  Topological growth control allows to produce a narrow distribution of
outgrown colloidal rods with defined and adjustable length. We use an external
magnetic field to assemble paramagnetic colloidal spheres into colloidal rods
of a chosen length. The rods reside above ametamorphic hexagonalmagnetic
pattern. The periodic repetition of specific loops of the orientation of an
applied external field renders paramagnetic colloidal particles and their
assemblies into active bipeds that walk on the pattern. The metamorphic
patterns allow the robust and controlled polymerization of single colloids to
bipeds of a desired length. The colloids are exposed to this fixed external
control loop that causes multiple simultaneous responses: Small bipeds and
single colloidal particles interpret the external magnetic loop as an order to
walk toward the active zone, where they assemble and polymerize. Outgrown
bipeds interpret the same loop as an order to walk away from the active zone.
The topological transition occurs solely for the growing biped and nothing is
changed in the environment nor in the magnetic control loop. As in many
biological systems the decision of a biped that reached its outgrown length to
walk away from the reaction site is made internally, not externally.
