Stretchable artificial muscles from coiled polymer fibers
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ft robots are being developed to mimic the movement of biological organisms and as wearable garments to assist human movement in rehabilitation, training, and tasks encountered in functional daily living. Stretchable artificial muscles are well suited as the active mechanical element in soft wearable robotics, and here the performance of highly stretchable and compliant polymer coil muscles are described and analyzed. The force and displacements generated by a given stimulus are shown to be determined by the external loading conditions and the main material properties of free stroke and stiffness. Spring mechanics and a model based on a single helix are used to evaluate both the coil stiffness and the mechanism of coil actuation. The latter is directly coupled to a torsional actuation in the twisted fiber that forms the coil. The single helix model illustrates how fiber volume changes generate a partial fiber untwist, and spring mechanics shows how this fiber untwist generates large tensile strokes and high gravimetric work outputs in the polymer coil muscles. These analyses highlight possible as yet unexplored means for further enhancing the performance of these systems.
I. INTRODUCTION
The development of wearable robotic suits for augmenting human movement or biomimetic soft robots that mimic the complex movements of biological organisms has gained considerable momentum recently through a paradigm shift in the materials used from hard and rigid to soft and compliant.1–3 This shift reflects that user acceptance is an essential requirement for wearable technology4–7 and any system that is in close and constant contact with the wearer’s body must be comfortable. As Fig. 1 illustrates, the evolution of exoskeletons has reflected these requirements with the latest systems utilizing textile attachments, cable-drives, or soft pneumatic systems to deliver torques to limb joints.8,9 While these lightweight exoskeletons have shown promise, benefits demonstrated to date have been small.10–12 Further reductions in size and weight of batteries and motor units are needed but achieving these goals with current technology comes at the cost of reduced mechanical assistance or shorter operating lifetimes. The next evolution in soft exosuits and soft robots seeks to replace the current motor systems with artificial muscle materials13 that can provide considerably higher power–weight ratios compared with similar sized electric motors14 thereby reducing motor mass.
Contributing Editor: Linda S. Schadler a) Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2016.316
The recently described spring-like polymer coil artificial muscles14 are highly stretchable and compliant and are of particular interest for use in robotic garments and soft robotic devices.15 As introduced by Baughman and co-workers, coiled polymer fiber artificial muscles are made by a simple twist insertion process using low-cost polymer textile fibers. The first reports of these thermally
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