Researchers at the Swiss Federal Laboratories for Materials Science and Technology (Empa; Dübendorf; www.empa.ch) and ETH Zurich (both Switzerland; www.ethz.ch) have made wood compressible and turned it into a piezoelectric generator. The research, reported last month in the journal Science Advances, has the potential for making wood-based bio-sensors or even for generating usable energy. The researchers have also replaced aggressive chemicals by using naturally occurring, wood-degrading fungi (Ganoderma applanatum) for the delignification process. The research was performed by professor Ingo Burgert and his team at Empa and ETH Zurich, together with the Empa research groups of Francis Schwarze and Javier Ribera.
Piezoelectric sensors often use materials that are unsuitable for use in biomedical applications, such as lead zirconate titanate (PZT), which cannot be used on human skin due to the lead it contains. Being able to use the natural piezoelectric effect of wood thus offers a number of advantages. Without special treatment, however, wood is not flexible enough and only a very low electrical voltage is generated in the deformation process.
To make the wood elastic, it must first be partially delignified, which results in a white wood elastic sponge, consisting of superimposed thin layers of cellulose. When squeezed together, differently charged areas are displaced against each other and the surface of the material becomes electrically charged.
Burgert’s team demonstrated that a (1.5 cm)3 test cube remained stable after about 600 load cycles. At each compression, the researchers measured a voltage of around 0.63 V, which is sufficient for sensor applications. In further experiments, 30 such wooden blocks under a load equivalent to the body weight of an adult can generate enough electricity to light up a simple LCD display. This opens the possibility of using wooden floors for converting the energy of people walking on it into electricity.
The researchers are already in talks with potential cooperation partners for adapting the technology for industrial applications.