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.
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