Functional Morphology and Hierarchy of Bioinspired Supercoiled Muscles

This talk will present a roadmap for elastic actuators inspired by natural muscles and their use in miniature robots requiring large mechanical work output.<br/>First, I will describe the material microstructure and geometric mechanics of polymeric twisted and coiled polymer actuators (TCPA) made by twisting nylon fishing lines, and how these actuators use internal strain energy to achieve record breaking performance. These artificial muscles generate large contractile mechanical work mimicking natural muscles, which makes them suitable for robotics. Understanding the mechanism of nylon TCPA remains challenging due to the interplay between their intricate geometry, chirality, residual stresses, and material microstructure. I will present a material microstructure model with rod theory to analytically predict the equilibrium helical shape of the nylon TCPA after fabrication, and to explain the observed contraction mechanism upon stimulation. The first ingredient of the model is to treat nylon as a two-phase thermomechanical microstructure system capable of storing strain energy and exchanging it among the two phases. This is validated by characterizing the torsional actuation response of twisted and annealed nylon fibers. The second ingredient of the model is to use the classic Kirchhoff Rod Theory and add a necessary term that couples the bending and twisting energy. Validation with experiments shows that the model captures the equilibrium and longitudinal stiffness of the TCPA in both active and passive states, and the stimulated contraction under external load. Importantly, the model quantifies the influence of the stored energy level on the actuation performance.<br/><br/>Next, I will describe the construction of a bio-inspired hierarchical supercoiled muscle which demonstrates a passive force-displacement J-curve. This passive nonlinear stretching makes these new muscles suitable to antagonistic arrangements similar to their biological counterparts. I will describe their use to actuate the rope climbing robots and antagonistic weightlifting robots.<br/><br/>These examples shed light on the future of robotics propelled by new bioinspired materials, nonlinear mechanics, and unusual manufacturing processes.