Competition between Bending and Internal Pressure Governs the Mechanics of Fluid Nanovesicles

Abstract

Nanovesicles (∼100 nm) are ubiquitous in cell biology and an important vector for drug delivery. Mechanical properties of vesicles are known to influence cellular uptake, but the mechanism by which deformation dynamics affect internalization is poorly understood. This is partly due to the fact that experimental studies of the mechanics of such vesicles remain challenging, particularly at the nanometer scale where appropriate theoretical models have also been lacking. Here, we probe the mechanical properties of nanoscale liposomes using atomic force microscopy (AFM) indentation. The mechanical response of the nanovesicles shows initial linear behavior and subsequent flattening corresponding to inward tether formation. We derive a quantitative model, including the competing effects of internal pressure and membrane bending, that corresponds well to these experimental observations. Our results are consistent with a bending modulus of the lipid bilayer of ∼14kbT. Surprisingly, we find that vesicle stiffness is pressure dominated for adherent vesicles under physiological conditions. Our experimental method and quantitative theory represents a robust approach to study the mechanics of nanoscale vesicles, which are abundant in biology, as well as being of interest for the rational design of liposomal vectors for drug delivery.

Nanomechanics of Extracellular Vesicles Reveals Vesiculation Pathways

“…We found that vesicles isolated from RBCs after 2 h of incubation are significantly softer than after longer incubation times, whereas they do not vary significantly between 20 h and 35 days (Figure A). Further, we find that there is a large difference in the size of vesicles: after 2 h, R 0 = 35 ± 1 nm, after 20 h, R 0 = 85 ± 3.5 nm and after 35 d: R 0 = 59 ± 3 nm (Figure B), with R 0 the original vesicle radius before adhesion assuming surface area conservation . This suggests that different vesicle generation mechanisms previously suggested theoretically, dominate at different incubation times.…”

“…Subsequently, we quantified the shape of EVs by measuring the height over the radius of curvature. From this data, we calculated the unperturbed radius of the vesicles (Figure S1B, Supporting Information), assuming surface area conservation . Figure S1 (Supporting Information) shows that the size distributions of vesicles as determined by atomic force microscopy (AFM) imaging are generally similar to those obtained from NTA.…”

“…Figure S1 (Supporting Information) shows that the size distributions of vesicles as determined by atomic force microscopy (AFM) imaging are generally similar to those obtained from NTA. We have previously demonstrated that our approach is applicable for vesicles within this size range . Second, force indentation curves (FDCs) were recorded; the curves were obtained by moving the tip to the center of the EV and indenting it until a preset force is reached (schematic drawing in Figure B).…”

“…We then estimate the tension in the membrane using the Young‐Laplace equation: ΔΠ = 2σ/ R c , with ΔΠ being the osmotic pressure difference over the membrane. With these measurements of the radius of curvature, the stiffness and the tether force, we used our recent model to fit the bending modulus of the vesicles . Importantly, this allows us to compare various vesicle populations, as a bending modulus is an intrinsic material property of the vesicles and does not depend on, e.g., vesicle size.…”

“…Very few previous pioneering studies addressed the importance of mechanical properties of extracellular vesicles and so far the pressurization of vesicles due to surface interactions has not been taken into account. Our recently developed approach does take pressurization into account. It allows reliable comparison of different vesicle populations, as we are able to estimate the bending modulus of the vesicles, an intrinsic material property that is independent of the measuring conditions.…”