Microfluidic Sperm Selection: Better Fertilization, Unclear Later Benefits

A randomized study compared microfluidic sperm selection with conventional density gradient centrifugation by dividing eggs from the same woman between the two methods. Fertilization rates were 82.3% versus 74.7%, but no statistically significant difference was found when embryos were grown to the blastocyst stage.

In IVF, only one sperm ultimately fertilizes an egg, yet an ejaculate may contain tens of millions of sperm. How should the one used for treatment be chosen? The question seems particularly important in intracytoplasmic sperm injection (ICSI), in which an embryologist injects a single sperm directly into an egg.

A randomized study published in Scientific Reports in June 2026 investigated this question. The researchers directly compared conventional density gradient centrifugation (DGC) with microfluidic sperm selection (MSS). Put simply, they compared selecting sperm by spinning a sample with selecting sperm by allowing them to swim through tiny channels.

Selecting sperm by letting them swim

A collected semen sample is not simply used as it is in IVF. It contains sperm with different levels of motility, nonviable sperm and other material, so sperm must be prepared and selected for treatment.

One widely used method is density gradient centrifugation. Semen is placed over layers of solutions with different densities and spun to separate a population of sperm with relatively favorable characteristics.

The method has a long history of use, but questions remain about whether centrifugation and repeated handling can expose sperm to unnecessary physical or oxidative stress.

Microfluidic sperm selection takes a different approach. Instead of spinning the sample, it allows motile sperm to swim through very small channels. The idea is to let sperm with good swimming ability reach the collection area themselves.

Does selecting sperm this way actually produce better results?

The study's comparison was particularly useful. Investigators recruited 55 couples in which the male partner had abnormal semen parameters. They did not assign different couples to the two methods. Instead, mature eggs obtained from each woman were randomly divided between the methods. Eggs in one group underwent ICSI with sperm prepared by MSS, while the other group underwent ICSI with sperm prepared by DGC.

This is called a sibling-oocyte comparison. Comparing eggs from different women introduces differences in age, ovarian function and egg characteristics. Dividing eggs retrieved from the same woman on the same day reduces many of these sources of variation and helps isolate the effect of the sperm-preparation method.

Fertilization favored microfluidic sperm selection

The proportion of eggs showing normal fertilization, identified by two pronuclei (2PN), was 82.3% with MSS and 74.7% with DGC. The difference of 7.6 percentage points was statistically significant.

The selected sperm also differed. MSS yielded a lower sperm concentration and a smaller total number of motile sperm, but the recovered population had higher total and progressive motility. Its strength was selecting sperm that moved well rather than recovering a larger number of sperm.

At this point, it might seem that replacing centrifugation with microfluidic selection would be the obvious next step. Following the embryos further, however, produced a more qualified picture.

The fertilization advantage did not clearly extend to blastocysts

Cleavage rates were very similar: 97.6% with MSS and 96.9% with DGC. Blastocyst formation rates were 61.8% and 65.2%, respectively, without a statistically significant difference. The rate of good-quality blastocyst formation also did not differ significantly.

MSS performed better at the first stage, fertilization, but the study did not show that the advantage continued as embryos developed. Selecting sperm with good motility and producing a good embryo are related questions, but they are not interchangeable outcomes.

Were there more chromosomally normal embryos?

For patients undergoing IVF, another question is whether the method produced more embryos with a normal chromosome number.

Preimplantation genetic testing for aneuploidy (PGT-A) was performed in 47 of the 55 cycles. Among tested blastocysts, the euploid rate was 35.8% with MSS and 24.8% with DGC. The numerical difference of 11 percentage points is striking.

However, the study's primary analysis of this outcome did not establish a statistically significant difference. It therefore does not justify concluding that MSS produces more euploid embryos. A larger study might confirm a difference, or the observed difference might turn out to reflect chance. The answer remains uncertain.

A sperm that swims well is not necessarily the one that leads to a birth

The message is that a “good sperm” is more complicated to define than it may first appear. Motility under a microscope, the ability to fertilize an egg, contribution to embryo development, implantation and eventual birth are distinct stages. A single visible characteristic cannot be assumed to predict all of them.

MSS showed promise at the fertilization stage, with an 82.3% rate compared with 74.7% after DGC. But this study did not demonstrate that it produced more good-quality blastocysts or more euploid embryos, or that it ultimately improved pregnancy and birth outcomes.

The sample of 55 couples was small. Because the men had abnormal semen parameters, the findings cannot be assumed to apply to all IVF patients, including those with normal semen analyses. Crucially, the study was not designed to compare implantation, pregnancy or final live-birth rates.

Nevertheless, the research shows how the process of selecting one sperm from many millions is evolving. A method that lets sperm swim through channels was compared directly with centrifugation using eggs from the same women, and it improved fertilization in that comparison.

The next question is whether the sperm selected in this way contribute to good embryos, successful implantation, an ongoing pregnancy and ultimately the birth of a healthy child. Those later outcomes were not established by this study.

Microfluidic sperm selection showed promise at an early stage of the IVF process. Whether that advantage persists through to live birth still needs to be tested.

This article is based on a randomized sibling-oocyte study by researchers at the Faculty of Medicine Ramathibodi Hospital, Mahidol University, Thailand, published in Scientific Reports on June 1, 2026. The study compared the two sperm-preparation methods using mature eggs from the same women in 55 couples.

Research references

Microfluidic sperm selection versus density gradient centrifugation in ICSI cycles with abnormal semen parameters: a randomized sibling-oocyte study. Scientific Reports, June 1, 2026. DOI: 10.1038/s41598-026-55209-1. PMID: 42230899.