Membrane Viscosity Plays Key Role in Cell Dynamics and Deformation

Cells and their internal structures are enclosed by membranes, which serve not only as boundaries but as dynamic components of cellular function. These membranes, composed of flexible lipid bilayers, can reshape and remodel themselves during processes like cell movement, division, and molecular trafficking. Until now, it was commonly assumed that the time scales of membrane deformation were set by the dissipation in the surrounding environment.

However, new research resulting from a collaboration between Northwestern Engineering’s Petia Vlahovska and Hammad Faizi (PhD ’22), and Rony Granek, a professor at Israel’s Ben-Gurion University of the Negev, reveals the internal properties of the membrane itself, particularly its viscosity, play a critical role in controlling its own deformation and dynamics.

By studying the equilibrium shape fluctuations of vesicles — small, cell-like sacs made of lipid bilayers — Vlahovska and the team during the Northwestern-BGU collaboration showed for the first time that dissipation within the membrane controls the undulation dynamics of highly curved membranes. 

“This finding emphasizes that membrane viscosity, previously overlooked, is a key factor in how membranes bend and reshape, offering new insights into cellular structure and function,” Vlahovska said. "These little droplets, or vesicles, are very soft and deformable, transporting cargo inside the cell. There’s great interest in understanding the mechanical properties of vesicles to grasp how cells operate and to design drugs that can enter cells."

Vlahovska is a professor of engineering sciences and applied mathematics at the McCormick School of Engineering.  The team reported its findings in the paper “Curvature Fluctuations of Fluid Vesicles Reveal Hydrodynamic Dissipation Within the Bilayer,” published in the Proceedings of the National Academy of Sciences.

The team’s research that combined theory and experiments shifts focus to the membrane itself.

"Membrane bending, driven by thermal or active forces, is commonly assumed to be dampened by viscous losses in the surrounding medium," Vlahovska said. "By examining the equilibrium shape fluctuations of vesicles, we showed that dissipation within the membrane controls the undulation dynamics of highly curved membranes."

“By leveraging optical microscopy and high-speed imaging of membrane’s thermal undulations, we were able to visualize the curvature fluctuations and utilize them as a novel approach to measure membrane viscosity non-invasively,” said Faizi, currently a senior research specialist at Dow.

Using the new method, the team could independently measure membrane bending rigidity and viscosity.

This new understanding of membrane behavior could shift how scientists think about the role of internal membrane viscosity in cellular processes and have important implications for interpreting experimental data and understanding how membranes function in living cells.

"Membrane fluidity is essential to life as it controls molecular mobility and structural rearrangements involved in cell functions, including signaling and molecular trafficking," Vlahovska said.

The discovery that internal viscosity plays a crucial role in membrane behavior adds to the understanding of how cells function, with potential implications for biomedical applications such as drug delivery systems or the design of synthetic membranes.

“Every cell, like all the cells that make up our bodies, is enveloped by membranes. All the cellular organelles are also enclosed by membranes,” Vlahovska said. “These membranes are constantly being deformed, reflecting the dynamic inner life of the cell.”