Sperm motion near surfaces plays a crucial role in fertilization, but the nature of this motion has not been resolved. Using total internal reflection fluorescence microscopy, we selectively imaged motile human and bull sperm located within one micron of a surface, revealing a distinct two-dimensional (2D) ‘slither’ swimming mode whereby the full cell length (50–80 μm) is confined within 1 μm of a surface. This behaviour is distinct from bulk and near-wall swimming modes where the flagellar wave is helical and the head continuously rotates. The slither mode is intermittent (∼1 s, ∼70 μm), and in human sperm, is observed only for viscosities over 20 mPa·s. Bull sperm are slower in this surface-confined swimming mode, owing to a decrease in their flagellar wave amplitude. In contrast, human sperm are ∼50% faster—suggesting a strategy that is well suited to the highly viscous and confined lumen within the human fallopian tube.

Sperm selection is essential to assisted reproductive technology (ART), influencing treatment outcomes and the health of offspring. The fundamental challenge of sperm selection is dictated by biology: a heterogeneous population of ~10⁸ sperm per milliliter with a short lifetime in vitro. However, conventional sperm selection approaches result in less than 50% improvement in DNA integrity. Here, a clinically applicable microfluidic device is presented that selects sperm based on the progressive motility in 500 parallel microchannels. The result is a one-step procedure for semen purification and high DNA integrity sperm selection from 1 mL of raw semen in under 20 minutes. Experiments with bull sperm indicate more than 89% improvement in selected sperm vitality. Clinical tests with human sperm show more than 80% improvement in human DNA integrity, significantly outperforming the best current practices. These results demonstrate the presence of a sub-population of sperm with nearly intact chromatin and DNA integrity, and a simple clinically-applicable lab-on-a-chip method to select this population.

The fallopian tube is lined with a highly complex folded epithelium surrounding a lumen that progressively narrows. To study the influence of this labyrinthine complexity on sperm behavior, we use droplet microfluidics to create soft curved interfaces over a range of curvatures corresponding to the in vivo environment. We reveal a dynamic response mechanism in sperm, switching from a progressive surface-aligned motility mode at low curvatures (larger droplets), to an aggressive surface-attacking mode at high curvatures (smaller droplets of <50 µm-radius). We show that sperm in the attacking mode swim ~33% slower, spend 1.66-fold longer at the interface and have a 66% lower beating amplitude than in the progressive mode. These findings demonstrate that surface curvature within the fallopian tube alters sperm motion from a faster surface aligned locomotion in distal regions to a prolonged physical contact with the epithelium near the site of fertilization, the latter being known to promote capacitation and fertilization competence.