Shape Transformations Of Vesicles Research Articles

Vesicle Geometries Enabled by Semiflexible Polymer.

Understanding and controlling vesicle shapes is fundamental challenge in biophysics and materials design. In this paper, we employ the Monte Carlo method to investigate the shape of soft vesicle induced by semiflexible polymer outside in two dimensions. The effect of bending stiffness of polymer and the strength of attractive interaction between vesicle and polymer on the shape of vesicle is discussed in detail in the present paper. It is found that the shape of vesicle is influenced by and . Typical shape of vesicles is observed, such as circular, cigar-like, double vesicle, and racquet-like. To engineer vesicle shape transformations is helpful for exploiting the richness of vesicle geometries for desired applications.

Investigation of Shape Transformations of Vesicles, Induced by Their Adhesion to Flat Substrates Characterized by Different Adhesion Strength.

The adhesion of lipid vesicles to a rigid flat surface is investigated. We examine the influence of the membrane spontaneous curvature, adhesion strength, and the reduced volume on the stability and shape transformations of adhered vesicles. The minimal strength of the adhesion necessary to stabilize the shapes of adhered vesicles belonging to different shape classes is determined. It is shown that the budding of an adhered vesicle may be induced by the change of the adhesion strength. The importance of the free vesicle shape for its susceptibility to adhesion is discussed.

Vesicle shape transformations driven by confined active filaments

In active matter systems, deformable boundaries provide a mechanism to organize internal active stresses. To study a minimal model of such a system, we perform particle-based simulations of an elastic vesicle containing a collection of polar active filaments. The interplay between the active stress organization due to interparticle interactions and that due to the deformability of the confinement leads to a variety of filament spatiotemporal organizations that have not been observed in bulk systems or under rigid confinement, including highly-aligned rings and caps. In turn, these filament assemblies drive dramatic and tunable transformations of the vesicle shape and its dynamics. We present simple scaling models that reveal the mechanisms underlying these emergent behaviors and yield design principles for engineering active materials with targeted shape dynamics.

Shape Transformations of Vesicles Induced by Their Adhesion to Flat Surfaces.

The shape transformations of lipid vesicles induced by the adhesion to a flat surface is investigated. We perform the calculations within the framework of the Helfrich spontaneous curvature model. The calculations were performed for a few values of the reduced volume and the spontaneous curvature. The range of stability for different shapes (oblate, prolate, and stomatocyte) of adhered vesicles is determined. New physical phenomena such as budding induced by the adhesion of vesicles are reported.

Membrane poration, wrinkling, and compression: deformations of lipid vesicles induced by amphiphilic Janus nanoparticles.

Building upon our previous studies on interactions of amphiphilic Janus nanoparticles with glass-supported lipid bilayers, we study here how these Janus nanoparticles perturb the structural integrity and induce shape instabilities of membranes of giant unilamellar vesicles (GUVs). We show that 100 nm amphiphilic Janus nanoparticles disrupt GUV membranes at a threshold particle concentration similar to that in supported lipid bilayers, but cause drastically different membrane deformations, including membrane wrinkling, protrusion, poration, and even collapse of entire vesicles. By combining experiments with molecular simulations, we reveal how Janus nanoparticles alter local membrane curvature and collectively compress the membrane to induce shape transformation of vesicles. Our study demonstrates that amphiphilic Janus nanoparticles disrupt vesicle membranes differently and more effectively than uniform amphiphilic particles.

Shape Transformations of Vesicles Induced by Swim Pressure.

While the behavior of vesicles in thermodynamic equilibrium has been studied extensively, how active forces control vesicle shape transformations is not understood. Here, we combine theory and simulations to study the shape behavior of vesicles containing active Brownian particles. We show that the combination of active forces, dimensionality, and membrane bending free energy creates a plethora of novel phase transitions. At low swim pressure, the vesicle exhibits a discontinuous transition from a spherical to a prolate shape, which has no counterpart in two dimensions. At high swim pressure it exhibits stochastic spatiotemporal oscillations. Our work helps researchers to understand and control the shape dynamics of membranes in active-matter systems.

Shape Transformations of Vesicles Self-Assembled from Amphiphilic Hyperbranched Multiarm Copolymers via Simulation.

The understanding of shape transformations of vesicles is of fundamental importance in biological and clinical sciences. Hyperbranched polymer vesicles (branched polymersomes) are newly emerging polymer vesicles with special structure and property. They have also been regarded as a good model for biomembranes. However, the shape transformations of hyperbranched polymer vesicles have not been studied from either an experimental or theoretical level. Herein, the shape transformations of vesicles self-assembled from amphiphilic hyperbranched multiarm copolymers (HMCs) in response to the interaction parameters between the hydrophobic core and hydrophilic arms and the polymer concentrations are investigated carefully through dissipative particle dynamics (DPD) simulations. In the morphological phase diagram, two types of vesicles are obtained: one type corresponds to vesicles without holes formed at low concentrations including unilamellar vesicles, double-lamellar vesicles, discocyte-shaped vesicles, and tubular vesicles, and the other type corresponds to vesicles with holes formed at high concentrations including stomatocyte-shaped vesicles, toroidal vesicles, genus-3 (G-3) toroidal vesicles with three holes, and genus-4 (G-4) toroidal vesicles with four holes. In addition, both the self-assembly mechanisms and the dynamics for the formation of these vesicles have been systematically studied. The current work will offer theoretical support for fabricating novel vesicles with various shapes from hyperbranched polymers.

Giant Vesicles Exposed to Aqueous Two‐Phase Systems: Membrane Wetting, Budding Processes, and Spontaneous Tubulation

In this review, we describe recent studies of giant unilamellar vesicles exposed to aqueous polymer solutions. These solutions undergo phase separation when the weight fractions of the polymers exceed a few percent. Depending on the lipid and polymer composition, the membrane may be completely or partially wetted by the aqueous phases. The deflation process induces a variety of vesicle shape transformations. One such transformation is droplet‐induced budding of the vesicles. Another, particularly striking transformation is the spontaneous tube formation, which reveals a substantial asymmetry and spontaneous curvature of the membranes, arising from the different polymer compositions across the membrane. The tubulation starts with the nucleation of small membrane buds which then grow into necklace‐like tubes. When the tube length reaches a certain critical value, the necklace‐like tube transforms into a cylindrical one. Analyzing different aspects of the observed vesicle shapes, quantitative estimates of the spontaneous curvature which is found to vary, depending on the lipid composition, between 1/(125 nm) and 1/(600 nm), are obtained. These curvatures are generated by the weak adsorption of poly(ethylene glycol) (PEG) molecules onto the lipid membranes, with a relatively small binding affinity of about 4 kJ mol−1 or 1.6 kBT per PEG molecule for either liquid‐ordered or liquid‐disordered membranes.

Vesicle Geometries Enabled by Dynamically Trapped States.

Understanding and controlling vesicle shapes is a fundamental challenge in biophysics and materials design. In this paper, we design dynamic protocols for enlarging the shape space of both fluid and crystalline vesicles beyond the equilibrium zone. By removing water from within the vesicle at different rates, we numerically produced a series of dynamically trapped stable vesicle shapes for both fluid and crystalline vesicles in a highly controllable fashion. In crystalline vesicles that are continuously dehydrated, simulations show the initial appearance of small flat areas over the surface of the vesicles that ultimately merge to form fewer flat faces. In this way, the vesicles transform from a fullerene-like shape into various faceted polyhedrons. We perform analytical elasticity analysis to show that these salient features are attributable to the crystalline nature of the vesicle. The potential to use dynamic protocols, such as those used in this study, to engineer vesicle shape transformations is helpful for exploiting the richness of vesicle geometries for desired applications.

Shape Transformations of Lipid Vesicles by Insertion of Bulky-Head Lipids.

Lipid vesicles, in particular Giant Unilamellar Vesicles (GUVs), have been increasingly important as compartments of artificial cells to reconstruct living cell-like systems in a bottom-up fashion. Here, we report shape transformations of lipid vesicles induced by polyethylene glycol-lipid conjugate (PEG lipids). Statistical analysis of deformed vesicle shapes revealed that shapes vesicles tend to deform into depended on the concentration of the PEG lipids. When compared with theoretically simulated vesicle shapes, those shapes were found to be more energetically favorable, with lower membrane bending energies than other shapes. This result suggests that the vesicle shape transformations can be controlled by externally added membrane molecules, which can serve as a potential method to control the replications of artificial cells.

Shape transformations of bilayer vesicles from amphiphilic block copolymers: a dissipative particle dynamics simulation study

Dissipative particle dynamics (DPD) simulations are employed to study the shape transformations of vesicles formed from amphiphilic triblock copolymers. The amphiphilic molecule is built from two different hydrophilic blocks on the sides and a hydrophobic block in the middle. To model the asymmetric membrane in the vesicle, spontaneous curvature is introduced by the difference in repulsive parameters between the two different hydrophilic blocks of the amphiphilic molecule. A plethora of complex vesicle shapes is revealed by the DPD simulations, including some shapes that have not been reported in other simulation studies on vesicles, such as starfish-shaped, toroidal, long rodlike, and inverted vesicles. These simulated vesicles agree with theoretically derived vesicle shapes based on the spontaneous curvature model and also with experimental observations.

Dynamic shape transformations of fluid vesicles

We incorporate a volume-control algorithm into a recently developed one-particle-thick mesoscopic fluid membrane model to study vesicle shape transformation under osmotic conditions. Each coarse-grained particle in the model represents a cluster of lipid molecules and the inter-particle interaction potential effectively captures the dual character of fluid membranes as elastic shells with out-of-plane bending rigidity and 2D viscous fluids with in-plane viscosity. The osmotic pressure across the membrane is accounted for by an external potential, where the instantaneous volume of the vesicles is calculated via a local triangulation algorithm. Through coarse-grained molecular dynamics simulations, we mapped out a phase diagram of the equilibrium vesicle shapes in the space of spontaneous curvature and reduced vesicle volume. The produced equilibrium vesicle shapes agree strikingly well with previous experimental data. We further found that the vesicle shape transformation pathways depend on the volume change rate of the vesicle, which manifests the role of dynamic relaxation of internal stresses in vesicle shape transformations. Besides providing an efficient numerical tool for the study of membrane deformations, our simulations shed light on eliciting desired cellular functions via experimental control of membrane configurations.

Shape Transformations of Membrane Vesicles from Amphiphilic Triblock Copolymers: A Dissipative Particle Dynamics Simulation Study

Dissipative particle dynamics (DPD) simulations are employed to study the shape transformations of vesicles formed from amphiphilic triblock copolymers. The amphiphilic molecule is built from two different hydrophilic blocks on the sides and a hydrophobic block in the middle. To model the asymmetric membrane in the vesicle, spontaneous curvature is introduced by the difference in repulsive parameters between the two different hydrophilic blocks of the amphiphilic molecule. A plethora of complex vesicle shapes is revealed by the DPD simulations, including some shapes that have not been reported in other simulation studies on vesicles, such as starfish-shaped, toroidal, long rodlike, and inverted vesicles. These simulated vesicles agree with theoretically derived vesicle shapes based on the spontaneous curvature model and also with experimental observations.

Vesicle Shapes from Molecular Dynamics Simulations

Lipid bilayer membranes are known to form various structures such as large sheets or vesicles. When the two leaflets of the bilayer have an equal composition, the membrane preferentially forms a flat sheet or a spherical vesicle. However, a difference in the composition of the two leaflets may result in a curved bilayer or in a wide variety of vesicle shapes. Vesicles with different shapes have already been shown in experiments and diverse vesicle shapes have been predicted theoretically from energy minimization of continuous curves. Here we present a molecular dynamics study of the effect of small changes in the phospholipid headgroups on the spontaneous curvature of the bilayer and on the resulting vesicle shape transformations. Small asymmetries in the bilayers already result in high spontaneous curvature and large vesicle deformations. Vesicle shapes that are formed include ellipsoids, discoids, pear-shaped vesicles, cup-shaped vesicles, as well as budded vesicles. Comparison of these vesicles with theoretically derived vesicle shapes shows both resemblances and differences.

Phospholipid membrane bending as assessed by the shape sequence of giant oblate phospholipid vesicles

Vesicle shape transformations caused by decreasing the difference between the equilibrium areas of membrane monolayers were studied on phospholipid vesicles with small volume to membrane area ratios. Slow transformations of the vesicle shape were induced by lowering of the concentration of lipid monomers in the solution outside the vesicle. The complete sequence of shapes consisted of a string of pearls, and wormlike, starfish, discocyte and stomatocyte shapes. The transformation from discocyte to stomatocyte vesicle shapes was analyzed theoretically to see whether these observations accord with the area difference elasticity (ADE) model. The membrane shape equation and boundary conditions were derived for axisymmetrical shapes for low volume vesicles, part of whose membranes are in contact. Calculated shapes were arranged into a phase diagram. The theory predicts that the transition between discocyte and stomatocyte shapes is discontinuous for relatively high volumes and continuous for low volumes. The calculated shape sequences matched well with the observed ones. By assuming a linear decrease of the equilibrium area difference with time, the ratio between the nonlocal and local bending constants is in agreement with reported values.

Influence of the shape of phospholipid vesicles on the measurement of their size by photon correlation spectroscopy.

The influence of shape transformation of large unilamellar vesicles (LUV) on their size measurement by photon correlation spectroscopy (PCS) has been investigated. The experimental size of vesicles after hyperosmotic contractions of increasing intensities have been compared to the theoretical volume decrease determined by applying Boyle Van't Hoff's law. The main observation is that PCS size measurement gives overestimated values when LUV have been subjected to a volume decrease of more than 20% of their initial volume. The PCS size overestimation is related to the influence of the shape transformation of the vesicles on their diffusion coefficient (D) as shown by modelling the evolution of D of a sphere which is transformed into an ellipsoid by internal volume reduction under constant area.

Dynamics of Giant Vesicles

The dynamics of vesicle shape transformations are governed by the competition between curvature energy, geometrical constraints and viscous dissipation in the surrounding liquid. After a discussion of the general principles, three examples illustrate these concepts, (i) Slow conformal diffusion of vesicles of higher genus arises from the invanance of the curvature elasticity with respect to conformal transformations, (ii) spinodal fluctuations indicate the onset of the budding instability, and (iii) the dynamic pearling instability of cylindrical vesicles is caused by the action of laser tweezers.

Shape transitions and shape stability of giant phospholipid vesicles in pure water induced by area-to-volume changes

Shape transformations of vesicles of dimyristoylphosphatidylcholine (= DMPC) and palmitoyloleylphosphatidylcholine (= POPC) in ion-free water were induced by changing the area-to-volume ratio via temperature variations. Depending on the pretreatment we find several types of shape changes for DMPC (in pure water) at increasing area-to-volume ratio: (a) budding transitions leading to the formation of a chain of vesicles at further increase of the area-to-volume ratio, (b) discocyte-stomatocyte transitions, (c) reentrant dumbbell-pear-dumbbell transitions, and (d) spontaneous blebbing and/or tether formation of spherical vesicles. Beside these transitions a more exotic dumbbell-discocyte transition (e) was found which proceeded via local instabilities. Pears, discocytes, and stomatocytes are stable with respect to small temperature variations unless the excess area is close to values corresponding to limiting shapes of budded vesicles where temperature variations of less than or equal to 0.1 degree C lead to spontaneous budding to the inside or the outside. For POPC we observed only budding transitions to the inside leading either to chains of vesicles or to distributions of equally sized daughter vesicles protruding to the inside of the vesicle. Preliminary experiments concerning the effect of solutes are also reported. The first three types of shape transitions can be explained in terms of the bilayer coupling model assuming small differences in thermal expansivities of the two monolayers. This does not hold for the observed instabilities close to the limiting shapes.

Shape transformations of vesicles: Phase diagram for spontaneous- curvature and bilayer-coupling models.

Vesicle shapes of low energy are studied for two variants of a continuum model for the bending energy of the bilayer: (i) the spontaneous-curvature model and (ii) the bilayer-coupling model, in which an additional constraint for the area difference of the two monolayers is imposed. We systematically investigate four branches of axisymmetric shapes: (i) the prolate-dumbbell shapes; (ii) the pear-shaped vesicles, which are intimately related to budding; (iii) the oblate-discocyte shapes; and (iv) the stomatocytes. These branches end up at limit shapes where either the membrane self-intersects or two (or more) shapes are connected by an infinitesimally narrow neck. The latter limit shape requires a certain condition between the curvatures of the adjacent shape and the spontaneous curvature. For both models, the phase diagram is determined, which is given by the shape of lowest bending energy for a given volume-to-area ratio and a given spontaneous curvature or area difference, respectively. The transitions between different shapes are continuous for the bilayer-coupling model, while most of the transitions are discontinuous in the spontaneous-curvature model. We introduce trajectories into these phase diagrams that correspond to a change in temperature and osmotic conditions. For the bilayer-coupling model, we find extreme sensitivity to an asymmetry in the monolayer expansivity. Both models lead to different predictions for typical trajectories, such as budding trajectories or oblate-stomatocyte transitions. Our study thus should provide the basis for an experimental test of both variants of the curvature model.

SHAPE TRANSFORMATIONS OF FREE, TOROIDAL AND BOUND VESICLES

Shapes and shape transformations of vesicles are considered theoretically within the spontaneous curvature model for three cases. (i) For free vesicles, the whole phase diagram is presented which includes pear-shaped vesicles and a line of limit shapes related to budding. (ii) For toroidal vesicles, three branches of solutions with low energy are found. (iii) An extension of this model to the adhesion of vesicles is briefly discussed.