Improving energy conversion efficiency of ion-driven artificial muscles based on carbon nanotube yarn

While artificial muscles provide giant work and power densities compared to natural muscles, their reported energy conversion efficiencies have so far been low. We here demonstrate a tension optimization process (TOP) for fabricating coiled carbon nanotube artificial muscles having record efficiencies. These TOP muscles were made by applying about 20 times higher tensile stress during pre-coiling twist insertion than the tensile stress applied during coiling, resulting in high twist density and high spring index. The TOP muscles driven by the tetrabutylammonium cation provide 6.1 J/g contractile work, which is ∼152 times the maximum capability of human skeletal muscles, and 13.1 % contractile energy efficiency. In addition, the contractile energy efficiency of the TOP muscles driven by the bis(trifluoromethanesulfonyl)imide anion is maximized to 38.8 % by minimizing side redox reactions. In the case of full-cycle actuation, which considers the whole cycle of contraction and relaxation, we increased the full-cycle energy conversion efficiency of TOP muscles to 6.7 %, which is 4.5 times that previously reported for ion-driven artificial muscles.