Mechanically adaptive materials with tunable stiffness for biomedical devices: Mechanisms, applications, and...

Variable-stiffness materials are engineered systems that can reversibly tune their mechanical stiffness through diverse mechanisms. In recent years, researchers have paid growing attention to these materials because they can adapt to different tasks, support multiple functions, and safely interact with soft biological tissues. For these reasons, they show strong potential for biomedical devices. However, most existing studies focus on single material systems or individual actuation modes, which limits the integration of different stiffness-control strategies within metamaterial-inspired architectures. As a result, cross-mechanism synergy remains underexplored, and unified system-level design rules are still lacking. This review summarizes recent progress in variable-stiffness materials for biomedical devices over the past decade. We divide current approaches into three main groups based on their working mechanisms: jamming-based systems, stimuli-responsive material-based systems, and antagonistic actuation-based systems. For each group, we explain the basic working principles and typical structural designs. We then describe how different activation methods control stiffness and support functional adaptation. Building on this, an application-oriented analysis is conducted across five representative biomedical device domains, including drug delivery systems, wearable assistive devices, minimally invasive surgical tools, bone repair applications, and adaptive sensing platforms, with a comparative evaluation of the suitability and prevalence of different stiffness modulation mechanisms in each domain. Finally, we discuss future research directions. These include AI-based real-time stiffness control, multimodal human-machine interfaces, 4D-printed adaptive structures, and bio-integrated hybrid materials. Together, these advances may help move Variable-stiffness materials from laboratory studies toward practical clinical use.