(172e) A Novel Method for Measuring Elasticity of Submicron-Sized Liposomes with ΑFΜ

Purpose: There are many useful colloidal drug delivery systems including liposomes. The elasticity is one of the important properties affecting their drug delivery functions such as their stability, encapsulated drug release and blood circulation. However, it is difficult to evaluate the elasticity of such fine particles and so far no useful methods have been reported to characterize it. In this report, we will demonstrate an unique method to evaluate the elasticity of liposomes by use of atomic force microscopy (AFM).

Methods: Liposomes were prepared with phospholipids (Egg phophatidylcholine (EPC), dimirystoyl phosphatidylcholine (DMPC) and distearoyl phosphatidylcholine (DSPC)), cholesterol and positively charged additive. The size of liposomes was controlled by extrusion. Submicron-sized latex beads were purchased from Sigma Co. Liposomes or latex beads were adsorbed onto mica or AP-mica after diluting to a suitable concentration for their observation. AFM observation was carried out in 10mM phosphate buffer with the tapping mode of Nanoscope IIIa. The vertical height (H) of submicron-sized particles was calculated by use of the depth analysis. Separately, the diameter of submicron-sized particles (P) was also measured by use of dynamic light scattering (DLS) method before they were adsorbed onto the substrate surface. These two parameters were introduced to describe the elasticity index (H/P value) of the submicron-sized particles (H/P value=Height average of adsorbed particles (H)/Particle diameter of particles (P)).

Results: The elasticity of latex beads as rigid polystyrene nanoparticles was analyzed by use of that equation. As a result, the H/P value of latex beads showed ca. 1 and it was confirmed the latex beads were rather rigid particles. Furthermore, when same analysis was carried out for liposomes, the image of liposomes was fully or partially flattened in cases of EPC and DMPC liposomes. On the other hand, a hemispherical shape was clearly observed for DSPC liposomes. The H/P value of the DSPC liposomes showed higher value compared with that of EPC or DMPC liposomes. Moreover, DSPC liposomes having different particle size or lipid composition, the elasticity of those DSPC liposomes was changed by those parameters. So, it was concluded the change of the elasticity of submicron-sized particles was determined as the change of their H/P value.

Conclusions: The combination of the two types of particle size measurements, the tapping mode of AFM in the buffer solution and other conventional method such as dynamic light scattering one is useful for evaluating the elasticity of submicron-sized particles such as liposomes.