Infertility, defined by the failure to conceive after at least 12 months of regular unprotected sexual intercourse, is estimated to affect around 17.5% of the adult population [1] or 8–12% of reproductive-age couples worldwide [2]. The etiology of infertility is multifactorial and complex and may involve factors relating to the male or female reproductive systems or a combination of both [3]. Infertility can significantly impact psychosocial well-being, prompting numerous couples to seek infertility treatment. Assisted reproductive technology (ART) has advanced considerably since the birth of Louis Brown—the first baby conceived through in vitro fertilization (IVF) in 1978 [4]. The utilization of ART is increasing as the effectiveness and safety of these technologies have improved over the years. However, there is still room for improvement in achieving higher success rates with fewer cycles [4].

Sperm selection is a vital part of all ART procedures, as sperm quality influences success rates and health in offspring [5]. However, since the start of ART, there has been little technological innovation for sperm preparation methods until the recent years [6,7,8]. During natural conception, sperm undergo a complex natural selection process, involving multiple stages during their long journey through the female reproductive tract, so that only the more motile, higher quality sperm reach the oocyte with the potential for successful fertilization. Only 1000 or less of the millions of sperm contained in a single ejaculate reach the fallopian tube, and only a small proportion of these reach the ampulla, or site of fertilization [9,10,11]. ART bypass stages of the natural sperm selection process in various ways. To avoid fertilization with a defective sperm, semen preparation techniques aim to select high-quality sperm by attempting to mimic aspects of the in vivo sperm selection process [10].

In conventional IVF, sperm and oocytes are mixed in a petri dish and left for fertilization to occur. Intracytoplasmic sperm injection (ICSI) is a technique in which a single sperm is injected into a mature oocyte, thereby bypassing the final barriers of sperm-oocyte interaction that the spermatozoa must overcome to achieve fertilization [12]. ICSI is the method of choice for male factor infertility, but has also been shown to be effective in other circumstances, such as unexplained infertility [13]. ICSI has become a more commonly used method of fertilization over conventional IVF in many areas of the world, including the United Arab Emirates, due to the technique’s consistency and versatility [14, 15]. However, sperm selection for ICSI is even more crucial than for other ART, in order to maximize the chance of fertilization with an optimal spermatozoon [14, 16].

Conventional methods of semen preparation include sperm washing, swim up, and density gradient centrifugation (DGC). DGC, the current standard sperm preparation method for IVF, utilizes differences in the density to select the best sperm, followed by washing using centrifugation [17]. However, DGC is time and labor intensive, with multiple operator-sensitive steps. In addition, centrifugation has been shown to increase reactive oxygen species (ROS) formation that may induce sperm DNA fragmentation (sDF) which could negatively impact sperm function and therefore fertilization and embryo development [8, 18, 19].

Advanced sperm preparation approaches have been developed to mimic the physiological selection that occurs in the female genital tract, simplify semen preparation, and avoid the use of centrifugation. Some sperm selection techniques include methods based on sperm membrane markers, such as hyaluronan or annexin [7]. The application of microfluidic-based technologies for sperm preparation and selection is a rapidly developing area in ART. Microfluidics is the science and technology of accurate manipulation of small amounts of fluids [20]. In microfluidic sorting devices, sperm typically travel through micro-channels whose dimensions hydrodynamically constrain the migration of compromised sperm while allowing motile sperm to progress to the outlet [21]. Microfluidic, motility-based sperm selection methods attempt to mimic the natural progression of sperm through the female reproductive tract, including physical aspects of the fallopian tubes [8, 10]. Microfluidic sorting selects sperm according to size, motility, and other characteristics, such as DNA integrity, without the need for centrifugation [18, 21, 22]. Microfluidic-based sperm separation (MSS) devices are proposed as a simple, reliable, and standardized method to improve ART outcomes, by mitigating against the production of reactive oxygen species and selecting high-quality, motile sperm [7, 10, 19, 23,24,25].

The FERTILE PLUS™ method is a standardized method with an easy-to-follow protocol that is far less dependent on the skill or experience of the embryologist than other methods, such as DGC. The FERTILE PLUS™ (850 µL) Sperm Sorting Chip is a single-use, flow-free, dual chambered, microfluidic-based sperm sorting device. FERTILE PLUS™ was previously known as Zymot, prior to a name change by the manufacturer. The lower chamber contains a sample inlet and fluid channel separated from the upper collection chamber by a microporous membrane with 8-μm pores, demonstrated as the optimal size for selection of sperm with higher motility and normal morphology [22, 26]. The design of the sperm sorting chip utilizes sperm forward motility to sort healthy motile sperm from compromised, poorly motile sperm present in the raw semen sample. After sperm enter the lower channel through the inlet, the more motile sperm swim through the micro-channel and up through the filter pores within the membrane to reach the outlet. The FERTILE PLUS™ (850µL) Sperm Sorting Chip produces a 500 µL sample that can be used for ICSI, IVF, or intrauterine insemination (IUI). This easy-to-use method with fewer sample manipulation steps offers increased reliability and significant time saving over traditional sperm preparation techniques. However, as microfluidic sorting produces lower sperm concentration yields than other techniques and sperm recovery is highly dependent on the quality of the semen sample, sperm concentration must be at least 10 million per milliliter for optimal use of FERTILE PLUS™.

Evaluating outcomes of MSS devices compared to other sperm preparation methods for ART is a relatively new area of research which has, so far, shown conflicting results [27, 28].