We study the interactions of vesicle membranes with nanoparticles of different shapes. The wrapping andinternalization of nanoparticles by biomembranes plays a critical role in drug delivery applications and nanomedicine.We begin with a single spherical particle adsorbed on a vesicle and solve the shape equations for rotationally symmetric vesicles to investigate the wrapping of the particle by the vesicle. These shape equations are based on the Helfrich bending energy of the vesicle membrane. We predict different regimes for partial and full wrapping of the nanoparticle and relate the wrapping transition and its energy barrier to the reduced volume of the vesicle and the relative size of the vesicle and particle.
We next study the cooperative wrapping of several spherical nanoparticles using simulated annealing Monte Carlo simulations of triangulated vesicles. We report novel tubular membrane structures induced by the nanoparticles, which we obtain from energy minimization. The membrane tubules enclose linear aggregates of particles and protrude into the vesicles. The high stability of the particle-filled tubules implies strongly attractive, membrane-mediated interactions between the particles. The tubular structures may provide a new route to encapsulate nanoparticles reversibly in vesicles.
Finally, we study the wrapping of single ellipsoidal nanoparticles by a vesicle via simulated annealing Monte Carlo simulations. We report two distinct regimes of spreading and internalization, which are separated by an energy barrier, and relate the success or failure of the internalization to the particle shape and orientation relative to the vesicle. We observe easier spreading yet more difficult internalization for ellipsoidal particles with lower aspect ratios, which may explain the high virulence of tubular viruses. We find that the wrapping of ellipsoidal particles is associated with an orientation change of the particle. While the spreading starts on the flat side of the ellipsoidal particles, the particle changes its orientation during wrapping, and internalization finally occurs in an orientation in which a tip of the ellipsoidal particles protrudes into the vesicles.