Fiber composite material from nature

Sustainably produced, biodegradable materials are an important focus of modern materials research. However, the processing of natural materials such as cellulose, lignin or chitin presents researchers with a compromise. In their pure form, natural materials are biodegradable, but often do not perform well enough. Chemical processing steps can be used to make them stronger, more resistant or more supple - but in doing so, they lose out on sustainability.

Empa researchers from the "Cellulose and Wood Materials" laboratory have now developed a bio-based material that cleverly avoids this compromise. Not only is it completely biodegradable, it is also tear-resistant and has versatile functional properties. All this with minimal processing steps and without any chemicals - you can even eat it. Its secret: it's alive.

Optimized by nature

As the basis for their novel material, the researchers used the mycelium of the common cleavers, a widespread edible fungus that grows on dead wood. Mycelia are root-like filamentous fungal structures that are already being actively researched as potential sources of material. Normally, the mycelial fibers - known as hyphae - are cleaned and, if necessary, chemically processed, which entails the well-known trade-off between performance and sustainability.

The Empa researchers chose a different approach. Instead of laboriously processing the mycelium, they use it as a whole. When growing, the fungus not only forms hyphae, but also a so-called extracellular matrix: a network of different fiber-like macromolecules, proteins and other biological substances that the living cells secrete. "The fungus uses this extracellular matrix to give itself structure and other functional properties. Why shouldn't we do the same?" explains Empa researcher Ashutosh Sinha. "Nature has already developed an optimized system," adds Gustav Nyström, head of the "Cellulose and Wood Materials" laboratory.

With a little targeted post-optimization, the researchers have given nature a helping hand. From the enormous genetic diversity of the common cleavers, they selected a strain that produces a particularly large amount of two specific macromolecules: the long-chain polysaccharide schizophyllan and the soap-like protein hydrophobin. Due to their structure, hydrophobins collect at interfaces between polar and apolar liquids, for example water and oil. Schizophyllan is a nanofiber: less than a nanometer thick, but more than a thousand times as long. Together, these two biomolecules give the living mycelium material properties that make it suitable for a wide range of applications.

A living emulsifier

The researchers demonstrated the versatility of their material in the laboratory. In their study, which was recently published in the journal "Advanced Materials", they presented two possible applications for the living material: a plastic-like film and an emulsion. Emulsions are mixtures of two or more liquids that normally cannot be mixed. If you want to see an example, all you have to do is open the fridge: Milk, salad dressing or mayonnaise are all examples. But various cosmetics, paints and varnishes are also available as emulsions.

One challenge is to stabilize such mixtures so that they do not "segregate" back into the individual liquids over time. This is where the living mycelium shows its best side: both the schizophyllan fibers and the hydrophobins act as emulsifiers. And the living fungus is constantly releasing more of these molecules. "This is probably the only type of emulsion that becomes more stable over time," says Sinha. Both the fungal filaments themselves and their auxiliary molecules are completely non-toxic, biologically compatible and even edible - the common split-leaf mushroom is considered an edible mushroom in many parts of the world. "Its use as an emulsifier in the cosmetics and food industry is therefore particularly interesting," says Nyström.

From compost bags to batteries

However, the living fungal network is also suitable for classic material applications. In a second experiment, the researchers produced thin films from their mycelium. The extracellular matrix with the long schizophyllan fibers gives the material very good tensile strength, which can be further enhanced by targeted alignment of the fungal and polysaccharide fibers.

"We combine the proven methods for processing fiber-based materials with the emerging field of living materials," explains Nyström. Sinha adds: "Our mycelium is a living fiber composite, so to speak." The researchers can control the properties of this material by changing the conditions under which the fungus grows. It would also be conceivable to use other fungal strains or species that produce other functional macromolecules.

However, working with the living material also presents certain challenges. "Biodegradable materials always react to their environment," says Nyström. "We want to find applications where this interaction is not a hindrance - or even an advantage." However, biodegradability is only part of the story for the mycelium. It is also biodegradable: the common cleavers can actively decompose wood and plant materials. Sinha sees a further potential application here: "Instead of compostable plastic bags for kitchen waste, it could be used to make bags that compost the organic waste themselves," says the researcher.

There are also promising applications for the mycelium in the field of sustainable electronics. For example, the fungal material reacts reversibly to moisture and could be used to produce biodegradable moisture sensors. Another application that Nyström's team is currently working on combines the living material with two other research projects from the Cellulose and Wood Materials laboratory: the mushroom battery and the paper battery. "We want to produce a compact, biodegradable battery whose electrodes consist of a living 'mushroom paper'," says Sinha.

Author

Anna Ettlin is a science editor and works in communications at Empa. www.empa.ch