Confined filaments in soft vesicles - case of sickle red blood cells

A semi-rigid filament confined in a soft vesicle of similar size can mutually deform each other. An important example from biological context is Hemoglobin-S (HbS) fibers which polymerize inside red blood cell (RBC). The fibers deform the healthy RBC into sickle-like shape causing difficulty in blood flow through capillaries. Using an area difference elasticity (ADE) model for RBC and a worm-like chain model for the HbS fibers, confined within RBC, we study the shape deformations at equilibrium. We also consider multiple filaments and find that confinement can generate multipolar RBC shapes and can also promote helical filament conformations. The same model, in different parameter regime, reproduces tubulation for phospholipid vesicles, as seen in experiments, when microtubules are confined in the vesicle. We conclude that with a decrease in the surface area to volume ratio, and membrane rigidity, the vesicle prefers tubulation over sickling. Our simulations can access various non-axisymmetric shapes, which have been observed experimentally, both in the context of sickle RBC and phospholipid vesicles, but have so far remained beyond the scope of variational methods.