A leading US-based fleece manufacturer has unveiled an innovative new product that it hopes will be the hottest garment in shops this Christmas - heated jackets that provide the wearer with warmth from light-weight batteries. Malden Mills has begun using "conductive fibres" in its fleece jackets powered by two rechargeable lithium-ion batteries that last around five hours and are sewn into waist pockets with a small control switch sewn into the chest pocket.