Tunable-impedance mechanisms can improve the adaptivity, robustness, and efficiency of a vast array of engineering systems and soft robots. In this study, we introduce a tunable-stiffness mechanism called a "sandwich jamming structure," which fuses the exceptional stiffness range of state-of-the-art laminar jamming structures (also known as layer jamming structures) with the high stiffness-to-mass ratios of classical sandwich composites. We experimentally develop sandwich jamming structures with performance-to-mass ratios that are far greater than laminar jamming structures (e.g., a 550-fold increase in stiffness-to-mass ratio), while simultaneously achieving tunable behavior that standard sandwich composites inherently cannot achieve (e.g., a rapid and reversible 1800-fold increase in stiffness). Through theoretical and computational models, we then show that these ratios can be augmented by several orders of magnitude further, and we provide an optimization routine that allows designers to build the best possible sandwich jamming structures given arbitrary mass, volume, and material constraints. Finally, we demonstrate the utility of sandwich jamming structures by integrating them into a wearable soft robot (i.e., a tunable-stiffness wrist orthosis) that has negligible impact on the user in the off state, but can reduce muscle activation by an average of 41% in the on state. Through these theoretical and experimental investigations, we show that sandwich jamming structures are a lightweight highly tunable mechanism that can markedly extend the performance limits of existing structures and devices.
Keywords: composite; jamming; orthosis; sandwich; tunable stiffness.