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Textiles: water-repellent thanks to fungal proteins

Outdoor lovers and athletes love them: water-repellent jackets and trousers. However, many consumers are unaware that the chemicals used to functionalise the textile surface often pollute the environment. Organic fluorine compounds (perfluorocarbons = PFC) are usually added to textiles to make them water-repellent. These compounds are extremely stable, almost impossible to biodegrade and potentially toxic. Scientists at the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB and the Hohenstein Group are researching an environmentally friendly and sustainable method for making textiles water-repellent.

Outdoor jackets: their water-repellent coating is often still harmful to the environment. © Juliette Irmer

PFCs accumulate in the environment worldwide due to their particular chemical properties. They accumulate in water, air, soil, plants and animal and human blood. The most dangerous PFCs, perfluorooctane sulphonic acid (PFOS) and perfluorooctanoic acid (PFOA), linger in the human body for up to nine years. They are considered carcinogenic and toxic to reproduction. Production and use of PFOS has already been banned worldwide, PFOA will be banned from 2020 onwards. The textile industry currently uses other long-chain fluorine compounds, which are converted to PFOA and similar substances in the environment. This is the reason why the precursors of PFOA and similar substances are heavily criticised. The German Environment Agency is preparing a proposal to restrict the use of these chemicals. Faced with increased regulatory pressure, companies are looking for a “green” solution to make surfaces water-repellent.

Hydrophobins as biological coating

Scientists from the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB’s Straubing branch “BioCat”, and researchers from the William Küster Institute life sciences department, which is part of the Hohenstein Group in Bönnigheim, are also researching ways to develop such a technique: "We work with proteins that are only found in fungi, so-called hydrophobins,” explains Michael Richter, a chemist from the Fraunhofer IGB. The name speaks for itself. Hydrophobins are water-repellent (= hydrophobic). Richter and his colleagues are looking for ways to transfer the properties of hydrophobins to textiles by permanently coating textiles with these proteins. The project was launched in April 2016 and is being closely followed by a committee of well-known textile companies.

In nature, hydrophobins occur on the surface of fungal spores. They stop the spores clumping and facilitate further growth, to name just two of their advantages. But how can these fungal proteins be coupled permanently to a textile surface? By using another protein with different properties, a so-called anchor, and coupling it to the hydrophobins. The anchor proteins are derived from bacterial cellulose-digesting enzymes that have a high natural affinity for cellulose, which in turn serves as test material. "The anchor adheres to cellulose by way of intermolecular interactions. The proteins have to be able to align correctly, i.e. the anchor proteins must always adhere to the textile side while the water-repellent hydrophobins form the new surface,” explains Richter. The ideal scenario is a wafer-thin protein layer on the textile surface.

Bacteria produce ideal fusion protein

To begin with, the researchers retrieved the sequences of the two protein types from a biodatabase. "We assembled the coding DNA segments of interest into a new gene and introduced this construct into bacteria," says Richter. The genetically modified bacteria read the new gene and produced the desired fusion protein. "The recovery and processing of proteins is simple, the technology for producing such proteins is a well-established, safe and standard practice in other industries. All we need to do is improve the yield,” says Richter. So far, everything has been tested in three-litre volumes. The researchers are hoping to upscale the fermentation process to ten litres.

The scientists tested combinations of five hydrophobins and six anchor proteins. The hydrophobins come from different organisms, but information on this cannot be disclosed for patent reasons. Drop tests were carried out, and two fusion proteins were found to be particularly water-repellent. This showed that the principle worked: “After applying the proteins to cellulose, we then apply a drop of water. If the drop of water remains for a defined period of time, it is a sign that functionalisation has worked well,” says Richter.

Modified wettability: coloured water droplets on hydrophobic textile.
Modified wettability: coloured water droplets on hydrophobic textile. © Fraunhofer IGB
Modified wettability: coloured water droplets on hydrophilic textile.
Modified wettability: coloured water droplets on hydrophilic textile. © Fraunhofer IGB

Green alone is not enough

But the list of requirements for an environmentally friendly, water-repellent coating is long: not only do the raw materials need to be cheap and easy to produce, they must also be practical, resistant to wear and tear, harmless to humans and the environment, and biodegradable. “Green alone is not enough; in comparison with conventional equipment, the new technology must enable the finished surface to perform as well as or better than existing surfaces. In addition, it should not cost any more,” says Richter.

The researchers still have a few experiments to carry out: for example, the researchers’ project partners from Hohenstein will be looking into aspects of ecotoxicological safety and biosafety. Amongst other things, they will be working with standardised, cell-based test systems to rule out cytotoxic properties or properties that have a sensitising effect. The researchers also need to assess whether the proteins adhere well and whether the textiles that have been developed can be washed without affecting their functionality. The protein coating also needs to be analysed to see whether it is compatible with common textile industry techniques such as staining and rolling, which are often associated with high temperatures, a high salt load and the use of detergents.

Consumer acceptance is expected to be quite high even though the organic coating will have some restrictions: “Such textiles cannot be washed with detergents containing proteases. But special care also needs to be taken when washing woollen sweaters,” says Richter.

In addition to the traditional textile industry, a water- and dirt-repellent coating is also of interest to many other industries including the medical technology and automotive sectors, in the form of washable car seat covers for example. Richter confirms this: “Although it is not yet market-ready, the hydrophobin biocoating has attracted a great deal of interest: “We have had enquiries from sectors that we had not even considered.” As the principle is very promising, further follow-up projects are planned.

The IGF project 18884N carried out by the research consortium “Forschungskuratorium Textil”, Reinhardtstraße 12-14,10177 Berlin, was funded through the German Federation of Industrial Research Associations (AiF) under the Industrial Collective Research (IGF) programme run by the German Ministry of Economics and Energy on the basis of a resolution passed by the German Bundestag.

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