  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.
