Effect of heat transfer coefficient on cooling rates during oocyte vitrification in liquid and slush nitrogen.

M. SANSINENA, M.V. SANTOS, N. ZARITZKY AND J. CHIRIFE

Rosario

Congreso; 49 Annual Meeting of the Society for Cryobiology; 2012

Society for Cryobiology

Advances in cryopreservation of gametes indicate that slow cooling protocols will soon be replaced by fast cooling methods. Vitrification, a process in which liquid water is converted into a glass-like amorphous solid without any ice formation, normally requires plunging directly liquid nitrogen in order to achieve high cooling rates. This leads to strong nitrogen vaporization around the sample surface forming a vapor film that acts as a heat insulation layer (Leidenfrost effect) resulting in limited heat transfer coefficient (h) between the surface of the sample and liquid nitrogen. Recently, slush nitrogen (SN2), a mixture of solid and liquid nitrogen obtained by applying negative pressure (average -207ºC) has been introduced in an attempt to minimize this effect. The objective of this study was to conduct a comparison of numerically calculated cooling rates of two small-volume devices plunged in liquid nitrogen versus slush nitrogen, and to analyze the individual effects of a lower temperature (-207ºC) and higher h values. A survey of literature h values for film boiling of small metal objects with different geometries plunged in liquid nitrogen, revealed a range between 125 to 1000 W/(m2 K). These h values were applied to a numerical simulation of cooling rates of two oocyte vitrification devices (open-pulled straw and Cryotop®), plunged in conditions representative of vitrification processes in liquid and slush nitrogen. The heat conduction equation with convective boundary condition was numerically solved using the finite element method to simulate the cooling process (Comsol Multiphysics® software). The cooling rate (ºC/min) was defined as the time needed to reduce initial core temperature (warmest point of the system) of the liquid from 20ºC to -150ºC while avoiding ice formation (vitrification); therefore the thermophysical properties of the vitrifying solution and the plastic device were considered independent of temperature. Numerical simulations were carried out with different h values likely to represent the stagnant and slush nitrogen conditions. The thermal properties for supercooled water at -23ºC were used; thermal conductivity (k) 0.50 W/m K, specific heat (Cp) 4218 J/kg K, and density (ρ) 983 kg/m3.