Emergence of macroscopic directional motion of deformable active cells in confined structures.

There is now growing evidence of collective turbulentlike motion of cells in dense tissues. However, how to control and harness this collective motion is an open question. We investigate the transport of deformable active cells in a periodically asymmetric channel by using a phase-field model. We demonstrate that collective turbulent-like motion of cells can power and steer the macroscopic directional motion through the ratchet channel. The active intercellular forces proportional to the deformation of cells can break thermodynamical equilibrium and induce the directional motion. This directional motion is caused by the ratchet effect rather than the spontaneous symmetry breaking. The motion direction is determined by the asymmetry of the channel. Remarkably, there exits an optimal nonequilibrium driving (depending on the active strength, the elasticity, and the packing fraction) at which the average velocity reaches the maximum. In addition, the optimized packing fraction and the optimized minimum width of the channel can facilitate the directional motion of cells. Our findings are relevant to understanding how macroscopic directional motion relates to the local force transmission mediated by cell-cell contacts in cellular monolayers.