Three-dimensional robotic manipulation and transport of micro-scale objects by a magnetically driven capillary micro-gripper

One major challenge for untethered micro-scale mobile robotics is the manipulation of external objects in the robot´s three-dimensional (3D) work environment. Here, we present a method to use the capillary force at a solid-liquid-gas interface to reversibly attach objects to a mobile magnetic microrobot. This is accomplished by the addition of a cavity in the hydrophobic microrobot, in which an air bubble is captured when the microrobot is placed in a water environment. The extension of the air bubble from the cavity is adjusted dynamically by controlling the pressure of the workspace environment. A peak switching ratio between the maximum and minimum gripping forces of 14:1 is shown for controlled attachment/detachment experiments, which allows for reliable pick-and-place operation. This work introduces an analytical capillary adhesion model and demonstrates control of the bubble size for pick-and-place gripping. A proof-of-concept demonstration of 3D manipulation in a fluidic environment shows the potential of capillary gripping for future use in confined environments such as inside microfluidic devices for transportation or assembly of hydrophobic objects.