Soft Actuators Based on Stimuli-Responsive Materials: From Material Design to Robotic Systems

Soft actuators are essential for compliant interaction, adaptive motion, and safe human-robot collaboration; yet, their performance is fundamentally constrained by energy conversion efficiency, structural amplification, and interfacial coupling stability. Recent advances in smart materials, soft matter mechanics, and advanced manufacturing have driven a shift from externally driven architectures toward material-structure-interface synergistic actuation. From a materials and interfacial science perspective, this review summarizes progress in soft actuators over the past five years. Conventional actuation mechanisms, including electrical, magnetic, thermal, optical, and fluidic driving, are comparatively analyzed with emphasis on intrinsic limitations in soft systems, such as material incompatibility, integration complexity, and scalability. Emerging strategies based on stimuli-responsive materials, including dielectric elastomers, ionic and hydrogel systems, liquid crystal elastomers, and multiresponsive composites, are then critically reviewed in terms of actuation mechanisms, structural design, and performance limits. The roles of multimaterial additive manufacturing and interfacial engineering are further discussed. Finally, future directions are outlined with regard to energy efficiency, environmental adaptability, and long-term reliability.