Bamboo underwear is made with a bamboo rayon and spandex mix. Rayon is a regenerated cellulose fiber, made from natural sources of cellulose. In this case, bamboo. The pulp from bamboo creates a soft, silky and smooth feel. It’s a premium luxury touch, making it perfect for underwear. The spandex allows the bamboo rayon to stretch, so that the underwear moulds perfectly to your body.