  Motivated by recent advances in vesicle engineering, we consider
theoretically the locomotion of shape-changing bilayer vesicles at low Reynolds
number. By modulating their volume and membrane composition, the vesicles can
be made to change shape quasi-statically in thermal equilibrium. When the
control parameters are tuned appropriately to yield periodic shape changes
which are not time-reversible, the result is a net swimming motion over one
cycle of shape deformation. For two classical vesicle models (spontaneous
curvature and bilayer coupling), we determine numerically the sequence of
vesicle shapes through an enthalpy minimization, as well as the fluid-body
interactions by solving a boundary integral formulation of the Stokes
equations. For both models, net locomotion can be obtained either by
continuously modulating fore-aft asymmetric vesicle shapes, or by crossing a
continuous shape-transition region and alternating between fore-aft asymmetric
and fore-aft symmetric shapes. The obtained hydrodynamic efficiencies are
similar to that of other low Reynolds number biological swimmers, and suggest
that shape-changing vesicles might provide an alternative to flagella-based
synthetic microswimmers.
