Enhancing the Work Capacity of Electrochemical Artiﬁcial Muscles by Coiling Plies of Twist-Released Carbon Nanotube Yarns Keon Jung Kim, † Jae Sang Hyeon, † Hyunsoo Kim, † Tae Jin Mun, † Carter S. Haines, ‡ Na Li, ‡ Ray H. Baughman, ‡ and Seon Jeong Kim* ,† † Center for Self-Powered Actuation, Department of Biomedical Engineering, Hanyang University, Seoul 04763, Korea ‡ Alan G. MacDiarmid NanoTech Institute, University of Texas at Dallas, Richardson, Texas 75080, United States *S Supporting Information ABSTRACT: Twisted-yarn-based artiﬁcial muscles can potentially be used in diverse applications, such as valves in microﬂuidic devices, smart textiles, air vehicles, and exoskeletons, because of their high torsional and tensile strokes, high work capacities, and long cycle life. Here, we demonstrate electrochemically powered, hierarchically twisted carbon nanotube yarn artiﬁcial muscles that have a contractile work capacity of 3.78 kJ/kg, which is 95 times the work capacity of mammalian skeletal muscles. This record work capacity and a tensile stroke of 15.1% were obtained by maximizing yarn capacitance by optimizing the degree of inserted twist in component yarns that are plied until fully coiled. These electrochemically driven artiﬁcial muscles can be operated in reverse as mechanical energy harvesters that need no externally applied bias. In aqueous sodium chloride electrolyte, a peak electrical output power of 0.65 W/kg of energy harvester was generated by 1 Hz sinusoidal elongation. KEYWORDS: carbon nanotube yarn, artiﬁcial muscle, electrochemical actuator, high work capacity, energy harvesting ■ INTRODUCTION The application of artiﬁcial muscles, from macro- to micro- scale, requires large-strokes, fast responses, high cycle life, and high mechanical work capacity. Many types of artiﬁcial muscles have been reported, such as piezoelectric ceramics, 1,2 shape- memory alloys, 3,4 conducting polymers, 5 and ionic-polymer metal composites. 6,7 Inserting twist into yarns comprising carbon nanotubes (CNTs) 8−21 or polymer, 22−27 graphene, 28 or metal 29 ﬁbers has resulted in muscles that provide both tensile and torsional actuation. Torsional CNT artiﬁcial muscles 8 fabricated by twisting CNT yarn have provided a similar speciﬁc torque as commercial electrical motors. By overtwisting CNT yarns, coiled muscles can be fabricated that provide large tensile strokes. Guest-ﬁlled, thermally powered coiled CNT yarn artiﬁcial muscle can generate ∼80 times higher mechanical work and power than natural muscle, 9,22 but have a cycle rate that is limited by the cooling needed to reverse actuation. To improve muscle performance, we have developed supertough, hierarchically twisted-yarn muscles that have a similar structure to straw ropes and elevator cables. 30 By using such muscles, which are able to lift heavy loads, we obtained very high gravimetric contractile work capacities. This coiled multiply structure introduces a new parameter that can be tuned to optimize muscle performance, the twist within the individual yarn plies. 31 In this communication, we demonstrate electrochemically driven, hierarchically twisted tensile artiﬁcial muscles that are fabricated from CNT yarns. We call the coiled, multiply CNT yarn a “hierarchically twisted CNT artiﬁcial muscle (HTAM)”. The HTAM has a high mechanical toughness (53 J/g), which is 3.5 times higher than for conventional single-ply coiled CNT yarn. A high contractile work capacity of 3.78 kJ/kg was demonstrated, which is 95 times higher than the 0.04 kJ/kg of mammalian skeletal muscle 32 and 1.72 times higher that of previous electrochemical muscles. 21 A tensile stroke of 15.1% was obtained using a low input voltage (3.25 V), by increasing capacitance by ∼30%, by optimizing the twist inserted in individual yarn plies. By operating the artiﬁcial muscle in reverse to convert mechanical energy to electrical energy, a new type of “Twistron” 33 energy harvester was demonstrated, which generated 0.65 W/kg of peak electrical power without the need for an externally applied bias voltage. As shown in the schematic images of Figure 1a, the HTAM contains three levels of the hierarchical structure; dual- Archimedean spun-twisted yarns that are plied and then coiled. Figure 1b−e shows the scanning electron microscope (SEM) images of a HTAM and its hierarchical components. The HTAM comprises dual-Archimedean spun-twisted CNT yarns that are fabricated from spinnable CNT forests, which were synthesized by chemical vapor deposition. 34 Dual- Archimedean CNT yarns (Figure 1b,c) were twisted under a Received: December 11, 2018 Accepted: March 20, 2019 Research Article www.acsami.org Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acsami.8b21417 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 03/29/19. For personal use only.