Characterization of bubble liposomes by measurements of ultrasound attenuation: Effects of shell materials

Two types of gas-encapsulated liposomes (Bubble liposomes) with different shell compositions were evaluated by measurements of pressure-dependent ultrasound attenuation of bubble suspensions and by high-speed observations of bubble behaviors under exposure to pulsed ultrasound. Attenuation peaks were observed at the frequency range of 16-19 MHz, suggesting that the size of Bubble liposomes is around 400 nm. There was a large difference between threshold pressures of bubble collapse in the two types of Bubble liposomes, indicating that decrease in shell fluidity by addition of cholesterol to a lipid bilayer and increase in surface area by bubble fragmentation have strong effects for facilitating bubble disappearance.