Mitchel C. Schiewe

At the beginning of this century, the application of vitrification technology was gaining momentum in clinical medicine with the use of various ‘open vitrification systems’ (i.e. direct embryo–solution contact with liquid nitrogen), which offered high cooling rate potential and high levels of completely intact embryo survival using mixed dimethyl sulfoxide (DMSO)/ethylene glycol (EG) solutions. Not surprisingly, the combined use of blastocoele collapsing proved effective at ensuring high rates of complete survival (>90%).1,2 By the mid-2000s, ‘closed vitrification systems’ (i.e. embryo–solution sealed in a container/device) began to be used efficiently and, to date, have achieved success rates comparable with and higher than those of other open-system devices.3,4 The latter aseptic closed systems completely eliminate risks and concerns associated with potential contaminants found in liquid nitrogen during storage.

It has been shown that previtrification blastocoele collapsing is not an essential prerequisite for vitrification solutions devoid of DMSO,4,5 as less permeable cryoprotectants (e.g. glycerol) and a more concentrated solution can induce and sustain blastocoele shrinkage during vitrification. Today we know that the efficacy of vitrification success is more highly dependent on warming rates than cooling rates.6,7 Independent of the vitrification device or open/closed system used, the warming rate must exceed the cooling rate to ensure high survival rates. High warming rates minimize the opportunity for any ice growth (i.e. re-crystallization of nucleated impurities in cryosolutions) during the devitrification phase of warming. Therefore, it is not surprising that device familiarity is an important factor regarding technical variation and successful outcomes. It is this concept of ‘technical signature’5 that explains why repeatability between programmes may be problematic.

When assessing the completeness of vitrification devices for your potential use, there are several quality control factors that should be taken into account, including

1. Recovery potential/survivability. Is the device design prone to potential problems in the guaranteed recovery of embryos, and will they reliably vitrify and maintain complete cellular integrity after warming?

2. Liquid nitrogen storage capacity. Does the device offer security and safety from possible contaminants as an aseptic closed system? Can the device be simply and safely handled and identified? Is its storage potential space efficient?