Mandrel-free fabrication of giant spring-index and stroke muscles for diverse applications

Methods for making high-spring-index polymer fiber or yarn muscles have required expensive fabrication by wrapping around a mandrel, which limits their practical applications. We demonstrate an inexpensive mandrel-free method for making polymer muscles that can have a spring index of >50 and a contractile tensile stroke exceeding 97%. This method enables the spring index to be varied along a muscle's length by varying the plying twist, resulting in muscles that transition between homochiral and heterochiral when either heated or cooled. We demonstrate use of these polymer muscles for robots and environmentally driven comfort-adjusting jackets. This mandrel-free method was used to make high-spring-index carbon nanotube yarns for mechanical energy harvesters, self-powered strain sensors, and solvent-driven and electrochemically driven artificial muscles.