  Many of the biological phenomena involve collective dynamics driven by
self-propelled motion and nonequilibrium force (i.e., activity) that result in
features unexpected from equilibrium physics. On the other hand, biological
experiments utilizing molecular motors, bacteria, and mammalian cells have
served as ideal setups to probe the effect of activity in materials and compare
with theory. As has been established, however, biomolecules are chiral in
nature, which can lead to the chiral patterning of cells and even to the
left-right symmetry breaking in our body. The general mechanism of how the
dynamics of bio-matters can couple with its own inherent chirality to produce
macroscopic patterns is yet to be elucidated. Here we report that cultured
neural progenitor cells (NPCs), which undergo self-propelled motion with
nematic cell-to-cell interactions, exhibit large scale chiral patterns when
flowing out from containers made by gel. Moreover, a robust chiral cell flow is
produced along the boundary when the NPCs are cultured on substrates with
edges. Perturbation by actomyosin inhibitors allowed control over the
chirality, resulting in the switching of the direction of the chiral patterning
and boundary flow. As predicted by a hydrodynamic theory analogous to the
non-Hermitian Schrodinger equation, we find an edge-localized unidirectional
mode in the Fourier spectrum of the cell density, which corresponds to the
topological Kelvin wave. These results establish a novel mechanism of flow that
emerges from a pool of bipolar cells, and demonstrate how topological concepts
from condensed matter physics can naturally arise in chiral active systems and
multi-cellular phenomena.
