Nanoparticles Binding to Lipid Membranes: from Vesicle-Based Gels to Vesicle Inversion and Destruction

Abstract

Cells offer numerous inspiring examples where proteins and membranes combine to form complex structures that are key to intracellular compartmentalization, cargo transport, and specialization of cell morphology. Despite this wealth of examples, we still lack the design principles to control membrane morphology in synthetic systems. Here we show that even the relatively simple case of spherical nanoparticles binding to lipid-bilayer membrane vesicles results in a remarkably rich set of morphologies that can be controlled quantitatively via the particle binding energy. We find that when the binding energy is weak relative to a characteristic membrane-bending energy, the vesicles adhere to one another and form a soft solid, which could be used as a useful platform for controlled release. When the binding energy is larger, the vesicles undergo a remarkable destruction process consisting first of invaginated tubules, followed by vesicles turning inside-out, yielding a network of nanoparticle-membrane tubules. We propose that the crossover from one behavior to the other is triggered by the transition from partial to complete wrapping of nanoparticles. This model is confirmed by computer simulations and by quantitative estimates of the binding energy. These findings open the door to a new class of vesicle-based, closed-cell gels that are more than 99% water and can encapsulate and release on demand. Our results also show how to intentionally drive dramatic shape changes in vesicles as a step toward shape-responsive particles. Finally, they help us to unify the wide range of previously observed responses of vesicles and cells to added nanoparticles.