  Investigating sperm locomotion in the presence of an external fluid flow and
geometries simulating the female reproductive tract can lead to a better
understanding of sperm motion during the fertilization process. In this study,
using a microfluidic device featuring a stricture that simulates the
biophysical properties of narrow junctions inside the female reproductive
tract, we observed the gate-like role the stricture plays to prevent sperm
featuring motility below a certain threshold from advancing towards the
fertilization site. At the same time, all sperm slower than the threshold
motility accumulate before the stricture and swim in a butterfly-shaped path
between the channel walls which maintains the chance of penetrating the
stricture and thus advancing towards the egg. Interestingly, the accumulation
of sperm before the stricture occurs in a hierarchical manner so that sperm
with higher velocities remain closer to each other and as the sperm velocity
drops, they spread further apart.
