Tailoring the properties of polymersomes [Discher et al., Science, 1999] for encapsulating functional loads and performing specific transport, target, and release operations is an active research area. For example, it has been demonstrated that incorporation of receptor proteins LamB into the polymersome shell allows λ-phages to bind and inject their DNA into the artificial container [Graff et al., PNAS, 2002]. These observations motivated this simulation study, focusing on how loading of polymersomes with long polymer molecules affects their equilibrium properties and stability in aqueous solutions.

The shell-forming AB amphiphiles and the loading polymer are represented by a bead-spring model. The size of the hydrophilic polymer (load) is 8x larger than the amphiphilic polymers forming the polymersome. The non-bonded interactions of the beads are given by a density-dependent functional derived from a third-order virial expansion of the free energy. The water is taken implicitly into account via the virial coefficients of the hydrophilic segments i.e. the model is “solvent free”. The coarse-grained Hamiltonian is then subjected to Monte Carlo simulations [Daoulas and Müller, J. Chem. Phys., 2006].

Figure 1 Left: The main panel shows a cut-through a loaded vesicle. The hydrophobic, A, and hydrophilic, B, beads are depicted in red and green. The blue color marks the C segments of the loading chains. The top inset demonstrates an intact, low-load, vesicle, while a high-load vesicle with a pore is shown at the bottom. Right: The volume of loaded polymersomes, V, referred to the volume, V_o, of an empty vesicle is shown as a function of the number of loading chains.

The simulations demonstrate that polymersomes can sustain a limited amount of loading while maintaining their integrity. Snapshots of an intact and a rupturing vesicle are shown on the left of Figure 1. For the considered polymersomes, loading does not have a significant effect on their volume and shell area, which is illustrated on the right graph of Figure 1. This is corroborated by simple calculations balancing the loading pressure with the Laplace pressure of the vesicle shell. The shell tension, estimated from the simple theory, is compared to the one obtained after fitting the fluctuation spectra with the functional form predicted by an interfacial Hamiltonian [Milner and Safran, Phys. Rev. A, 1987]. The later approach also allows the calculation of the shell membrane bending rigidity, which is similar to the simple, planar, bilayer case.