Influence of geometric and flow complexity on sperm migration dynamics

Successful fertilization depends on the ability of sperm to navigate the complex and dynamic microenvironment of the female reproductive tract, shaped by geometric confinement and fluid flow. In this study, we employ microfluidic platforms to investigate bovine sperm migration across three levels of geometrical and hydrodynamic complexity. First, we examine rheotactic swimming through tapered microchannel strictures with varying aperture angles, showing that narrower angles (45°) promote upstream progression, while orthogonal constrictions (90°) inhibit it. Second, we explore sperm motion in outward radial flow fields using both theoretical modeling and experiments, uncovering a novel behavior termed rotary rheotaxis, where sperm exhibit stable curved upstream trajectories around the flow origin. This behavior is leveraged in a microfluidic selection device for isolating highly motile sperm. Third, we study sperm interactions with periodic pillar lattices under viscoelastic fluid conditions and chemical hyperactivation, revealing a complex interplay between cell motility state and environmental topology. These findings shed light on sperm-fluid-structure interactions in biologically relevant settings and inform the design of bioinspired selection platforms and microswimmer navigation models.