Our bodies are made up of countless cells, and each one is enclosed by a thin layer called the cell membrane. This membrane is not a rigid wall, but a soft, flexible sheet made of lipids. Beneath it lies a supportive network of proteins known as the cytoskeleton, which helps the cell maintain its shape.

The cell membrane is not only soft but also possesses a property called fluidity, allowing it to flow like the surface of the sea. This fluidity, quantified as membrane viscosity, plays a crucial role in membrane transport and cellular functions. Viscosity measurements have largely relied on model membranesーartificial systems composed only of lipidsーdue to technical limitations. That is until now.

A collaborative research team from Tohoku University, the National Institute of Genetics, and Hokkaido University has successfully measured the viscosity of living cell membranes directly using a newly developed method.

"By applying force to the membrane and inducing flow across the entire cell - at the micrometer scale - we succeeded in measuring the flow patterns, finding that the viscosity of living cell membranes is four orders of magnitude higher than that of model membranes," explains Yuka Sakuma, an associate professor at the Graduate School of Science. "This increased viscosity arises from complex structures unique to living cells, such as the cytoskeleton and membrane proteins, which hinder membrane flow."

Based on these findings, Sakuma and her colleagues proposed a new concept: in addition to the well-known short-range viscosity derived from molecular thermal motion, living cells also exhibit a long-range viscosity that emerges from cell-scale structures.

"This discovery represents a significant advance in our understanding of the physical properties of living cells and is expected to contribute to future studies on cellular functions and pathological mechanisms," add Sakuma.

The next goal of the team is to compare the membrane viscosity of fertilized and unfertilized eggs.

Details of the findings were published in the Biophysical Journal, the academic journal of the American Biophysical Society, on September 5th, 2025.