“Thick and Viscous” or “Light and Easy”: An Oil Overlay Comparison

Abstract

Background: Oil overlay in IVF culture media is essential in the maintenance of pH and prevention of evaporation. The recent proliferation of “Heavy oil” raises questions of how oil density and viscosity affect CO2 diffusion into and out of culture media. Aim: To compare the effects of “Heavy oil” versus standard oil on pH stability in embryo culture dishes. Method: Density (g/ml) readings of each oil (Ovoil, Ovoil Heavy and LifeGuard) were obtained in triplicate using a volumetric flask and analytical balance in both refrigerated (4[Formula: see text]C) and warmed state (37[Formula: see text]C). Viscosity testing was performed externally by Intertek laboratory. pH was measured at regular intervals using a blood gas analyser (EPOC system) in culture dishes under standard and ‘Heavy’ oil overlays. Results: Density measurements showed Ovoil Heavy to be the least dense (0.8227g/ml) followed by Ovoil (0.8476g/ml), with LifeGuard the most dense at 0.8640g/ml. There was a significant difference (p<0.0001) when comparing densities between oil types in both 4[Formula: see text]C and 37[Formula: see text]C. Viscosity testing reported LifeGuard the most viscous at 153.6mm2/s followed by Ovoil Heavy at 57.98mm2/s and Ovoil at 30.68mm2/s. Media equilibrated more slowly under denser oil, requiring 5 hours to read pH 7.4 under LifeGuard with lighter oil requiring 3 hours to reach equilibration. Equilibrated dishes exposed to reduced CO2 concentration (4%) showed quicker pH loss in Ovoil at 0.049/hour when compared to LifeGuard at 0.034/hour (p=0.006). Culture dishes placed in a humidicrib with open cuffs, emulating real world settings, showed slower pH loss under LifeGuard (0.06/hour) compared to Ovoil (0.08/hour). Conclusion: Higher density oil overlay takes longer to equilibrate to optimal pH but can slow down loss of pH when dishes are exposed to ambient conditions.