Faster sperm selected by rheotaxis leads to superior early embryonic development in vitro.

To understand the impact of sperm speed as they swim against the flow on fertilization rates, we created conditions similar to the female reproductive tract (FRT) on a microfluidic platform for sperm selection. Selected sperm were evaluated based on early development of fertilized embryos. Bovine and human spermatozoa were selected at various fluid flow rates within the device. We found that the speed of bovine spermatozoa increases as the flow rate increases and that the amount of DNA fragmentation index is lowered by increasing the flow rate. Bovine spermatozoa selected by our platform at low (150 µL h-1, shear rate 3 s-1), medium (250 µL h-1, shear rate 5 s-1), and high flow rates (350 µL h-1, shear rate 7 s-1) were used for fertilization and compared to sperm sorted by centrifugation. The samples collected at the highest flow rate resulted in the formation of 23% more blastocysts compared to the control. While selecting for higher quality sperm by increasing the flow rate does result in lower sperm yield, quality improvement and yield may be balanced by better embryonic development.