A Review of Variable Stiffness in Continuum Robots: Mechanisms, Modeling and Control

Variable stiffness endows continuum robots with both compliance and tunable rigidity, making them promising alternatives to traditional rigid manipulators in confined and unstructured environments. Over the past decade, great progress has been made in variable stiffness technologies involving structural design, actuation, modeling, and control. However, current research is fragmented and mostly focuses on individual aspects, lacking a systematic review and a unified framework integrating structure, modeling, and control. This paper presents a comprehensive review of variable stiffness in continuum robots, emphasizing the interrelationships among stiffness principles, modeling, and control strategies. We summarize classical and emerging variable stiffness methods, analyze their integration with control approaches, and evaluate the evolution of control strategies, especially multi-modal fusion of actuation, sensing, and control. Such fusion can improve control accuracy and robustness in human-centered environments and is regarded as a key driver for next-generation intelligent continuum robots. Finally, we outline future directions, highlight the “actuation–stiffness–control” paradigm, and discuss existing challenges and open research opportunities for high-performance intelligent control.