Flash cooling holds promise for egg storage

Droplet size, temperature and composition can affect the vitrification process, according to research published in PNAS (Proceedings of the National Academy of Sciences) last month. The findings could be used to improve the vitrification techniques used to store biological materials, including eggs for IVF treatment.

So-called 'egg freezing' involves cooling eggs removed from a woman's ovaries so they can be stored. At a later date, these eggs can be thawed, artificially inseminated and introduced back into the womb. Two main methods are used to cool eggs for IVF: slow cooling and vitrification. Both aim to minimise the formation of harmful ice crystals in eggs during cooling or warming, while also minimising any harmful effects of the cryoprotectants. In slow cooling, eggs are cooled slowly using lower concentrations of cryoprotectants. During vitrification, eggs are cooled quickly using higher concentrations of cryoprotectants.

Until now, no one had found a way to vitrify cells with low levels of cryoprotectants, according to Wired magazine. But this new research into droplet vitrification may provide some answers to this problem and has implications for fertility treatment.

During vitrification, tissues or cells are suspended in cryoprotectant liquid before being immersed in liquid nitrogen at temperatures of about -196 degrees Celsius. This results in the rapid removal of water. Vitrification keeps cells metabolically dormant and helps avoid the formation of damaging ice crystals. Dr Utkan Demirci from Harvard Medical School's Brigham & Women's Hospital led the study into droplet vitrification. The team studied the physical phenomena that occur when a droplet of cryoprotectant is injected into liquid nitrogen.

They found a droplet rises to the surface when injected into liquid nitrogen, buoyed by a vapour cloud which generates around it. After skimming around on the surface, the droplet sinks as its temperature reaches that of liquid nitrogen. This process is similar to the Leidenfrost effect, which is seen when water is sprinkled onto a hot pan.

According to Wired magazine, the team measured the droplets as they sank back into the nitrogen. They found that small droplets were completely vitrified, but larger droplets formed damaging ice crystals because their larger surface area prevented heat from escaping. This meant they froze more slowly.

After droplets froze and sank back into the nitrogen, they measured the droplets and used a microscope to determine how crystallized they were.

They found smaller droplets were almost completely vitrified, while larger droplets formed damaging ice crystals. That was because the larger droplets had more surface area to prevent heat from escaping, so they froze more slowly, Demirci said. Using smaller droplets of about the width of a human hair prevents the cells from crystallizing, which means they are more likely to survive the process.

This new research into droplet vitrification has implications for fertility treatments. According to Wired, Dr Demirci's team believe using droplets smaller than a human hair prevents cells from forming ice crystals so they are more likely to survive the vitrification process. They are currently applying the new droplet-based biopreservation technology to mouse eggs as an initial step towards clinical trials in the future. The findings could enable the development of high-throughput, automated methods of cryopreservation of biological samples such as blood and eggs.