The density-gradient sperm separation is one of the most popular sperm processing methods.
Density gradients refer to a multilayered solution composed of liquids that do not mix well because of their different densities. A density gradient is formed by the physical property that a less dense liquid is lighter than a denser one, so that the lightest liquid is always on top and the heaviest, at the bottom. These two liquid phases are divided by a clearly visible barrier, or interphase.
With this method, sperms are isolated based on their density after centrifugation. Morphologically normal, abnormal or dead sperms have different densities. At the end of centrifugation, each sperm will be located at the gradient level that corresponds to its density.
In other words, each density gradient functions as a sieve separating the normal from the abnormal and dead sperms, white blood cells or other waste products that can be normally found in the semen. The normal sperms sediment at the bottom of the tube after centrifugation, while all the rest become trapped in the top layer.
Sperms isolated by this method can then be re-suspended in the appropriate medium and utilized for a fertility treatment.
Density-gradient sperm separation is performed by one of our andrology technologists in the laboratory following sperm collection for fertility treatment.
Initial stages of semen preparation include liquefaction (the ability of semen to turn from the gel-like state of the ejaculate into a liquid state), semen washing and centrifugation. Then the technologist prepares the density gradient by carefully layering the two solutions of different densities, one on top of the other. The liquid semen is added last, and with a 20-minute centrifugation step, the normal, good-quality sperms are allowed to pass through the layers, leaving behind debris, abnormal and dead sperms.
After centrifugation, the top density-gradient layer is discarded and the good quality sperms are washed by re-suspension in medium and re-centrifugation. A final re-suspension into a medium for the subsequent fertility treatment brings this method at an end, after about 1 hour of processing.