(432c) Encapsulation of Vesicles and Particles by Open Bilayers – Statics and Dynamics

Engineers have exploited the natural compartmentalizing functionality of vesicles to achieve specific objectives such as imaging, separations and drug delivery. Some of these implementations require particles or vesicles to be captured within bigger unilamellar vesicles, which can be achieved by triggering the closure of flat sheets of bilayers [Kisak et al. (2002)]. Application of hypotonic osmotic stresses, electric pulses and freezing thawing on vesicle systems also create open bilayers which subsequently may encapsulate other vesicles. However, there has been limited theoretical work in the literature to understand encapsulation by bilayers. The present work represents an effort towards remedying that deficiency.

In this paper, we address two aspects of the encapsulation of entities by open bilayers. First, we look at the steady state configurations for a vesicle being engulfed by a bilayer patch, where both the vesicle and bilayer are composed of charged lipids. We also consider electrolyte concentrations such that vesicle and bilayer surfaces are purely repelling at all separations. Any steady configuration of the bilayer around the vesicle, which is characterized in our simplified model by a curling angle θ, is a result of the balance between the line tension of the bilayer patch attempting to curl around the vesicle and the repulsion between the two attempting to keep them apart. Using a simple description of the energy of the bilayer, it is demonstrated that the bilayer either encapsulates the vesicle completely, or assumes a steady curling angle θ_s around the vesicle with an open line. These steady state calculations can be used to predict the maximum size of a stable hole in a bilamellar (or multilamellar) vesicle, which may be induced by osmosis or electric pulses.

The second problem we consider is the dynamics of encapsulation of a particle by an open bilayer. As the bilayer curls up around the particle, it squeezes fluid out, thus pushing the particle further away. It can be shown that under certain conditions, the capture of a particle depends solely on its initial position, irrespective of the rate at which the bilayer curls up. We identify the parameters necessary for particle capture by simulating the bilayer and particle motions using a boundary integral method.

References

1. Kisak E. T., B. Coldren and J. A. Zasadzinksi, ?Nanocompartments Enclosing Vesicles, Colloids, and Macromolecules via Interdigitated Lipid Bilayers?, Langmuir 18, 284-288 (2002).