Numerical simulation of cooling rates in vitrification systems used for oocyte cryopreservation

Oocyte cryopreservation is of key importance in the preservation and propagation of germplasm. Interest in oocyte cryopreservation has increased in recent years due to the application of assisted reproductive technologies in farm animals such as in vitro fertilization, nuclear transfer and the need for the establishment of ova/gene banks worldwide. However, the cryopreservation of the female gamete has been met with limited success mainly due to its small surface-area:volume ratio. In the past decade, several vitrification devices such as open pulled straws (OPS), fine and ultra fine pipette tips, nylon loops and polyethylene films have been introduced in order to manipulate minimal volumes and achieve high cooling rates. However, experimental comparison of cooling rates presents difficulties mainly because of the reduced size of these systems. To circumvent this limitation, a numerical simulation of cooling rates of various vitrification systems immersed in liquid nitrogen was conducted solving the non-stationary heat transfer partial differential equation using finite element method. Results indicate the nylon loop (Cryoloop®) is the most efficient heat transfer system analyzed, with a predicted cooling rate of 180,000°C/min for an external heat transfer coefficient h= 1000 W/m(2)K when cooling from 20 to -130°C; in contrast, the open pulled straw method (OPS) showed the lowest performance with a cooling rate of 5521°C/min considering the same value of external heat transfer coefficient. Predicted cooling rates of Miniflex® and Cryotop® (polyethylene film system) were 6164 and 37,500°C/min, respectively, for the same heat transfer coefficient.