These researchers in Switzerland can get electricity from wood

Researchers at ETH Zurich and Empa chemically modified wood and made it compressible and turned it into a mini-generator. When compressed, it generates an electrical voltage. Such wood can serve as a biosensor or as a building material that harvests energy.

Ingo Burgert and his team at the public research university ETH Zurich and the Swiss federal laboratory Empa have proven that wood is much more than just a building material. Their research improves the properties of wood to use it for new applications. For example, they have already developed high-strength, water-repellent and magnetizable wood.

Now, together with the Empa research group led by Francis Schwarze, the team used one chemical and one biological process to generate electrical voltage from a type of wood sponge. In doing so, they reinforce what is known as the “piezoelectric effect” of wood.

Compression creates tension

When a piezoelectric material is elastically deformed, it generates an electrical voltage. Measurement technology utilizes this phenomenon especially by using sensors that generate a charge signal when mechanically stressed. However, many of the materials often used for these sensors are not suitable for biomedical applications. For example, lead zirconate titanate (PZT) cannot be used on the skin due to the toxic lead and must be specially disposed of.

Wood also has a natural piezoelectric effect, but produces only a very low electrical voltage. If you want to increase the tension, the chemical composition of the wood must be changed – and this also makes it more compressible.

From wooden block to sponge

To transform wood into an easily malleable material, one component of the cell walls must be dissolved. Wood cell walls consist of three basic substances: lignin, hemicellulose and cellulose. ‘Lignin is the stabilizing agent that trees need to grow tall. Without lignin, which binds the cells and prevents the stiff cellulose fibrils from bending, it would not be possible, ”says Burgert.

A few months ago, Jianguo Sun, a doctoral student in Burgert’s team, along with colleagues from ETH and Empa, published a study in ACS Nano which explained how wood can be made deformable if the lignin is chemically removed. As a result, its piezoelectric effect is enhanced.

The researchers achieved this “delignification” by placing wood in a mixture of hydrogen peroxide and acetic acid. The acid dissolves the lignin and leaves a framework of cellulose layers behind. “The process preserves the hierarchical structure of wood and prevents the disassembly of the individual fibers,” explains Burgert.

In this way, a piece of balsa wood becomes a white, wood sponge consisting of layer upon layer of thin cellulose. The sponge can simply be compressed and then return to its original shape. ‘The wooden sponge delivers an electrical voltage 85 times higher than that of indigenous [untreated] wood, ”says Sun.

A mini generator in the wood floor

The team subjected a test block with a side length of about 1.5 cm to about 600 loading cycles. The wooden sponge was surprisingly stable: for each charge, the researchers measured a voltage of about 0.63 volts, which would be suitable for a sensor. In further experiments, the team tested the scalability of this mini generator. When 30 such wooden blocks are connected and charged evenly with the body weight of an adult, enough electricity is generated to offer a simple LCD screen.

Treatment with fungi instead of chemicals

In a follow-up study that fits in Scientific progress, the ETH-Empa research team went a step further to manufacture the wood sponge without using chemicals. The researchers found the solution in nature: the fungus Ganoderma applanatum causes white rot in wood and breaks down the lignin and hemicellulose gently. “Although the electrical voltage generated in the first tests was lower than with chemically treated wood, the fungal process is more environmentally friendly,” says Burgert.

There are clear benefits to such a simple, renewable piezoelectric system. The researchers see different potential applications for the wood sponges – for example as sustainable building materials that harvest energy in the use phase or skin-friendly pressure sensors for medical purposes.

However, a few more steps need to be taken before piezo wood can be used as a biosensor, or even as a parquet floor that harvests electricity. Burgert and his colleagues are now investigating with various partners how to adapt the technology for industrial applications.

News source: ETH Zurich editorial staff

Main photo: Joel & Jasmin Førestbird / Unsplash

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