Hierarchical Artificial Muscle with Nonlinear Elasticity for Antagonistic and Cyclic Robotics

A key design motif of skeletal muscles is their arrangement in pairs to enable the cyclic, contra-lateral contractions necessary for motion. This mechanism may initially appear inefficient, since the contraction of a muscle group stretches the antagonist, increasing resistance and energy consumption. However, the hierarchical architecture of muscles provides a clever solution. By giving rise to J-shaped stress-strain responses, muscle tissue is soft at small strains, thus minimizing resistance, while it stiffens at large strains to enable economical energy release and prevent excessive elongation and damage. Here, we develop hierarchical supercoiled artificial muscles by plying fishing line fibers that recapitulate this behavior and thus allow antagonistic actuation. Computational models based on Cosserat rods reveal the physical mechanisms underlying the observed J-shaped responses. The artificial muscles are used in an antagonistic biceps/triceps arm mechanism and a vertical rope-climbing robot that weighs 14.4 grams and carries a payload 14.6 times heavier than its own weight.