Various aspects of cryopreservation of small numbers of sperm in assisted reproductive technology.

In cases of severe oligospermia and non-obstructive azoospermia, few spermatozoa with appropriate motility and viability are detected in epididymal and testicular samples. Sperm cryopreservation is performed to avoid multiple testis biopsies and to maintain samples. Consequently, various sperm retrieval methods are employed together with intracytoplasmic sperm injection. Common cryopreservation techniques are not ideal if there are few spermatozoa because sperm may be lost during the freezing and thawing procedures. Single sperm cryopreservation was first developed in 1997 and has since evolved in various aspects, including cryo-devices, freezing methods, cryoprotectants, and clinical and neonatal outcomes, as discussed in this comprehensive review. The CENTRAL, Embase, MEDLINE, PubMed, Scopus, and Web of Science databases were searched for relevant studies published during 1997-2023. Single sperm cryopreservation has been applied successfully for individuals with a limited number of spermatozoa. First, cryo-devices and their advantages and disadvantages are discussed. Furthermore, different aspects of this technique-including the use of testicular sperm rather than ejaculated sperm, proper cryoprotectants with ideal freezing outcomes, optimal cooling and warming rates, and testicular sperm selection prior to freezing-require more attention. This technique has been applied in clinical settings, and clinical outcomes have been reported using various carriers such as empty zonae pellucidae, straws, cell sleepers, cryotops, cryopieces, the sperm vitrification device (Sperm VD), and stripper tips. Furthermore, delivery rates have also been reported using this method. Nonetheless, more clinical trials are required to facilitate biological offspring through a practical platform.