Replacing the gel-like buffer between our spinal discs following an injury, soaking up leaked oil, or filtering carbon dioxide from the air – trees provide an environmentally friendly material that can be used for all these purposes: nanocellulose. Then there are the fungi that make the wood stick better, or sound better. Empa is one of the world-leaders in research into the extraordinary abilities of this material.

Passion for wood: Tanja Zimmermann, head of the Applied Wood Materials laboratory and her colleague Sebastien Josset.

Trees as high as 30 metres still stand up to the wind, even if their stems are relatively thin. Tanja Zimmermann has meticulously studied the structure of wood for years in order to understand this surprising phenomenon. At present she is directing the Department of Applied Wood Materials at the Empa in Dübendorf. “The substance which gives wood its tensile strength is cellulose – an excellent material for lightweight construction”, the expert explains. If one grinds the cellulose until the individual filaments are less than a hundred nanometres thick, the material has even better mechanical properties, because imperfections are removed. “Originally we used the nano-fibres to reinforce (bio-) plastics”, Tanja Zimmermann says. “Then we noticed that the cellulose can do much more.”

If one stirs just two percent by weight of nanocellulose in water, a solid gel is formed. This could be suitable as a specially well-tolerated and compatible material in the biomedical field. In cooperation with EPFL, the Empa researchers developed a hydrogel using nanocellulose that is intended as a replacement for the gelatinous core of intervertebral discs. The depletion of the gelatinous core often leads to back pain or even forms of spinal disc herniation (“slipped disc”). The scientists at the EPFL are currently experimenting on dead bovine tails to see how the hydrogel can be inserted in the vertebrae and how it behaves there. The cells in the remains of animals that have recently died are still active, so that the researchers expect to find results of importance for future uses on human beings. “We have published our development work and already patented it as well”, Tanja Zimmermann says. “But it will probably take some time before it is actually used in medicine.”

Outdoor weathering of a wood covering featuring functionalised nanocellulose. Shown in picture: Martin Arnold, head of the Wood Technology and Surface Technology Group.

In general, Empa’s main areas of research are focusing on five different topics: nanostructured materials, sustainable built environment, health and performance, natural resources and pollutants, and energy. “We can use nanocellulose in all these research areas, it spans the whole range of Empa research”, claims Tanja Zimmermann delightedly. “I am burning with enthusiasm for this material. We have so many exciting projects in which we would like to use nanocellulose.”

Miracle substance against oil pollution

One of these started making headlines in 2014: sponges made of nanocellulose could be used to clear away oil pollution in bodies of water. If one removes the water from the nanocellulose, a specially absorbent material results; but it equally absorbs both water and oil. Only when the researcher team added an additional substance to the starting gel, did it lose its “water-loving” property and then it only absorbed the oil. In the laboratory test the sponge absorbed fifty times its weight in oil in a matter of seconds. What is more, it could be rinsed out and used again up to ten times.

“In contrast to other groups, we used a relatively simple, environmentally friendly method of manufacture”, says Tanja Zimmermann. Although many interested potential users, including the Zurich lake police, flooded the Empa researchers with enquiries, it took some time before a company was found which was willing to invest in the production of the new materials on a scale of tons.

The chemically modified nanocellulose sponge can absorb fifty times its own weight in oil.

Now the firm Wicor Weidmann in Rapperswil-Jona, in collaboration with Empa, is planning to develop marketable products using the nanocellulose sponge. “We have mastered the material itself and the chemistry needed”, says Tanja Zimmermann. But the drying process has to be made more economical. The freeze-dry method they used in the laboratory is too costly for mass production. Still, the scientist is confident that they will find a cheaper process. When the nanocellulose sponge comes to market, it could be used to soak up motor oil, or all types of solvents. If one does not wish to collect and reuse the nanocellulose, it can be quite easily incinerated. Tanja Zimmerman says it is a bit early at this stage to say whether this method will be used to effectively combat major oil disasters on the ocean: “I would rather not make too many promises.”

A network of tiny spaghetti threads

The material that the Empa researchers are experimenting with consists of threads five to one hundred nanometre wide, whose length is in the order of micrometres. “It is a spaghetti-like material that forms dense networks, which means there are no nanoparticles left that might be a risk to health”, the expert explains. One can manufacture sponges, gels, foams and membranes from the nanocellulose. These can be used, for example, to remove contaminants such as humic acid or heavy metals from water – or filter out carbon dioxide from the surrounding air. This is the goal of a system produced by Climeworks, a spin-off company of the ETH Zurich.

A suitably designed and activated nanocellulose foam captures the carbon dioxide from the air flowing through and releases it again when the material is heated to 70 to 90 degrees. The CO2 obtained in this way can be used in the manufacture of beverages, or for the accelerated growth of plants in greenhouses. The CO2 collector is close to being ready for the market. “Here our nanocellulose is competing with a synthetic material”, says Tanja Zimmermann, and she hopes that the environmentally friendly alternative from Dübendorf will win the race.

“Green chemistry” is also the goal of Francis Schwarze and Mark Schubert, wood researchers at the Empa in St.Gallen. They are studying how one can use fungi as beneficial organisms to modify and activate special functions of wood. “This is, to my knowledge, unique in the world”, says Francis Schwarze, who is also Professor of forest botany at the Albert Ludwigs University in Freiburg. “For this we use all the stages of the fungus from myce-lium and fruiting body to spores, and on to the enzymes or polymers that the fungi make.” One enzyme that has proved to be specially advantageous is called laccase, which was first obtained from the sap of the Japanese lacquer tree in the 19th century.

Laccase reacts with many molecules, and only needs oxygen for this, producing just water as a waste product. “By using laccase we can bind various molecules to the surface of the wood, and in this way activate the special functions of wood in a cost-effective and environmentally friendly way”, says Mark Schubert. For example, in this way one can make wood surfaces permanently impervious to water. Indeed, the opposite effect can also be achieved with the aid of the enzyme: wood surfaces that are readily wettable so that they can be easily glued together. In a collaboration with the fibre board manufacturer Pavatex, the Empa team is using laccase to produce more environmentally friendly products. “The laccase makes the adhesive stick better. This results in either a better cohesiveness or one needs less of the adhesive”, explains the enthusiastic scientist. “This is really grand.”

The best-sounding wood

The topic that the researchers found has struck the greatest chord in the public mind, however, is their refinement of musical wood. “Even when I was doing my doctoral project I was already investigating how various fungi decompose wood”, says Francis Schwarze. He found a strain of fungus that grows into tree trunks and which makes the wood lighter, and it does this without changing the speed of sound in the material. The scientist had the idea that wood treated by the fungus could be specially suitable for constructing violins: “Because for high-quality musical wood it is desirable if it is light in weight”. For his famous instruments Antonio Stradivari thus used wood that had grown in the so-called Maunder Minimum from 1645 to 1715, a period of long winters and cool summers, which combined a high flexural strength with a low density.

In a blind test, where the master violinist Matthew Trusler played on various instruments – including a Stradivari – behind a curtain, the judgement of the 200 listeners was clear: The violin “Opus 58”, made of wood that had been treated for nine months with a fungus, produced the best result. “I was sitting there and thought that must be the Stradivari”, recalls Francis Schwarze, about the experiment in 2009. Since then the researchers have accelerated and standardised the method to manufacture the musical wood. “The procedure is now reproducible and the quality assurance is provided”, says the Empa researcher, “Today we have a much better understanding of why these fungus-treated instruments sound so similar to an over 300 year-old violin.”

After the success with “Opus 58”, the Walter Fischli Foundation took on the financing of the project. Their aim is the promotion of young musicians. In one or two years the first fungus-treated violins should come to market. They will cost 5,000 to 10,000 Swiss francs more than a conventional master violin, for which one has to pay from 15,000 to 50,000 Swiss francs. But compared to a Stradivari that costs two million US dollars this is quite a small extra charge. “Everyone can hear the difference between a new, untreated violin and a fungus-treated instrument”, says Schwarze enthusiastically, and he is happy to offer his audio files as evidence.