Millimeter-scale fluid-driven soft robots

ABSTRACT Millimeter-scale soft robots (milli-SRs) promise significant advancements in biomedical engineering and inspection, enabling precise navigation in confined spaces. However, fabricating miniaturized fluid-driven soft robots is hindered by microscale forces. Here, we introduce a new universal design and fabrication approach (referred to as the mini bubble casting method) to create high-quality multifunctional fluid-driven milli-SRs. By injecting a bubble into pre-modified silicone liquid under high-stability conditions, we achieve submillimeter internal voids, overcoming interfacial-tension-induced instability. The modification strategy is guided by our theoretical model, which explains the influence of viscous resistance and interfacial tension on the dynamic behavior of the bubble-silicone interface. We successfully fabricate soft milli-actuators ten times smaller than existing works with low surface smoothness (${\rm Ra}=11.2$ nm). We demonstrate a milli-gripper handling delicate insects and a thrombus extractor for narrow vessels. We present a miniature steerable tip for bronchial navigation, improving safety and dexterity over traditional tools, showing the tremendous biomedical potential of these devices.