Two 20-μm-diameter beads were passed between four independently controlled OFB microrobots. The four OFB microrobots are marked as A, B, C, and D. The two 20-μm beads are marked by 1 and 2. The time stamp format is minutes: seconds. (a-c) OFB microrobot A transferred the no. 1 bead to microrobot C; at the same time, microrobot D transferred the no. 2 bead to microrobot B. (d) The beads are transported to the center of the field of view. (e) The beads were exchanged between microrobots B and C. (f) The OFB microrobots then transported the exchanged beads to the side opposite their starting locations in (d).