Rheotactic alignment and collective swimming of spermatozoa in shear-thinning fluids

抄録

Mammalian spermatozoa navigating through the female reproductive tract to reach the egg comprises a fundamental step in achieving successful fertilization. Within the oviduct, sperms encounter oviductal mucus, which is a highly viscous and shear-thinning non-Newtonian fluid whose rheological characteristics strongly influence sperm motility and guidance. During their journey, sperms are subjected to fluid flow generated by the peristaltic motion of the oviductal wall, prompting them to employ behavioral mechanisms such as thigmotaxis and rheotaxis to efficiently migrate toward the oocyte. In the present study, we experimentally examined the interplay between rheotactic behavior and collective swimming of sperm in shear-thinning fluids. To reproduce the physiological flow conditions in the oviduct, we designed and fabricated microchannels. These microchannels mimic the geometry and flow environment of the oviduct using a combination of high-resolution 3D printing and soft lithography techniques. These microfluidic systems allow precise control of the flow field and viscosity distribution, thereby enabling quantitative analysis of sperm motility under conditions relevant to in vivo reproduction. Our results demonstrate that spermatozoa exhibit pronounced rheotaxis within regions near the channel wall, where strong velocity gradients are present. The shear-thinning nature of the medium enhances this orientation response, which results in improved alignment of swimming directions and spontaneous formation of sperm clusters. Furthermore, sperms engaged in collective motion exhibit significantly higher swimming velocities than those swimming individually, suggesting hydrodynamic cooperation among neighboring cells. These findings provide new insights into the physical and biological mechanisms underlying sperm transport in complex reproductive environments. The enhanced rheotactic alignment and collective behavior observed in shear-thinning fluids may confer evolutionary advantages by increasing the probability of successful fertilization under physiological flow conditions.