Magnetorheological phase-change McKibben actuators for frequency-controlled actuation and high force output.

State-of-the-art soft robots face a critical trade-off: high-performance pneumatic actuators require bulky external compressors, whereas emerging smart-material actuators often lack sufficient force output. Here, we present a magnetorheological elastomer (MRE)-based phase-change McKibben actuator actuated via induction heating. The inclusion of ferromagnetic particles contributes to improved force output and thermal response of the elastomeric matrix. Furthermore, the design enables wireless operation, where the induction coil's excitation frequency directly regulates the electromagnetic heating and actuation timing. A multiphysics modeling framework, integrating finite element electromagnetic analysis with analytical fluid-pressurization and structural models, guides the actuator design. Prototypes operating at 127 kHz and 150 kHz are fabricated and tested. They produce up to 70 N blocked force at a 23 g mass, representing an increase over prior phase-change actuators. The force output is 48% higher, with faster heating and cooling times compared to identically dimensioned hyperelastic actuators. Increasing the induction frequency to 150 kHz reduces the time to reach a 35 N load by nearly 25%. With heating times of 10-20 s and wireless operation, this actuator represents a promising platform for lightweight soft robotic systems.