Athermal artificial muscles with drastically improved work capacity from pH-Responsive coiled polymer fibers

Abstract

• We demonstrated anisotropic pH-sensitive actuation response of fibers which reflects the anisotropic morphology of the polymer. • We showed that coiling amplifies both actuation stroke and work capacity by benefitting from anisotropic fiber actuation. • Mechanics-based models were shown to reveal the contribution of different deformation mechanisms to actuation. • Our work elucidated the relative significance of various mechanistic mechanisms that control the rate of actuation and their dependence on length scale. Polymeric artificial muscles are great candidates to replace traditional rigid actuators due to their lightweight nature and high actuation stroke. However, the actuation mechanism of many polymer artificial muscles relies on large temperature changes which may cause polymer degradation. Besides, having remotely detectable thermal IR signals is not suitable for some applications. Chemical stimulants, on the other hand, can operate polymer artificial muscles and address these limitations. In this paper, we evaluated the actuation response of athermal pH-responsive artificial muscles made of aligned and coiled fibers. We demonstrated that a coiled architecture can be utilized to simultaneously amplify actuation stroke and work capacity by benefitting from the anisotropic length and diameter changes of individual fibers in response to pH changes. Our coiled pH-responsive fibers can deliver up to 43 % contractive actuation stroke. The maximum obtained work capacity of 393 J/kg is 5 times higher than that of the aligned fiber counterpart. At an actuation stroke of 22 %, the polymer muscle lifts weights over 2000 times heavier relative to its own weight. A mechanistic model of coiled fibers revealed that the amplification of stroke and work capacity are owed to significant changes in the bending and torsional stiffness stemming from changes in elastic modulus and fiber diameter.