Researchers from ETH Zurich and Empa have chemically modified wood and made it more compact, turning it into a mini generator. When compressed, it generates an electrical voltage. Such wood could serve as a biosensor or as a building material that harvests energy.
Ingo Burgert and his team from 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 in order to use it for new applications. For example, they have already developed high-quality, water-repellent and magnetizable wood.
Now the team, along with the Empa research group led by Francis Schwarze, has used one chemical and one biological process to generate electrical voltage from a type of wood sponge. In doing so, they enhance what is known as the “piezoelectric effect” of wood.
Compression creates tension
When a piezoelectric material is deformed elastically, it generates an electrical voltage. Measurement technology in particular makes use of this phenomenon by using sensors that generate a charge signal under mechanical stress. However, many of the materials commonly used for these sensors are not suitable for biomedical applications. For example, lead zirconate titanate (PZT) cannot be used on the skin because of its toxic lead and must be specially disposed of.
Wood also has a natural piezoelectric effect, but only generates a very low electrical voltage. If one wants to increase the tension, the chemical composition of the wood has to be changed – and this also makes it more compact.
From log to sponge
To convert wood into an easily deformable material, a component of the cell walls must be dissolved. Wooden cell walls consist of three basic materials: lignin, hemicellulose and cellulose. “Lignin is the stabilizing substance that trees need to grow tall. That would not be possible without lignin, which connects the cells and prevents the stiff cellulose fibers from kinking, ”says Burgert.
A few months ago, Jianguo Sun, a PhD 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 malleable when the lignin is chemically removed. As a result, the 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. “The process preserves the hierarchical structure of wood and prevents the individual fibers from falling apart,” Burgert explains.
In this way, a piece of balsa wood becomes a white, wooden sponge, built up from layer upon layer of thin cellulose. The sponge can be simply compressed and then returns to its original shape. “The wooden sponge generates an electrical voltage 85 times higher than native [untreated] wood, ”says Sun.
A mini generator in the wooden floor
The team subjected a test cube with a side length of approximately 1.5 cm to approximately 600 loading cycles. The wooden sponge turned out to be surprisingly stable: for each load 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 together and evenly loaded with the body weight of an adult, enough electricity is generated to power a simple LCD display.
Treatment with mold instead of chemicals
In a follow-up study just published in Science Advancesthe research team at ETH-Empa went one step further and tried to produce the wooden sponge without using chemicals. The researchers found the solution in nature: the fungus Ganoderma applanatum causes white rot in wood and gently breaks down the lignin and hemicellulose. “Although the electrical voltage generated in the first tests was lower than with chemically treated wood, the mold process is more environmentally friendly,” says Burgert.
There are clear benefits to such a simple, renewable piezoelectric system. The researchers see various possible applications of 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, there are still a number of steps to be taken before piezo wood can be used as a biosensor, or even as a parquet floor that stores electricity. Burgert and his colleagues are now investigating with various partners how the technology can be adapted for industrial applications.
News source: editors ETH Zurich
Main picture: Joel & Jasmin Førestbird / Unsplash
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