In this modern era of glass and steel, wood might seem like an old-school material. However, researchers say that they have given wood a makeover. It is not only a sturdy materials but transparent and able to both store and release heat.

This could aid in the construction of energy-efficient homes. Researchers hope to develop a biodegradable version. This would increase the eco-friendly credentials, and the material would become an alternative for plastics, glass, or (maybe) even cement!
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"We prepared a material that is multifunctional—it can transmit light very well and also it can store heat. We combined these two functions in a single material."

Céline Montanari of the KTH Royal Institute of Technology in Stockholm, Sweden

Montanari will be presenting this work at the national meeting of the American Chemical Society in Orlando, Florida, this spring.

To produce this material, previous work was built upon; the lignin was removed from balsa wood. Lignin is component that gives wood its strength and color. Acrylic was introduced into the remaining tissue to fill the pores left over from the lignin removal and the hollow vessels that carry water. While acrylic is water repellent, it is not biodegradable. Montanari said this not only helped maintain the wood's structure but also restored its strength and improved its optical properties. The end result was a frosted-looking wood-based material.

This time around, acrylic was mixed with polyethylene glycol. The polyethylene penetrates wood well but has another crucial feature as well: when heated, it absorbs energy and melts; when later cooled, it hardens and releases energy in the process. The team says that this means their material, which would go from semi-transparent to transparent as it heats, could be used to make buildings more energy-efficient. Energy captured from the sun during the day would later be released into the building's interior.

"If you take 100g [about 3.5 ounces] of this transparent wood material with the polyethylene glycol inside, it can absorb up to 8,000J [Joules] of heat, which corresponds to what a 1W [watt] bulb could produce in two hours."

Céline Montanari of the KTH Royal Institute of Technology in Stockholm, Sweden

She added that different types of polyethylene glycol melt at different temperatures. This means that the team could tweak the wood's properties according to its application(s).

Materials that can trap and release heat in this way are not a new idea in construction. Various types are available as an innovative form of insulation to reduce energy consumption and costs. The team says that their approach is different, however, as the natural material reduces the need for oil-based materials and related CO2 emissions.

Montanari says there is plenty of work to still be done. This includes: replacing the acrylic with a biodegradable alternative for certain applications, scaling up production, and carrying out computer models to see how the wood compares with glass.

Materials and Society Professor Mark Miodownik from University College London, in London, England, cautioned against biodegradable engineering, despite not being involved in research. He said it would make the wood less environmentally sustainable, not more.

"We need construction materials to be carbon sinks and so they need to be recyclable and reusable, not biodegradable."

Materials and Society Professor Mark Miodownik

He added that one possibility would be for the new type of lumber to be recovered from old buildings, similar to the way steel is used in "modular construction."
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While he did add that the materials appeared to be a "solution looking for a problem," he said that this approach has precedent. He noted that this is "how many materials have got invented in the past."