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

## TL;DR

This study shows how neural progenitor cells move directionally in ratchet-shaped environments using active nematic behavior, avoiding jamming in dense conditions.

## Contribution

The study introduces a mechanism for directional cell flow in dense environments using ratchet geometries and active nematic theory.

## Key findings

- NPCs exhibit directional migration in ratchet geometries without jamming due to active nematic behavior.
- Agent-based simulations reveal how ratchet asymmetry and active forces synergistically reinforce directional flow.
- The findings explain how cells navigate crowded 2D environments through topotaxis and collective behavior.

## Abstract

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.

This study demonstrates enhanced directional migration of neural progenitor cells in micro‐fabricated ratchet geometry, taking advantage of active collective motions. Jamming is eased by cells' spontaneous back‐and‐forth motion as active nematics, answering the long‐standing puzzle of how cells navigate crowded environments over long distances. Agent‐based simulations and active nematic theory provide deeper insights into the mechanisms of collective cell migration.