Blastocyst cryopreservation: ultrarapid vitrification using cryoloop technique.

Human embryos have been cryopreserved mainly by slow freezing, but vitrification has also proven effective for embryos at early cleavage stages. However, clinical results on blastocyst cryopreservation have not been consistent. A feasible option appears to be ultrarapid vitrification, in which embryos are vitrified with a reduced amount of solution to achieve extremely high rates of cooling and warming. The cryoloop is a tiny nylon loop connected to the lid by a small metal tube; a metal insert on the lid enables the use of a stainless steel handling rod with a small magnet, and the loop can be stored in the cryovial. In the HART Clinic group, of 444 supernumerary human blastocysts that were vitrified by cryoloops 79% survived after warming, and of 126 recipients 36% became pregnant. The outline of ultrarapid vitrification using cryoloops is described.