Experimental Analysis and Physics-Based Analytical Model on Twisted and Coiled Artificial Muscles

This work presents a comprehensive experimental and analytical study of twisted and coiled artificial muscles (TCAMs) fabricated from three types of silver-coated nylon 6,6 precursor fibers. The coupled thermo-electromechanical response of these actuators is investigated through systematic characterization and a physics-based analytical model. Built upon Castigliano's theorem, the model captures nonlinear contraction behavior with high accuracy while remaining computationally efficient compared to more complex formulations. Experimental validation demonstrates maximum contractions up to 19.3%, strongly influenced by precursor type and the applied prestrain. Beyond displacement, actuator performance is quantified through additional metrics. The specific mechanical work per cycle reaches values as high as 8 kJ kg−1, highlighting the excellent work density achievable with minimal actuator mass. Conversely, electromechanical efficiency is found to be limited (0.15 ± 0.2% under optimized conditions), primarily due to thermal losses resulting from convection, radiation, and conduction. Time-constant analysis reveals fiber-dependent trends: Thinner fibers exhibit faster overall dynamics but slower heating than cooling, while thicker fibers display longer relaxation times due to their larger thermal mass. The combined experimental and analytical approach provides both a detailed understanding and a predictive tool for TCAMs, offering insights into their strengths and limitations for future integration into wearable and soft robotic systems.