Effect of Temperature on Stability of Lipid Microbubbles

Abstract

The effect of temperature on stability of lipid microbubble shell containing polyethyleneoxide-40-stearate (PEG40St) as emulsifier was investigated. Microbubbles at 4 °C were subjected to differenttemperatures up to 48 ºC (down-to-up) and it was found that both the number and the size of microbubblesremained unchanged in the population up to a certain time, so called “onset time”. The onset time was about6 hours at 10 °C, 2 hours at 20 °C and shorter at elevated temperatures, exhibiting an exponential decreasewith increasing temperature. Once the onset time was reached, the number of microbubbles started todecrease and the average size of the population started to increase. Observation of single microbubbles on aconstant temperature heating stage exhibited that each microbubble had its own onset time, with the smallermicrobubbles vanishing earlier than the larger ones. The Langmuir monolayer studies showed that hydrationdegree of the emulsifier PEG chains decreased with temperature, causing them go through conformationalchanges and subsequently destabilization of the shell. By subjecting the freshly produced microbubbles directlyto the desired temperatures in up-to-down fashion, more stable microbubbles were able to be produced, withtheir onset time increased 40% at 10 °C to 500% at 38 °C. Overall, the results suggest that the new strategiesneed to be developed to control the collapse process in the microbubble shell resulting from the conformationalchanges in the PEG chains of the emulsifier for the design of more stable microbubbles.