Development of a novel cervix-inspired tortuous microfluidic system for efficient, high-quality sperm selection

Microfluidic systems have been extensively studied in recent years as potential alternatives for problematic conventional methods of sperm selection. However, despite the widespread use of simple straight channels in these systems, the impact of channel geometry on selected sperm quality has not been thoroughly investigated. To explore this further, we designed and fabricated serpentine microchannels with different radii of curvature, inspired by the tortuous structure of the cervix. Our results showed that in the presence of gentle backflow, microfluidic channels with a 150 μm radius of curvature significantly enhanced the quality of selected sperms when compared to straight channels. Specifically, we observed significant improvements of 7% and 9% in total motility and progressive motility, respectively, as well as 13%, 18%, and 19% improvements in VCL, VAP, and VSL, respectively. Through careful observation of the process, we discovered a unique near-wall sperm migration pattern named boundary detachment-reattachment (BDR), that was observed exclusively in curved microchannels. This pattern, which is a direct consequence of the special serpentine geometry and sperm boundary-following characteristic, contributed to the superior selection performance when combined with a fluid backflow. After determining the best channel design, we fabricated a parallelized chip consisting of 85 microchannels capable of processing 0.5 ml of raw semen within 20 minutes. This chip outperformed conventional methods of swim-up and density gradient centrifugation (DGC) in terms of motility (9% and 25% improvements, respectively), reactive oxygen species (18% and 15% improvements, respectively), and DNA fragmentation index (14% improvement to DGC). Outstanding performance and advantages such as user-friendliness, rapid selection, and independence from centrifugation make our microfluidic system a prospective sperm selection tool in clinical applications.