Stretchable artificial muscles from coiled polymer fibers


Soft 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.

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  1. 1.

    H. Shen: The soft touch. Nature 530, 24 (2016).

    CAS  Article  Google Scholar 

  2. 2.

    R. Pfeifer, M. Lungarella, and F. Iida: Self-organization, embodiment, and biologically inspired robotics. Science 318, 1088 (2007).

    CAS  Article  Google Scholar 

  3. 3.

    D. Rus and M.T. Tolley: Design, fabrication and control of soft robots. Nature 521, 467 (2015).

    CAS  Article  Google Scholar 

  4. 4.

    S. Viteckova, P. Kutilek, and M. Jirina: Wearable lower limb robotics: A review. Biocybern. Biomed. Eng. 33, 96 (2013).

    Article  Google Scholar 

  5. 5.

    R. Cowan, B.J. Fregly, M.L. Boninger, L. Chan, M.M. Rodgers, and D.J. Reinkensmeyer: Recent trends in assistive technology for mobility. J. Neuroeng. Rehabil. 9, 20 (2012).

    Article  Google Scholar 

  6. 6.

    H. Herr: Exoskeletons and orthoses: Classification, design challenges and future directions. J. Neuroeng. Rehabil. 6, 21 (2009).

    Article  Google Scholar 

  7. 7.

    J.L. Pons: Rehabilitation exoskeletal robotics. IEEE Eng. Med. Biol. Mag., 29 (3), 57 (2010).

    Article  Google Scholar 

  8. 8.

    A.T. Asbeck, S.M.M. de Rossi, K.G. Holt, and C.J. Walsh: A biologically inspired soft exosuit for walking assistance. Int. J. Robot. Res. 34, 744 (2015).

    Article  Google Scholar 

  9. 9.

    P. Polygerinos, Z. Wanga, K.C. Galloway, R.J. Wood, and C.J. Walsh: Soft robotic glove for combined assistance and at-home rehabilitation. Robot. Autonom. Syst. 73, 135 (2015).

    Article  Google Scholar 

  10. 10.

    L.M. Mooney, E.J. Rouse, and H.M. Herr: Autonomous exoskeleton reduces metabolic cost of human walking during load carriage. J. Neuroeng. Rehabil. 11, 80 (2014).

    Article  Google Scholar 

  11. 11.

    G.S. Sawicki and D.P. Ferris: Powered ankle exoskeletons reveal the metabolic cost of plantar flexor mechanical work during walking with longer steps at constant step frequency. J. Exp. Biol. 212, 21 (2009).

    Article  Google Scholar 

  12. 12.

    P. Malcolm, W. Derave, S. Galle, and D. De Clercq: A simple exoskeleton that assists plantarflexion can reduce the metabolic cost of human walking. PLoS One 8, e56137 (2013).

    CAS  Article  Google Scholar 

  13. 13.

    E. Mattar: A survey of bio-inspired robotics hands implementation: New directions in dexterous manipulation. Robot. Autonom. Syst. 61, 517 (2013).

    Article  Google Scholar 

  14. 14.

    C.S. Haines, M.D. Lima, N. Li, G.M. Spinks, J. Foroughi, J.D.W. Madden, S.H. Kim, S. Fang, M.J. de Andrade, F. Goktepe, O. Goketpe, S.M. Mirvakili, S. Naficy, X. Lepro, J. Oh, M.E. Kozlov, S.J. Kim, X. Xu, B.J. Swedlove, G.G. Wallace, and R.H. Baughman: Artificial muscles from fishing line and sewing thread. Science 343, 868 (2014).

    CAS  Article  Google Scholar 

  15. 15.

    M.C. Yip and G. Niemeyer: High-performance robotic muscles from conductive nylon sewing thread (IEEE Int. Conf. Robot. Autom., Seattle, May 2015), 2313–2318.

  16. 16.

    A.B. Zoss, H. Kazerooni, and A. Chu: Biomechanical design of the Berkeley lower extremity exoskeleton (BLEEX). IEEE ASME Trans. Mechatron. 11, 128 (2006).

    Article  Google Scholar 

  17. 17.

    V. Giurgiutiu, C.A. Rogers, A. Rogers, and Z. Chaudhry: Energy-based comparison of solid-state induced-strain actuators. J. Intell. Mater. Syst. Struct. 7, 4 (1996).

    Article  Google Scholar 

  18. 18.

    G.M. Spinks, L. Liu, G.G. Wallace, and D. Zhou: Strain response from polypyrrole actuators under load. Adv. Funct. Mater. 12, 437 (2002).

    CAS  Article  Google Scholar 

  19. 19.

    A. Cherubini, G. Moretti, R. Vertechy, and M. Fontana: Experimental characterization of thermally-activated artificial muscles based on coiled nylon fishing lines. AIP Adv. 5, 067158 (2015).

    Article  Google Scholar 

  20. 20.

    W. Zheng, G. Alici, P.R. Clingan, B.J. Munro, G.M. Spinks, J.R. Steele, and G.G. Wallace: Polypyrrole stretchable actuators. J. Polym. Sci., Part B: Polym. Phys. 51, 57 (2013).

    CAS  Article  Google Scholar 

  21. 21.

    G.M. Spinks, T.E. Campbell, and G.G. Wallace: Force generation from polypyrrole actuators. Smart Mater. Struct. 14, 406 (2005).

    CAS  Article  Google Scholar 

  22. 22.

    S. Aziz, S. Naficy, J. Foroughi, H.R. Brown, and G.M. Spinks: Controlled and scalable torsional actuation of twisted nylon 6 fiber. J. Polym. Sci., Part B: Polym. Phys. 54, 1278 (2016).

    CAS  Article  Google Scholar 

  23. 23.

    S. Aziz, S. Naficy, J. Foroughi, H.R. Brown, and G.M. Spinks: Characterisation of torsional actuation in highly twisted yarns and fibres. Polym. Test. 46, 88 (2015).

    CAS  Article  Google Scholar 

  24. 24.

    J. Foroughi, G.M. Spinks, G.G. Wallace, J. Oh, M.E. Kozlov, S. Fang, T. Mirfakhrai, J.D.W. Madden, M.K. Shin, S.J. Kim, and R.H. Baughman: Torsional carbon nanotube artificial muscles. Science 334, 494 (2011).

    CAS  Article  Google Scholar 

  25. 25.

    M.D. Lima, N. Li, M.J. de Andrade, S. Fang, J. Oh, G.M. Spinks, M.E. Kozlov, C.S. Haines, D. Suh, J. Foroughi, S.J. Kim, Y. Chen, T. Ware, M.K. Shin, L.D. Machado, A.F. Fonseca, J.D.W. Madden, W.E. Voit, D.S. Galvão, and R.H. Baughman: Electrically, chemically, and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles. Science 338, 928 (2012).

    CAS  Article  Google Scholar 

  26. 26.

    S.M. Mirvakili, A. Pazukha, W. Sikkema, C.W. Sinclair, G.M. Spinks, R.H. Baughman, and J.D.W. Madden: Niobium nanowire yarns and their application as artificial muscles. Adv. Funct. Mater. 23, 4311 (2013).

    CAS  Article  Google Scholar 

  27. 27.

    C.L. Choy, F.C. Chen, and K. Young: Negative thermal expansion in oriented crystalline polymers. J. Polym. Sci., Polym. Phys. Ed. 19, 335 (1981).

    CAS  Article  Google Scholar 

  28. 28.

    Y. Kobayashi and A. Keller: The temperature coefficient of the c lattice parameter of polyethylene; an example of thermal shrink age along the chain direction. Polymer 11, 114 (1970).

    CAS  Article  Google Scholar 

  29. 29.

    L.R.G. Treloar: Rubber Elasticity (Oxford University Press, Oxford, 1975).

    Google Scholar 

  30. 30.

    A.L. Ross: Cable kinking analysis and prevention. Trans. ASME, 99, 112 (1977).

    Google Scholar 

  31. 31.

    F.B. Fuller: The writhing number of a space curve. Proc. Natl. Acad. Sci. U.S.A. 68, 915 (1971).

    Article  Google Scholar 

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The author thanks Dharshika Kongahage and Dr. Sina Naficy for providing the experimental data of Figures 4 and 5. The author also thanks Professors Ray Baughman (University of Texas at Dallas), John D.W. Madden (University of British Columbia), and Seon Jeong Kim (Hanyang University) for valuable discussions. Financial support for this work was provided by the Australian Research Council (DP 110101073).

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Correspondence to Geoffrey M. Spinks.

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Spinks, G.M. Stretchable artificial muscles from coiled polymer fibers. Journal of Materials Research 31, 2917–2927 (2016).

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