Wood that generates electrical voltage

Ingo Burgert's team at ETH Zurich and Empa has often shown that wood is a surprisingly versatile material. In his work, the professor of wood-based materials is often concerned with extending the existing properties of wood to make it suitable for new applications. For example, he has already created exceedingly strong, water-repellent or magnetizable wood.

Now Burgert's team, together with the Empa research group led by Francis Schwarze, has used one chemical and one biological process to generate electrical voltage in a type of wooden sponge. In doing so, they amplify the so-called piezoelectric effect of wood.

Compression generates electrical voltage

When a piezoelectric material is elastically deformed, it generates an electrical voltage. Measurement technology in particular takes advantage of this phenomenon by using sensors that generate a charge signal when subjected to mechanical stress. However, such sensors often use materials that are unsuitable for biomedical applications. For example, lead zirconate titanate (PZT), which is unsuitable for use on the skin due to its toxic lead and must be specially disposed of.

Wood also has a natural piezoelectric effect, but only a very low electrical voltage is generated. If you want to increase the voltage generated, you have to change the chemical composition of the wood, which also makes it more compressible.

From block to sponge

To convert wood into an easily formable material, one component of the cell walls can be removed. Wood cell walls consist of three basic substances: Lignin, hemicellulose and cellulose. "Lignin is the stabilizing substance that trees need to grow far into the air. Without lignin, which binds the cells together and prevents the tensile cellulose fibrils from buckling, this would not be possible," Burgert knows.

A few months ago, Jianguo Sun, a doctoral student in Burgert's team, together with colleagues from ETH and Empa, was able to show in a study published in the journal ACS Nano how wood can be made deformable by chemically removing the lignin - with the result that the piezoelectric effect is enhanced.

The researchers achieved the so-called delignification by placing the wood in a mixture of hydrogen peroxide and acetic acid. The acid dissolves out the lignin. What remains is a framework of cellulose layers. "We take advantage of the hierarchical structure of the wood without having to first completely dissolve it, as in paper production, for example, and then reconnect the fibers," Burgert explains.

A piece of balsa wood is thus turned into a white wooden sponge consisting of thin cellulose layers on top of each other. These can simply be pressed together and then return to their original shape. "The wood sponge generates an 85-fold increase in electrical voltage compared to native wood," says Sun.

A mini-generator in the wood floor

The team subjected a test cube with a side length of about 1.5 cm to about 600 stress cycles. In the process, the wooden sponge proved to be surprisingly resistant: With each load, the researchers measured a voltage of around 0.63 volts, which would be useful for an application as a sensor. In further experiments, the team tested the scalability of this mini-generator. If 30 such wooden blocks are connected to each other and evenly loaded with the body weight of an adult, a simple LCD display can be operated.

Treatment with fungus instead of chemicals

In a follow-up study, which has just been published in the journal Science Advances, the ETH-Empa research team went one step further: the aim was to produce the wooden sponge without chemicals. The researchers found a suitable candidate capable of delignification in nature: the fungus Ganoderma applanatum causes white rot in wood and degrades the lignin and hemicellulose particularly gently. "Although the tension generated in initial tests was lower than with chemically treated wood, the fungal process is more environmentally friendly," says Burgert.

The advantages of such a simple and renewable piezoelectric system are obvious. The researchers see possible future applications for the wood sponges, such as sustainable building materials that generate energy during the use phase, or skin-friendly pressure sensors in the medical field.

However, there are still several steps to be taken before piezo wood can effectively be used as a biosensor or even as an electricity-generating parquet floor. In order to adapt the technology for industrial applications, Burgert and his colleagues are already in talks with possible cooperation partners.

Scientific publiactions:
ACS Nano: Sun et al. (2020) - Sustainable and Biodegradable Wood Sponge Piezoelectric Nanogenerator for Sensing and Energy Harvesting Applications
Science Advances: Sun et al. (2021) - Enhanced mechanical energy conversion with selectively decayed wood

Source: ETH Zürich - Holz das elektrische Spannung erzeugt

Image source: Wikimedia Commons: Brataffe - Füsse auf einem Holzboden