During World War II, when real wool was scarce, the synthetic fabric Aralac became popular as a substitute. It was made from cheese! Maggie Koerth-Baker at Boing Boing tells us how Aralac is made, and although you don't see it much anymore, an improved fabric using the technique is still produced by a German company. Link
A commenter noted at the article site that there is yarn made from milk. They are making yarns from all manner of plants and foodstuffs, including soy, bamboo (which washes beautifully, the fibers becoming softer over time) and corn. There's a show that's in reruns on either HGTV or DIY called Knitty Gritty. They had an entire episode dedicated to new types of yarns, along with the recommended needles and uses. I knit casually, but even I found it really interesting.
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