A vesicle microrheometer for high-throughput viscosity measurements of lipid and polymer membranes

TL;DR

This paper introduces a novel, high-throughput, non-invasive method to measure the viscosity of lipid and polymer membranes using vesicle deformation, revealing how membrane viscosity varies with composition, phase, and polarization.

Contribution

The study presents a new vesicle microrheometer technique for rapid, probe-independent viscosity measurements across diverse membrane systems, including phase behavior and polarization effects.

Findings

Pure lipid bilayers behave as Newtonian fluids with strain-rate independent viscosity.

Phase-separated and diblock-copolymer membranes exhibit shear-thinning behavior.

Electrically polarized bilayers are significantly more viscous than charge-neutral ones.

Abstract

Viscosity is a key property of cell membranes that controls mobility of embedded proteins and membrane remodeling. Measuring it is challenging because existing approaches involve complex experimental designs and/or models, and the applicability of some is limited to specific systems and membrane compositions. As a result there is scarcity of data and the reported values for membrane viscosity vary by orders of magnitude for the same system. Here, we show how viscosity of bilayer membranes can be obtained from the transient deformation of giant unilamellar vesicles. The approach enables a non-invasive, probe-independent and high-throughput measurement of the viscosity of bilayers made of lipids or polymers with a wide range of compositions and phase state. Pure lipid and single-phase mixed bilayers are found to behave as Newtonian fluids with strain-rate independent viscosity, while phase-separated and diblock-copolymers systems exhibit shear-thinning in the explored range of strain rates 1-2000 $s^{-1}$. The results also reveal that electrically polarized bilayers can be significantly more viscous than charge-neutral bilayers. These findings suggest that biomembrane viscosity is a dynamic property that can be actively modulated not only by composition but also by membrane polarization, e.g., as in action potentials.