Coil Formation and Biomimetic Performance Characterization of Twisted Coiled Polymer Artificial Muscles.

Objective: A biological muscle's force is nonlinearly constrained by its current state (force, length, and speed) and state history. To investigate if artificial muscles can mimic the complete mechanical state spectrum of biological muscles, this study uses a novel method to characterize twisted coiled polymer actuators (TCPAs) mechanically. Thus, comprehensive and reproducible test procedures are established to verify artificial muscle biomimetics regarding stress, strain, and strain rate combinations intrinsic to biological muscle. Approach: A rheometer performs novel high-precision mechanical characterization methods that comprehensively verify biomimetic performance. Sample twist level, torque, length, force, and temperature were controlled and measured during twist-induced coiling, heatsetting/annealing, and mechanical testing. TCPAs were formed from linear low-density polyethylene monofilament. Main Results: LLDPE TCPAs generate larger stresses than biological muscle through the entire spectrum of strains - contracting more than 40%, exerting more than 0.3 MPa at rest length, and withstanding tension of 8 MPa without damage. Thus, the LLDPE TCPAs attained biological muscle performance statically, but additional tests are required to assess this dynamically. Significance: The mechanical performance of LLDPE TCPAs enables biomimetic actuation with an intelligent control and measurement system. Their high-throughput textile manufacturability positions them for advanced biomechatronic applications - including prosthetics and exoskeletons.