  Active materials are capable of converting free energy into mechanical work
to produce autonomous motion, and exhibit striking collective dynamics that
biology relies on for essential functions. Controlling those dynamics and
transport in synthetic systems has been particularly challenging. Here, we
introduce the concept of spatially structured activity as a means to control
and manipulate transport in active nematic liquid crystals consisting of actin
filaments and light-sensitive myosin motors. Simulations and experiments are
used to demonstrate that topological defects can be generated at will, and then
constrained to move along specified trajectories, by inducing local stresses in
an otherwise passive material. These results provide a foundation for design of
autonomous and reconfigurable microfluidic systems where transport is
controlled by modulating activity with light.
