Actuators and variable stiffness of flexible surgical actuators: A review

Flexible surgical robots have gained attention for their ability to perform minimally invasive interventions on lesions in confined areas within the human body through a single port, marking a significant step towards the goal of non-invasive surgery. However, the narrow and tortuous human cavities impose higher demands on the compliance, operational precision, and force output of the actuators. Therefore, designing actuators with variable stiff capabilities is particularly crucial. Since the coordination between actuators and variable stiffness design is not a simple universal or direct correspondence but involves complex interplay, it is essential to delve into their mechanisms and integration schemes. This paper focuses on the progress of research in flexible surgical actuators over the past decade, first discussing the actuation principles and latest designs of various types of actuators. It then reviews the mechanisms of variable stiffness, analyzing the performance characteristics of different mechanisms in terms of stiffness adjustment range, response time, and space occupation. Finally, it summarizes and analyzes the integration schemes of actuators and variable stiffness approaches and points out that composite variable stiffness actuators will be a key direction for future development in this field.