Enzymes from fungi, bacteria and plants render wood resistant to decomposing and pathogenic microorganisms. Possible applications might be germ-resistant wood surfaces for hospitals and nursing homes, but also fungus-resistant fašades that last longer even without a protective coating.
Weather-beaten wooden fašades on exteriors are often a sorry sight: gray with fungi, eaten away by bacteria, the handsome material loses much of its original splendor, which discourages many home-owners and building contractors from using wood as a construction material. And wood is also regarded as an absolute no-no in kitchens or nursing homes, where hygiene is paramount. This is because, as a natural material, wood is an ideal breeding ground for many pathogenic microorganisms.
Although wood varnishes or glazes can prevent an infestation, they frequently cause new problems as they often contain toxic substances. These pose a health risk during production and application, and can be washed out of the wood by rain or through wear and tear. Copper can get into the soil, for instance, where it pollutes the environment.
Finding a method that protects wood from fungi and bacteria without any toxic additives would therefore be ideal. And a team of Empa researchers from the Laboratory for Applied Wood Materials has now hit the jackpot. The trick: the scientists used a biochemical method that involves a substance taken from fungi themselves. Forest and environmental scientist Mark Schubert is a specialist in wood fungi. For him, fungi are more than just "pests" that damage wooden fašades; some species of fungus also contain enzymes that can give the material useful properties. For instance, the many-colored polypore, a wood-dwelling species of fungus found all over the world, has enzymes that "arm" the wood with antimicrobial iodine protection.
These enzymes - known as laccases - act as catalysts in their natural environment and ensure the oxidation of phenolic substances. In woody plants, for example, laccases are involved in the synthesis and breakdown of lignin, one of the main components in woody cell walls.
The researchers’ idea: in an "artificial" environment, the laccase obtained from white rot fungi is supposed to ensure that iodine is joined covalently - i.e. chemically "bound". In an aqueous solution, the laccase oxidizes the iodide (I?) to form highly reactive iodine (I2), which creates a bond with the lignin on the surface of spruce. "The advantage," explains Schubert, "is that the chemically bound iodine can’t be washed out and is therefore permanent."
The researchers have already patented the eco-friendly, simple and affordable application that doesn’t alter the wood’s haptic properties. They are now in talks with various partners from the furniture, construction and paper industry, who want to use the technique for their own purposes - whether it be to produce furniture with antiseptic surfaces for hospitals, provide wooden fašades that are immune to bacterial and fungal infestations, or replace toxic binding agents for fibers in the production of paper.
Two long-term tests with iodized wood are just getting underway at NEST, Empa and Eawag’s modular research and innovation building. Local fir and spruce is being used for the fašade, while the door handles in the interior are made of oak. The wood is initially iodized with the aid of laccase from the fungus Trametes versicolor. The treated timber will then be exposed to everyday conditions for several years to give the researchers an idea of how bacterial and fungal infestations might be curbed or even prevented in practice.
But it doesn’t stop there: not only are laccase-catalyzed techniques just the ticket for iodizing surfaces; damping plates are also used at NEST, which were improved by Empa within the scope of a CTI project in collaboration with industrial partner Pavatex. They succeeded in reducing the synthetic binding agent before eventually replacing it completely with sustainable, environmentally friendly biopolymers thanks to laccase-catalyzed reactions.
Laccases are already deployed in numerous branches of industry: in the food sector, the enzyme is used to remove toxic polyphenol from orange juice or beer mash, for instance. The textile industry uses laccases to dye jeans or create smoother textile surfaces. And in cosmetics, laccases are used to produce aromas and fragrances. The biocatalysts are interesting for industry because they are robust, don’t need any special additives and can be obtained in large amounts and at affordable prices. Moreover, enzymes work under mild conditions, i.e. in aqueous solutions, at room temperature and under normal pressure.