A Wide Range of Multi-Stage Stiffness Regulation via Harnessing Phase Variability of Low-Melting-Point Alloy

The high rigidity of the low-melting-point alloy (LMPA) renders it suitable for soft robots with heavy loads and extensive workspace requirements. However, its two-stage stiffness adjustment and prolonged response time have limited further application. This research proposes a novel approach to achieve multi-stage stiffness by precisely controlling the heat to selectively melt or solidify the LMPA layer-by-layer, thereby regulating the solid-liquid phase ratio. We fabricate a multi-layer stiffness regulation device consisting of a unidirectional heat source and cooling channel to enable unidirectional multi-stage stiffness adjustment and seamless transition between different levels of rigidity. The incorporation of liquid alloys enhances the versatility and applicability of stiffness adjustment. The cooling time was reduced by 72.4% by controlling the phase transition temperature in the interval centered on the melting point. The various factors involved in achieving rapid multistage stiffness adjustment are investigated. Experiments demonstrate the adaptability and integration potential of the alloy. We demonstrate LMPA's wide range of stiffness and dynamic stiffness adjustability with the soft gripper. Our research endeavors to contribute towards advancing soft robotics by integrating rigidity and flexibility.