Living Cells Respond to the Interface Tension of Soft Solids

For decades, scientists have known that living cells are sensitive to the hardness of the ground beneath them. Just as a person might walk differently on a paved road compared to a muddy field, cells change their shape, movement, and internal structure depending on whether they are sitting on a stiff surface or a soft one. This ability to feel and react to mechanical stiffness is called mechanosensing. It is a fundamental process that helps cells decide whether to grow, divide, or move, and it plays a critical role in how tissues form and how diseases like cancer spread. To study this, researchers typically grow cells on artificial surfaces that mimic the body's natural environment. Two of the most common materials used for these surfaces are hydrogels, which are water-rich gels similar to the tissue inside our bodies, and elastomers, which are rubber-like plastics.

However, a long-standing puzzle has persisted in the laboratory: cells behave differently on these two materials even when they are made to have the same measured stiffness. On a soft rubber surface, cells often act as if the material is much stiffer than it actually is, refusing to spread out as they would on a soft gel. For years, scientists suspected that factors like surface roughness, porosity, or water-repelling properties were to blame. But the true reason for this discrepancy remained hidden, leaving a gap in our understanding of how cells truly perceive their world.

A team of researchers at the University of Tübingen has now uncovered the missing piece of the puzzle. They discovered that the rubber-like surfaces used in these experiments possess a hidden physical property called interface tension. This is a force that acts along the very edge where the solid material meets the liquid environment, similar to the tension you feel on the surface of a drop of water. The researchers found that on soft rubber substrates, this interface tension is surprisingly strong, ranging from 10 to 20 millinewtons per meter. This force is so significant that it effectively stiffens the surface from the perspective of the cell. When a cell tries to push against a soft rubber surface, it encounters not just the softness of the rubber itself, but also this additional resistance from the interface tension. The result is that the cell senses a surface that is much harder than the rubber actually is.

To reveal this invisible force, the team used a specialized imaging tool called scanning ion conductance microscopy. Unlike traditional microscopes that might press down on a sample and squish it, this technique hovers just above the surface, measuring the flow of electricity through a tiny glass tip filled with liquid. By gently probing the rubber surfaces with this method, the researchers could measure how much the material deformed under a microscopic force. They found that while the rubber felt soft when measured with large-scale tools, it appeared remarkably stiff under the tiny tip of the microscope. This discrepancy occurred because the interface tension was resisting the deformation, making the soft rubber feel like a much harder material.

The researchers then tested whether removing this tension would change how cells behaved. They treated the rubber surfaces with a substance known as an emulsifier, which is designed to lower surface tension. After this treatment, the interface tension dropped significantly, falling to around 3 to 4 millinewtons per meter. When they grew cells on these treated surfaces, the behavior changed dramatically. The cells no longer acted as if the surface were stiff; instead, they spread out and behaved exactly as they would on a soft hydrogel. This confirmed that the cells were not reacting to the rubber's inherent hardness, but rather to the combined effect of that hardness and the interface tension.

By combining these observations, the team developed a simple physical model to explain the phenomenon. They determined that cells sense an "effective stiffness" that is the sum of the material's actual hardness and a value derived from the interface tension. This relationship depends on a specific length scale, which the researchers calculated to be about 3 micrometers. This size corresponds roughly to the scale of the tiny anchors cells use to grip onto surfaces. The model suggests that whenever a cell interacts with a soft material, it is essentially feeling a combination of the material's bulk stiffness and the tension at its surface.

This discovery reshapes how scientists should interpret experiments involving soft materials. It explains why cells on rubber substrates have historically behaved differently than those on hydrogels, even when the materials were intended to be identical in stiffness. The study shows that interface tension is a crucial, yet often overlooked, factor in mechanobiology. For researchers, this means that simply matching the stiffness of a material is not enough; they must also account for the tension at the surface. By understanding and controlling this property, scientists can create more accurate models of the human body, leading to better insights into how cells function in health and disease. The work highlights that the physical world cells inhabit is more complex than a simple measure of hardness, involving a delicate balance of forces that dictate life at the microscopic level.