  Detailed control over the motion of colloidal particles is relevant in many
applications in colloidal science such as lab-on-a-chip devices. Here, we use
an external magnetic field to assemble paramagnetic colloidal spheres into
colloidal rods of several lengths. The rods reside above a square magnetic
pattern and are transported via modulation of the direction of the external
magnetic field. The rods behave like bipeds walking above the pattern.
Depending on their length, the bipeds perform topologically distinct classes of
protected walks above the pattern. We demonstrate that it is possible to design
parallel polydirectional modulation loops of the external field that command up
to six classes of bipeds to walk on distinct predesigned paths. We use such
parallel polydirectional loops to induce the collision of reactant bipeds,
their polymerization addition reaction to larger bipeds, the separation of
product bipeds from the educts, the sorting of different product bipeds, and
also the parallel writing of a word consisting of several different letters.
