Teng Li at the University of Maryland and his colleagues developed a material called hardened wood which is 23 times harder than raw wood and can be carved to make knives three times sharper than standard steel ones. A coating of mineral oil makes the knives resistant to water.
“The hardened wood knives can be washed, dried and resharpened if needed, so that an extended lifetime is expected,” says Teng.
The key to making the knives so sharp was using strong cellulose fibres found in plant cell walls, which make up almost half the mass of wood, rather than weaker polymers in cell walls called lignin and hemicellulose.
The team removed these weaker polymers from raw wood by soaking it in a solution containing sodium hydroxide and sodium sulphite, before boiling the solution at 100°C for a few hours. They then squished the remaining cellulose fibres together at a pressure of 20 megapascals for 6 hours before drying them at around 100°C to produce hardened wood.
Making commercially available steel and ceramic knives often requires at least 10 times higher temperatures and pressures, making these wooden knives a less energy intensive form of cutlery.
The team compared the sharpness of knives made from hardened wood with those made from steel by measuring how easily the blades sliced through a standard electrical wire. They found that the wooden knives required around three times less force to slice through the wire compared with steel knives.
The team found that the wooden knives felt comparable with steel knives when cutting through medium-well done steaks.
“There are more than 3 trillion mature trees on Earth. They are abundant and renewable. Revealing and realising the full potential of wood, beyond its conventional use, holds promise towards a better and greener future. We’d love to see its commercialisation in the near future,” says Teng.
Journal reference: Matter, DOI: 10.1016/j.matt.2021.09.020
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