Modulating vesicle adhesion by electric fields

We introduce an experimental setup for modulating adhesion of giant unilamellar vesicles to a planar substrate. Adhesion is induced by the application of an external potential to a transparent indium tin oxide-coated electrode (the substrate), which enables single-vesicle studies. We demonstrate tunable and reversible adhesion of negatively charged vesicles. The adhesion energy at different potentials is calculated from the vesicle shape assessed with confocal microscopy. Two approaches for these estimates are employed: one based on the whole contour of the vesicle and a second based on the contact curvature of the membrane in the vicinity of the substrate. Both approaches agree well with each other and show that the adhering vesicles are in the weak adhesion regime for the range of explored external potentials. Using fluorescence quenching assays, we detect that, in the adhering membrane segment, only the outer bilayer leaflet of the vesicle is depleted of negatively charged fluorescent lipids, while the inner leaflet remains unaffected. We show that depletion of negatively charged lipids is consistent Poisson-Boltzmann theory, taking into account charge regulation from lipid mobility. Finally, we also show that lipid diffusion is not significantly affected in the adhering membrane segment. We believe that the approaches introduced here for modulating and assessing vesicle adhesion have many potential applications in the field of single-vesicle studies and research on membrane adhesion

Ultrasonic triggering of giant magnetocaloric effect in MnAs thin films

Mechanical control of magnetic properties in magnetostrictive thin films offers the unexplored opportunity to employ surface wave acoustics in such a way that acoustic triggers dynamic magnetic effects. The strain-induced modulation of the magnetic anisotropy can play the role of a high frequency varying effective magnetic field leading to ultrasonic tuning of electronic and magnetic properties of nanostructured materials, eventually integrated in semiconductor technology. Here, we report about the opportunity to employ surface acoustic waves to trigger magnetocaloric effect in MnAs(100nm)/GaAs(001) thin films. During the MnAs magnetostructural phase transition, in an interval range around room temperature (0{\deg}C - 60{\deg}C), ultrasonic waves (170 MHz) are strongly attenuated by the phase coexistence (up to 150 dB/cm). We show that the giant magnetocaloric effect of MnAs is responsible of the observed phenomenon. By a simple anelastic model we describe the temperature and the external magnetic field dependence of such a huge ultrasound attenuation. Strain-manipulation of the magnetocaloric effect could be a further interesting route for dynamic and static caloritronics and spintronics applications in semiconductor technology

Particle engulfment by strongly asymmetric membranes with area reservoirs

Biological cells are capable of undergoing extensive shape transformations thanks to the existence of membrane area reservoirs from which they can pull out membrane when required. A particularly relevant example of such membrane remodelling is given by endocytic and phagocytic processes, during which the cell membrane engulfs nano- and micrometer sized particles. Recently, it was shown that cell-like membrane reservoirs can be mimicked in giant vesicles with nanotubes stabilized by strong bilayer asymmetry, as quantified by the membrane's spontaneous curvature. Here, we theoretically investigate particle engulfment by such strongly-asymmetric membranes. We find that, depending on the sign of the spontaneous curvature, the engulfment transition may be continuous or discontinuous. Moreover, we find that, in the case of particle engulfment, the presence of asymmetry-stabilized reservoirs is not well captured by the constant-tension model typically used to describe cell-membrane deformations. This highlights the need for a better understanding of the nature of cellular membrane reservoirs, in order to accurately describe membrane remodelling processes

Engulfment of ellipsoidal nanoparticles by membranes: full description of orientational changes

We study the engulfment of ellipsoidal nanoparticles by membranes. It has been previously predicted that wrapping by the membrane can induce reorientation of the particle, however, previous studies only considered the wrapping process constrained to either side-oriented or tip-oriented particles. In contrast, we consider here the full two-dimensional energy landscape for engulfment, where the two degrees of freedom represent (i) the amount of wrapping and (ii) the particle orientation. In this way, we obtain access to the stability limits of the differently-oriented states, as well as to the energy barriers between them. We find that prolate and oblate particles undergo qualitatively different engulfment transitions, and show that the initial orientation of the particle at first contact with the membrane influences its fate

Self-assembled vesicle–colloid hybrid swimmers: Non-reciprocal strokes with reciprocal actuation

We consider a self-assembled hybrid system, composed of a bilayer vesicle to which a number of colloids are adhered. Based on known results of membrane curvature elasticity, we predict that, for sufficiently deflated prolate vesicles, the colloids can self-assemble into a ring at a finite distance away from the vesicle equator, thus breaking the up–down symmetry in the system. Because the relative variation of the position of the colloidal ring along the vesicle endows the system with an effective elasticity, periodic cycles of inflation and deflation can lead to non-reciprocal shape changes of the vesicle–colloid hybrid, allowing it to swim in a low Reynolds number environment under reciprocal actuation. We design several actuation protocols that allow control over the swimming direction

Uniform and Janus-like nanoparticles in contact with vesicles: Energy landscapes and curvature-induced forces

Biological membranes and lipid vesicles often display complex shapes with non-uniform membrane curvature. When adhesive nanoparticles with chemically uniform surfaces come into contact with such membranes, they exhibit four different engulfment regimes as recently shown by a systematic stability analysis. Depending on the local curvature of the membrane, the particles either remain free, become partially or completely engulfed by the membrane, or display bistability between free and completely engulfed states. Here, we go beyond stability analysis and develop an analytical theory to leading order in the ratio of particle-to-vesicle size. This theory allows us to determine the local and global energy landscapes of uniform nanoparticles that are attracted towards membranes and vesicles. Whereas the local energy landscape depends only on the local curvature of the vesicle membrane and not on the overall membrane shape, the global energy landscape describes the variation of the equilibrium state of the particle as it probes different points along the membrane surface. In particular, we find that the binding energy of a partially engulfed particle depends on the 'unperturbed' local curvature of the membrane in the absence of the particle. This curvature dependence leads to local forces that pull the partially engulfed particles towards membrane segments with lower and higher mean curvature if the particles originate from the exterior and interior solution, respectively, corresponding to endo- and exocytosis. Thus, for partial engulfment, endocytic particles undergo biased diffusion towards the membrane segments with the lowest membrane curvature whereas exocytic particles move towards segments with the highest curvature. The curvature-induced forces are also effective for Janus particles with one adhesive and one non-adhesive surface domain. In fact, Janus particles with a strongly adhesive surface domain are always partially engulfed which implies that they provide convenient probes for experimental studies of the curvature-induced forces that arise for complex-shaped membranes

Stabilization of membrane necks by adhesive particles, substrate sur- faces, and constriction forces

Membrane remodelling processes involving the formation and fission of small buds require the formation and closure of narrow membrane necks, both for biological membranes and for model membranes such as lipid bilayers. The conditions required for the stability of such necks are well understood in the context of budding of vesicles with bilayer asymmetry and/or intramembrane domains. In many cases, however, the necks form in the presence of an adhesive surface, such as a solid particle or substrate, or the cellular cortex itself. Examples of such processes in biological cells include endocytosis, exocytosis and phagocytosis of solid particles, the formation of extracellular and outer membrane vesicles by eukaryotic and prokaryotic cells, as well as the closure of the cleavage furrow in cytokinesis. Here, we study the interplay of curvature elasticity, membrane-substrate adhesion, and constriction forces to obtain generalized stability conditions for closed necks which we validate by numerical energy minimization. We then explore the consequences of these stability conditions in several experimentally accessible systems such as particle-filled membrane tubes, supported lipid bilayers, giant plasma membrane vesicles, bacterial outer membrane vesicles, and contractile rings around necks. At the end, we introduce an intrinsic engulfment force that directly describes the interplay between curvature elasticity and membrane-substrate adhesion