A Novel Hybrid Variable Stiffness Mechanism: Synergistic Integration of Layer Jamming and Shape Memory Polymer

Soft robots have garnered considerable attention recently due to their versatility, compliance, and myriad applications. However, the inherent low stiffness of soft robots also limits their stability and force output capability. Hence, variable stiffness technology has emerged as a solution, which enables soft robots to modulate stiffness according to the application scenario. Two primary methods have been developed to regulate stiffness: material phase transition (MPT) based method and geometric reconfiguration (GR) based method. However, these approaches have not achieved miniaturization while maintaining a wide range of stiffness change. This work introduces a novel hybrid variable stiffness (HVS) concept that combines the MPT-based and GR-based variable stiffness methods for the first time. Specially, the HVS structure leverages shape memory polymer (SMP) method and layer jamming (LJ) to get a simultaneous response. Bending tests reveal that the compact bi-layer designed HVS structure can achieve a wide stiffness range (0.31 N/mm ∼ 4.86 N/mm, 15.7 times) and load-bearing capacity (1.76 N∼ 28.1 N, 16.0 times), which has been simulated by finite element analysis. Response tests show that the jamming response is rapid (∼ms) while the maximum heating rate is 2.77 ± 0.16 °C/s, indicating that the HVS structure can achieve a relatively fast response. Furthermore, a soft gripper equipped with the HVS structure is developed to illustrate the enhanced grasping ability. Several performed grasping tests reveal that the variable stiffness soft gripper can grasp various objects with diverse shapes (40.0 mm ∼ 190 mm, 4.75 times) and materials while lifting a weight up to 650 g, which provides an effective solution for the complex application requirements of soft robots.