02 April 2021

Transparent Wood

Welcome back. I don’t know about you, but I had no idea that wood could be made transparent.

Unbeknownst to me, various researchers have been working to optimize see-through wood. The products have promising properties for buildings and structural applications, including their renewable and abundant source, optical properties, outstanding mechanical performance, low density, low thermal conductivity and potential for multiple applications.

I suppose I shouldn’t be surprised. A few years ago, I owned up to gaps in my material sciences education. If you recall, I blogged how amazed I was that a team of researchers developed a way to transform wood into a product that’s about as strong as steel (see reprinted post, Steel-Like Wood).

Well, some of those same University of Maryland researchers are back to amaze me again, this time with transparent wood. If you’re befuddled by what transparent wood is, don’t be. It’s just what you think it is.

Viewing the sky through a 1 mm (0.04 in) thick sheet of transparent wood (from Fig. 1 of Univ. of Maryland study, advances.sciencemag.org/content/7/5/eabd7342).

Making Wood Transparent
To understand how transparent wood is fabricated, it helps to know a bit about wood’s cellular structure, particularly the organic polymer lignin. In Steel-Like Wood, I wrote:

Wood cells consist of cellulose (about 50%), lignin (about 20% to 25% in hardwoods, 25% to 33% in softwoods) and hemicellulose (the residual). In general, cellulose is wood’s fibrous bulk. Lignin holds the fibers together, filling the cell wall spaces between cellulose and hemicellulose and conferring mechanical strength as well as a more hydrophobic barrier for water transport.

The usual approach to producing transparent wood is to remove the lignin with a solution-based immersion, then add a refractive-index matching polymer to the delignified wood matrix to minimize light absorption and scattering.

The problem, of course, is that removing lignin reduces wood’s mechanical strength. Moreover, employing a solution-based immersion is relatively energy and chemical intensive, lengthens the processing time and generates liquid waste that’s difficult to recycle.

More recent approaches, as exemplified by the University of Maryland study, have tried to address these issues by controlling the degree of delignification.

Solar-Assisted Chemical Brushing
Instead of immersing the wood, the Maryland researchers brush hydrogen peroxide across its surface to modify the lignin, then illuminate the wood with ultraviolet light (sunlight or artificial) to remove the lignin chromophores responsible for its color. Finally, they infiltrate the wood with a refractive-index matching epoxy. 

Schematic illustration of fabricating transparent wood by brushing with hydrogen peroxide and illuminating with sunlight to modify the lignin and make wood colorless, then adding an epoxy (from Fig. 1 of Univ. of Maryland study, advances.sciencemag.org/content/7/5/eabd7342).

Natural wood’s porous structure promotes rapid infiltration and diffusion of the hydrogen peroxide, ultraviolet light trapping and epoxy infiltration. The resulting product is a dense microstructure with low light absorption and scattering and high light transmittance (up to about 90%) whether the wood is cut longitudinally or transversely. The lignin-modified wood is some 50 times stronger than lignin-removed wood.

Compared with solution-based immersion processes, the chemical brushing and UV light approach requires fewer chemicals and less energy, greatly reducing the cost and liquid waste. One very cool benefit is that designable patterns can be easily added to the transparent wood.

Image of transparent wood patterned with a tree-leaf shape and word “Wood” (from Fig. 1 of Univ. of Maryland study, advances.sciencemag.org/content/7/5/eabd7342).

Wrap Up
The study demonstrated a rapid, cost-effective and sustainable approach to fabricating patternable transparent wood that has optical and mechanical properties favorable for energy-efficient building applications and light management. That the approach can use solar energy in the processing opens the application to large-scale industrial production.

As much as I appreciate advances in metals, polymers, ceramics and composites, there’s something comforting about advances in wood. Thanks for stopping by.

Study of solar-assisted fabrication of transparent wood in Science Advances journal: advances.sciencemag.org/content/7/5/eabd7342
Example articles on study:
Examples of earlier reports on transparent wood:

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