Kinked Air Channel Enables Retraction of Small-Scale Soft Everting Robots

Retraction of soft everting robots via material inversion enables minimal interaction between the robot and the environment, as well as repeated growth and retraction for navigating through complex paths. However, existing retraction strategies are limited to larger-scale robots and often require integration of rigid components. In this work, we present a retraction mechanism for millimeter-scale, soft everting robots enabled by a kinked air channel. The flat air channel is attached to the robot body and kinked at the tip. This kink blocks the air flowing through the channel, allowing the pressure to effectively apply a retraction force at the tip for buckling-free retraction. We developed and validated a model by characterizing the robot at various scales and studying the effectiveness of the kinked air channel for retraction. We demonstrated the robot's retraction capability in several challenging environments, highlighting the benefits of the small size and softness of the retraction mechanism.