Fish-like magnetic microrobots for microparts transporting at liquid surfaces

Magnetic microrobots have tremendous potential applications due to their wireless actuation and fast response in confined spaces. Herein, inspired by fish, a magnetic microrobot working at liquid surfaces was proposed in order to transport microparts effectively. Different from other fish-like robots propelled by flexible caudal fins, the microrobot is designed as a simple sheet structure with a streamlined shape. It is fabricated monolithically utilizing polydimethylsiloxane doped with magnetic particles. The unequal thicknesses of different parts of the fish shape enable the microrobot to move faster via a liquid level difference around the body under an oscillating magnetic field. The propulsion mechanism is investigated through theoretical analysis and simulations. The motion performance characteristics are further characterized through experiments. It is interesting to find that the microrobot moves in a head-forward mode when the vertical magnetic field component is upward, whereas it moves in a tail-forward mode when the component is downward. Relying on the modulation of capillary forces, the microrobot is able to capture and deliver microballs along a given path. The maximum transporting speed can reach 1.2 mm s^-1, which is about three times the microball diameter per second. It is also found that the transporting speed with the microball is much higher than that of the microrobot alone. The reason for this is that when the micropart and microrobot combine, the increased asymmetry of the liquid surfaces caused by the forward movement of the gravity center can increase the forward driving force. The proposed microrobot and the transporting method are expected to have more applications in micromanipulation fields.