Membrane properties of swollen vesicles: growth, rupture, and fusion

A vesicle can be swollen in response to the imposed osmotic pressure gradient and vesicular membrane properties are changed. The swelling of small unilamellar vesicles is explored by dissipative particle dynamics, including the growth, rupture, and fusion processes. The spontaneously formed vesicles are inflated by water addition into the vesicle lumen. As the vesicle size grows due to the increment of lumenal contents, the bilayer gets thinner and the area density of the inner lipid head declines much more significantly than that of the outer does. Both area densities converge to the same value before membrane rupture. The packing parameters for both leaflets also decrease and the shape of lipids in the inner leaflet changes from an inverted truncated cone to a truncated cone. The distribution of lipid tails indicates that an interdigitated structure takes place within the bilayer of an inflated vesicle. The outcome of local order parameter demonstrates that the orientation order decays with inflation, revealing that vesicular membrane stretching is accompanied with entropy increment, which is opposite to stretched elastic membrane with entropy reduction. The Young-Laplace equation is adopted to estimate tension, which rises with inflation. Although vesicles of different sizes rupture at distinct degrees of inflation, membrane properties, such as membrane thickness, packing parameter, order parameter, and tension increment must reach the same critical values for all vesicle sizes before rupture. It is also found that the vesicle fusion process is greatly facilitated by the increment of tension owing to the substantial reduction in the time periods for adhesion and hemifusion processes.