Double helix artificial muscles (Conference Presentation)

High performance artificial muscles based on twisted and coiled polymer fibers have attracted considerable attention since their discovery in 2014. These artificial muscles generate tensile strokes as a result of a torsional actuation occurring within the twisted fiber. The torsion is due to a volume expansion and is related to the helical topology of the twisted polymer fiber or fiber composite. The volume expansion can be induced thermally, electrochemically, photonically or by absorption of small molecules, such as water. The magnitude of the torsional stroke and/or the torque generated has been successfully modeled using a single helix approximation. This paper presents a new type of tensile artificial muscle that exploits the properties of the double helix. Two fibers are plied to form the double helix structure and diameter expansion of the fibers generates a large lengthwise contraction in the plied structure. The process is successfully modeled using the single helix approach. The example plied double helix actuators were fabricated from cotton yarn impregnated with hydrogel. The cotton was pultruded through a solution containing the polyurethane based hydrogel. Actuators were made by air drying followed by co-twisting two lengths of the hydrogel-cotton and heat-setting at 60 degrees C. The samples were tested in isotonic mode by tensioning and fully immersing in water. The samples contracted in length when wet and the process was reversed on drying. The effect of hydrogel content and twist density have been investigated. Single hydrogel-cotton yarn samples showed negligible change in length when immersed in water, but two-ply double helix samples showed contractions of more than 10% in length. The length contraction of the double helices was attributed to the increase in fiber diameter during water absorption. The geometry changes were successfully modeled using a single helix approach.