The Safety of Cryopreservation Devices in Protecting Against Liquid Nitrogen

Currently, in Japan, the most commonly used embryo vitrification method is “ultra-rapid vitrification” which is designed to achieve the maximum cooling rate by exposing embryos directly to LN2 in a tiny droplet of cryoprotectant. Though this method has yielded acceptable embryo viability rates, questions have been raised by various countries' regulatory agencies regarding the effects of LN2 exposure. The 0.25 ml plastic straw has long been used as an entirely closed vitrification system. However, the cooling rate is slower using the 0.25 ml straw than when using the ultra-rapid cooling method, resulting in injury to embryos exposed to a high concentration of cryoprotectant. Recently, a new system has been developed which addresses both the issues of rates of cooling and safeguards against LN2. This paper compares the clinical results of blastocyst vitrification and warming using the conventional 0.25 ml straw method with those of the Rapid-i™ method. The survival rate after warming the group cryopreserved using Rapid-i™ was significantly higher than that of the group of frozen-thawed cryopreserved using the 0.25 ml straw. Moreover, the on-going pregnancy rate after transfer blastocysts into recipients' uteri was higher after cryopreservation with the Rapid-i™ method. These results suggest that the new closed system is safe to use in cryopreservation.