A study suggests that even the parquet floor could one day supply green electricity: when deformed, wood can generate usable voltage due to the piezoelectric effect, scientists report. The results show that the natural power of a wood-degrading fungus can also be used to make the material more elastic for this purpose. As can be seen from performance tests with the piezo wood elements, they could be suitable for use as medical sensors and even for generating energy.
Turbine, rotor, bicycle dynamo … In addition to the usual options for generating electrical energy from mechanical processes, there is another interesting alternative: So-called piezoelectric elements can convert deformation energy into electricity. Pressure shifts the positive and negative center of gravity in solids. This creates electrical voltage that can be used to generate energy. So far, however, the phenomenon has been mainly used in measurement technology: Piezoelectric elements can serve as sensors for mechanical loads. In addition to technology, this also offers application potential in medicine.
Stress due to deformation
However, the usual piezo elements mostly consist of lead-zirconate-titanate, which is not suitable for use in connection with body tissues due to the lead content. In addition, the disposal of conventional piezo elements is problematic. That is why biologically and ecologically compatible alternatives are in demand. Wood moved into the focus of the researchers working with Ingo Burgert from the Eidgenössische Materialprüfungs- und Forschungsanstalt (Empa) near Zurich. It has long been known that a piezoelectric effect also occurs when this natural solid is subjected to pressure. Due to the low deformability of conventional wood, there is only a very low tension that can hardly be used.
To solve this problem, the researchers use a process that can take the hardness out of wood: delignification. As they explain, wood cell walls are made up of three basic substances: lignin, hemicelluloses and cellulose. “A tree needs lignin above all in order to be able to grow high up. This would not be possible without lignin as a stabilizing substance that connects the cells and prevents the stiff cellulose fibrils from buckling, ”explains Burgert. By loosening the lignin, wood can, conversely, be converted into a deformable material. And that is exactly what the researchers show. First, they eliminated the lignin by soaking balsa wood in a suspension of hydrogen peroxide and acetic acid. The chemicals dissolve the lignin from the material – what remains is a whitish framework made of elastic cellulose layers.
“Fungal attack” also makes it elastic
In order to make the concept completely natural, the researchers also found a way to get by without the aggressive chemicals: The biological lignin remover is the Ganoderma applanatum fungus, which is known to cause so-called white rot in wood. “This fungus breaks down the lignin in the wood very gently,” explains co-author Javier Ribera from Empa. In addition, the process can be easily controlled in the laboratory, which has proven to be an ecological alternative to chemical treatment.
As the researchers report, both treatment methods lead to a wood-sponge fabric, which consists of superimposed, thin layers of cellulose with favorable material properties: They can simply be pressed together, but then expand again to their original shape. “With this concept, we make use of the hierarchical structure of the wood without first having to break it down, as is the case in paper production, for example, and then having to reconnect the fibers,” explains Burgert.
Electricity from the parquet floor?
To show the potential, the researchers carried out tests with cubes with a side length of about 1.5 centimeters. As they report, the material showed astonishing stability during the 600 or so load cycles. The measurements showed that at a pressure of 45 kilopascals, a voltage of up to 0.87 volts is generated. As the scientists explain, this effect would already be well suited for use as a sensor. In further experiments, the team then sounded out what scaling options there would be for the nanogenerators. If, for example, 30 of the wooden blocks are loaded in parallel with the weight of an adult, a simple LCD display can be made to glow. With a further optimization of the system, a functionalized parquet floor would also be conceivable, which converts step energy into electricity. In addition, the researchers gathered initial indications that the system is suitable for use as a pressure-sensitive sensor on human skin.
Before the piezo wood can be used as a sensor or power generator, some development work is still required, which the researchers are now devoting themselves to. But according to them, the advantages of such an uncomplicated and at the same time biological piezoelectric system are obvious. In order to adapt the technology for industrial use, the researchers are already in talks with possible cooperation partners, Empa concludes.
Source: Federal Materials Testing and Research Institute, specialist article: Science Advances, doi: 10.1126 / sciadv.abd9138