MAGTWIST: A Magnetically-Driven Rotary Actuator Using a Traveling-Wave With Integrated Stiffness Tunability

Robotic systems in confined environments rely on compact and dexterous actuators. In minimally invasive surgery, cable-driven mechanisms remain standard but require high locking forces, causing tendon fatigue and friction. Magnetic actuation eliminates the need for cable transmissions but suffers from low load-bearing capacity and lacks coaxial rotation. We present a magnetic rotary actuator (MAGTWIST) that exploits a traveling wave to achieve a large motion range, stepless rotation, and controllable (un)locking. MAGTWIST features an undulating belt, inspired by peristalsis, and made from a thermo-responsive polymer. Torque generated by an applied magnetic field engages sliders with the undulation, inducing localized deformation that forms a traveling wave around a circular path. Switching between (un)locked states is done by Joule heating. Heating softens the belt within 3.0 s, allowing the wave to travel in a circle arc of up to 270°. Cooling restores stiffness within 5.3 s, halting the wave and achieving stepless shape-locking. Once stiffened, actuation torque rises from 4.5 to 250 N·mm, securing the rotary position and increasing load-bearing capacity. The tubular design with an inner and outer diameter of 4 and 8 mm integrates as an end-effector in a robotic manipulator, as demonstrated through gripping and in situ shape transitions during minimally invasive surgical tasks.