Active Nematics Reinforce the Ratchet Flow in Dense Environments Without Jamming.

The past decade witnessed a surge in discoveries where biological systems, such as bacteria or living cells, inherently portray active polar or nematic behavior: they prefer to align with each other and form local order during migration. Although the underlying mechanisms remain unclear, utilizing their physical properties to achieve controllable cell-layer transport will be of fundamental importance. In this study, the ratchet effect is harnessed to control the collective motion of neural progenitor cells (NPCs) in vitro. NPCs travel back-and-forth and do not specify head or tail, and therefore regarded as nematics alike liquid crystals. Ratchet and splay-shaped confinements are crafted to modulate collective cell dynamics in dense environments, while jamming is not explicitly spotted. The adaptation of an agent-based simulation further revealed how the ratchet's asymmetry and active forces from nematic order synergistically reinforce the directional cell flow. These findings provide insights into topotaxis in cell populations when restricted to crowded 2D ratchets and the mechanisms that regulate collective behavior of the cells.