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Dynamics of microparticles inside lipid vesicles: movement in confined spaces

Microparticles and nanoparticles used in drug delivery frequently depend on their movement in confined spaces such as cells. Liposomes containing small numbers of 1-µm diameter polystyrene particles were used to study the dynamics of their movement within the confined space of the liposome interior. The analysis of the trajectories of single and multiple entrapped particles revealed that the particles were largely localized toward the periphery of the liposome with a rare presence in the centre. Interparticle interactions were studied by calculating interparticle distances, ranging from close to zero to around 8 µm with a mean of ∼4 µm. The diffusion coefficient of a single entrapped particle was D = 0.27 × 10−9 cm2 s−1 when compared with 5.1 × 10−9 cm2 s−1 free in water. When more than one particle was entrapped, the calculated diffusion coefficients were D = 0.61 × 10−9 cm2 s−1 for two particles, D = 1.26 × 10−9 cm2 s−1 for three particles, and D = 1.3 × 10−9 cm2 s−1 for multiple particles). Particle movement was found to be distinctly faster at the periphery (average velocity 21.4 μm s−1) than at the centre of the vesicle (average velocity 14.2 μm s−1). These results demonstrate the significance of particle–particle interactions as well as particle–surface interactions, which is evident here in some systems by particle aggregation close to the liposome membrane.