Matrix assembly. A 4 × 4 matrix of 20-μm-diameter polystyrene beads in 1% agarose solution was assembled by column of four OFB microrobots. The time stamp format is minutes: seconds. (a) Start of assembly. A magnified photo of an OFB microrobot with a bead is shown in the inset. (b) The leftmost column of the matrix has been assembled, and the four OFB microrobots are transporting the second column of microbeads. (c) Two columns of the matrix have been assembled, and the third column is undergoing transportation. (d) The assembled 4 × 4 matrix of beads.