There’s a reason humans have been building homes from wood for 10,000 years. It’s an excellent building material—durable, renewable, good at insulating. But since it’s opaque, we’re still in the habit of cutting windows in our houses and installing panes of glass.

But what if wood could be made transparent?

Researchers in Sweden have been trying to do just that. They’ve created a clear wood material they hope could one day be used for more energy-efficient building.

“We chemically modified wood to make it transparent without losing the mechanical properties,” says Céline Montanari, a researcher at the KTH Royal Institute of Technology in Stockholm who recently presented her work at the national meeting of the American Chemical Society.

Montanari and her team took balsa wood and removed its lignin, the polymer that helps make wood rigid, and filled the resulting microscopic holes with acrylic. The resulting wood looked rather like the frosted glass of a shower door. Then they took the material one step further, mixing it with polyethylene glycol, a so-called “phase-change material” that melts at 80 degrees Fahrenheit. When it melts, it absorbs energy and becomes transparent, then solidifies and releases energy when cooled.

In theory, this means the polyethylene glycol-infused wood material could be used to make windows that absorb energy during the hot part of the day and release it into the home when it got cold at night.

“Transparent wood is already a great insulating material when compared with glass,” Montanari says. Adding the polyethylene glycol makes it an even more potentially eco-friendly material by turning it into a “thermal battery” to keep indoor temperatures constant.

The transparent wood is also strong and durable, and could potentially bear far more weight than glass. The team is also working on making it entirely biodegradable. This could make it an appealing material for things like skylights, with the potential to reduce energy costs in homes and other buildings.

Making transparent wood by removing the lignin and refilling the pores with polymer is not a new technique, says Mark Swihart, a professor of chemical and biological engineering at the University at Buffalo. Materials that store and release energy are not new either.

“The thing that seems to be new is their approach to the energy storage,” Swihart says.

Using wood as a base material is attractive because it has strength and mechanical resistance that’s hard to reproduce in synthetic materials at the same cost, Swihart says. Whether or not the new transparent wood-polymer blend will be successful will depend in large part on how cheaply it can be made, he says. It’s also difficult to get new materials widely adopted by the building industry, as there are existing supply chains for things like glass.

“[The researchers] have to find the case where early adopters are going to pay extra for this,” Swihart says.

Montanari and her team are currently looking at increasing the heat storage capacity of the transparent wood, for greater energy savings. They’re also investigating how to ramp up production to an industrial scale. They hope to have a product ready for market within five years.

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