Four years after Stella McCartney and the Japanese biotech firm Spiber began developing it, a fermented alternative to cashmere and wool has finally arrived.
Stella McCartney’s Winter 2026 collection weaves in Brewed Protein, a fermented alternative to wool and cashmere that McCartney and the Japanese biotech company Spiber spent more than four years developing together, the two companies said in a release.
“Their fine fibers, which can be controlled as fine as the finest cashmere or silk, offer a unique, luxurious, soothing feel,” Kazuhide Sekiyama, Spiber’s founder and chief executive, told the Fédération de la Haute Couture et de la Mode in 2024. “They offer solutions to various issues such as microplastic pollution from synthetic fibres, desertification caused by cashmere goat farming, and greenhouse gas emissions from animal agriculture.”
Brewed Protein can emit up to 75 percent fewer emissions than cashmere while requiring 94 percent less water and 86 percent less land to produce. The fibers used in McCartney’s collection measured 12 to 14 microns in diameter, fine enough, the companies said, to deliver a smooth, refined texture without a single fleece behind it. Spiber describes the resulting material as fully bio-based, biodegradable, and cruelty-free, grown through fermentation rather than drawn from petroleum feedstocks.
Spiber grows its protein fiber through precision fermentation rather than raising the goats, sheep, or silkworms that conventional luxury textiles depend on, a shift the companies described as “a structural transformation in the way materials are produced.” Stella McCartney has long challenged the conventional notion that luxury must depend on animal-derived materials, the release noted.
McCartney founded her label in 2001 after working at Chloé, committed from the outset to sustainable materials and a ban on animal products, according to Sortiraparis. The Brewed Protein debut follows her addition in January 2025 to the BioCircular Materials Alliance, the Spiber-founded coalition working toward industry-wide adoption of biobased materials. The Winter 2026 collection draws 93 percent of its materials from sustainable sources overall, including recycled denim, responsibly sourced wool, and GRS-certified blends, plus forest-friendly viscose, organic cotton, Econyl, Vegea, and vegan fur.
Spiber has also supplied Brewed Protein to Yuima Nakazato’s 2021 haute couture collection, to Sacai’s runway in 2019 and Junya Watanabe’s in 2022, to outdoor labels including The North Face and Woolrich, and to Kering Group’s Material Innovation Lab, according to Sekiyama’s 2024 interview. The McCartney collection puts that fiber in front of a wider luxury audience than most of those releases reached.
Pieces in the winter collection made with Spiber include Crewneck Spiber Knit Jumper and a Shawl-Collar Spiber Knit Jumper.
“Similar to artisans who dedicate years to perfecting their techniques,” Sekiyama told the Fédération de la Haute Couture et de la Mode, “we spent nearly two decades refining our technology and fibres to attain the necessary level of precision and innovation.”
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