Fibrous stretchable actuators, distinguished by their lightweight, flexibility, and high robustness, are capable of operating in confined, complex environments and can be seamlessly integrated into wearable textiles, demonstrating significant potential for applications in soft robotics, smart healthcare, and prosthetics. Shape memory alloys (SMAs) offer high energy density, large mechanical outputs, and versatile shape changes, but their inherent rigidity limits their use in soft actuators. Significant challenges remain to achieve stretchable SMA actuators with high conformability, excellent reversibility, large mechanical outputs, and the ability to achieve different morphing modes. In this study, we present an artificial muscle fiber designed by combining the phase transition of SMAs under temperature changes and the polymer chain realignment of stretchable elastomer under mechanical strains to tackle the issue. The fibrous actuators exhibit high stretchability (120%), robust reversibility under cyclic thermal activation, and large actuation strain (32%), producing bidirectional forces (1.2 N extension, 4.3 N contraction). Importantly, the actuators can undergo repetitive shape programming for different morphing modes. With excellent output performance, the fibrous actuators enabled various soft robotic applications, including a fibrous robot for curvilinear pipe navigation, a stretchable wearable exoskeleton, and an untethered soft actuator driven by external electromagnetic fields.
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