SpermSlow is a viscous medium containing hyaluronic acid, which is naturally occurring and biodegradable. Hyaluronic Acid (HA) acts as a "physiological selector" for spermatozoa prior to intracytoplasmic sperm injection (ICSI). As the sperm suspension mixes with SpermSlow, the more mature and viable sperm slow down and the immature sperm keep moving.

It has been suggested that mature sperm possess less fragmented DNA than immature forms, and as well as helping to identify these SpermSlow also makes the actual sperm injection procedure smoother.

That Injection of HA-bound spermatozoa (HA-ICSI) significantly improves embryo quality and implantation. The use of SpermSlow does appear to lower the early pregnancy loss. This may be because there was a reduction in the DNA fragmentation.

In nature, human oocytes are surrounded by Hyaluronic Acid (HA), which is then involved in the mechanism of sperm selection. In fact, only mature spermatozoa which have extruded their specific receptors to bind to and digest HA can reach the oocyte and fertilize it. HA’s role as “physiologic selector” is now well recognized also in vitro: it has been demonstrated that spermatozoa able to bind HA in vitro are those that have completed plasma membrane remodelling, cytoplasmic extrusion and nuclear maturation. Furthermore, these HA-bound spermatozoa show low chromosomal aneuploidies and DNA fragmentation, and good nuclear morphology

Since the chromosomal status and the chromatin integrity are not predictable by the observation of sperm dimension and shape when performing conventional ICSI, the injection of aneuploid spermatozoa may generate chromosome aberrations in ICSI offspring; likewise, oocyte fertilization with spermatozoa with damaged DNA may lead to an increased risk of pregnancy loss. Nowadays, viscous media containing synthetic plastic polyvinylpyrrolidone (PVP) are routinely used to reduce sperm motility during ICSI procedure in the majority of AR centers. Nevertheless, some authors maintain that PVP may be toxic for the gametes and the developing embryo .HA-bound spermatozoa are easily recovered by an injecting pipette; furthermore, HA-containing products have no negative effects on post-injection zygote development and can be metabolised by the oocyte.