  Using novel micro-printing techniques, we develop a versatile experimental
setup that allows us to study how lateral confinement tames the active flows
and defect properties of the microtubule/kinesin active nematic system. We
demonstrate that the active length scale that determines the self-organization
of this system in unconstrained geometries loses its relevance under strong
lateral confinement. Dramatic transitions are observed from chaotic to vortex
lattices and defect-free unidirectional flows. Defects, which determine the
active flow behavior, are created and annihilated on the channel walls rather
than in the bulk, and acquire a strong orientational order in narrow channels.
Their nucleation is governed by an instability whose wavelength is effectively
screened by the channel width. All these results are recovered in simulations,
and the comparison highlights the role of boundary conditions.
