While some bras are machine-washable, it is always best to leave your bras out to air dry. Bras made from synthetic materials, such as nylon, polyester, or spandex should not be put in the dryer unless necessary. Placing these undergarments in the dryer will wear down the fabric and its elasticity. If you do this regularly, the bras can lose their shape quickly and you will need to replace them more often. Cotton bras are more likely to shrink if placed in the dryer.