Power generating parquet floor

Ingo Burgert's team at Empa and ETH Zurich has often demonstrated that wood can be used for more than just construction. The research work often involves extending the existing properties of wood so that it is suitable for completely new areas of application. For example, high-strength, water-repellent or magnetizable wood has already been created. Now, together with the Empa research group led by Francis Schwarze and Javier Ribera, the team has developed a simple, environmentally friendly process to generate electrical voltage using a type of wooden sponge, as they reported last week in the renowned journal Science Advances.

Stress due to deformation

If you want to generate an electrical voltage with wood, the so-called piezoelectric effect comes into play. Piezoelectricity means that the elastic deformation of solids 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, for example. However, such sensors often use materials that are unsuitable for use in the biomedical field, such as lead zirconate titanate (PZT), which is not suitable for use on the skin because of the lead. Being able to use the natural piezoelectric effect of wood therefore offers certain advantages. If thought through further, the effect could also be used for sustainable energy generation. But first, wood must be given the appropriate properties. This is because without special treatment of the wood, only a very low electrical voltage is generated in the deformation process when it is subjected to mechanical stress.

 From block to sponge

Jianguo Sun, a doctoral student in Burgert's team, applied a process that forms the basis for various further developments of wood: delignification. Wood cell walls consist of three basic substances: Lignin, hemicelluloses and cellulose. "Lignin is primarily needed by a tree to grow far into the air. Without lignin as a stabilizing substance that connects the cells and prevents the tensile cellulose fibrils from buckling, this would not be possible," Burgert explains.

In order to convert wood into an easily formable material, the lignin must be at least partially "dissolved out". This is achieved by placing the wood in a mixture of hydrogen peroxide and acetic acid. In the acid bath, the lignin is dissolved out, leaving a framework of cellulose layers. In this process, Burgert's team was concerned with working with relatively simple and environmentally friendly processes: "We make use of the hierarchical structure of the wood without first having to dissolve it, as in paper production, for example, and then having to reconnect the fibers." The resulting white wood sponge consists of superimposed, thin cellulose layers that can be easily pressed together and then expand back to their original shape.

Electricity from the wooden floor

The research group subjected the test cube with a side length of about 1.5 cm to around 600 load cycles. The material showed astonishing stability in the process. With each load, the researchers measured a voltage of around 0.63 V - a voltage that would be useful for an application as a sensor. In further experiments, the team tried to explore the potential scalability of this nanogenerator. For example, they were able to show that 30 such wooden blocks, when loaded in parallel with the body weight of an adult, could already cause a simple LCD display to light up. If the process were further optimized, it would also be conceivable to use a functionalized parquet floor that converts the kicking energy into electricity. The researchers tested the suitability as a pressure-sensitive sensor on human skin, demonstrating that an application in the medical field would also be possible.

Application in preparation

The work on the latest publication by the Empa-ETH Zurich team, however, goes one step further: the aim was to modify the process so that it does not require aggressive chemicals. The researchers found a suitable candidate that can carry out the delignification in the form of a biological process in nature: the fungus Ganoderma applanatum causes white rot in wood. "The fungus breaks down the lignin and hemicellulose in the wood in a particularly gentle way," says Empa researcher Javier Ribera, explaining the environmentally friendly process. In addition, the process can be easily controlled in the laboratory.
There are still some steps to be taken before the "piezo" wood can be used as a sensor or as a power-generating parquet floor. But the advantages of such a simple and at the same time renewable and biodegradable piezoelectric system are obvious - and are now being investigated by Burgert and his colleagues in further research projects. And in order to adapt the technology for industrial application, the researchers are already in talks with potential cooperation partners.