Multimodal sperm motility and surface navigation across diverse flow regimes

Sperm motility in complex flows is crucial for mammalian fertilization, but individual sperm behavior under varying flow conditions is poorly understood. Here, we developed a uniform planar Poiseuille flow system to examine how bovine sperm navigate near surfaces across diverse flow intensities. Trajectory analysis reveals a sequential transition in near-surface motility patterns: from circular swimming in the absence of flow, to stable upstream rheotaxis in low-speed flows, and to oscillatory motion in high-speed flows. Despite increasing flow strength, sperm exhibit remarkable resilience, maintaining proximity to surfaces and resisting washout. Using a mechanistic model, we reproduce our experimental findings and identify six distinct three-dimensional motility states that emerge with flow variation. Notably, even in high-speed flows, sperm periodically approach the surface in a wall-modified oscillatory trajectory. Our findings uncover a hydrodynamic buffer zone that stabilizes sperm near surfaces at high shear strengths, representing a critical adaptation for navigation in fluctuating flow environments. These insights advance our understanding of sperm transport mechanisms in vivo and inform the design of bioengineering devices for reproductive technologies.