Bio-Friendly Artificial Muscles for Electroactive Textiles Based on Carbon Nanotube Yarns

Electrochemically driven coiled yarn actuators, which provide fast actuation and large contractile stroke, are promising soft transducers and could be considered precursors of artificial muscles for applications in smart textiles, prosthetics, soft robotics, and exoskeletons. These actuators are built on a coiled electroactive yarn and an electrolyte, serving respectively as the electrode and the ion source. Charging the electrochemical double layer (EDLC) at the interface of the yarn and the electrolyte promotes a local increase in volume as the ions swell the yarn’s surface, leading to transversal expansion and longitudinal contraction. The most utilized type of yarns for these actuators is made of carbon nanotubes (CNTs) due to their high conductivity, high surface area, light weight, and excellent mechanical robustness. Air operation can be achieved by associating two coiled yarns with a gel electrolyte coating. The gel electrolyte plays a critical role in the ultimate performance of such actuators but usually suffers from issues such as air-drying for hydrogels, toxicity, or low conductivity in the case of organogels or ionic liquid-based gels.<br/>In this work, we present eutectogels as gel electrolytes for fabricating bio-friendly, air-operating, rapid, coiled yarn actuators from commercially available CNT yarns. The eutectogels are synthesized using a deep eutectic solvent (DES), which is a multicomponent mixture of a hydrogen bond donor (HBD) and acceptor (HBA). DES presents properties similar to ionic liquids, such as air stability and high ionic conductivity, with additional desired features: biocompatibility, low cost, and facile synthesis. Coiled yarns are initially electromechanically characterized in a 3-electrode electrochemical setup using various DES dilutes. The effects of CNT surface treatments (chemical, thermal, etc.) and the composition of the DES are investigated to optimize the electromechanical performances, reaching a maximal strain rate of 1%/s and a maximal stroke of 5.3% in 15 seconds.<br/>To enable air actuation, novel eutectogels were directly coated on the surface of coiled CNT yarns using UV-polymerization of liquid precursors. More specifically, two oppositely charged gels were developed to promote the so-called unipolar charging of yarns. The polyanionic gel contained grafted anions, while the polycationic one contained grafted cations, meaning that the anions and cations were respectively, blocked from participation in the intercalation process at the negative and the positive electrodes. This enabled unidirectional contractile stroke over the full electrochemical window. First characterizations of the resulting actuator in open air showed a contractile stroke of 3.6% in 30 seconds and a volumetric work capacity of up to 100 kJ/m^3.<br/>The demonstrated coiled CNT yarn/gel actuators can be fabricated rapidly and can operate in open air, at an applied potential difference of less than 3V, using bio-friendly and biocompatible electrolytes and commercially available CNT yarns. Further development will include their integration into e-textile garments as soft actuators.