Researchers at the Swiss Federal Laboratories for Materials Science and Technology (Empa) and the Swiss Federal Institute of Technology in Zürich (ETH Zurich) have made wood compressible and turned it into a micro-generator. When it is loaded, an electrical voltage is generated. In this way, the wood can serve as a bio-sensor or generate usable energy. The latest highlight: to ensure that the process does not require aggressive chemicals, naturally occurring wood-degrading fungi take over the task of modifying the wood. Ingo Burgert and his team at Empa and ETH Zurich have proven it time and again: wood is so much more than “just” a building material. Their research aims at extending the existing characteristics of wood in such a way that it is suitable for completely new ranges of application. For instance, they have already developed high-strength, water-repellent and magnetisable wood. Now, together with the Empa research group comprising Francis Schwarze and Javier Ribera, the team has developed a simple, environmentally friendly process for generating electricity from a type of wood sponge, as reported in the journal Science Advances.
Voltage through deformation
If you want to generate electricity from wood, the so-called piezoelectric effect comes into play. Piezoelectricity means that an electric voltage is created by the elastic deformation of solids. This phenomenon is mainly exploited by metrology, which uses sensors that generate a charge signal, say, when a mechanical load is applied. However, such 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. It also makes the ecological disposal of PZT and Co rather tricky. Being able to use the natural piezoelectric effect of wood thus offers a number of advantages. If thought further, the effect could also be used for sustainable energy production. But first of all, wood must be given the appropriate properties. Without special treatment, wood is not flexible enough; when subjected to mechanical stress, therefore, only a very low electrical voltage is generated in the deformation process.
From block to sponge
Jianguo Sun, a PhD student in Burgert’s team, used a chemical process that is the basis for various “refinements” of wood the team has undertaken in recent years: delignification. Wood cell walls consist of three basic materials: lignin, hemicelluloses and cellulose.
“Lignin is what a tree needs primarily in order to grow to great heights. This would not be possible without lignin as a stabilising substance that connects the cells and prevents the rigid cellulose fibrils from buckling,” explained Burgert. In order to transform wood into a material that can easily be deformed, lignin must at least partially be “extracted”. This is achieved by placing wood in a mixture of hydrogen peroxide and acetic acid. The lignin is dissolved in this acid bath, leaving a framework of cellulose layers.
“We take advantage of the hierarchical structure of wood without first dissolving it, as is the case in paper production, for example, and then having to reconnect the fibres,” said Burgert. The resulting white wood sponge consists of superimposed thin layers of cellulose that can easily be squeezed together and then expand back into their original form – wood has become elastic.
This article appeared in the July / August issue of Panels & Furniture Asia. To read the article, click here.