| 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. |