44. Mathematical modeling of the heat transfer process in vitrification devices used for oocyte cryopreservation

Abstract

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. Oocyte cryopreservation is of key importance in the preservation and propagation of germplasm, however, the cryopreservation of the female gamete has been met with limited success. 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 overcome this limitation, the cooling rates of various vitrification systems immersed in liquid nitrogen, (Cryoloop®, Cryotop®, Miniflex® and Open Pulled Straw ) were mathematically modeled. Thermal histories were obtained by numerically solving the unsteady-state heat conduction partial differential equation for the appropriate geometries of the devices, using the finite element method. The thermo-physical properties, thermal conductivity, density and specific heat, which are involved in the partial differential equations were considered constant since during vitrification there is no phase change of water into ice crystals. The Cryoloop® system (Hampton Research, USA) consists of a mounted loop of 20 μm diameter nylon. Oocytes are loaded with a volume The Open Pulled Straw (OPS) are normally manufactured in the laboratory from 0.25 mL polyethylene French straws (I.M.V., Orsay, France), typically by softening over a hot plate and pulling manually until the inner diameter decreases from 1700 to approximately 800 μm and wall thickness of the central part decreases from approximately 150–70 μm. The straws are air-cooled and cut at the narrowest point with a razor blade. Their commercial version, the Cryotip® (Irvine Scientific, USA) are specially designed drawn plastic straws with ultra-fine tips and protective metal cover sleeves. Results showed that at a constant heat transfer coefficient the highest cooling rate (180,000 °C/min for h = 1000 W/m2K ) was observed for the Cryoloop® system and the lowest rate (5521 °C/min, for the same h value) corresponded to the OPS. The Cryotop® exhibited the second best cooling rate of 37,500 °C/min, whereas the Miniflex® only achieved a cooling rate of 6164 °C/min. It can be concluded that in cryopreservation systems, the information obtained using the numerical model enables biotechnologist in reproductive areas to determine the performance of each oocyte vitrification device under different operating conditions. Funding sources: CONICET- Universidad Nacional de La Plata – Agencia de Promocion Cientifica y Tecnologica (ANPCYT) – Universidad Catolica Argentina.