Variable Stiffness Flexure Structures Enabled by Phase‐Change Gallium and Adhesive Interfacial Locking for Soft Robotic Applications

Variable stiffness structures are essential for robotic systems that require both compliance for safe interactions with humans and rigidity for load‐bearing tasks. However, existing approaches—whether based on interfacial locking (e.g., jamming) or material‐based stiffness modulation—often struggle to achieve a wide range of stiffness variation without sacrificing either soft‐state compliance or rigid‐state strength. We propose a variable stiffness structure with gallium embedded in a flexure core, combining phase‐change modulation and oxide‐induced interfacial adhesion for enhanced performance. Upon solidification, gallium adheres to the surrounding surfaces, constraining internal motion and significantly increasing structural stiffness. Experimental results demonstrate a stiffness gain of approximately 200×, from 0.10 N mm ⁻¹ in the liquid phase to 20 N mm ⁻¹ in the solid phase. Parametric studies reveal that stiffness values in both states can be independently tuned through geometric design, reaching as low as 0.04 N mm ⁻¹ and as high as 21.4 N mm ⁻¹ . Additionally, the integration of aluminum oxide (Al ₂ O ₃ ) into the silicone cover improves thermal conductivity, helping reduce the phase transition time. The application of the proposed method is demonstrated through a wearable glove capable of maintaining finger postures under external loads. The proposed design enables enhanced and tunable stiffness variability, offering new opportunities in soft wearable devices and robotic manipulation.