Cells in our bodies move in groups during biological processes such as wound healing and tissue growth—but because of resistance, or viscosity, these cells can’t just slide past each other.
Or can they?
Using a pioneering method they developed to directly measure viscosity in a group of cells, University of Wisconsin-Madison engineers made a surprising discovery that upends our understanding of how cells move.
It’s called “negative viscosity” and it promotes cells instead of hindering them.
“This advance may allow researchers to develop better models of cell movement, which could lead to future applications in human health, such as ways to speed up wound healing or facilitate key processes in tissue development,” says Jacob Notbohm, an associate professor of mechanical engineering who led the research with doctoral student Molly McCord.
Notbohm and McCord discuss their findings in a paper published Dec. 4, 2025, in the journal PRX Life.
Cells generate forces that cause them to move, but it is not clear how the forces balance between groups of cells to generate movement. So McCord and Notbohm wanted to find a way to measure the viscosity in the system. the magnitude of viscosity has been a missing part of the equation for understanding the collective movement of cells.
In experiments, the researchers used optical imaging to analyze how a single layer of epithelial cells deformed a soft gel surface as they migrated across it. This allowed them to calculate how much force the cells produced.
McCord then developed a new approach to analyzing the data that involved examining various multicellular areas or groups of cells. Her analysis revealed that there were areas of cells where viscosity, unexpectedly, was negative.
“This surprising discovery of negative effective viscosity implies injection—rather than dissipation—of energy into the flow,” says Notbohm. “For example, if you were driving a car and the air had negative viscosity, air resistance would push the car forward instead of resisting it, which goes against the standard laws of physics.”
However, Notbohm says that negative viscosity is possible for systems with an energy source—like cells that convert nutrients into energy. Both he and McCord found that regions of cells with negative viscosity had increased metabolic activity—reflecting an increased energy demand on those cells.
“When we started this project, our question was how big is the number for viscosity,” says Notbohm. “But we now learned that we should be asking a different question: Is this number positive—or negative? This discovery redefines the problem and shows that it is important to treat this viscosity as positive or negative, which was not considered before.”
This work was supported by the National Science Foundation (grant no. CMMI-2205141) and the National Institutes of Health (grant no. R35GM151171).
