Even though vitrification is increasingly applied in ART, slow freezing has some advantages especially when handling large numbers of samples.
Our colleagues at Vitrolife have recently added to their range of products with a new media range for slow rate freezing. This move was aided by important work Dr David Edgar has been doing recently at Melbourne IVF Clinic and Melbourne University Hospital.
Data published by Dr. Edgar's group demonstrated that modifications to traditional slow freezing methodology for cleavage stage embryos based on physiological considerations can result in significantly higher proportions of surviving embryos and significantly higher proportions of surviving blastomeres.
You can see this discussed on the Vitrolife website at: http://www.vitrolife.com/en/Fertility/Together/Stories/Does-slow-freezing-still-have-a-role-in-embryo-cryopreservation--/ from a symposium which took place in London in July 2013.
As Dr Edgar points out the stakes are well worthwhile for increasing proportions of embryos and blastomeres surviving cryopreservation. However the methodology used for slow controlled rate cooling of human cleavage-stage embryos has remained largely unchanged for over two decades. Some studies point to the value of increasing the extent of intracellular dehydration by increasing the concentration of non-permeating cryoprotectant prior to slow cooling of oocytes and embryos previously biopsied for preimplantation genetic diagnosis. The Edgar, Karani & Gook publication in Reproductive Medicine Online extends that idea to the slow cooling of non-biopsied day-2 embryos. Dehydration in the presence of 0.2mol/l sucrose significantly increased the proportions of surviving embryos, surviving blastomeres and fully intact embryos (92.6%, 91.1%, and 80.4%, respectively) relative to those observed after dehydration in 0.1mol/l sucrose (78.5%, 74.1%, and 54.6%, respectively, all P<0.001). Post-thaw implantation was not adversely affected by the increased prefreeze dehydration. This improved method for slow cooling of cleavage-stage embryos should have a major impact on clinical outcome.
You can read the full Paper at http://www.rbmojournal.com/article/S1472-6483(09)00016-9/fulltext