selectively targets the cellulose fiber tubes while leaving other cell walls intact. It also utilizes a polymer with a higher refractive index than the lignin in wood, making it more transparent and eliminating the color of the underlying lignin.
The resulting material is both flexible and strong enough to be used in construction. It has the potential to drastically reduce energy costs in buildings by providing sufficient natural light and improving thermal insulation. It also has the potential to be used in a variety of applications that require a combination of transparency, light-emitting functionality, and rigidity, including photovoltaic cells and electrochromic devices.
Mechanical Properties
Researchers at the University of Maryland have developed a way to make wood transparent, without losing its strength or insulating properties. The team chemically stripped away lignin—the polymer that makes up most of the material in a tree—and then filled the resulting microscopic holes with acrylic, creating a see-through wood that looks like frosted glass. The researchers believe the material could be used to build entire houses that are lighter and more energy efficient than conventional buildings.
Current approaches to transparent wood fabrication usually focus on complete or nearly complete delignification, thereby removing light-absorbing components such as chromophore and extractives. However, such intensive chemical treatments cause degradation of the original wood structure, including cell walls and growth ring patterns. Furthermore, the polymer infiltration procedure is a time-consuming and laborious process. Effective prevention of polymer shrinkage during polymerization is a key question for scaled-up production.
In addition, the mechanical performance of transparent wood is significantly influenced by the alignment of the cellulose nanofibres within the cell wall. As a result, the tensile strength of aesthetic wood is greatly enhanced along the fiber direction but lower perpendicular to it, due to the anisotropic nature of its cell wall structure and geometry. Therefore, achieving the optimum tensile properties of aesthetic wood requires further optimization.
Thermal Properties
Researchers led by Professor Lars Berglund from Swedish KTH University and Professor Liang Bing Hu from the University of Maryland have developed a way to eliminate colour and some chemicals from small wood samples before adding polymers to make them transparent. The material, known as aesthetic transparent wood, boasts a high optical transmittance and a tunable haze while maintaining a low thermal conductivity compared to glass. It is also able to harvest sunlight effectively due to its light-guiding properties. This makes it a promising engineering component in green energy-efficient buildings and wooden architecture.
To achieve these impressive results, the scientists used a combination of spatially selective delignification and epoxy infiltration to alter the cellulose structure of the wood. They managed to keep the original wood patterns while achieving an average transparency of about 80% and a haze of about 93%. The team’s work opens the door for more transparent wood applications and could eventually lead to a whole new generation of sustainable construction materials.
The ability to incorporate functional particles into the cellular structures of wood opens the possibility for a variety of multifunctional transparent wood applications such as magnetic wood, UV-stabilized wood, stimuli responsive wood, and conductive wood [41]. However, the incorporation of inorganic particles negatively affects the mechanical properties of aesthetic transparent wood. Therefore, it is necessary to develop effective strategies to improve the mechanical strength of multifunctional transparent wood.
Electrical Properties
Researchers at the University of Maryland have turned ordinary sheets of wood into transparent material that looks like glass but is stronger and better insulating. The team used a chemical that removes molecules called chromophores from the walls of wood cells to make it clear, and then soaked it in epoxy. The result is a block of clear wood that lets 90 per cent of light pass through it.
The scientists developed a fast, spatially selective delignification procedure that makes the natural wood transparent while preserving its original patterns at the same time. They also designed a polymer that is refractive index-matched with the wood’s lignin to reduce the amount of light scattering inside the material. The resulting aesthetic transparent wood exhibits high average transparency, tunable haze and excellent light-guiding effect over visible wavelengths.
Aesthetic transparent wood can be used to make buildings with a greener design. Because it has low thermal conductivity compared with glass and can harvest sunlight effectively due to its light-guiding properties, it can help save energy and provide comfortable indoor environment. In addition, the mechanical properties of transparent wood are promising for various engineering applications. However, there are challenges in terms of improved transparency and mechanical performance, fabrication process optimization, functionalization routes and advanced applications.