Sonoporation is achieved by ultrasound-mediated destruction of ultrasound contrast agents (UCA) microbubbles. For this, UCAs must be tissue specific and have good echogenicity and also function as drug carriers. Previous studies have developed acoustic liposomes (ALs), liposomes that encapsulate phosphate buffer solution and perfluoropropane (C3F8) gas and function as both UCAs and drug carriers. Few studies have examined the co-existence of gas and liquid in ALs. This study aims to elucidate AL structure using TEM. The size, zeta potential and structure of ALs were compared with those of two other UCAs, human albumin shell bubbles (ABs; Optison) and lipid bubbles (LBs). ABs and LBs encapsulate the C3F8 gas. Particle size was measured by dynamic light scattering. The zeta potential was determined by the Smoluchowski equation. UCA structure was investigated by TEM. ALs were ~200 nm in size, smaller than LBs and ABs. ALs and LBs had almost neutral zeta potentials whereas AB values were strongly negative. The negative or double staining TEM images revealed that ~20% of ALs contained both liquid and gas, while ~80% contained liquid alone (i.e. nonacoustic). Negative staining AB images indicated electron beam scattering near the shell surface, and albumin was detected in filament form. These findings suggest that AL is capable of carrying drugs and high-molecular-weight, low-solubility gases.

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