A sperm racetrack to separate sperm by swim speed.

Abstract

The genetic basis for sperm swimming ability in humans is underinvestigated. In particular, selfish genes known as segregation distorters dramatically influence sperm swimming in organisms like mice. Segregation distorters are difficult to screen for in humans because no tool exists to compete sperm by swimming ability and collect the separated fast and slow sperm for further study. We designed, built, and tested a "sperm racetrack", a microfluidic device that races sperm against each other and allows collection of sperm based on the distance travelled. This biologically-inspired PDMS device contains a long, straight channel with counterflow against which sperm swim naturally, and contains regularly-spaced ports along the channel for collection of sperm after the race. We performed a series of test races and found that sperm in the fourth channel segment are significantly faster swimming according to video analysis than those in the second channel segment. Sperm from these same segments are significantly different in speed after extraction from the channel as well. We characterized several swimming behaviors for these faster and slower sperm, including linearity (ability to swim in a straight line) and wobble (amount of side-to-side head movement per beat), and found a significant association between higher sperm wobble and distance travelled along the channel in extracted sperm. Together, we show that the sperm racetrack separates sperm according to swimming speed, and this sorting of sperm is faithfully maintained after extraction from the device as fast- and slow-swimming groups. The sperm racetrack may prove useful in understanding the genetic and physiological causes of sperm immotility, and in identifying swimming-based segregation distorters in humans. The sperm racetrack may also serve as a tool for enriching fast-swimming sperm for selection in assisted reproductive technologies such as <i>in vitro</i> fertilization.