# Millimeter-scale fluid-driven soft robots

## TL;DR

A new method for making tiny soft robots is introduced, enabling precise biomedical tasks like handling delicate objects and navigating narrow spaces.

## Contribution

The mini bubble casting method enables fabrication of high-quality, submillimeter fluid-driven soft robots with low surface roughness.

## Key findings

- The mini bubble casting method successfully creates submillimeter internal voids in soft robots.
- Soft milli-actuators fabricated using this method are ten times smaller than existing works with smooth surfaces.
- Applications include a milli-gripper for insects and a thrombus extractor for narrow vessels.

## 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 (\documentclass[12pt]{minimal}
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${\rm Ra}=11.2$\end{document} 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.

Mini Bubble Casting technique is developed to robustly fabricate millimeter-scale fluid-driven soft robots for biomedical applications.