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Variable Stiffness Flexure Structures Enabled by Phase-Change Gallium and Adhesive Interfacial Locking for Soft Robotic Applications
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DOI：10.1002/adem.202502415.png)
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Abstract
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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 (Al2O3) 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.
Keywords:
flexure-based structure
gallium adhesion
liquid metals
stiffness modulation
variable stiffness mechanism
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